# Acenaphthene Derivatives as Signatures of C11H9⁺ Reactivity with Methylated Naphthalenes

Ana Isabel Lozano, Anthony Bonnamy, Aude Simon, Christine Joblin

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Ana Isabel Lozano, Anthony Bonnamy, Aude Simon, Christine Joblin. Acenaphthene Derivatives as Signatures of C11H9⁺ Reactivity with Methylated Naphthalenes. 2025. hal-05447383

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# Acenaphthene Derivatives as Signatures of
$C_{11}H_9^+$ Reactivity with Methylated
Naphthalenes

Ana I. Lozano,† Anthony Bonnamy,† Aude Simon,‡ and Christine Joblin*,†

†Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse,
CNRS, CNES, 9 Avenue du Colonel Roche, F-31028 Toulouse, France

‡Laboratoire de Chimie et Physique Quantiques LCPQ/FeRMI, Université de Toulouse,
CNRS, 118 Route de Narbonne, 31062 Toulouse, France

E-mail: christine.joblin@cnrs.fr

## Abstract

The $-H$ cation of methyl-naphthalene (MeNp), $C_{11}H_9^+$, consists of three long-lived isomers: 1-naphthylmethylium, 2-naphthylmethylium, and benzyltropylium. Here we show that this species is also the dominant fragment cation formed from dimethyl-naphthalene (diMeNp), regardless of the methyl-substitution pattern. Using the multiplex capabilities of PIRENEA, a setup dedicated to laboratory astrophysics, we studied the reactivity of $C_{11}H_9^+$ with diMeNp under isolated conditions relevant to radiative association. Two reaction products are observed, $C_{12}H_{11}^-$ also formed in the reaction with MeNp - and $C_{13}H_{13}^+$, with branching ratios that depend on the specific diMeNp isomer. The reaction products were subsequently exposed to UV-visible irradiation to gain insight into their structures. The acenaphthylene radical cation, $C_{12}H_8^+$, was identified as the most stable photofragment. We show that this experimental approach,
supported by density functional theory calculations and molecular dynamics simula-
tions, provides new constraints on the chemistry of benzylium-type species. It reveals
a chain-growth mechanism followed by ring closure and H migration, which allows us
to assign each reaction product to a specific naphthylmethylium isomer. Moreover, the
chemistry uncovered here highlights new pathways for the formation of pentagonal rings
during PAH growth under low-pressure and cold conditions. In particular, it can lead
to efficient formation of acenaphthylene-like species, recently detected in the TMC-1
cold cloud.

# 1 Introduction

A recent milestone in astrochemistry is the identification of individual gas-phase polycyclic aromatic hydrocarbons (PAHs) in the cold, dense cloud TMC-1. Using the Green Bank Telescope (GBT), McGuire et al. reported the detection of 1- and 2-cyanonaphthalene.¹ Shortly afterward, the pure bicyclic aromatic indene was detected by Cernicharo et al.² with the highly sensitive receivers of the Yebes 40 m radio telescope, and this detection was soon confirmed at the GBT.³ Since then, additional PAHs have been identified, including cyano-acenaphthylene C₁₂H₇CN (1- and 5-isomers),⁴ phenalene C₁₃H₁₀,⁵ cyano-pyrene C₁₆H₉CN (1-, 2- and 4-isomers)⁶,⁷ and cyano-coronene C₂₄H₁₁CN, the largest PAH identified so far.⁸ Most PAHs detected to date are cyano-substituted species, whose large dipole moments significantly enhance their radio detectability.

The formation of cyano-PAHs under the cold conditions of dark molecular clouds is thought to result from reactions between PAHs and CN radicals; for benzene and toluene, this reaction has been shown to be barrierless.⁹⁻¹¹ This connection makes it possible to infer the abundances of the unsubstituted parent PAHs from the observed abundances of their cyano derivatives.⁷,⁸ The inferred values indicate that PAHs must be produced efficiently in cold clouds.¹² Although the underlying processes remain uncertain, current evidence favors a bottom-up gas-phase chemistry.⁴,⁵,¹³ Moreover, consistent with the pioneering work of
Herbst and Klemperer on dense-cloud chemistry,¹⁴ these findings further highlight the key role of ion-molecule reactions in building molecular complexity.

