Ultrafast solvent-modulated roaming mechanism in bromoform revealed by femtosecond X-ray solution scattering

Domain: neuroscience
Published: 2026-02-09
Editorial outcome: major_revision

Abstract
Roaming-mediated isomerization is a universal reaction mechanism in photochemistry, yet solvent-dependent pathways of roaming intermediates remain poorly understood, particularly for environmentally relevant halogen compounds involved in ozone depletion. Here, using femtosecond time-resolved X-ray solution scattering, we resolve the solvent-dependent roaming dynamics of CHBr3 in methanol and methylcyclohexane. By combining multi-method experimental analysis, machine learning-assisted ab initio molecular dynamics simulations, and density functional theory calculations, we uncover distinct solvent-steered reaction pathways. In methanol, roaming enhances solute-solvent interactions, leading to solvolysis before a stable isomer forms. In methylcyclohexane, roaming facilitates isomerization to a long-lived iso-CHBr2-Br product. Direct dissociation into CHBr2 + Br competes with both pathways in either solvent. By tracking bond-length oscillations and angular dynamics in real time, we visualize how the condensed-phase environment governs the branching ratio between competing pathways. Our findings establish solute-solvent interactions as key factors controlling roaming-mediated reactions in CHBr3, with broad implications for photochemical outcomes in solution.

Introduction
Ultraviolet (UV) irradiation of poly-halomethanes releases reactive halogens, driving catalytic ozone depletion in the atmosphere. Among these compounds, bromoform (CHBr3) is the most abundant natural source of bromine (Br), a halogen that is over 100 times more destructive to ozone than other halogens1,2. As photoactivation is indispensable in Br release from CHBr3,3,4, understanding its photochemical reaction mechanism is essential for elucidating its role in atmospheric photochemistry. In the gas phase, CHBr3 undergoes UV-induced roaming-mediated isomerization in less than 500 fs—a non-traditional dissociation pathway where transiently dissociated fragments reorient via roaming trajectories to form a metastable species, iso-CHBr2–Br, before Br release5–8. First identified in formaldehyde, roaming has since been observed in triatomic and tetratomic molecules and has redefined mechanistic understanding across a broad range of photochemical systems9–15. A recent study even proposed roaming between water vapor and Criegee intermediates, linking it to atmospheric aerosol formation16.

However, the current mechanistic picture of CHBr3, derived largely from gas-phase studies, overlooks a critical environmental reality: CHBr3 is primarily emitted from marine biogenic sources, such as phytoplankton and macroalgae17,18. Consequently, a fraction of atmospheric CHBr3 resides in condensed-phase environments, including stratospheric aerosols and marine boundary layer droplets19–21, where solvent interactions are expected to significantly reshape photochemical pathways22–26. Indeed, ultrafast optical spectroscopic and theoretical investigations have revealed that the isomerization mechanism is highly dependent on the surrounding environment5–7,27–30. The iso-CHBr2-Br can be generated through roaming-mediated isomerization in a few hundreds of femtoseconds or through solvation cage induced germinate recombination of the CHBr2 and Br radicals on picosecond timescales5,28. In addition, the lifetime of the iso-CHBr2-Br relies strongly on the properties of solvents; it is metastable in non-polar solvents such as methylcyclohexane and cyclohexane from several nanoseconds to microseconds28, but relaxes quickly within one nanosecond in polar solvents such as acetonitrile28 and within a few picoseconds in CCl431.

While optical probes are valuable for tracking excited-state dynamics, their indirect sensitivity to structural changes imposes fundamental limitations. In contrast, time-resolved X-ray solution scattering (TRXSS) offers Ångström-scale spatial resolution, enabling direct visualization of transient structural dynamics during photochemical reactions32–35. Previous picosecond TRXSS studies of CHBr3 in methanol could not capture the iso-CHBr2–Br intermediate and instead revealed the formation of CH3OCHBr2 and HBr, suggesting an ultrafast solvent-mediated reaction pathway in which the isomer rapidly reacts with the solvent36. A similar reaction with water has been observed by time-resolved Raman spectroscopy with a timescale of 1.5 ps30. Conversely, the isomer remains stable in methylcyclohexane, as confirmed by our picosecond TRXSS results and previous transient optical spectroscopic studies28,36. These previous studies have shown that the stable or hot CHBr2-Br isomer can undergo an O-H insertion/HBr elimination reaction with CH3OH or H2O. However, the reported lifetimes of this isomer, less than 1.5 ps in H2O30 and less than 100 ps in methanol36, remain ambiguous. This ambiguity arises partly from the limited temporal resolution of synchrotron-based TRXSS (~100 ps), which cannot access the sub-picosecond timescales where key branching events are likely to occur.

Here, we employ femtosecond TRXSS at the European X-ray Free-electron Laser (EuXFEL), in conjunction with machine-learning-supported ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations, to directly visualize the photochemical dynamics of CHBr3 in methanol and methylcyclohexane. Our TRXSS results, which are directly sensitive to molecular structures of reacting species in solution, confirm that roaming dynamics initiates universally at the onset of photochemical reaction in both solvents. However, solvent-specific interactions lead to distinct outcomes. In the hydroxyl-rich environment of methanol, solute-solvent interactions quench the roaming-mediated pathway of the hot CHBr2-Br isomer, preventing it from forming the equilibrium stable iso-CHBr2, the hot CHBr2-Br isomer then undergoes subsequent methanolysis reaction, producing secondary CH3OCHBr2 and HBr30,37–39. In contrast, the nonpolar environment of methylcyclohexane stabilizes the roaming-mediated isomers without solvolysis26,33. By determining a ~385 fs lifetime for the hot CHBr2-Br isomer in methanol, our findings reveal how solvent interactions steer CHBr3’s photochemical trajectories from the first few hundreds of femtoseconds, providing atomic-scale insight into how condensed-phase environments determine the fate of photochemical products.

Results
We excited CHBr3 in methanol and methylcyclohexane using 267 nm UV pulses and conducted TRXSS measurements at the FXE instrument of the EuXFEL (Fig. 1)40,41. Photoexcitation of CHBr3 pumps it from the ground state (S0) to a mixed manifold occupied by both the singlet (S1) and triplet (T, and possibly higher b̃3E(T1) and c̃3A1(T1)) states, with S1 likely dominating. Both S1 and T1 states are repulsive towards CHBr2· + Br·, with the former having a stronger dissociative character. The T1 state is populated either directly upon excitation or indirectly via an intersystem crossing (ISC) from the S1 state and internal conversion from possible higher triplet states1,5,8,28,42–45. From T1, the molecule undergoes structural rearrangement leading to alternative isomeric products5. Experimental details are provided in the Supplementary Information (SI).

The TRXSS signals of photoexcited CHBr3 in methanol show clear shifts in momentum transfer (q) within the first ps (Fig. 2a), indicative of ultrafast structural rearrangements. Linear combination fitting (LCF) of theoretical solute/cage signals and experimental solvent scattering to the experimental data33 rules out isomer formation and identifies CH3OCHBr2 and HBr as the primary products after 400 fs (Supplementary Fig. 3; see Supplementary Note 3 for LCF details). The concentration profiles (Supplementary Fig. 3b) reveal ultrafast photodissociation of CHBr3 upon excitation, generating CHBr2 and Br within 150 fs. These species decay rapidly, while the methanolysis products CH3OCHBr2 and HBr (CHBr2* + CH3OH → CH3OCHBr2 + HBr) start to appear at delays >200 fs. This time offset implies the involvement of transient intermediates between excited CHBr3* and the methanolysis products, suggesting that the early reaction dynamics extend beyond simple photodissociation and methanolysis pathways. After 300 fs, the decay of CHBr2 and Br slows down markedly, coinciding with a gradual accumulation of CH3OCHBr2 and HBr at a comparably reduced rate. This correlation suggests that methanolysis is driven by CHBr2 and Br at late time delays. By 1 ps, the concentration of the majority of species begin to stabilize, with the remaining slower methanolysis component persisting for several picoseconds. Notably, the formation of CH3OCHBr2 and HBr exhibits biphasic kinetics within the first picosecond, suggesting multiple types of mechanisms in the reaction pathway.

To further elucidate competing photodissociation pathways, we applied Singular Value Decomposition (SVD) to the TRXSS data of photoexcited CHBr3 in methanol (see Supplementary Note 4 for SVD details). The analysis yielded three characteristic timescales: τ1 = 462 ± 45 fs, τ2 = 386 ± 25 fs, and τ3 = 4.75 ± 2.4 ps, with an instrument response function (IRF) of 151 ± 22 fs (Supplementary Figs. 7, 8). The first component (τ1 ≈ 460 fs) corresponds to bulk solvent heating, showing good agreement with both our LCF analysis (471 fs; Supplementary Fig. 5) and previously reported value46. The second, somewhat shorter timescale (τ2 = 386 fs), may be associated with a roaming-mediated process of CHBr3, potentially involving transient structural evolution prior to methanolysis. This timescale is comparable to values reported for similar roaming dynamics46–48. The longest timescale (τ3 = 4.75 ps) appears to correspond to a slower methanolysis pathway involving diffusing CHBr2 and Br fragments. In this case, fragment separation may enable stochastic solute–solvent encounters that ultimately lead to the formation of CH3OCHBr2 and HBr.

Preliminary analysis using LCF and SVD suggested the presence of multiple reaction pathways and provided initial estimates for their lifetimes and branching ratios. Based on these parameters and according to the proposed scheme in Fig. 2b, a kinetic model was constructed, which served as the basis for global fitting analysis (GFA, see Supplementary Note 5 for details). By fitting TRXSS data to this model, GFA refined the initial estimates and offered further support for the proposed framework. Photoexcitation triggers ultrafast C–Br bond cleavage (<150 fs), producing CHBr2 and Br, which subsequently bifurcate into two competing pathways: (1) Roaming-mediated methanolysis (52 ± 18% branching), where CH3OCHBr2 and HBr are formed within 385 ± 80 fs; (2) Direct dissociation (48 ± 16% branching), where 65% of CHBr2 and Br undergo solvent-cage-induced methanolysis in 4.4 ± 0.9 ps, while the remaining 35% persist beyond the nanosecond timescale27,28,36. The dissociation-to-methanolysis ratio (1:2.5) at 1 ps is comparable to previous synchrotron-based TRXSS results36. The kinetic traces derived from GFA show good agreement with those obtained from LCF (Supplementary Fig. 11a) as well.

Previous study indicated that CHBr3 isomerization passes through the S1/S0 conical intersection in ~29 fs5, while our data illustrate that the resulting hot CHBr2-Br isomer reacts with methanol at ~385 fs. This prolonged timescale confirms that the methanolysis reaction occurs after the system undergoes an electronic transition from the excited state to the S0 ground state.

With the kinetic model, we aimed to characterize the transient intermediates bridging the excited CHBr3* and the methanolysis products. These hot intermediates are likely not at the local minimum on the potential energy surface (PES), which are challenging to model theoretically. To address this point, we performed atomic motion fitting (AMF) of the TRXSS data over the critical first 600 fs with numerically constructed transient structures involving various geometries along the reaction PES8. This AMF approach (see Supplementary Note 6 for details) enabled direct insight into ultrafast structural reorganizations along distinct photoreaction coordinates. Specifically, we considered structural configurations representative of the three proposed mechanistic channels — roaming-mediated methanolysis, direct dissociation, and cage-induced methanolysis — as guided by the GFA results. AMF was focused on transient changes in key reactive structural motifs involving carbon and bromine atoms, including C–Br bond lengths, Br–C–Br bond angles, and Br–C–Br–Br dihedral angles8. To comprehensively explore the relevant configurational space, chemically reasonable ranges were defined for each degree of freedom. A random sampling approach was employed to generate ~1,000 structurally diverse geometries within these constraints46.

The dissociation pathway exhibits two distinct behaviors (Fig. 2c). In a direct dissociation reaction, C–Brc increases from 1.94 Å to ~6.5 Å by 150 fs. In cage-induced methanolysis, limited C–Brc separation (~4.5 Å at 200 fs) and unexpected contraction to ~3.5 Å by 600 fs illustrate the collisions between the fragments and solvent molecules, facilitating solvent-mediated reorganization to CH3OCHBr2 and HBr. In roaming-mediated methanolysis (Fig. 2d), the ground state CHBr3 (C–Brc: 1.94 Å; Brb–Brc: 3.22 Å; C–Brb–Brc: 34°) undergoes ultrafast structural reorganization within 100 fs after excitation: C–Brc bond elongates to 3.75 Å, while Brb–Brc contracts to ~2.6 Å, accompanied by a sharp expansion of the C–Brb–Brc angle to 108°. These changes in internal coordinates indicate that the CHBr2 and Br fragments undergo a roaming motion, which forms a hot CHBr2-Br isomer. Subsequent oscillations (100–600 fs) in bond lengths (C–Brc: 3-3.8 Å; Brb–Brc: 2.2-3 Å) and C–Brb–Brc angle (80–120°) of the hot CHBr2-Br isomer enhance methanol encounters, progressively driving methanolysis. Similar roaming-mediated behaviors have been observed in previous studies5,8. Detailed structural dynamics of CHBr3 in methanol at early time delays are displayed in Supplementary Fig. 13.

The transient population dynamics of the key structures involved in the three mechanistic pathways are shown in Fig. 2e. The concentrations of the representative structures associated with the direct dissociation and roaming-mediated methanolysis pathways rapidly stabilized at 0.5 mM and 0.8 mM, respectively, within 200 fs. In contrast, the population associated with the cage-induced methanolysis pathway gradually increased to ~0.3 mM at 300 fs, suggesting a slower interaction with the cage environment. The combined AMF-GFA analysis resolves the transient structural dynamics during the reaction’s critical initiation phase (Supplementary Fig. 14). This approach provides substantially better agreement between theoretical and experimental scattering signals (Supplementary Fig. 14c), with residuals (Supplementary Fig. 14b) markedly reduced at early times compared to pure global fitting (Supplementary Fig. 11c).

Figure 3a shows the experimental TRXSS signal of photoexcited CHBr3 in methylcyclohexane. The LCF analysis (Supplementary Fig. 4) reveals the formation of the CHBr2-Br isomer and direct dissociation products CHBr2 + Br, with both populations peaking within 400 fs. Notably, the CHBr2-Br isomer appears ~100 fs later than the dissociation products, indicating precursor-mediated isomer formation49. This time delay results from the formation of the T1 potential energy surface, which is populated either directly upon excitation or indirectly via ISC from S1 state and internal conversion from higher triplet excited states within the mixed excitation manifold. The concurrent temporal evolution of the product concentrations demonstrates that the growths of the dissociated radicals and the CHBr2-Br isomer are coupled over picoseconds.

We performed SVD analysis of the TRXSS data in Fig. 3a, isolating a dominant component (Supplementary Fig. 9). Fitting the corresponding first right singular vector (RSV) (Supplementary Fig. 10) gave an IRF of 153 ± 33 fs, matching our methanol measurements. Two lifetimes were resolved: a fast component (<150 fs) and a slower one (373 ± 66 fs), corresponding to different product formation timescales.