PAHs – including methylated and, more broadly, alkylated PAHs – have been found to be abundant in laboratory analyses of primitive Solar System matter, particularly in carbonaceous chondrite meteorites¹⁵⁻¹⁹ and, more recently, in samples from the asteroid Ryugu returned by the Hayabusa-2 mission.²⁰,²¹ Zeichner et al.²² investigated the ¹³C isotopic composition of several small PAHs in Ryugu material and concluded that these molecules were formed through distinct chemical processes: some at the very low temperatures characteristic of molecular clouds (~10 K), and others at much higher temperatures (>1000 K) that may prevail in circumstellar environments or within the parent body.

Methylated PAHs may contribute to the 3.4 µm emission band,²³ which appears as a satellite feature of the 3.3 µm aromatic infrared band (AIB) – both of which are key targets of James Webb Space Telescope (JWST) observations in the Milky Way and in external galaxies.²⁴⁻²⁶ The 3.4 µm band is known to be particularly sensitive to UV irradiation,²³,²⁷ consistent with its carriers containing aliphatic C–H bonds, either in alkyl side groups or in the form of superhydrogenated PAHs with additional H atoms.²⁸,²⁹

Previously, we investigated the properties of the C₁₁H₉⁺ ions – the H-loss fragments of methyl-naphthalene cations – and highlighted the relevance of these ions in environments exposed to mild UV irradiation due to their photophysical properties.³⁰ Three long-lived isomers were identified: 1- and 2-naphthylmethylium (1- and 2-NpCH₂⁺) and benzyltropylium (BzTr⁺). Their reactions with the parent neutral lead to two products of unknown structure, C₁₂H₁₁⁺ and C₂₂H₁₉⁺, the latter corresponding to an adduct. It was further shown that BzTr⁺ is unreactive, whereas the other two isomeric populations – very likely 1- and 2-NpCH₂⁺ – display reaction rate constants differing by an order of magnitude.

This article investigates the reactivity of C₁₁H₉⁺ ions with several isomers of dimethyl-naphthalene (diMeNp), namely 2,3-, 1,4-, and 1,5-diMeNp. Using the multiplex capabilities of PIRENEA, a setup dedicated to laboratory astrophysics, we studied these reactions under
isolated conditions relevant to radiative association. Moreover, the reaction products were exposed to UV-visible light to gain further insight into their structures and to identify fragments with enhanced photostability. We show that this experimental approach, supported by DFT calculations, not only improves our understanding of the chemistry of these species – including isomer-specific effects – but also reveals new chemical pathways for the formation of cosmic PAHs under cold and mild-UV irradiation conditions.

<figure><img src="image_3.png" /><figcaption>Figure 1: Top row: molecular structures of 1,4-, 1,5-, and 2,3-dimethylnaphthalene cations (1,4-, 1,5, and 2,3-diMeNp•⁺). Bottom row: molecular structures of the three expected long-lived isomers of C₁₁H₉⁺ cations: benzyltropylium (BzTr⁺), 1-, and 2-naphthylmethylonium (1- and 2-NpCH₂⁺).</figcaption></figure>

# 2 Methodologies

## 2.1 Experimental Methods

The experiments were performed using the PIRENEA setup,³¹ operated under the configuration described in detail elsewhere.³⁰ The apparatus consists of a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR-MS) maintained under ultra-high vacuum conditions (~ 10⁻⁹ mbar at room temperature), enabling ion trapping under isolated conditions and for long time scales. PIRENEA can also be run at cryogenic temperatures but
this capability was not used in this study due to the sticky nature of the injected molecules.
When required, specific ions in the ICR cell (e.g., ¹²C isotopologs) were isolated by applying
selective excitation of their cyclotron motion.

$C_{11}H_9^+$ cations were produced from 2,3-, 1,4-, and 1,5-diMeNp (Figure 1), all of which possess sufficient vapor pressure at room temperature to be used in our experiments. All precursors were ionized using the fourth harmonic (266 nm, 4.66 eV) of a Nd:YAG Surelite I laser (Continuum), and the resulting ions were trapped in the ICR cell. The production of $C_{11}H_9^+$ was optimized by photoprocessing of the trapped parent ions using a high-pressure Xe arc lamp equipped with a 610 nm color filter (CF). Typical mass spectra illustrating the production of $C_{11}H_9^+$ from 1,4-diMeNp is shown in Figure S1.