The fittings (solid lines, Supplementary Fig. 4) of the LCF-derived kinetics for the two reaction pathways confirm photoproduct formation within the instrument resolution (150 fs). This observation rules out a stable precursor and supports a roaming-mediated isomerization mechanism involving transient structural rearrangements26,46, although slightly delayed compared to the competing direct dissociation. The fitted lifetime agrees with the shorter component resolved by SVD. LCF further quantified solvent heating dynamics, revealing a single-exponential temperature rise (408 ± 27 fs, Supplementary Fig. 6), which matches the slower SVD component (373 ± 66 fs). Together, LCF and SVD analysis show that both dissociation and isomerization proceed on the ultrafast timescale, though isomer formation involves a more complex pathway, with a delayed appearance due to roaming-mediated rearrangements (Supplementary Fig. 4). The observed ~100 fs delay reflects the early-time electronic and nuclear dynamics of the spin-mixed n(Br)→σ*(C–Br) excited-state manifold populated at 267 nm. Following photoexcitation, the wavepacket rapidly departs the Franck-Condon region, undergoes ultrafast spin equilibration and internal conversion among nearly degenerate singlet and triplet states, and approaches regions of strong nonadiabatic coupling between T1 and S0. Formation of the CHBr2-Br roaming configuration thus occurs only after this joint electronic–nuclear evolution, naturally accounting for the measured temporal offset.

To probe the transient structural evolution leading to isomer and radical formation, we analyzed the TRXSS data within the first 600 fs using AMF method (Supplementary Fig. 18). In the direct dissociation pathway (Fig. 3c), C–Brc bond elongation occurs in three distinct phases: 0-50 fs, rapid elongation from 1.94 Å to 4.39 Å (0.049 Å/fs), approaching the C–Br dissociation threshold. 50–150 fs, slowed extension to 5.8 Å (0.0141 Å/fs), indicating solvent drag effects. 150-300 fs, further deceleration to 6.5 Å (0.0047 Å/fs) as energy dissipates through solvent collisions. Beyond 300 fs, the separation distance of C–Brc reaches a plateau, indicating loss of geminate correlation between CHBr2 and Br, which is in close agreement with LCF-derived product formation timescales.

In contrast, isomerization proceeds via changes in three primary coordinates: C–Brc and Brb–Brc distances, and the C–Brb–Brc angle (Fig. 3d). 0–50 fs, the angle increases from 34° to 57° (0.46°/fs), while C–Brc elongates to 3.31 Å (0.0274 Å/fs) and Brb–Brc expands to 3.93 Å (0.0142 Å/fs), consistent with an early-time reorientation of the CHBr2 and Br fragments. 50-100 fs, the angle rapidly rises to 118° (1.22°/fs), while Brb–Brc sharply contracts to 2.4 Å (-0.031 Å/fs), forming a hot CHBr2-Br isomer. 100-150 fs, the angle decreases to 98° (–0.46°/fs), and Brb–Brc modestly increases to 2.67 Å (0.0054 Å/fs), indicating early stabilization of the hot CHBr2-Br isomer. The observed non-monotonic dynamics trace a roaming-mediated transition to a vibrationally hot configuration by 150 fs (C–Brc ≈ 3.44 Å, Brb–Brc ≈ 2.67 Å, ∠C–Brb–Brc ≈ 95°), thereby confirming that a delayed structural rearrangement precedes and ultimately leads to the formation of a hot isomer. Our observations are consistent with previous reports on roaming-mediated dynamics in similar systems5,8. The subsequent weak oscillations (150–600 fs) reflect vibrational relaxation around the stabilized isomer.

The roaming dynamics on T1 ultimately relax to the ground-state (S0) surface via T1/S0 interaction, as evidenced by our DFT calculations (Supplementary Fig. 27; see Supplementary Note 8 for details). Our structural analysis shows that the Br atom and CHBr2 fragment roaming most likely proceeds on T1 surface within the first 100 fs and yields a hot CHBr2-Br isomer by ~150 fs in the S0 state. Consequently, the weak oscillations observed from 150 to 600 fs, which reflect vibrational relaxation around the stabilized isomer, are assigned to dynamics on the S0 potential energy surface.

AMF also quantifies the temporal evolution of key structural populations (Fig. 3e). Both pathways contribute simultaneously at early time delays, reconciling the initial temporal (~100 fs) offset between isomerization and dissociation. The excellent agreement between LCF, AMF, and experimental TRXSS data (Supplementary Fig. 18) robustly support these mechanistic conclusions.

To elucidate the solvent-dependent roaming behavior of CHBr3 observed experimentally, we complemented our studies with ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations in methanol and methylcyclohexane. These calculations offer atomistic insights into the mechanistic origins of the solvent-controlled reactivity contrast.

We employed the ab initio molecular dynamics (AIMD) based on the unrestricted Kohn-Sham method, combined with machine learning potentials, to probe the triplet excited-state (T1) dynamics of CHBr3 in methanol and methylcyclohexane5,28,45 (see Supplementary Note 7 for details). Our simulations were conducted on the T1 excited state due to the ultrafast ISC from S1 to T1, which makes the triplet excited state the dominant channel for the subsequent photochemical dynamics, specifically relevant for the roaming process. Correlation plots of interatomic distances, specifically Brb–Brc versus Bra–Brc (Fig. 4a, b), effectively capture the trajectory-dependent behavior of the dissociating bromine atom (Brc). Trajectories along the diagonal correspond to symmetric dissociation, where Bra and Brb remain equidistant from Brc, indicative of direct dissociation and negligible roaming. Off-diagonal excursions indicate a transient asymmetry in the Br-Br separations, signifying a reorientation of both the CHBr2 and Br fragments. These excursions are clear signatures of CHBr2-Br isomer structures formed via roaming-mediated dynamics, and their subsequent oscillations represent the vibrational motion of these transient intermediates5,8.

In methanol (Fig. 4a), simulations reveal a rapid dissociation of the Br radical from the CHBr2 fragment. A representative trajectory (red trace) illustrates structural evolution at 50 fs (point B), 200 fs (point C), and 300 fs (point D). Within the first 50 fs, rapid Br dissociation occurs, followed by distinctive roaming behavior that leads to transient configurations. Notably, this involves an expansion of the C–Brb–Brc angle from 32° to 104° and an increase in the Bra–Brc distance from 3.2 Å to 5.4 Å, indicative of roaming-mediated reaction. Although explicit solvent coordination between CHBr3 and methanol was not directly modeled due to computational constraints, the simulated structural dynamics align well with the experimental observations (Supplementary Fig. 21). The simulation also illustrates a significantly smaller solvation cage for CHBr3 in methanol (Supplementary Fig. 23), indicating a stronger solute-solvent interaction. These AIMD results suggest that vibrational motions of isomer-like structures facilitate methanolysis by promoting solute–solvent collisions.

Simulations of CHBr3 in methylcyclohexane (highlighted in red in Fig. 4b) reveal pronounced roaming-mediated isomerization. By 240 fs, the geometry converges distinctly toward iso-CHBr2-Br, consistent with the experimentally determined isomerization timescale. Simultaneously, in the direct dissociation channel, the Brb–Brc distance increases from 3.2 Å to 5.4 Å by 200 fs, while the C–Brc distance shortens from 4.7 Å (100 fs) to 4.0 Å (200 fs), indicating rotation of the CHBr2 fragment around the Bra–Brb axis. A detailed comparison between AIMD simulations and AMF results further confirms our experimental findings (Supplementary Fig. 22).

To evaluate the influence of solvent-cage environments, we analyzed the simulated radial distribution function (RDF) of ground-state CHBr3 in methanol and methylcyclohexane (Supplementary Fig. 23). The cage radius in methanol (3.1 Å) is significantly smaller than in methylcyclohexane (5.66 Å), indicating a more confined environment in methanol. This distinction explains the observed mechanistic differences: the spacious, gas-phase-like cage in methylcyclohexane facilitates roaming-mediated isomerization, while the compact hydroxyl-rich environment of methanol hinders relaxation of the hot CHBr2-Br isomer to the equilibrated ground-state iso-CHBr2 and instead promotes methanolysis.

DFT calculations reveal the energetic landscape of the roaming-mediated methanolysis reaction (Fig. 4c). The dissociated Br atom dynamically reorients relative to the CHBr2 radical and the methanol molecule, eventually forming a stable reaction complex (RC). This RC overcomes a modest 1.8 kcal/mol energy barrier via a transition state, yielding CH3OCHBr2 and HBr (Supplementary Figs. 24–26 and Supplementary Table 2 for detailed structural parameters and reaction dynamics). Together with experimental results, these computations establish a unified mechanistic picture that explains the solvent-dependent competition between dissociation, isomerization, and methanolysis pathways.

Methods
Femtosecond time-resolved X-ray solution scattering (fs-TRXSS)
Time-resolved X-ray solution scattering (TRXSS) experiments were performed at the FXE instrument of the European XFEL using a pump-probe configuration40,41. Solutions of bromoform in methanol or methylcyclohexane (40 mM) were used to probe reaction dynamics, while 4-bromo-4′-(N,N-diethylamino)-azobenzene (2 mM) in the same solvents was used to measure solvent heating signals50. Photoexcitation was achieved with 267 nm laser pulses of 70 fs duration, focused to 70 × 70 μm2 (FWHM) at the liquid jet. X-ray probe pulses (~50 fs) were focused to 15 × 15 μm2 (FWHM), and scattered signals were recorded using a Large Pixel Detector (LPD) positioned 238 mm from the jet. Momentum transfer was defined as q = 4πsin(θ)/λ (where θ is the scattering angle and λ the X-ray wavelength) and reached 6 Å⁻1. Two-dimensional scattering patterns of CHBr3 in methanol or methylcyclohexane were collected over time delays ranging from −0.2 ps to 1 ps with a step size of 50 fs. To probe the long-time kinetics, scattering data were acquired at selected longer delays up to 10 ps (Supplementary Figs. 3 and 4). Two-dimensional scattering patterns of the dye were recorded at a fixed delay of 10 ps. Each dataset was measured in five independent runs to ensure reproducibility. Detailed information on data acquisition is provided in Supplementary Note 1.

Extraction of difference scattering signals
One-dimensional scattering profiles S(q) were obtained by azimuthal integration of 2D scattering maps. Difference scattering signals ΔS(q) were computed for each laser-on/off pulse pair and averaged across all pairs to give the final difference profile. The averaged ΔS(q) was weighted by q (as qΔS(q)) to highlight high-angle features.

2D maps were decomposed into isotropic and anisotropic components using a second-order Legendre polynomial expansion51. The isotropic component reflects solute structural changes and solute–solvent interactions (Figs. 2a, 3a). The anisotropic component reflects ultrafast solvent reorientation along the laser polarization axis (Supplementary Fig. 1a, c). The temporal evolution of the anisotropic signals was fitted with a Gaussian–exponential model to determine solvent reorientation times and the instrument response function (IRF) (Supplementary Fig. 1b, d). Details of signal extraction and processing are provided in Supplementary Note 1.

Linear combination fitting analysis of difference scattering signals
The isotropic difference scattering signals were first analyzed using Linear Combination Fitting (LCF) to extract the time-dependent coefficients of different transient species. The data were modeled as a linear combination of solute, cage (the solute-solvent interaction), and solvent-heating contributions. The solvent-heating term ∂S/∂T was derived from the TRXSS measurement of a zo-dye50. Theoretical scattering curves for all species were calculated based on density functional theory optimized structures and molecular dynamics simulated cages36,52–55. LCF was performed at each time delay by minimizing the figure of merit (χ2) following the formula:

χ2 = Σn i=1 [(ΔS experiment i - ΔS theory i) / σ i]2

where σi is the standard deviation of the scattering signal at the i-th q value for the given time delay. The full description of the LCF strategy is provided in Supplementary Note 3.

Singular value decomposition analysis of difference scattering signals
Singular value decomposition (SVD) was applied to the isotropic TRXSS data to determine the number of statistically significant components and their associated kinetics. The significance of components was evaluated through singular value magnitudes and the autocorrelation of the singular vectors. The right singular vectors (RSVs) were modeled using multi-exponential functions convolved with IRF. Further details on the SVD and RSV fitting are provided in Supplementary Note 4.

Global fitting analysis of difference scattering signals
Global fitting analysis (GFA) was performed to construct a kinetic model that consistently describes the TRXSS signals of CHBr3 in methanol across all momentum-transfer space and time-delay ranges. The kinetic model was constructed by integrating LCF concentration profiles with time constants identified by SVD. This approach provides physically informed constraints on both the reaction pathways and the associated time scales. The model treats roaming-mediated isomerization and direct bond dissociation as competing parallel channels, therefore containing multiple adjustable parameters, including the branching ratio between the two dissociation pathways, the characteristic time constants associated with secondary reaction steps, and the fraction of unreacted species. A total of 500 sets of initial parameters for the model were randomly sampled within these bounds and used as initial points for global fitting. Detailed kinetic fitting process and the parameter sampling strategy are described in Supplementary Note 5.

Atomic motion fitting of early (≤600 fs) difference scattering signals
To resolve ultrafast structural evolution, Atomic Motion Fitting (AMF) was employed to refine transient molecular geometries within the first 600 fs. Transient structures were constructed by varying heavy-atom positions (C and three Br atoms) while constraining the hydrogen atom, which has a negligible X-ray scattering contribution. This strategy reduces the number of independent degrees of freedom while preserving the physically relevant structural changes. The geometry was parameterized using spherical coordinates to maintain molecular symmetry. Structural models representing the distinct pathways identified by GFA were refined by minimizing the figure of merit χ2. Structural uncertainties were estimated by randomly sampling 1000 initial structures at each time delay and retaining optimized geometries with χ2 values within a narrow threshold within 1.05 times the global minimum46. A comprehensive description of the structural representation and fitting constraints is provided in Supplementary Note 6. All LCF, SVD, GFA, and AMF were performed using MATLAB R2024a.

Ab-initio molecular dynamics simulations
Ab initio molecular dynamics (AIMD) simulations were performed to investigate the photochemical dynamics of CHBr3 in solution. Electronic-structure calculations were carried out using the van der Waals–corrected hybrid functional PBEh(40%)-rVV1056. Simulations were carried out for CHBr3 in methanol and methylcyclohexane at room temperature, considering both the electronic ground state and the triplet excited state.

Machine-learning potentials based on the MACE framework57 were trained on configurations sampled from ground-state and triplet-state (T1) AIMD simulations and used to enable long-timescale molecular dynamics. Ground-state trajectories of CHBr3 solvated in methanol and methylcyclohexane were propagated for 100 ps under NPT conditions at 300 K and 1 bar. Excited-state dynamics were initiated from configurations sampled along the equilibrated ground-state trajectories and propagated on the triplet-state potential energy surface for 2 ps in the NVE ensemble. Molecular dynamics simulations were performed with i-PI, and electronic-structure calculations were carried out using CP2K. Details of the simulation parameters are provided in Supplementary Note 7.

Density functional theory (DFT) calculations
DFT calculations were performed to investigate the roaming-mediated reaction between bromoform and methanol. Geometry optimizations, vibrational frequency analyses, and transition-state searches were carried out using Gaussian 16 at the CAM-B3LYP/6-311++G(3df,3pd) level of theory58–60 (Supplementary Fig. 24). Transition states were validated by frequency and intrinsic reaction coordinate (IRC) calculations connecting the corresponding reactants and products (Supplementary Figs. 25, 26). Potential energy profiles along the IRC were calculated for the S₀ and T₁ surfaces of CHBr3 at the same level of theory (Supplementary Fig. 27). Detailed computational procedures and structural parameters are provided in Supplementary Note 8.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Ultrafast solvent-mediated roaming...\nSu et al\nThis paper compares the dissociation dynamics of bromoform in methanol and methyl cyclohexane, polar and non-polar\nsolvents, using ultrafast time-resolved X-ray solution scattering (TRXSS). Roaming dynamics are found to play an important\nrole in both, but distinct behavior is seen with methanol undergoing reaction facilitated by roaming, while the methyl\ncyclohexane case shows a long-lived intermediate formed by roaming-mediated isomerization.\nRoaming reactions are the subject of considerable ongoing interest with broad implications across many aspects of\nchemical reaction dynamics. Roaming reactions in the condensed phase are far less studied as the techniques required are\nchallenging and the situation far more complex than in the gas phase. This paper represents a spectacular example of how\nto meet these challenges. Although the results from diffraction experiments are low--dimensional and underdetermined,\nwhen combined with state-of-the-art theory and careful analysis, deep insight can be obtained with some confidence.\nThe quality of the scientific investigation, the analysis, and the clarity of the presentation make this paper eminently suitable\nfor Nature Comm. I have one concern I think the authors should address. Almost nothing is said about the photoexcitation\nprocess, the excited state lifetime or the decay dynamics. In the SI I find they are modeling the triplet, which I presume is the\nrelevant excited state, but this should be stated and justified in the main text. Then it seems they are not actually doing any\nnonadiaatic dynamics, rather, for the excited state dynamics they sample from configurations on the ground state over the\n100 ps of the simulation. Are the AIMD calculations entirely in the triplet state? Clearly the methanolysis reaction is on the\nground electronic state. Some clarity on this point, in the main text as well as the SI, would be helpful. Once the authors\naddress this point I recommend publication.", "reviewer_2": "Ultrafast solvent-modulated roaming mechanism in bromoform revealed by femtosecond X-ray solution scattering\nAuthors: by P. Su et al.\nBy combining the results from femtosecond time-resolved x-ray scattering, machine learning, DFT and molecular dynamics,\nthe authors came to conclusion that CHBr3 undergoes roaming-mediated structural dynamics in both methanol and\nmethylcyclohexane. To my knowledge, it is the first work (after ref. [3]) which gives support to roaming-mediated structural\ndynamics in CHBr3. The significance of this reaction mechanism is very high, and the results are noteworthy and significant.\nThe manuscript may be publishable, but it requires major revisions. The revised manuscript should be reviewed again. My\ndetailed comments are below, please see #1-17.\nOverall, the Conclusions (line 290) state that “CHBr3 undergoes roaming-mediated structural dynamics within 150 fs in both\nmethanol and methylcyclohexane”, but the oscillatory, non-monotonic angle/bond distance behavior (taken to be signatures of\nroaming, #9) seems to be present within the first 600 fs (see #17). In the original work on CHBr3, ref. [3] it is a light C atom\nof a CHBr2 fragment that roams, but in the manuscript, despite bong angle and bond distance change being similar to what\nwas described in ref. [3], it seems to be proposed that it is the Br-atom fragment that roams. Br is heavy, and much heavier\nthat a hydrogen atom roaming of which in formaldehyde in the gas phase was initially suggested by Bowman, so forces to\nmake it wander should be much, much stronger. This is hard to believe, so the difference C vs. Br should be explained. The\nmanuscript data clearly show, in my opinion, the formation of CHBr2-Br, which is the isomer (the structure on the isomer\npotential) in both methanol and methylcyclohexane, so this primary pathway, which is isomerization, is the same in both\nsolvent environments, Fig, 2b,d vs. Fig. 3b,d, and Fig. 4a,b. Nevertheless, the Abstract says that “In methanol …it is\nsolvolysis without isomerization”, lines 35-36. Secondary products are different in methanol as hot CHBr2-Br undergoes a\nfacile reaction with solvent; this has been well known, #5. Solvent coordination is a key parameter for roaming reactions,\nlines 40-41, but the meaning of solvent coordination is unclear (H-bonding with CHBr3?) and no evidence is given.\nComments\n1. Abstract, lines 35-36\n“In methanol, roaming enhances solute-solvent interactions, leading to solvolysis without isomerization.” This statement\nappears further (lines 277-278): “ the compact hydroxyl-rich environment of methanol hinders isomerization and instead\npromotes methanolysis”.\nThe formation of CHBr2-Br, which is the vibrationally hot structure on the isomer potential in clear both methanol and\nmethylcyclohexane, Fig, 2b,d vs. Fig. 3b,d, and Fig. 4a,b. It is the formation of the equilibrated ground-state iso-CHBr3 that\nthe methanol environment ‘hinders’.\n2. Abstract, lines 40-41\n“Our findings establish solvent coordination as a key parameter for roaming reactions, with broad implications for\nphotochemical outcomes.” However, lines 261-262 state: “Although explicit solvent interactions between CHBr3 and\nmethanol were not directly modelled…”\nI wonder than how one can state that solvent coordination as a key parameter if explicit solvent interactions were not\nmodelled? The experimental data in the manuscript are silent regarding solvent coordination, apart from the statement\n(based on simulation of the radial distribution function of ground state CHBr3, lines 273-274), that the cage radios in\nmethanol is significantly smaller than in methylcyclohexane. How this related to solvent coordination being a key parameter\nneeds to be explained.\n3. line 47\n“As photoactivation is indispensable in Br2 release from CHBr3…”. Is this a unimolecular reaction? Has it been ever\nexperimentally observed? Reference(s) must be given.\n4. lines 65-68\n“…the lifetime of the iso-CHBr2-Br …. relaxes within a few picoseconds in neat bromoform, ref. [3]”. I do not think that ref. [3]\nis relevant for iso-CHBr2-Br relaxation in neat bromoform. Ref. [25] is a poor choice in my opinion because most\nobservations in that work were due to a Br-atom interacting with the surrounding CHBr3 solvent on a sub-ps time scale.\nHowever, ref. [3], and ref. [26] specifically, are relevant: iso-CHBr3 being metastable in methylcyclohexane and the lifetime\nof iso-CHBr3 being within one nanosecond in acetonitrile. Instead of ref. [25], I recommend the authors to refer to the\nsubsequent paper by Crim and co-workers, J. Phys. Chem. A 2013, 117, 14, 2899\n(https://pubs.acs.org/doi/abs/10.1021/jp310737d), where the deficiencies of ref. [25] were addressed and the conclusion\nbecame very similar to those in ref. [26].\n5. lines 78-79\nThe authors state that “there is the ultrafast solvent-mediated reaction pathway” (in methanol) “in which the isomer rapidly\nreacts with the solvent”, referring to their previous work, ref. [33] and to time-resolved Raman spectroscopy results by Phillips\ngroup, ref. [28]. They continue: “…the mechanistic origin of the contrasting solvent-dependent reactivity remains unsolved.”\nHowever, in my opinion, the mechanistic origin has already been firmly established: isomers of di- and polyhalogenated\nalkanes (X = Br, I) readily react with CH3OH via O-H insertion/HX elimination reactions. This reaction may be fast, so that\nthe formation of the product species is already observed on at 1.5 ps after excitation (ref. [28], CHBr2-Br or iso-CHBr3 in\nmethanol), or slower, e.g., iso-CHI3 decays with a 740-ps lifetime by reacting with CH3OH via O-H insertion/HI elimination\nreactions, ChemPhysChem, 2009, 10, 1895, doi: 10.1002/cphc.200900122, ref. [34] in the current manuscript. Radical\nspecies react via the same reaction but much slower, ref. [28, 34] and other work by Phillips group. So, the mechanistic\norigin is established; what is not determined is the iso-CHBr3 lifetime in methanol. Apparently, it is < 1.5 ps.\n6. lines 86-91\n“…our results reveal that roaming dynamics initiates universally at the beginning of photochemical reaction …”\nThis has been shown before, ref. [3,4], and with superior temporal resolution.\n“However, solvent-specific interactions leas to distinct (probably distinctly different is meant here?) outcomes. The hydrogen-\nbonding capacity of methanol mediates proton transfer, quenching isomerization via solvent interactions, ref. [28,34-36]\nwhereas the non-polar environment of methylcyclohexane stabilizes roaming-mediated isomers, ref. [26,33].”\nThere is some confusion here. The authors should distinguish between different products, perhaps, called primary and\nsecondary. Their Abstract said there is no isomerization in methanol, but the sentence above reads as if there is\nisomerization, but it is quenched. Figure 2b,d (methanol) and Figure 3b,d (methylcyclohexane) show the isomer-like\nstructures CHBr2-Br, which look very similar: C-Brc and Brb-Brc distances look similar. Do the authors restrict themselves to\ncalling only the equilibrium CHBr2-Br structure as the isomer? The CHBr2-Br structures shown in Figure 2b and Figure 3b\nare certainly located on the isomer potential energy surface (hot isomer). This is the primary product species in both\nsolvents, along with the radical pair. In methanol, the isomer undergoes the methanolysis reaction, producing secondary\nCH3OCHBr2 and HBr products, whereas this reaction is not viable in methylcyclohexane.\n7. line 118\n“By 1 ps, the concentration of all species tends to stabilize”.\nThis is surprising, and unexpected. The radical species remained confined to the same solvent cage within several\npicoseconds, ref. [26], and are expected to undergo cage-induced recombination (primary recombination). Also, they can\nundergo cage-induced isomerization on the same timescale. Do the authors suggest that these reactions are absent?\nFurther, the SVD analysis (lines 129-140) finds a significant (one of the two related to the reaction dynamics) component\nwith  3 = 4.75 ps, which the authors ascribe to a slower methanolysis pathway. How then “all concentrations tend to stabilize\nby 1 ps” if there is a 4.75-ps component? How reliable is the 4.75 ps determination: the experiments were performed only\nto a 1 ps delay time (Supporting Information, line 47).\n8. lines 131-133\nFor CHBr3 in methanol,  1 = 462 45 fs and  2 = 386 25 fs were ascribed to bulk solvent heating and “a roaming-mediated\nprocess of CHBr3”. These time constants are more or less within the error bar. How reliably they can be differentiated and\nassigned?\nFurther, with 9 microJ/pulse excitation energy focused into a 70x70 micrometer2 area, I would expect significant\nphotoproduction of solvated electrons. Their solvation and thermalization occur on a 150 fs and ~5 ps time scales, e.g., J.\nChem. Phys. 2015, 142, 234501 and references therein. These processes do not happen when azo dyes are used to obtain\nthe solvent heating signals (azo dyes were used to obtain the solvent heating signals in the current work, Supporting\nInformation, lins 49-50). How reliable is then the solvent heating subtraction? Significant solvent photochemistry at this\nexcitation level should also occur in methylcyclohexane, e.g., J. Phys. Chem. 1993, 97, 32, 8378.\n9. lines 225-233\nThis text describes the C-Brb-Brc angle change as well as non-monotonic elongation and contraction of the chemical bonds\nin CHBr3 which the authors take as being characteristic of roaming. Reference of the previous work (ref. [3]) which saw\nsimilar behavior should be given here. Lines 173-180 describe similar changes occurring for CHBr3 in methanol.\n10. line192\n“Notably the isomer appears ~100fs later than the dissociation products…”\nDo the authors mean that the equilibrium CHBr2-Br structure isomer appear at ~100 fs? What the author mean under\n‘isomer’ needs to be clarified in detail in the main text. Lines 233-234 read as “…a delayed structural rearrangement\npreceding isomerization”: this means preceding population of the isomer ground-state minimum? See also my comment #6.\n11. line 194\n“The independent evolution of product concentrations further rules out solvent-induced radical recombination…”\nI do not see how this process can be ruled out in view of the experiment being conducted only to 1 ps delay times. See also\nmy comment #7.\n12. line 218\n‘Fig. S15’.\nPossibly, it is another figure?\n13. line 243\nFigure 4a,b shows roaming and dissociation dynamics in methanol and methylcyclohexane. Roaming trajectories appear to\nbe very similar; so, in my opinion it is roaming-mediated isomerization in both solvents, but with the isomer lifetime\ndramatically shortened in methanol by the reaction with the solvent.\n14. line 255\n“…represent hallmark signatures of roaming-mediated dynamics, ref. 43.” I do not think that Toulson and et al. in ref. [43]\nobserved any roaming in CHBr3. Can you please explain better what you mean by ‘hallmark signatures’ in reference to ref.\n[43].\n15. line 293\n“Hydrogen bonding in methanol drives ultrafast methanolysis, forming CH3OCHBr2…”\nI do not see any evidence given in this work for hydrogen bonding in methanol.\n16. Supporting Information, line 91\nThe 0,0-coordinate onset is missing in Figure S2 (left). The linear fit should intercept the (0,0) point in the absence of\nmultiphoton excitation, but currently it is impossible to see. Please re-plot.\n17. Supporting Information, lines 397-398 (and also lines 402-407 with Fig. 18b,c)\n“… (Fig. S17b,c), indicating oscillatory motion of the free Br atoms near CHBr2 fragments. This roaming likely enhances\nsolute-solvent interaction…”\nThe timescale in Fig. S17b,c reaches 0.6 ps; for t>150 fs Br atoms form charge-transfer complexes with solvent molecules,\ne.g., ref. [26] in the main txt and references therein, so Br cannot be ‘free’.\nOverall, this reads as if Br atoms roam about CHBr2 fragments. However, in the original roaming work on CHBr3, ref. [3],\nmain text, it was a much lighter carbon atom of the CHBr2 fragment that roamed. What the authors mean by roaming should\nbe better explained through the manuscript, see also my note at the beginning of this review.