Reactions between the $C_{11}H_9^+$ ions and their neutral precursors were performed at a typical pressure of ~ $10^{-8}$ mbar. The reaction time was adjusted to ensure complete conversion of the reactive ion population.

Photodissociation kinetics were recorded under broadband visible irradiation from a Xe arc lamp. A 400 nm CF filter was used, with or without a neutral-density filter (NDF) providing 37% transmission.

Multiple-photon dissociation (MPD) spectroscopy was performed with a mid-band tun-
able optical parametric oscillator (OPO) laser (Panther EX, Continuum), operating at 10 Hz
with 5 ns pulse duration. The dissociation yield was recorded as a function of OPO wave-
length using a fixed irradiation time of 10 s and a pulse energy of 5.5 mJ.

For all experiments, the recorded intensities were normalized to the summed ion intensity
to account for variations in absolute ion intensities across different measurements.

**2.2 Computational Methods**

Following the methodology proposed by Simon et al.³², we performed molecular dynamics
(MD) simulations to investigate the dissociation of the three diMeNp cations studied here.
In this approach, molecular dissociation is modeled in the electronic ground state, with the
internal energy represented as vibrational excitation. This regime is consistent with the
building of internal energy in the cations by absorption of multiple photons from the Xe
lamp.³³ However, the energies need to be sufficiently high for dissociation to occur within
the nanosecond timescale accessible to the simulation. This approach has been recently
applied to rationalize the dissociation dynamics of both MeNp radical cations ($C_{11}H_{10}^{+}$) and
their $C_{11}H_{9}^{+}$ fragments.³⁰

Briefly, the MD simulations were carried out in the Born-Oppenheimer approximation; the electronic structure is computed on-the-fly using the self-consistent charge density functional based tight-binding (SCC-DFTB) method,³⁴ employing the recently revised C-H parameter set.³⁵ This explicit treatment of the electronic structure enables the description of chemical evolution processes such as isomerization and dissociation. For simplicity, this approach will hereafter be referred to as MD/DFTB.

Each simulation was initiated from the ground-state optimized geometry with random-
ized initial velocities, propagated in the microcanonical (NVE) ensemble. Energies and
gradients were computed every 0.2 fs. All MD/DFTB simulations were performed using the
deMonNano code.³⁶ In the present work, a total of 180 simulations were run for the three
diMeNp cations at internal energies of 15.0 and 16.5 eV, for simulation durations of 2 ns and
1 ns, respectively. This strategy enables insight into reaction kinetics, branching ratios, and
underlying mechanisms and thus contributes to the rationalization of experimental results.

In addition to MD/DFTB simulations, DFT local optimisations and all atoms harmonic
frequency calculations were performed at the B3LYP/6-31G(d,p) level of theory to support
and interpret experimental results. Unless it is explicitly mentioned, all reported molecular
structures were obtained at this level of theory and all reported energy values include zero-
point energy corrections. In specific cases, wavefunction (CCSD(T)/cc-pvtz) single point
energy calculations or vertical Time-Dependent DFT calculations (PBE0 functional³⁷) were
performed. All these calculations were achieved with the Gaussian16 suite of programs.³⁸
# 3 Results

We present our results on the production of $C_{11}H_9^+$ species from the various investigated dimethylnaphthalene ions (Figure 1), providing evidence for the formation of multiple isomers. These observations are interpreted in the context of previous experimental studies and MD simulations. We then examine the reactions between $C_{11}H_9^+$ and their neutral di-MeN precursors, followed by a detailed characterization of the photophysics of the resulting products.

## 3.1 $C_{11}H_9^+$ as a common photofragment

### 3.1.1 Experimental Insights

Our first key experimental finding is that photodissociation of the diMeNp cations yields $C_{11}H_9^+$ at $m/z$ 141 as the dominant fragment, regardless of the methyl substitution pattern. A similar behavior is observed for the MeNp cations, for which $C_{11}H_9^+$ is produced as the exclusive photofragment.30,39 In all cases, the $C_{11}H_9^+$ cation further dissociates by losing $C_2H_2$, producing $C_9H_7^+$ cations, which were previously assigned to indenylium when 1-MeNp was used as the precursor.30 Unlike the photodissociation of MeNp cations, a very minor H-loss channel was detected for diMeNp when irradiated at 266 nm (see Figure S1), accounting for at most ~3% of the total dissociation yield.