\nThe conclusion section in the main text reads as “CHBr3 undergoes roaming-mediated structural dynamics within 150 fs in\nboth methanol and methylcyclohexane”, lines 290-292. However, Fig. S17,18 seem to discuss a longer time scale, up to 600\nfs. The oscillations are present Fig. S17,18 up to 600 fs. Time constants in Figure 4, main text, of 300 fs (Fig. 4a) and\n240 fs (Fig. 4b) looks like associated with roaming motion.  2 = 386 25 fs in methanol is ascribed to “a roaming-mediated\nprocess of CHBr3, main text, comment #8. In view of these discrepancies it is not clear what timescale is associated with roaming.", "reviewer_3": "The manuscript presents a well-executed and insightful study on solvent-modulated reaction pathways in bromoform,\nemploying femtosecond X-ray solution scattering (TRXSS) to resolve structural dynamics on ultrafast timescales. The work\nlays a solid foundation for further investigations into solvent coordination effects and roaming-mediated reaction channels in\nsolution-phase chemistry.\nThe authors convincingly demonstrate the essential role of solvation in determining reaction mechanisms, motivating the\nstudy of organic solvents through connections to aqueous-phase behavior and atmospheric gas-phase processes,\nparticularly those involving halogens.\nFurthermore, the manuscript is well structured with the experimental setup described concisely, followed by a detailed\nanalysis of the results. Key methodological information is provided in the Supporting Information, which enhances\nreproducibility and credibility.\nThis is a high-quality manuscript with significant insight into ultrafast solvent-modulated reaction dynamics. With minor\nclarifications and additions, I strongly support its publication.\n1. Figure 2 is rich in information and instrumental in illustrating the results of the study. Enhancing clarity by explicitly stating\nthe three distinct pathways (direct dissociation, cage-induced methanolyis of the direct dissociated molecules, and roaming-\nmediated methanolysis of short-lived isomers) in the figure caption would aid comprehension. Aligning the color scheme of\nthe arrows in b) with the colors used in panels c)–e) could further improve visual mapping.\n2. The integration of ATM, GFA, and SVD analyses with AIMD simulations forms a robust analytical framework. The\ncongruence of timescales and findings across methods, particularly in methanol, demonstrates the strength of this multi-\nmodal approach. A comparative summary table of results from ATM, GFA, SVD, and AIMD (e.g. associated time constants)\nwould emphasize methodological convergence and provide readers with a quick reference for cross-validation.\n3. All TRXSS data are shown for Q > 1 Å⁻¹. The low-Q regime is the region with residual as seen in Supplementary Figure\nS1. Is this potentially linked to the optical Kerr effect?\n4. In methylcyclohexane, the observation of two temporally offset reaction pathways (offset by ~100 fs) suggests that these\nshould be identifiable via SVD decomposition as well. Is the lack of SVD traces due to the signal-to-noise of the data?\nIn general, the manuscript would benefit from quantitative comparisons of the magnitudes of signal components described in\nEquation 1 of the Supporting Information. Adding chi-square values as well as visualizations of individual signal\ncontributions (e.g. heating vs. solute-related signals), would improve interpretability.\n5. Fitting Results in S8: The tri-exponential fitting model yields three distinct timescales, yet the raw RSV traces (RSV1 and\nRSV2) appear qualitatively to reflect fewer distinct transitions. Clarifying how each timescale maps onto these signals would\nenhance the fit's interpretive value.", "reviewer_4": "This is a tour-de-force paper reporting on roaming dynamics in the condensed phase for the centrally important system\nCHBr3. The experiments are state-of-the-art and the computational work supporting the experiments is well done and\nconvincing. I strongly recommend publication.\nJust a minor point about some material in the SI. Imaginary frequencies are not -xyz cm-1 but xyz i. This is a common error\none sees in the literature, please correct this."}, "author_response": "1 REVIEWER COMMENTS\n2\n4\n8 cyclohexane, polar and non-polar solvents, using ultrafast (cid:415)me-resolved X-ray solu(cid:415)on\n9 sca(cid:425)ering (TRXSS). Roaming dynamics are found to play an important role in both, but\n10 dis(cid:415)nct behavior is seen with methanol undergoing reac(cid:415)on facilitated by roaming, while\n11 the methyl cyclohexane case shows a long-lived intermediate formed by roaming-mediated\n12 isomeriza(cid:415)on.\n13\n15 across many aspects of chemical reac(cid:415)on dynamics. Roaming reac(cid:415)ons in the condensed\n16 phase are far less studied as the techniques required are challenging and the situa(cid:415)on far\n17 more complex than in the gas phase. This paper represents a spectacular example of how to\n18 meet these challenges. Although the results from diffrac(cid:415)on experiments are low--\n19 dimensional and underdetermined, when combined with state-of-the-art theory and careful\n20 analysis, deep insight can be obtained with some confidence.\n21\n23 condensed-phase roaming mechanisms is both challenging and underexplored, and we are\n24 commi(cid:425)ed to conduc(cid:415)ng further research to contribute to this field.\n25\n26 1. The quality of the scien(cid:415)fic inves(cid:415)ga(cid:415)on, the analysis, and the clarity of the presenta(cid:415)on\n27 make this paper eminently suitable for Nature Comm. I have one concern I think the authors\n28 should address.\n29\n31 for publica(cid:415)on in Nature Communica(cid:415)ons. We will address all comments point by point. We\n32 believe that your comments will significantly improve the manuscript.\n33\n34 2. Almost nothing is said about the photoexcita(cid:415)on process, the excited state life(cid:415)me or the\n35 decay dynamics.\n36\n3 1\n38 relaxes to the ground state or forms the isomer on an ultrafast (cid:415)mescale. As reported in ref.\n39 [5], theore(cid:415)cal simula(cid:415)ons indicated that CHBr isomeriza(cid:415)on passes through the S /S conical\n3 1 0\n40 intersec(cid:415)on in ~29 fs and reaches isomeric geometries by ~80 fs. Given that our (cid:415)me\n41 resolu(cid:415)on, determined by the instrument response func(cid:415)on (IRF), is ~150 fs, it is difficult to\n42 directly resolve the decay dynamics and life(cid:415)me of the excited CHBr .\n3\n43\n44 3. In the SI I find they are modeling the triplet, which I presume is the relevant excited state,\n45 but this should be stated and jus(cid:415)fied in the main text.\n46\n48 manuscript to explicitly state that our structural modeling is based on the triplet exited state\n49 with corresponding jus(cid:415)fica(cid:415)on. Since UV (240–270 nm) excita(cid:415)on generates the triplet\n50 excited state of CHBr [8], modeling the triplet state is appropriate and consistent with the\n3\n51 experimental condi(cid:415)ons.\n52\n54 1. Lines 101-103, first sentence in Results and discussions:\n55 “TRXSS measurements of CHBr in methanol and methylcyclohexane were conducted at the\n3\n3\n57 methylcyclohexane using 267 nm UV pulses and conducted TRXSS measurements at the FXE\n58 instrument of the EuXFEL (Fig. 1)40,41. Photoexcita(cid:415)on of CHBr₃ pumps it from the ground state\n59 (S₀) to the first singlet excited state (S₁). The molecule then undergoes ultrafast intersystem\n60 crossing to the triplet excited state (T₁), driven by the strong spin-orbit coupling of the bromine\n61 atoms42-44. From T₁, the molecule can either dissociate via C–Br bond elonga(cid:415)on to yield Br\n62 radicals, or undergo structural rearrangement leading to alterna(cid:415)ve isomeric products5.”\n63\n64\n65 4. Then it seems they are not actually doing any nonadiaba(cid:415)c dynamics, rather, for the\n66 excited state dynamics they sample from configura(cid:415)ons on the ground state over the 100 ps\n67 of the simula(cid:415)on. Are the AIMD calcula(cid:415)ons en(cid:415)rely in the triplet state?\n68\n71 photodynamics of CHBr have been characterized in previous work. Semiclassical simula(cid:415)ons\n3\n72 at the CASPT2 level [5] reveal that CHBr reaches an S₁/S₀ conical intersec(cid:415)on approximately\n3\n73 29 fs a(cid:332)er photoexcita(cid:415)on and forms a near-isomerized geometry within 80 fs. These results\n74 confirm that electronic transi(cid:415)ons between poten(cid:415)al energy surfaces occur on a (cid:415)mescale\n75 shorter than 100 fs. Given that the temporal resolu(cid:415)on of our experiment is ~150 fs, the direct\n76 observa(cid:415)on of the sub-100fs electronic-state transi(cid:415)ons is challenging. Therefore, we did not\n77 perform nonadiaba(cid:415)c dynamics simula(cid:415)ons. Instead, our study focuses on the structural\n78 dynamics associated with molecular dissocia(cid:415)on and roaming-mediated isomeriza(cid:415)on\n79 following photoexcita(cid:415)on. For this purpose, all of our AIMD simula(cid:415)ons were conducted on\n80 the triplet-state poten(cid:415)al energy surface, which appropriately captures the post-excita(cid:415)on\n81 structural evolu(cid:415)on accessible within our experimental condi(cid:415)ons.\n83 trajectories. We extracted 200 configura(cid:415)ons from the 100 ps ground-state trajectory and\n84 used them as star(cid:415)ng points for triplet (T₁) simula(cid:415)ons in the NVE ensemble, yielding 2 ps\n85 trajectories of CHBr on the triplet-state poten(cid:415)al energy surface.\n3\n86\n88 1. Lines 263-267:\n89 “We probe the excited-state dynamics of CHBr in methanol and methylcyclohexane using\n3\n91 molecular dynamics (AIMD) based on the unrestricted Kohn-Sham method, combined with\n92 machine learning poten(cid:415)als, to probe the triplet excited-state (T1) dynamics of CHBr3 in\n93 methanol and methylcyclohexane. Our simula(cid:415)ons were conducted on the T1 excited state\n94 due to the ultrafast intersystem crossing from S1 to T1, which makes the triplet excited state\n95 the dominant channel for the subsequent photochemical dynamics5 (see SI Sec(cid:415)on 7 for\n96 details).”\n97\n99 1. Lines 431-433: “using the machine learning poten(cid:415)als (MLP) trained on the ground state\n100 under an NVT ensemble at 300 K with a CSVR thermostat.” was changed to “using the\n101 machine learning poten(cid:415)als (MLP) trained on the ground state under an NPT ensemble at\n102 300 K and 1 bar with a CSVR thermostat and barostat.”\n103\n104 2. Lines 433-437: “For excited-state dynamics, we employ …the 100 ps simula(cid:415)ons.” was\n105 changed to “The dataset used to train the triplet excited state (T₁) MLP was sampled from\n107 mul(cid:415)plicity 3 in the NVT ensemble at 300 K. To inves(cid:415)gate the dynamics following\n108 excita(cid:415)on with MLP, we extracted 200 configura(cid:415)ons from the 100 ps ground state\n109 trajectory as star(cid:415)ng points to run T₁ simula(cid:415)ons in the NVE ensemble, obtaining 2 ps\n110 trajectories of CHBr on the triplet-state poten(cid:415)al energy surface.”\n3\n111\n112\n113 5. Clearly the methanolysis reac(cid:415)on is on the ground electronic state. Some clarity on this\n114 point, in the main text as well as the SI, would be helpful. Once the authors address this\n115 point I recommend publica(cid:415)on.\n116\n119 (sec(cid:415)on 8), which illustrates the poten(cid:415)al energy surfaces (PES) for CHBr isomeriza(cid:415)on. This\n3\n120 figure supports our proposed reac(cid:415)on mechanism for CHBr in methylcyclohexane:\n3\n121 photoexcita(cid:415)on is firstly followed by a roaming-mediated isomeriza(cid:415)on on the triplet (T )\n1\n122 surface, the T /S intersec(cid:415)on then facilitates a nonadiaba(cid:415)c transi(cid:415)on from the T₁ to the S₀\n1 0\n123 state and ul(cid:415)mately isomerizing to a stable ground-state isomer.\n124\n126 on the triplet (T ) surface then on the ground state (S ) surface. Previous study indicated that\n1 0\n3 1 0\n128 illustrate that the resul(cid:415)ng hot CHBr -Br isomer reacts with methanol at ~385 fs. This\n2\n129 prolonged (cid:415)mescale confirms that the methanolysis reac(cid:415)on proceeds a(cid:332)er the system has\n130 undergone an electronic transi(cid:415)on from the excited state to the S ground state. We agree\n0\n131 with the reviewer that the methanolysis reac(cid:415)on is on the ground electronic state.\n132\n3 1 0\n135 conical intersec(cid:415)on in ~29 fs [5], while our data illustrate that the resul(cid:415)ng hot CHBr -Br isomer\n2\n136 reacts with methanol at ~385 fs. This prolonged (cid:415)mescale confirms that the methanolysis\n137 reac(cid:415)on proceeds a(cid:332)er the system has undergone an electronic transi(cid:415)on from the excited\n138 state to the S ground state.” was added.\n0\n139\n140\n142 1. Lines 495-496:\n143 “The S0 and T1 poten(cid:415)al energy surfaces for CHBr3 isomeriza(cid:415)on were calculated at the CAM-\n146\n147\n3\n149 ground-state (S , blue) and the ver(cid:415)cal triplet excited state (T₁, red).\n0\n150\n151\n152\n154\n155 [Also a(cid:425)ached as pdf file]. Please use the pdf.\n156\n158 ray solu(cid:415)on sca(cid:425)ering\n159\n163 roaming-mediated structural dynamics in both methanol and methylcyclohexane. To my\n164 knowledge, it is the first work (a(cid:332)er ref. [3]) which gives support to roaming-mediated\n165 structural dynamics in CHBr3. The significance of this reac(cid:415)on mechanism is very high, and\n166 the results are noteworthy and significant.\n167\n169\n171 should be reviewed again. My detailed comments are below, please see #1-17.\n172\n174 comments, which will greatly improve the manuscript. Our point-by-point responses are\n175 provided below.\n176\n178 dynamics within 150 fs in both methanol and methylcyclohexane”, but the oscillatory, non-\n179 monotonic angle/bond distance behavior (taken to be signatures of roaming, #9) seems to\n180 be present within the first 600 fs (see #17).\n181\n183 roaming-mediated forma(cid:415)on of a hot CHBr -Br isomer occurs on an ultrafast (cid:415)mescale (<150\n2\n184 fs), which is consistent with prior results [3, new ref. 5; and 6, new ref. 8]. As correctly pointed\n185 out by the Reviewer, this newly formed hot CHBr -Br isomer is expected to be ini(cid:415)ally in a\n2\n186 vibra(cid:415)onally hot state on the S poten(cid:415)al energy surface. As indicated by a recent femtosecond\n0\n187 ultrafast electron diffrac(cid:415)on study (see Figure 4b of new ref. 8), this hot isomer is expected to\n188 exhibit non-monotonic, oscillatory varia(cid:415)ons in its bonds and angles. The delayed relaxa(cid:415)on\n189 implied by this behavior is consistent with the roaming mechanism, which proceeds through\n190 a conical intersec(cid:415)on on a flat region of the poten(cid:415)al energy surface. A similar delay in the\n191 relaxa(cid:415)on of an “early-isomer” to a stable equilibrium isomer in solu(cid:415)on has been reported\n192 for bismuth triiodide using femtosecond TRXSS method [39, new ref. 45]. As described in Lines\n193 237-248 of main text, the major roaming-mediated structural dynamics are complete by 150\n194 fs, weak vibra(cid:415)onal relaxa(cid:415)on to a stable equilibrium isomer con(cid:415)nues un(cid:415)l 600 fs.\n195\n198 193 of page 8 and lines 237-248 of page 10.\n199\n3\n201 methanol and methylcyclohexane” was changed to “CHBr undergoes roaming-mediated\n3\n202 isomer forma(cid:415)on within 150 fs in both methanol and methylcyclohexane”.\n203\n205 but in the manuscript, despite bong angle and bond distance change being similar to what\n206 was described in ref. [3], it seems to be proposed that it is the Br-atom fragment that roams.\n208 the gas phase was ini(cid:415)ally suggested by Bowman, so forces to make it wander should be\n209 much, much stronger. This is hard to believe, so the difference C vs. Br should be explained.\n210\n212 that the original work [3, new ref. 5] provides the relevant descrip(cid:415)on in the introduc(cid:415)on's\n213 second paragraph:\n214\n215 “This (here, the iso-CHBr isomer forma(cid:415)on) occurs via a photochemical reac(cid:415)on path in which\n3\n216 the C–Br bond of CHBr par(cid:415)ally dissociates and the incipient Br fragment migrates to one of\n3\n217 the other two bromine sites to form the Br-HC–Br–Br isomeric species. This isomeriza(cid:415)on\n218 pathway could be termed ‘roaming-mediated’, because it involves the migra(cid:415)on of the\n219 incipient bromine fragment and leads to the iso product species.”\n220\n222 “The data suggest that the intramolecular S /S poten(cid:415)al surfaces of CHBr force the migra(cid:415)ng\n1 0 3\n223 bromine atom towards another.”\n224\n226 “High-level ab ini(cid:415)o trajectory computa(cid:415)ons suggest that the direct isomeriza(cid:415)on reac(cid:415)on\n227 involves ‘wandering’ of the carbon atom and can be characterized as roaming”.\n228\n230 par(cid:415)ally dissociated fragments. This process involves the concurrent migra(cid:415)on of both Br and\n2\n2\n233 reasonable to refer to it as a rela(cid:415)ve migra(cid:415)on of Br.\n235 the AMF analysis, the carbon atom was used as a central atom to define a body-fixed reference\n236 frame. This conven(cid:415)on was adopted to improve parameter iden(cid:415)fiability and to transparently\n237 track the (cid:415)me-dependent C–Br and Br–Br distances and the Br–C–Br angle that descript the\n238 rearrangement of the iso-CHBr –Br. This modeling conven(cid:415)on does not imply that the carbon\n2\n239 atom is sta(cid:415)onary or that “only bromine roams.” The roaming process involves the correlated\n240 mo(cid:415)on of both the CHBr and Br fragments.\n2\n241\n242\n246 describe the rela(cid:415)ve mo(cid:415)on of the CHBr and Br groups.\n2\n247 a) Lines 181 : “C–Br ruptures from ...” was changed to “C–Br increases from ...”\nc c\n248 b) Lines 188-189: “This angular shi(cid:332) reorients Br toward Br , forming a quasi-stable\nc b\n249 [CHBr ···Br] intermediate characteris(cid:415)c of roaming dynamics.” was changed to “These\n2\n250 changes in internal coordinates indicate that the CHBr and Br fragments undergo a\n2\n251 roaming mo(cid:415)on, which forms a hot CHBr -Br isomer.”\n2\n252 c) Lines 234-236: “Beyond 300 fs, bond length reaches a plateau as the Br atom fully escapes\n253 the solva(cid:415)on cage, in close agreement with LCF derived product forma(cid:415)on (cid:415)mescales.”\n254 was changed to “Beyond 300 fs, the separa(cid:415)on distance of C–Br reaches a plateau,\nc\n255 indica(cid:415)ng the loss of geminate correla(cid:415)on between CHBr and Br, which is in close\n2\n256 agreement with LCF derived product forma(cid:415)on (cid:415)mescales.”\n257 d) Lines 237-248, were revised to:\n258 “In contrast, isomeriza(cid:415)on proceeds via changes in three primary coordinates: C–Br and\nc\nc b b c\n260 to 57° (0.46°/fs), while C–Br elongates to 3.31 Å (0.0274 Å/fs) and Br–Br expands to\nc c b\n261 3.93 Å (0.0142 Å/fs), consistent with an early-(cid:415)me reorienta(cid:415)on of the CHBr and Br\n2\n262 fragments. 50-100 fs, the angle rapidly rises to 118° (1.22°/fs), while Br –Br sharply\nc b\n263 contracts to 2.4 Å (-0.031 Å/fs), forming a hot CHBr -Br isomer. 100-150fs, the angle\n2\n264 decreases to 98° (–0.46°/fs), and Br –Br modestly increases to 2.67 Å (0.0054 Å/fs),\nc b\n265 indica(cid:415)ng early stabiliza(cid:415)on of the hot CHBr -Br isomer. The observed non-monotonic\n2\n266 dynamics trace a roaming-mediated transi(cid:415)on to a vibra(cid:415)onally hot configura(cid:415)on by 150\n267 fs (C–Br  3.44 Å, Br–Br  2.67 Å, C–Br –Brc  98°), thereby confirming that a delayed\nc c b b\n268 structural rearrangement precedes and ul(cid:415)mately leads to the forma(cid:415)on of a hot isomer.\n270 similar systems [5,8]. The subsequent weak oscilla(cid:415)ons (150–600 fs) reflect vibra(cid:415)onal\n271 relaxa(cid:415)on around the stabilized isomer.”\n272\n273\n275 1. Lines 295-301 of SI: A descrip(cid:415)on was added to explain the reference-frame conven(cid:415)on:\n276\n279 does not imply that the carbon atom is sta(cid:415)onary. The conven(cid:415)on is physically neutral for\n280 solu(cid:415)on sca(cid:425)ering, which is sensi(cid:415)ve to interatomic distance distribu(cid:415)ons and is invariant to\n281 global transla(cid:415)on/rota(cid:415)on. Accordingly, the reported evolu(cid:415)ons of the C-Br and Br-Br\n282 distances and Br-C-Br angle quan(cid:415)fy the rela(cid:415)ve mo(cid:415)on of the two fragments in iso-CHBr -Br\n2\n283 without ascribing immobility to any specific atom.\n284\n285 2. Lines 458-462 of SI:\n286 “indica(cid:415)ng oscillatory mo(cid:415)on of … methanolysis reac(cid:415)on.” was changed to “A hot CHBr -Br\n2\n287 isomer forms within 150fs via roaming of the Br and CHBr fragments. Subsequently, the non-\n2\n288 monotonic mo(cid:415)ons of the internal coordinates persist to 600fs, corresponding to vibra(cid:415)onal\n289 oscilla(cid:415)ons of the hot isomer. These oscilla(cid:415)ons are likely to enhance transient solute-solvent\n290 interac(cid:415)on, thereby facilita(cid:415)ng the methanolysis reac(cid:415)on.”\n291\n292 3. Lines 468-472 of SI:\n293 “C-Br cleavage (<150fs) …the stabiliza(cid:415)on of the Br–Br distance at ~2.9 Å and the C–Br –Br\nc b b c\n294 angle at ~110°.” was changed to “reorienta(cid:415)on of the CHBr and Br fragments that leads to\n2\n295 the forma(cid:415)on of a hot CHBr -Br isomer within 150 fs. The internal coordinates stabilize with\n2\n296 ~3.44 Å for the C–Br distance, ~2.67 Å for the Br–Br distance and ~98° for the C–Br –Br\nc c b b c\n297 angle. Subsequent weak oscilla(cid:415)ons of these internal coordinates (150 fs-600 fs) reflect\n298 vibra(cid:415)onal relaxa(cid:415)on around the stabilized isomer.”\n299\n301 isomer (the structure on the isomer poten(cid:415)al) in both methanol and methylcyclohexane, so\n302 this primary pathway, which is isomeriza(cid:415)on, is the same in both solvent environments, Fig,\n303 2b,d vs. Fig. 3b,d, and Fig. 4a,b. Nevertheless, the Abstract says that “In methanol …it is\n304 solvolysis without isomeriza(cid:415)on”, lines 35-36.\n305\n307 an isomer-like intermediate in methanol before 400 fs. However, its structure differs from the\n308 equilibrium CHBr -Br isomer. For this reason, we ini(cid:415)ally described the process as occurring\n2\n309 “without isomeriza(cid:415)on”. We acknowledge the reviewer’s point that this species indeed meets\n310 the defini(cid:415)on of an isomer, and we have revised the abstract accordingly.\n311\n314 “In methanol, roaming enhances solute-solvent interac(cid:415)ons, leading to solvolysis without\n315 isomeriza(cid:415)on.” was changed to “In methanol, roaming enhances solute-solvent interac(cid:415)ons,\n316 leading to solvolysis before a stable isomer forms.”\n317\n319 with solvent; this has been well known, #5. Solvent coordina(cid:415)on is a key parameter for\n320 roaming reac(cid:415)ons, lines 40-41, but the meaning of solvent coordina(cid:415)on is unclear (H-\n321 bonding with CHBr3?) and no evidence is given.\n322\n325 solvent coordina(cid:415)on directly, since X-ray solu(cid:415)on sca(cid:425)ering is sensi(cid:415)ve to interatomic\n326 distances rather than chemical bonding. AIMD simula(cid:415)ons indicate a stronger solute-solvent\n327 interac(cid:415)on in methanol than in methylcyclohexane (Fig. S23). DFT calculated the reac(cid:415)ons of\n328 intermediates with methanol (Fig. 4c in main text and Figs. S24, S25 in SI). While our\n329 calcula(cid:415)ons provide valuable insight of solute-solvent interac(cid:415)on rather than clear solvent\n330 coordina(cid:415)on, we have revised the abstract to state this more carefully.\n331\n333 1. Lines 40-42:\n334 “Our findings establish solvent coordina(cid:415)on as a key parameter for roaming reac(cid:415)ons, with\n335 broad implica(cid:415)ons for photochemical outcomes.” was changed to “Our findings establish the\n336 solute-solvent interac(cid:415)on as a key factor controlling roaming-mediated reac(cid:415)ons in CHBr ,\n3\n337 with broad implica(cid:415)ons for photochemical outcomes, such as solvolysis in methanol and\n338 isomeriza(cid:415)on in methylcyclohexane solu(cid:415)ons.”\n339\n340\n342\n343 1. Abstract, lines 35-36\n344 “In methanol, roaming enhances solute-solvent interac(cid:415)ons, leading to solvolysis without\n345 isomeriza(cid:415)on.” This statement appears further (lines 277-278): “ the compact hydroxyl-rich\n346 environment of methanol hinders isomeriza(cid:415)on and instead promotes methanolysis”.\n348 in clear both methanol and methylcyclohexane, Fig, 2b,d vs. Fig. 3b,d, and Fig. 4a,b. It is the\n349 forma(cid:415)on of the equilibrated ground-state iso-CHBr3 that the methanol environment\n350 ‘hinders’.\n351\n353 manuscript as follows.\n354\n356 1. Lines 35-36\n357 “In methanol, roaming enhances solute-solvent interac(cid:415)ons, leading to solvolysis without\n358 isomeriza(cid:415)on.” was changed to “In methanol, roaming enhances solute-solvent interac(cid:415)ons,\n359 leading to solvolysis before a stable isomer forms.”\n360\n361 2. Lines 299-301:\n362 “while the compact hydroxyl-rich environment of methanol hinders isomeriza(cid:415)on and instead\n363 promotes methanolysis” was changed to “while the compact hydroxyl-rich environment of\n364 methanol hinders relaxa(cid:415)on of the hot CHBr -Br isomer to the equilibrated ground-state iso-\n2\n3\n366\n367\n368 2. Abstract, lines 40-41\n369 “Our findings establish solvent coordina(cid:415)on as a key parameter for roaming reac(cid:415)ons, with\n370 broad implica(cid:415)ons for photochemical outcomes.” However, lines 261-262 state: “Although\n371 explicit solvent interac(cid:415)ons between CHBr3 and methanol were not directly modelled…”\n373 solvent interac(cid:415)ons were not modelled? The experimental data in the manuscript are silent\n374 regarding solvent coordina(cid:415)on, apart from the statement (based on simula(cid:415)on of the radial\n375 distribu(cid:415)on func(cid:415)on of ground state CHBr3, lines 273-274), that the cage radios in methanol\n376 is significantly smaller than in methylcyclohexane. How this related to solvent coordina(cid:415)on\n377 being a key parameter needs to be explained.\n378\n381 regarding solvent coordina(cid:415)on, apart from the statement (based on simula(cid:415)on of the radial\n382 distribu(cid:415)on func(cid:415)on of ground state CHBr , lines 273-274), that the cage radio in methanol is\n3\n383 significantly smaller than in methylcyclohexane”, please see our responses above (lines 323-\n384 338). We have revised the abstract and the relevant text to address this point.\n385\n388 “Although explicit solvent interac(cid:415)ons between CHBr and methanol were not directly\n3\n389 modeled due to computa(cid:415)onal constraints, the simulated structural dynamics align well with\n390 the experimental observa(cid:415)ons” was changed to “Although explicit solvent coordina(cid:415)on\n391 between CHBr and methanol was not directly modeled due to computa(cid:415)onal constraints, the\n3\n392 simulated structural dynamics align well with the experimental observa(cid:415)ons (Fig. S21). The\n393 simula(cid:415)on also illustrates a significantly smaller solva(cid:415)on cage for CHBr in methanol (Fig. S23),\n3\n394 indica(cid:415)ng a stronger solute-solvent interac(cid:415)on.”\n395\n396 3. line 47\n397 “As photoac(cid:415)va(cid:415)on is indispensable in Br2 release from CHBr3…”. Is this a unimolecular\n398 reac(cid:415)on? Has it been ever experimentally observed? Reference(s) must be given.\n399\n2\n401 reported using (cid:415)me-of-flight mass spectrometry following excita(cid:415)on at 267 and 234 nm [Xu et\n402 al., J. Chem. Phys. 117, 2578–2585 (2002) h(cid:425)ps://doi.org/10.1063/1.1491877] and using\n403 cavity ring-down absorp(cid:415)on spectroscopy at 248 nm [ Huang et al., J. Chem. Phys. 121, 5253–\n404 5260 (2004) h(cid:425)ps://doi.org/10.1063/1.1777211]. The respec(cid:415)ve references have been added\n405 to the manuscript (Line 48).\n406\n407 4. lines 65-68\n408 “…the life(cid:415)me of the iso-CHBr2-Br …. relaxes within a few picoseconds in neat bromoform,\n409 ref. [3]”. I do not think that ref. [3] is relevant for iso-CHBr2-Br relaxa(cid:415)on in neat bromoform.\n411 a Br-atom interac(cid:415)ng with the surrounding CHBr3 solvent on a sub-ps (cid:415)me scale. However,\n412 ref. [3], and ref. [26] specifically, are relevant: iso-CHBr3 being metastable in\n413 methylcyclohexane and the life(cid:415)me of iso-CHBr3 being within one nanosecond in\n414 acetonitrile. Instead of ref. [25], I recommend the authors to refer to the subsequent paper\n415 by Crim and co-workers, J. Phys. Chem. A 2013, 117, 14, 2899\n416 (h(cid:425)ps://pubs.acs.org/doi/abs/10.1021/jp310737d), where the deficiencies of ref. [25] were\n417 addressed and the conclusion became very similar to those in ref. [26].\n418\n420 addi(cid:415)onal reference. The text has been revised accordingly, and the new reference has been\n421 added at Lines 67-69 in main text.\n422\n423\n424 5. lines 78-79\n426 methanol) “in which the isomer rapidly reacts with the solvent”, referring to their previous\n427 work, ref. [33] and to (cid:415)me-resolved Raman spectroscopy results by Phillips group, ref. [28].\n429 remains unsolved.” However, in my opinion, the mechanis(cid:415)c origin has already been firmly\n430 established: isomers of di- and polyhalogenated alkanes (X = Br, I) readily react with CH3OH\n431 via O-H inser(cid:415)on/HX elimina(cid:415)on reac(cid:415)ons. This reac(cid:415)on may be fast, so that the forma(cid:415)on\n432 of the product species is already observed on at 1.5 ps a(cid:332)er excita(cid:415)on (ref. [28], CHBr2-Br\n433 or iso-CHBr3 in methanol), or slower, e.g., iso-CHI3 decays with a 740-ps life(cid:415)me by reac(cid:415)ng\n434 with CH3OH via O-H inser(cid:415)on/HI elimina(cid:415)on reac(cid:415)ons, ChemPhysChem, 2009,10, 1895, doi:\n435 10.1002/cphc.200900122, ref. [34] in the current manuscript. Radical species react via the\n436 same reac(cid:415)on but much slower, ref. [28, 34] and other work by Phillips group. So, the\n437 mechanis(cid:415)c origin is established; what is not determined is the iso-CHBr3 life(cid:415)me in\n438 methanol. Apparently, it is < 1.5 ps.\n439\n442 literature, which we have incorporated into the revised manuscript.\n443\n445 1. Lines 79-81:\n446 “Despite these advances, …do they simply accelerate solvolysis?” was changed to “These\n447 previous studies have shown that the stable or hot CHBr -Br isomer can undergo an O-H\n2\n448 inser(cid:415)on/HBr elimina(cid:415)on reac(cid:415)on with CH OH or H O. However, the reported life(cid:415)mes of this\n3 2\n449 isomer, less than 1.5 ps in H O30 and less than100 ps in methanol36, remain ambiguous.”\n2\n450\n451 2. Line 95:\n452 “By determining a ~380 fs life(cid:415)me for the hot CHBr -Br isomer in methanol,” was added.\n2\n453\n454 3. Lines 159-160\n455 “The equilibrium dissocia(cid:415)on-to-methanolysis ra(cid:415)o (1:2.5) agrees with previous synchrotron-\n456 based TRXSS results” was changed to “The dissocia(cid:415)on-to-methanolysis ra(cid:415)o (1:2.5) at 1 ps is\n457 comparable to previous synchrotron-based TRXSS results.”\n458\n459 6. lines 86-91\n460 “…our results reveal that roaming dynamics ini(cid:415)ates universally at the beginning of\n461 photochemical reac(cid:415)on …”\n463\n465\n468 “Our results reveal that … in both solvents” was changed to “Our TRXSS results, which are\n469 directly sensi(cid:415)ve to molecular structures of reac(cid:415)ng species in solu(cid:415)on, confirm that roaming\n470 dynamics ini(cid:415)ates universally at the onset of photochemical reac(cid:415)on in both solvents”\n471\n472\n473 “However, solvent-specific interac(cid:415)ons leas to dis(cid:415)nct (probably dis(cid:415)nctly different is meant\n474 here?) outcomes. The hydrogen-bonding capacity of methanol mediates proton transfer,\n475 quenching isomeriza(cid:415)on via solvent interac(cid:415)ons, ref. [28,34-36] whereas the non-polar\n476 environment of methylcyclohexane stabilizes roaming-mediated isomers, ref. [26,33].”\n478 perhaps, called primary and secondary. Their Abstract said there is no isomeriza(cid:415)on in\n479 methanol, but the sentence above reads as if there is isomeriza(cid:415)on, but it is quenched.\n482 restrict themselves to calling only the equilibrium CHBr2-Br structure as the isomer? The\n484 poten(cid:415)al energy surface (hot isomer). This is the primary product species in both solvents,\n485 along with the radical pair. In methanol, the isomer undergoes the methanolysis reac(cid:415)on,\n486 producing secondary CH3OCHBr2 and HBr products, whereas this reac(cid:415)on is not viable in\n487 methylcyclohexane.\n488\n490 accordingly. Please refer to our responses to lines 306-316 and 352-365 above for details.\n491\n493 1. Lines 90-94:\n494 “The hydrogen-bonding capacity of methanol mediates proton transfer, quenching\n495 isomeriza(cid:415)on via solvent interac(cid:415)ons” was changed to “In the hydroxyl-rich environment of\n496 methanol, solute-solvent interac(cid:415)ons quench the roaming-mediated pathway of the hot\n2 3 2\n3 2\n500 stabilizes the roaming-mediated isomers without solvolysis”\n501\n502 7. line 118\n503 “By 1 ps, the concentra(cid:415)on of all species tends to stabilize”.\n505 solvent cage within several picoseconds, ref. [26], and are expected to undergo cage-induced\n506 recombina(cid:415)on (primary recombina(cid:415)on). Also, they can undergo cage-induced isomeriza(cid:415)on\n507 on the same (cid:415)mescale. Do the authors suggest that these reac(cid:415)ons are absent? Further, the\n509 dynamics) component with t3 = 4.75 ps, which the authors ascribe to a slower methanolysis\n510 pathway. How then “all concentra(cid:415)ons tend to stabilize by 1 ps” if there is a 4.75-ps\n511 component? How reliable is the 4.75 ps determina(cid:415)on: the experiments were performed\n512 only to a 1 ps delay (cid:415)me (Suppor(cid:415)ng Informa(cid:415)on, line 47).\n513\n516 concentra(cid:415)on tend to stabilize by 1 ps” could be misinterpreted to mean that all dynamical\n517 processes cease. Our intended meaning was that the concentra(cid:415)ons of the majority of species\n518 enter a phase of much slower change a(cid:332)er 1ps. As the reviewer correctly notes and as\n519 supported by Figure S3, S8 and S11, slower recombina(cid:415)on and methanolysis processes\n520 con(cid:415)nue beyond this point. The experimental measurements were performed with a fine step\n521 size of 50 fs from -0.2ps to 1ps, and with a coarser step size from 1 ps to 10 ps (lines 47-48 in\n523\n525 exponen(cid:415)al func(cid:415)on convolved with a Gaussian instrument response func(cid:415)on (IRF), and fits\n526 both RSVs simultaneously with shared (cid:415)me constants, as detailed in SI Sec(cid:415)on 4 and Figures\n528 a shared ini(cid:415)al ultrafast process. We therefore assign the first kine(cid:415)c component to solvent\n529 hea(cid:415)ng. Between 300-700 fs, the traces deviate from simple monotonic behavior—RSV1\n530 transi(cid:415)ons to a shallow rise, while RSV2 displays a clear decay—features captured by the\n531 second kine(cid:415)c component and a(cid:425)ributed to the fast methanolysis channel. At late (cid:415)me delays\n532 (0.7–1 ps), the two RSVs display coordinated late-(cid:415)me behavior: RSV1 exhibits a renewed,\n533 modest rise rather than reaching a plateau, while RSV2 shows a persistent slow decay toward\n534 1 ps. Within the IRF-convolved framework, these joint, non-plateauing trends cannot be\n535 reproduced by just two sub-picosecond components. This indicates the need to introduce a\n536 third, longer component whose onset lies within our experimental window and measurably\n537 governs the late curvature extending up to 1 ps. The simultaneous fit yields τ1 = 462 ± 45 fs,\n538 τ2 = 386 ± 25 fs, and τ3 = 4.75 ± 2.4 ps, with an IRF of 151 ± 22 fs. The slowest term (τ3)\n539 specifically accounts for the secondary slow rise in RSV1 and the sustained slow decay of RSV2\n540 beyond 700 fs (Figure S8). Thus, τ3 does not represent an over-extrapola(cid:415)on of the\n541 experimental data but is constrained by the late-(cid:415)me curvature and non-flat trends observed\n542 in both RSVs within the IRF-convolved kine(cid:415)c model.\n543\n545 large uncertainty, with an error margin of approximately 2.4 ps (over 50%). Nevertheless, the\n546 presence of a slow component extending beyond 1 ps is supported by independent fits of the\n548 3.6 ps). The close agreement between these values, despite large error bars, indicates that\n549 late-(cid:415)me dynamics are indeed present.\n550\n552 constant of 7.99 ps with a larger uncertainty (±6.06 ps, R²= 0.883) due to limited data points.\n554 on the several-picosecond (cid:415)me range. The (cid:415)me constant of slow methanolysis is inherently\n555 limited by the coarser (cid:415)me step at longer delays, introducing picosecond-level uncertainty.\n556\n558 1. Lines 126-127:\n559 “By 1 ps, the concentra(cid:415)on of all species tends to stabilize” was changed to “By 1 ps, the\n560 concentra(cid:415)on of the majority of species begins to stabilize with remaining slower\n561 methanolysis component persis(cid:415)ng for several picoseconds.”\n562\n564 1. Lines 138-152: We add detailed descrip(cid:415)ons to the LCF sec(cid:415)on of revised SI correla(cid:415)ng\n565 the fi(cid:425)ed (cid:415)mescales with the discernible kine(cid:415)c features, as follows:\n566\n567 “To resolve the fast and slow methanolysis reac(cid:415)on kine(cid:415)cs, we fi(cid:425)ed the LCF-derived\n568 concentra(cid:415)on profiles (Fig. S3b) with a bi-exponen(cid:415)al decay model convolved with the IRF.\n570 (0.6–1 ps): CH₃OCHBr₂/HBr exhibits a sustained, gradual rise, whereas CHBr₂/Br displays a\n571 slower decay. Within the IRF-convolved framework, these coordinated late-(cid:415)me trends cannot\n572 be captured by a single sub-picosecond process, promp(cid:415)ng us to incorporate a slower\n573 component. The fit yields two (cid:415)mescales—τ_fast = 330 ± 57 fs and τ_slow = 5.15 ± 3.6 ps,\n574 with IRF = 157 ± 28 fs. This mul(cid:415)-picosecond constant does not represent an extrapola(cid:415)on\n575 beyond experimental data, but rather quan(cid:415)fies the early evolu(cid:415)on of the slow component\n576 that already exists within 1 ps. Although τ_slow extends our (cid:415)me window and is subject to\n577 large uncertainty, it provides a preliminary es(cid:415)mate of the (cid:415)mescale for the slow methanolysis.\n579 ± 6.06 ps, with an IRF of 150 ± 29 fs (Fig. S3c). The uncertainty, par(cid:415)cularly for the longer\n580 life(cid:415)me component, is increased by the limited number of data points available at long (cid:415)me\n581 delays. Nevertheless, the results confirm that the slower methanolysis persists on the\n582 picosecond (cid:415)mescale.”\n583\n584 2. We have updated Figure S3 in SI.\n585\n586\n588 without the isomer component within 1 ps; c. without the isomer component extended to 10\n589 ps.\n590\n591\n592 8. lines 131-133\n594 hea(cid:415)ng and “a roaming-mediated process of CHBr3”. These (cid:415)me constants are more or less\n595 within the error bar. How reliably they can be differen(cid:415)ated and assigned?\n596\n598 would expect significant photoproduc(cid:415)on of solvated electrons. Their solva(cid:415)on and\n599 thermaliza(cid:415)on occur on a 150 fs and ~5 ps (cid:415)me scales, e.g., J. Chem. Phys. 2015, 142, 234501\n600 and references therein. These processes do not happen when azo dyes are used to obtain\n601 the solvent hea(cid:415)ng signals (azo dyes were used to obtain the solvent hea(cid:415)ng signals in the\n602 current work, Suppor(cid:415)ng Informa(cid:415)on, lins 49-50). How reliable is then the solvent hea(cid:415)ng\n603 subtrac(cid:415)on? Significant solvent photochemistry at this excita(cid:415)on level should also occur in\n604 methylcyclohexane, e.g., J. Phys. Chem. 1993, 97, 32, 8378.\n605\n607 sca(cid:425)ering is a structurally sensi(cid:415)ve technique that can effec(cid:415)vely separate solute dynamics\n608 from the solvent hea(cid:415)ng response based on the dis(cid:415)nct shapes of their difference signals as\n609 shown in the newly added Fig. S15 and Fig. S19 in SI. The two (cid:415)me constants, τ₁ (bulk solvent\n610 hea(cid:415)ng) and τ₂ (methanolysis), which differen(cid:415)ate the dominant le(cid:332) singular vectors (principal\n611 components) of the sca(cid:425)ering data, can describe the kine(cid:415)cs reliably.\n612\n614 their thermaliza(cid:415)on on the TRXSS signal is a cri(cid:415)cal factor to interpre(cid:415)ng the data. Previous\n615 studies have shown that the contribu(cid:415)on of solvated electrons to TRXSS signals is either absent\n616 or below the detec(cid:415)on limit. For example, femtosecond TRXSS studies, such as those on I- in\n617 water by Vester et al. [J. Chem. Phys. 157, 224201 (2022) h(cid:425)ps://doi.org/10.1063/5.0107224],\n618 on I- and Br- in water by Markmann et al. [Chem. Sci., 2024,15, 11391-11401\n619 h(cid:425)ps://doi.org/10.1039/D4SC01912A] and on larger molecular systems by Haldrup et al. [J.\n621 consistently show that the contribu(cid:415)on of solvated electrons is negligible. In these works, the\n623 solva(cid:415)on cage), and the structural dynamics of the solute. The excellent fit of our CHBr\n3\n624 photolysis data (Figs. S15 and S19) using the solvent hea(cid:415)ng measured with azo dyes, solva(cid:415)on\n625 cage and solute dynamics indicates that the contribu(cid:415)on of solvated electrons to the TRXSS\n626 signal is minimal and can be neglected. The comparison in Figs. S15 and S19 also illustrates\n627 that the solvent hea(cid:415)ng signal is located in the low q range (low sca(cid:425)ering angle), which allows\n628 it to be readily separated from the solute signals.\n629\n630\n633 sca(cid:425)ering signals into solute, cage and solvent hea(cid:415)ng.\n634\n635\n636\n3\n638 curves (black) at (a) 150 fs and (b) 1 ps are shown. The simulated traces (red) is a sum of the\n639 solute (blue), solva(cid:415)on cage (green) and the solvent (purple).\n640\n641\n3\n643 sca(cid:425)ering curves (black) at (a) 150 fs and (b) 1 ps are shown. The simulated traces (red) is a\n644 sum of the solute (blue), solva(cid:415)on cage (green) and the solvent (purple).\n645\n646\n647 9. lines 225-233\n649 contrac(cid:415)on of the chemical bonds in CHBr3 which the authors take as being characteris(cid:415)c\n650 of roaming. Reference of the previous work (ref. [3]) which saw similar behavior should be\n651 given here. Lines 173-180 describe similar changes occurring for CHBr3 in methanol.\n652\n654 and [6, now 8].\n655\n657 1. Lines 191-192:\n658 “Similar roaming-mediated behaviors have been observed in previous studies5,8.” was added.\n659 2. Lines 246-247:\n660 “Our observa(cid:415)ons are consistent with previous reports on roaming-mediate dynamics in\n661 similar systems5,8.” was added.\n662\n663\n664 10. line192\n665 “Notably the isomer appears ~100fs later than the dissocia(cid:415)on products…”\n668 233-234 read as “…a delayed structural rearrangement preceding isomeriza(cid:415)on”: this means\n669 preceding popula(cid:415)on of the isomer ground-state minimum? See also my comment #6.\n670\n2\n672 the direct dissocia(cid:415)on products CHBr and Br, due to the roaming-mediated process. The\n2\n673 forma(cid:415)on of equilibrium CHBr -Br isomer completes ~400 fs as shown in Fig. S4 in SI.\n2\n674 “…a delayed structural rearrangement preceding isomeriza(cid:415)on” means preceding popula(cid:415)on\n675 of a vibra(cid:415)onally hot CHBr -Br isomer, which subsequently cools to form the equilibrium\n2\n676 ground-state isomer on a (cid:415)mescale exceeding 400 fs.\n677\n679 1. Lines 243-246:\n680 “These non-monotonic dynamics reveal a roaming-mediated transi(cid:415)on, with the system\n681 reaching a stabilized configura(cid:415)on by 150 fs (C–Br  4.1 Å, Br –Br  2.9 Å, C–Br –Br  110°),\nc c b b c\n682 confirming a delayed structural rearrangement preceding isomeriza(cid:415)on” was changed to “The\n683 observed non-monotonic dynamics trace a roaming-mediated transi(cid:415)on to a vibra(cid:415)onally hot\n684 configura(cid:415)on by 150 fs (C–Br  3.44 Å, Br –Br  2.67 Å, C–Br –Br  98°), thereby confirming\nc c b b c\n685 that a delayed structural rearrangement precedes and ul(cid:415)mately leads to the forma(cid:415)on of a\n686 hot isomer.”\n687\n688\n689 11. line 194\n690 “The independent evolu(cid:415)on of product concentra(cid:415)ons further rules out solvent-induced\n691 radical recombina(cid:415)on…”\n693 only to 1 ps delay (cid:415)mes. See also my comment #7.\n694\n696 indeed extend beyond 1 ps, and we have added new fi(cid:427)ng results up to 10 ps (see Fig. S4b).\n698 limited data points at long delays prevent us from en(cid:415)rely ruling out processes like radical\n699 recombina(cid:415)on or cage-induced isomeriza(cid:415)on between 1.6 and 10 ps, we have revised the text\n700 to clarify this point.\n701\n703 1. Lines 206-208:\n705 solvent-cage-induced radical recombina(cid:415)on, as such a process would require coupled\n706 forma(cid:415)on-deple(cid:415)on kine(cid:415)cs of dissociated radicals and iso-CHBr -Br.” has been changed to\n2\n707 “The concurrent temporal evolu(cid:415)on of the product concentra(cid:415)ons demonstrates that the\n708 growths of the dissociated radicals and the CHBr –Br isomer are coupled over picoseconds.”\n2\n709\n712 analysis for CHBr in methylcyclohexane in the revised Suppor(cid:415)ng Informa(cid:415)on.\n3\n713\n714 1. Lines 154-158\n715 “The extended LCF analysis up to 10 ps shows no clear monotonic change in the popula(cid:415)ons\n716 of the CHBr –Br isomer or the CBr + Br dissocia(cid:415)on products (Fig. S4b). However, given the\n2 2\n717 sparse temporal sampling at these delays, the poten(cid:415)al for slower processes, such as radical\n718 recombina(cid:415)on or cage-induced isomeriza(cid:415)on between 1.6 ps and 10 ps, cannot be fully\n719 excluded.” was added.\n720\n721\n3\n723 extended to 10 ps.\n724\n725\n726 12. line 218\n727 ‘Fig. S15’. Possibly, it is another figure?\n728\n730 is now Figure S18 in the revised suppor(cid:415)ng informa(cid:415)on. We have updated the main text\n731 accordingly (Line 230).\n732\n733 13. line 243\n736 isomeriza(cid:415)on in both solvents, but with the isomer life(cid:415)me drama(cid:415)cally shortened in\n737 methanol by the reac(cid:415)on with the solvent.\n738\n740 that the roaming-mediated isomeriza(cid:415)on pathway is effec(cid:415)ve in both methanol and\n741 methylcyclohexane, as supported by the roaming-mediated trajectories in Figures 4a and 4b.\n743 methylcyclohexane, the equilibrium iso-CHBr persists stably, whereas in methanol the hot\n3\n744 isomer reacts directly with the solvent, leading to a significantly shortened life(cid:415)me. We have\n745 revised the manuscript to clarify that while the ini(cid:415)al reac(cid:415)on mechanism is iden(cid:415)cal,\n746 the isomer’s persistence is governed by subsequent solvent interac(cid:415)ons.\n747\n748\n749 14. line 255\n750 “…represent hallmark signatures of roaming-mediated dynamics, ref. 43.” I do not think that\n752 what you mean by ‘hallmark signatures’ in reference to ref. [43].\n753\n756 methodology for analyzing our MD trajectories, specifically the use of atomic distances to track\n757 roaming-mediated mo(cid:415)ons, not to imply that Ref [4] by Toulson et al. provided experimental\n758 evidence for CHBr roaming. We agree that the original cita(cid:415)on was poten(cid:415)ally confusing and\n3\n759 have therefore replaced it with more relevant references [5,8], and have adjusted the text for\n760 clarity.\n761\n763 1. Lines 271-275:\n764 “Devia(cid:415)ons from this diagonal, highligh(cid:415)ng asymmetric behavior where Br preferen(cid:415)ally\nc\n765 approaches either Br or Br , represent hallmark signatures of roaming-mediated dynamics”\na b\n766 was changed to “Off-diagonal excursions indicate a transient asymmetry in the Br-Br\n767 separa(cid:415)ons, signifying a reorienta(cid:415)on of both the CHBr and Br fragments. These excursions\n2\n768 are clear signatures of CHBr -Br isomer structures formed via roaming-mediated dynamics,\n2\n769 and their subsequent oscilla(cid:415)ons represent the vibra(cid:415)onal mo(cid:415)on of these transient\n770 intermediates5,8”\n771\n772 15. line 293\n773 “Hydrogen bonding in methanol drives ultrafast methanolysis, forming CH3OCHBr2…”\n775\n778 directly evidenced. Our intent was to describe the reac(cid:415)on of the hot CHBr -Br isomer with\n2\n779 methanol via O–H inser(cid:415)on/HBr elimina(cid:415)on. We have removed the statements regarding\n780 hydrogen bonding and have revised the corresponding text in the manuscript to clarify the\n781 reac(cid:415)on mechanism.\n782\n784 1. Lines 90-93:\n785 “The hydrogen-bonding capacity of methanol mediates proton transfer, quenching\n786 isomeriza(cid:415)on via solvent interac(cid:415)ons.” was changed to “In the hydroxyl-rich environment of\n787 methanol, solute-solvent interac(cid:415)ons quench the roaming-mediated pathway of the hot\n2 3 2\n789 isomer then undergoes subsequent methanolysis reac(cid:415)on, producing secondary CH OCHBr\n3 2\n790 and HBr30,37-39.”\n791 2. Lines 316-317:\n792 “Hydrogen bonding in methanol drives the ultrafast methanolysis, forming CH₃OCHBr and HBr\n2\n793 with ~385 fs” was changed to “The solvent interac(cid:415)ons and cage effect in methanol facilitate\n794 the ultrafast methanolysis, leading to the forma(cid:415)on of CH₃OCHBr and HBr within ~385 fs”.\n2\n795\n796\n797 16. Suppor(cid:415)ng Informa(cid:415)on, line 91\n799 point in the absence of mul(cid:415)photon excita(cid:415)on, but currently it is impossible to see. Please\n800 re-plot.\n801\n805\n806\n807\n3\n809 a linear absorp(cid:415)on profile, confirming that single-photon excita(cid:415)on dominates the\n810 photodissocia(cid:415)on process.\n811\n812\n813 17. Suppor(cid:415)ng Informa(cid:415)on, lines 397-398 (and also lines 402-407 with Fig. 18b,c)\n814 “… (Fig. S17b,c), indica(cid:415)ng oscillatory mo(cid:415)on of the free Br atoms near CHBr2 fragments.\n817 complexes with solvent molecules, e.g., ref. [26] in the main txt and references therein, so\n819\n821 in SI accordingly.\n822\n824 1. Lines 458-462:\n825 “… (Fig. S17b,c, new Fig. S21b, c), indica(cid:415)ng oscillatory mo(cid:415)on of the free Br atoms near CHBr\n2\n826 fragments. This roaming likely enhances solute-solvent interac(cid:415)on…” was changed to “A hot\n2 2\n828 the non-monotonic mo(cid:415)ons of the internal coordinates persist to 600fs, corresponding to\n829 vibra(cid:415)onal oscilla(cid:415)ons of the hot isomer. These oscilla(cid:415)ons are likely to enhance transient\n830 solute-solvent interac(cid:415)on, thereby facilita(cid:415)ng the methanolysis reac(cid:415)on.”\n831\n832 2. Lines 468-472:\n833 “C-Br cleavage (<150 fs) …the stabiliza(cid:415)on of the Brc–Brb distance at ~2.9 Å and the C–Brb–\n835 that leads to the forma(cid:415)on of a hot CHBr2-Br isomer within 150 fs. The internal coordinates\n836 stabilize with ~3.44 Å for the C–Brc distance, ~2.67 Å for the Brc–Brb distance and ~98° for the\n838 reflect vibra(cid:415)onal relaxa(cid:415)on around the stabilized isomer.”\n839\n841 roaming work on CHBr3, ref. [3], main text, it was a much lighter carbon atom of the CHBr2\n842 fragment that roamed. What the authors mean by roaming should be be(cid:425)er explained\n843 through the manuscript, see also my note at the beginning of this review.\n844\n846\n848 structural dynamics within 150 fs in both methanol and methylcyclohexane”, lines 290-292.\n850 present Fig. S17,18 up to 600 fs. Time constants in Figure 4, main text, of 300 fs (Fig. 4a) and\n851 240 fs (Fig. 4b) looks like associated with roaming mo(cid:415)on. 2 = 38625 fs in methanol is\n852 ascribed to “a roaming-mediated process of CHBr3, main text, comment #8. In view of these\n853 discrepancies it is not clear what (cid:415)mescale is associated with roaming.\n854\n856 628 above.\n857\n858\n859\n861\n863 reac(cid:415)on pathways in bromoform, employing femtosecond X-ray solu(cid:415)on sca(cid:425)ering (TRXSS)\n864 to resolve structural dynamics on ultrafast (cid:415)mescales. The work lays a solid founda(cid:415)on for\n865 further inves(cid:415)ga(cid:415)ons into solvent coordina(cid:415)on effects and roaming-mediated reac(cid:415)on\n866 channels in solu(cid:415)on-phase chemistry.\n868 mechanisms, mo(cid:415)va(cid:415)ng the study of organic solvents through connec(cid:415)ons to aqueous-\n869 phase behavior and atmospheric gas-phase processes, par(cid:415)cularly those involving halogens.\n871 concisely, followed by a detailed analysis of the results. Key methodological informa(cid:415)on is\n872 provided in the Suppor(cid:415)ng Informa(cid:415)on, which enhances reproducibility and credibility.\n874 reac(cid:415)on dynamics. With minor clarifica(cid:415)ons and addi(cid:415)ons, I strongly support its publica(cid:415)on.\n875\n877 comments on our manuscript. These sugges(cid:415)ons are invaluable in strengthening the\n878 manuscript. The manuscript has been revised to address all points raised and have provided a\n879 detailed point-by point response below.\n880\n881 1. Figure 2 is rich in informa(cid:415)on and instrumental in illustra(cid:415)ng the results of the study.\n883 induced methanolyis of the direct dissociated molecules, and roaming-mediated\n884 methanolysis of short-lived isomers) in the figure cap(cid:415)on would aid comprehension.\n886 further improve visual mapping.\n887\n890 pathways. In addi(cid:415)on, in Figure 2b, each pathway is color-coded: red represents the direct\n891 dissocia(cid:415)on pathway, green denotes the cage-induced methanolysis pathway, and blue\n892 signifies the roaming-mediated methanolysis pathway. This labeling scheme is consistent with\n893 the diagrams in Figures 2c-e.\n894\n897\n3\n899 photoexcited CHBr in methanol. b. Proposed photoinduced kine(cid:415)c model of CHBr in\n3 3\n900 methanol. Carbon (teal-blue), bromine (red-pink), hydrogen (grey), oxygen (red). c. Atomic\n901 mo(cid:415)on fi(cid:427)ng (AMF) data of the C–Br distance in the direct dissocia(cid:415)on pathway and cage-\nc\n902 induced methanolysis pathway. d. AMF data of the roaming-mediated methanolysis pathway:\n903 the C-Br distance, Br -Br distance and C-Br -Br angle evolu(cid:415)on. e. Time-dependent transient\nc b c b c\n904 popula(cid:415)on changes obtained from AMF analysis.\n905\n906 2. The integra(cid:415)on of ATM, GFA, and SVD analyses with AIMD simula(cid:415)ons forms a robust\n907 analy(cid:415)cal framework. The congruence of (cid:415)mescales and findings across methods,\n908 par(cid:415)cularly in methanol, demonstrates the strength of this mul(cid:415)-modal approach. A\n909 compara(cid:415)ve summary table of results from ATM, GFA, SVD, and AIMD (e.g. associated (cid:415)me\n910 constants) would emphasize methodological convergence and provide readers with a quick\n911 reference for cross-valida(cid:415)on.\n912\n914 and AIMD indicate that a non-sta(cid:415)onary isomer forms at approximately 150fs a(cid:332)er CHBr\n3\n915 excita(cid:415)on. We have added a summary table in the SI presen(cid:415)ng the relevant (cid:415)me constants\n916 obtained from LCF, SVD, and GFA analyses.\n917\n3\nMethods Roaming-mediated Solvent hea(cid:415)ng Cage-induced\nmethanolysis methanolysis\nLCF 330 ± 57 fs 471 ± 78 fs 5.15 ± 3.6 ps\nSVD 386 ± 25 fs 462 ± 45 fs 4.75 ± 2.4 ps\nGFA 385 ± 80 fs 460 fs (fixed) 4.4 ± 0.9 ps\n921\n922\n923 3. All TRXSS data are shown for Q > 1 Å⁻¹. The low-Q regime is the region with residual as\n924 seen in Supplementary Figure S1. Is this poten(cid:415)ally linked to the op(cid:415)cal Kerr effect?\n925\n927 signals in the low-Q range (Q < 1 Å⁻¹) are unreliable due to two experimental setup limita(cid:415)ons.\n929 protect the detector from damage in case of accidental beam dri(cid:332)s, or increased forward\n930 sca(cid:425)ering off bubbles or aerosols formed from jet instabili(cid:415)es. Second, even with a stable jet,\n931 the signals below 1 Å⁻¹ o(cid:332)en exhibit significant ar(cid:415)factual fluctua(cid:415)ons due to possible\n932 varia(cid:415)ons of the He gas sca(cid:425)ering background around the beam (the solu(cid:415)on flows in a He-\n933 purged chamber) and the low number of pixels available for integra(cid:415)on in low Q region is o(cid:332)en\n934 resul(cid:415)ng in ar(cid:415)ficially large fluctua(cid:415)ons of the signals below 1 Å⁻¹.\n935\n937 the op(cid:415)cal Kerr effect, giving rise to anisotropic sca(cid:425)ering. Figure S1 in Supplementary\n939 protocol from reference 6 in SI, these anisotropic sca(cid:425)ering signals are extracted to generate\n940 the isotropic sca(cid:425)ering signals, which are used in our data analysis to obtain the structural\n941 dynamics. The extrac(cid:415)on is based on the nearly periodic varia(cid:415)on of the difference signal over\n942 the azimuthal angle at each Q value. All difference sca(cid:425)ering signals presented in the main\n943 text and SI, except for Fig. S1, are isotropic.\n944\n945\n946 4. In methylcyclohexane, the observa(cid:415)on of two temporally offset reac(cid:415)on pathways (offset\n947 by ~100 fs) suggests that these should be iden(cid:415)fiable via SVD decomposi(cid:415)on as well. Is the\n948 lack of SVD traces due to the signal-to-noise of the data?\n949\n951 applicability of SVD is subject to certain limita(cid:415)ons. The LCF approach, employing difference\n952 sca(cid:425)ering traces for the dissociated and isomer species, is sensi(cid:415)ve to their temporal\n953 appearance. It reveals a delayed forma(cid:415)on of the isomer, consistent with the (cid:415)me scale\n954 required for roaming-mediated structural rearrangements. Since the intermediate structures\n955 are not included in the model, the SVD method intrinsically favors global consistent signal\n956 shapes and decomposes them based on different temporal evolu(cid:415)on. Signals from transient\n957 intermediates are highly localized in (cid:415)me and appear as “rare events”, SVD is not sensi(cid:415)ve to\n958 resolve them. Therefore, one possible reason for SVD’s inability to dis(cid:415)nguish the dissocia(cid:415)on\n959 and isomeriza(cid:415)on components is the high similarity of their temporal evolu(cid:415)on for the\n960 majority of the data set, with significant differences occurring at only 2-3 of the 25 measured\n961 delays. Thus, in this case SVD appears to overlook these local differences, modeling the data\n962 with a single component that combines the dissocia(cid:415)on and isomer signals and evolves with\n963 a slower rise.\n964\n966 of signal components described in Equa(cid:415)on 1 of the Suppor(cid:415)ng Informa(cid:415)on. Adding chi-\n967 square values as well as visualiza(cid:415)ons of individual signal contribu(cid:415)ons (e.g. hea(cid:415)ng vs.\n968 solute-related signals), would improve interpretability.\n969\n972 solvent hea(cid:415)ng contribu(cid:415)ons at 150 fs and 1 ps and have presented this analysis in SI (Fig. S15,\n973 and Fig. S19). In addi(cid:415)on, we have included the figure of merits (χ²) as a func(cid:415)on of (cid:415)me\n974 obtained from the LCF analysis in the SI (Fig. S16, and Fig. S20).\n975\n977\n978\n3\n980 curves (black) at (a) 150 fs and (b) 1 ps are shown. The simulated traces (red) is a sum of the\n981 solute (blue), solva(cid:415)on cage (green) and the solvent (purple).\n982\n3\n984\n985\n986\n3\n988 difference sca(cid:425)ering curves (black) at (a) 150 fs and (b) 1 ps are shown. The simulated traces\n989 (red) is a sum of the solute (blue), solva(cid:415)on cage (green) and the solvent (purple).\n990\n991\n3\n993 methylcyclohexane.\n994\n995 5. Fi(cid:427)ng Results in S8: The tri-exponen(cid:415)al fi(cid:427)ng model yields three dis(cid:415)nct (cid:415)mescales, yet\n996 the raw RSV traces (RSV1 and RSV2) appear qualita(cid:415)vely to reflect fewer dis(cid:415)nct transi(cid:415)ons.\n998 value.\n999\n1002 and RSV2 with the components resolved by the tri-exponen(cid:415)al model as shown in Fig. S8. Both\n1005 which we assign to solvent hea(cid:415)ng. Following the ini(cid:415)al rise (300-700 fs), both traces deviate\n1006 from a simple monotonic behavior: RSV1 shows a slowed ascent, whereas RSV2 displays a clear\n1007 decay. We a(cid:425)ribute these non-monotonic features to a fast methanolysis process, as captured\n1008 by the second kine(cid:415)c component. At later delays (700 fs-1 ps), the emergence of a third, slower\n1009 process is evident. This is indicated by a renewed, slower rise in RSV1 and a con(cid:415)nued gradual\n1010 decay of RSV2 into nega(cid:415)ve values. We a(cid:425)ribute these concurrent late-(cid:415)me behaviors to a\n1011 slower methanolysis pathway, as described by the third kine(cid:415)c component.\n1012\n1014 components. Accordingly, we performed a simultaneous tri-exponen(cid:415)al fit to RSV1 and RSV2,\n1015 which yielded three life(cid:415)mes corresponding to the processes described above. For\n1016 transparency, we have now annotated all exponen(cid:415)al fits within the figures (including Fig. S8)\n1017 with their corresponding R² values. These addi(cid:415)ons directly illustrate the goodness of fit and\n1018 enhance the interpretability of the models against the experimental data.\n1019\n1021 1. Lines 201-210: We have added an explanatory paragraph to the SVD Method sec(cid:415)on:\n1022\n1024 386 (± 25) fs, and 4.75 (± 2.4) ps, with an instrument response func(cid:415)on width of 151 (± 22)\n1025 fs at the FWHM. The earliest component (462 fs) corresponds to the ini(cid:415)al sub-300fs rise\n1026 observed in both RSV1 and RSV2 and is assigned to solvent hea(cid:415)ng, consistent with the\n1027 independent azo-dye-referenced analysis, which extracted a solvent hea(cid:415)ng life(cid:415)me of 470 fs\n1028 (Fig. S5). The intermediate component (386 fs) captures the non-monotonic evolu(cid:415)on from\n1029 300 to 700 fs, which is manifested as a suppressed rising in RSV1 concurrent with a clear decay\n1030 in RSV2. The component is a(cid:425)ributed to the fast methanolysis process. The slowest\n1031 component (4.75 ps) corresponds to a secondary slow rise in RSV1 and a persistent slower\n1032 decay in RSV2 beyond 700 fs, reflec(cid:415)ng a slower methanolysis process.\n1033\n1034\n1036\n1038 centrally important system CHBr . The experiments are state-of-the-art and the\n3\n1039 computa(cid:415)onal work suppor(cid:415)ng the experiments is well done and convincing. I strongly\n1040 recommend publica(cid:415)on.\n1041\n1043 recommenda(cid:415)on for publica(cid:415)on.\n1044\n1045 1. Just a minor point about some material in the SI. Imaginary frequencies are not -xyz cm-\n1046 1 but xyz i. This is a common error one sees in the literature, please correct this.\n1047\n1049 imaginary frequencies have been corrected in SI.\n1050\n1052 1. Lines 492-493:\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nI am sa(cid:415)sfied with the changes made and recommend publica(cid:415)on.\nAuthors’ response:\nWe thank the reviewer for recommending the revised manuscript for publica(cid:415)on.\nReviewer #2 (Remarks to the Author):\nUltrafast solvent mediated roaming.\nSu et al.\nThe authors addressed all my previous concerns. The quality of the manuscript has improved,\nand I strongly support its publica(cid:415)on. I have two comments, however, I think the authors should\naddress further and clarify. once these points are addressed i recommend publica(cid:415)on.\nAuthors’ response: We appreciate the Reviewer’s posi(cid:415)ve evalua(cid:415)on of our revisions and will\nprovide a point-by-point response to their new comments.\nComment:\nLines 103-105 of the revised version state: ‘The molecule (Reviewer: from S1, the Franck-Condon\nstate, line 103) then undergoes ultrafast intersystem crossing to the triplet state (T1), driven by\nthe strong spin-orbit coupling of the bromine atoms, ref. [42-44]. Lines 205-206 state ‘the CHBr2-\nBr appears ~100 fs later than dissocia(cid:415)on products…’.\nThis puts ISC at ~100 fs. I inspected refs. [42-44]. Ref. [44] reports efficient intersystem crossing\nin Br-NPB with ISC rate constant of 1.97 10^10 s^-1, which is ~500 fs. Ref. [44] calculates the\nfastest ISC (cid:415)me constant of ~330 fs for one ortho-Br compound. This is s(cid:415)ll much slower than\n100 fs. Ref. [42] reports ultrafast ISC (<100 fs) in PDI-Br4 but evidence comes from global analysis\nof very congested transient absorp(cid:415)on spectra and, in my opinion, is inconclusive.\nI would appreciate if the authors can strengthen their argument (100-fs ISC in Br-containing\nmolecules). SOC of Br-atom is 1130 cm-1 and much smaller than in iodine-containing systems (I-\natom, SOC 5069 cm-1), which indeed is known to undergo sub-100 fs ISC.\nAuthors’ response:\nWe thank the Reviewer for this construc(cid:415)ve sugges(cid:415)on. A systema(cid:415)c review of previous reports\nindicates that UV-vis excita(cid:415)on of CHBr3 pumps it into different excita(cid:415)on states: Ref. 5 showed\nthat 250 nm excita(cid:415)on launches the molecular wave packet on the S1 state with subsequent S1/S0\nconical intersec(cid:415)on; Ref. 8 indicated that excita(cid:415)on above 240 nm generates a triplet-singlet mixed\nmanifold, while 267 nm excita(cid:415)on predominantly populates the lowest lying triplet (T1), possibly\nwith contribu(cid:415)ons from slightly higher lying triplet states (T2, T3); Ref. 28 found that 255 nm\nexcita(cid:415)on predominantly produces singlet excited state (Ã ¹A₂ and ˜B ¹E), with contribu(cid:415)ons from\na triplet state (˜c ³A₁); the newly added Ref. 45 (Struct. Dyn. 6, 054304 (2019)) indicated that 268\nnm excita(cid:415)on populates the T1 (˜a ³A ) electronic surface; and an early gas-phase ab ini(cid:415)o\n2\ncalcula(cid:415)on study (Ref. 1) showed that the first triplet excited state (T1) is accessible at  < 297 nm,\nwhile the lowest singlet excited state (e.g. S1 (Ã ¹A₂) and S2 (˜B ¹E)) require wavelengths of 243 nm\nand shorter. Both S1 and T1 states are repulsive towards CHBr2· + Br· with the former having a\nstronger dissocia(cid:415)ve character.\nIn summary, the 267 nm excita(cid:415)on used in our experiment is expected to populate a mixed\nmanifold occupied by both the singlet (S1) and triplet (T1, and possibly higher b(cid:518) 3E (T2) and ˜c ³A₁\n(T3)) states, with S1 likely domina(cid:415)ng. In this scenario, T1 state is populated either directly and\nimmediately upon excita(cid:415)on due to the spin-mixed character of the manifold (allowed transi(cid:415)on\nwith spin inversion due to the strong SOC), or indirectly via ISC from S1 state and internal\nconversion from poten(cid:415)al higher triplet excited states, which could introduce a delay of several\nhundred fs for the isomer forma(cid:415)on, as correctly noted by the Reviewer. While the ISC and internal\nconversion lead to delayed roaming dynamics, the directly populated T1 state within the mixed\nmanifold can immediately ini(cid:415)ate ultrafast roaming dynamics without addi(cid:415)onal ISC or internal\nconversion steps. Therefore, the observed ~100 fs delay in the isomer forma(cid:415)on could represent a\ncombina(cid:415)on of the delayed ISC/internal conversion processes and the ultrafast roaming dynamic\nfrom directly populated T1 state. Although our TRXSS measurement is not directly spin-sensi(cid:415)ve,\nour structural analysis shows that the excited-state popula(cid:415)on departs from the ini(cid:415)al Franck–\nCondon geometry, and the isomer species appear with an observable delay, which is consistent\nwith this interpreta(cid:415)on. Moreover, the asymmetric rise of the roaming-mediated isomer species in\nFigure 3e may indicate the combined process of gradual popula(cid:415)on of the roaming intermediate\non T1.\nChanges in manuscript:\n1. Lines 102-108 “Photoexcita(cid:415)on of CHBr3 pumps it from the ground state (S₀) to the first singlet\nexcited state (S₁). The molecule then undergoes ultrafast intersystem crossing to the triplet excited\nstate (T₁), driven by the strong spin-orbit coupling of the bromine atoms42-44. From T₁, the molecule\ncan either dissociate via C–Br bond elonga(cid:415)on to yield Br radicals, or undergo structural\nrearrangement leading to alterna(cid:415)ve isomeric products5” was changed to “Photoexcita(cid:415)on of\nCHBr pumps it from the ground state (S₀) to a mixed manifold occupied by both the singlet (S )\n3 1\nand triplet (T , and possibly higher b(cid:518) 3E (T ) and ˜c ³A₁ (T )) states, with S likely domina(cid:415)ng. Both\n1 2 3 1\nS and T states are repulsive towards CHBr · + Br· with the former having a stronger dissocia(cid:415)ve\n1 1 2\ncharacter. The T state is populated either directly upon excita(cid:415)on or indirectly via an intersystem\n1\ncrossing (ISC) from the S state and internal conversion from possible higher triplet states1,5,8,28,42-\n1\n45. From T₁, the molecule undergoes structural rearrangement leading to alterna(cid:415)ve isomeric\nproducts5.”\n2. Lines 208-211 “This (cid:415)me delay results from the forma(cid:415)on of the T poten(cid:415)al energy surface,\n1\nwhich is populated either directly upon excita(cid:415)on or indirectly via ISC from S state and internal\n1\nconversion from higher triplet excited states within the mixed excita(cid:415)on manifold.” was added.\n3. Lines 283-286 “Our simula(cid:415)ons were conducted on the T1 excited state due to the ultrafast\nintersystem crossing from S1 to T1, which makes the triplet excited state the dominant channel for\nthe subsequent photochemical dynamics” was changed to “Our simula(cid:415)ons were conducted on\nthe T1 excited state due to the ultrafast ISC from S1 to T1, which makes the triplet excited state the\ndominant channel for the subsequent photochemical dynamics specifically relevant for the\nroaming process.”\nComment:\nThe CHBr2-Br roams on the T1 surface where it appears with a delay cf. to radical dissocia(cid:415)on\nproducts (lines 205-206, methylcyclohexane) and, I think, this observa(cid:415)on provides a ra(cid:415)onale\nfor the authors to introduce ISC from S1. Although the authors of ref. [8] express an opinion that\n240-270 nm excita(cid:415)on ini(cid:415)ally produces the triplet excited state of CHBr3, singlet-to-singlet\nexcita(cid:415)on character may s(cid:415)ll be predominant in this region, and especially, in the short-\nwavelength part of it. Excita(cid:415)on at 267 nm used in the present work may also promote CHBr3\ninto high-energy E or A2 triplets internal conversion from which may lead to the delayed\npopula(cid:415)on of the T1.\nThe methanolysis reac(cid:415)on is on the S0 surface (Sec(cid:415)on 8 of the SI), which is reached from T1 via\nT1/S0 intersec(cid:415)on. Does this intersec(cid:415)on play a role in methylcyclohexane and when does the\niso-form reach the ground electronic state in this solvent? Line 248 states….weak oscilla(cid:415)ons\n(150-600 fs) reflect vibra(cid:415)onal relaxa(cid:415)on around the stabilized isomer…. Does this mean in S0?\nFigure 3b says roaming con(cid:415)nues within 300 fs and the roaming is a(cid:425)ributed to the T1 surface, I\nunderstood.\nAuthors’ response:\nWe thank the Reviewer for this percep(cid:415)ve observa(cid:415)on. We agree that the ~100 fs delay observed\nfor the CHBr2-Br roaming provides a ra(cid:415)onale for introducing an ISC step from S1 and internal\nconversion from other excited state like b(cid:518) 3E (T2), as discussed in our response above.\nWe also appreciate this opportunity to clarify the roaming dynamics in methylcyclohexane. Our\nDFT calculated T /S intersec(cid:415)on (SI Sec(cid:415)on 8) is applicable to the roaming dynamics in both\n1 0\nmethanol and methylcyclohexane, and we believe that it plays a similar role in methylcyclohexane.\nOur structural analysis indicates that the roaming mo(cid:415)on of the Br atom and CHBr fragments likely\n2\nproceeds on the T1 excited surface within the first 100 fs, and a hot CHBr2-Br isomer is formed by\n~150 fs in the S0 state. Consequently, the weak oscilla(cid:415)ons observed from 150 to 600 fs, which\nreflect vibra(cid:415)onal relaxa(cid:415)on around the stabilized isomer, are on the S poten(cid:415)al energy surface.\nroaming pathway from the ini(cid:415)ally excited CHBr to the stabilized isomer, which includes both the\n3\nroaming dynamics on the T excited state (before 150 fs) and the subsequent vibra(cid:415)onal relaxa(cid:415)on\n1\non S (a(cid:332)er 150 fs). We appreciate the Reviewer for highligh(cid:415)ng the ambiguity and have revised\nChanges in manuscript:\n1. Fig. 3b has been revised as shown below.\n2. Lines 234-240: “The observed ~100 fs delay reflects the early-(cid:415)me electronic and nuclear\ndynamics of the spin-mixed n(Br)→σ*(C–Br) excited-state manifold populated at 267 nm. Following\nphotoexcita(cid:415)on, the wavepacket rapidly departs the Franck–Condon region, undergoes ultrafast\nspin equilibra(cid:415)on and internal conversion among nearly degenerate singlet and triplet states, and\napproaches regions of strong nonadiaba(cid:415)c coupling between T₁ and S₀. Forma(cid:415)on of the CHBr₂–\nBr roaming configura(cid:415)on thus occurs only a(cid:332)er this joint electronic–nuclear evolu(cid:415)on, naturally\naccoun(cid:415)ng for the measured temporal offset.” was added.\n3. Lines 261-266: “The roaming dynamics on T1 ul(cid:415)mately relax to the ground-state (S0) surface\nvia T1/S0 interac(cid:415)on, as evidenced by our DFT calcula(cid:415)ons (Fig. S27; see SI Sec(cid:415)on 8 for details).\nOur structural analysis shows that the Br atom and CHBr2 fragment roaming most likely proceeds\non T1 surface within the first 100 fs and yields a hot CHBr2-Br isomer by ~150 fs in the S0 state.\nConsequently, the weak oscilla(cid:415)ons observed from 150 to 600 fs, which reflect vibra(cid:415)onal\nrelaxa(cid:415)on around the stabilized isomer, are assigned to dynamics on the S0 poten(cid:415)al energy\nsurface.” was added.\nReviewer #3 (Remarks to the Author):\nAll ques(cid:415)ons and clarifica(cid:415)ons have been thoroughly addressed. The responses provided were\naccurate, comprehensive, and appropriate to the context. I am sa(cid:415)sfied with the resolu(cid:415)on of\nall points raised and fully support the publica(cid:415)on.\nAuthors’ response:\nWe thank the reviewer for recommending the revised manuscript for publica(cid:415)on.\nREVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nAll clarifications have been satisfactorily made. I am satisfied with\nthe resolution of all issues brought up and support the publication.\nAuthors’ response: We thank the reviewer for recommending publication\nof the revised manuscript."}]

Decision letter