Figure 2 shows the photofragmentation kinetics under irradiation with the Xe lamp of the $C_{11}H_9^+$ species produced from all diMeNp investigated. All curves display a common pattern: an initial fast decay followed by a slower one, regardless of the precursor. A similar behavior was previously observed for $C_{11}H_9^+$ ions produced from 1-MeNp30 and was attributed to the presence of three long-lived isomers –1-NpCH$_2^+$, 2-NpCH$_2^+$, and BzTr$^+$– each exhibiting distinct absorption features in the visible range.30,40 One could therefore expect the presence of the same isomers for the $C_{11}H_9^+$ species produced from diMeNp, given the structural similarities with MeNp. Indeed, this is further corroborated by the MD/DFTB simulations,
as discussed in Section 3.1.2.

<figure><img src="image_4.png" /><figcaption>Figure 2: Normalized intensity of C₁₁H₉⁺ species as a function of the irradiation time using the Xe arc lamp with a CF at 400 nm. Dashed lines represent fits obtained with a two-exponential decay model (Table S1). The C₁₁H₉⁺ species were produced from different precursors (see text for further details).</figcaption></figure>

As noted previously,³⁰ the photofragmentation kinetics curves must be interpreted with caution, since they reflect the temporal evolution of isomer populations governed by the competition between dissociation and isomerization, which in turn strongly depends on the internal energy distribution of the absorbing cations. The slower decay component is expected to reflect the dissociation of the BzTr⁺, since this isomer exhibits the lowest absorption cross section in the visible range.³⁰,⁴⁰ Conversely, the faster decay is attributed to both NpCH₂⁺ isomers, as their photoabsorption dominates in this spectral region.³⁰,⁴⁰

### 3.1.2 Theoretical Insights

To gain further insights into the dissociation dynamics of the three investigated diMeNp cations, we performed MD/DFTB simulations following the procedure described in Section 2.2. At the two examined energies (15 and 16.5 eV), CH₃ loss is the dominant dissociation pathway, leading to the formation of different isomers of C₁₁H₉⁺. The branching
ratios (BRs) between the different isomers of $C_{11}H_9^+$ are summarized in Tables 1 and S2. The 1-NpCH$_2^+$ isomer is found to be the most abundant isomer when fragmenting 1,4- and 1,5-diMeNp cations at the two studied energies. The predominance of this isomer is even more striking in the case of 1,5-diMeNp$^{•+}$. A representative trajectory of the MD simulations of 1,5-diMeNp$^+$ resulting in 1-NpCH$_2^+$ is given in Figure 3. It starts with a H migration from the CH$_3$ group to a carbon belonging to the carbon ring (step (1)). The introduction of an sp$^3$ carbon into the ring weakens the C–C bond, leading to ring opening (step (2)). This is followed by ring closure, facilitated by the electronic unsaturation of the free carbon chain (step (3)). In the intermediate formed, the CH$_3$ group is attached to an sp$^3$ carbon within the ring, resulting in a weaker C–CH$_3$ interaction compared to 1,5-diMeNp$^{•+}$. Its detachment restores aromaticity, yielding 1-NpCH$_2^+$.

<table><caption>Table 1: Branching ratios for the dissociation of 1,4-, 1,5-, and 2,3-diMeNp cations, obtained from MD/DFTB simulations over a 2 ns timescale at 15 eV internal energy. The values were averaged over 180 simulations with randomly assigned velocity distributions. Note: # denotes the number of occurrences.</caption><thead><tr><th>Precursor</th><th>1,4-diMeNp</th><th>1,5-diMeNp</th><th>2,3-diMeNp</th></tr></thead><tbody><tr><td>#1-NpCH<sub>2</sub><sup>+</sup>/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.59</td><td>0.79</td><td>0.11</td></tr><tr><td>#2-NpCH<sub>2</sub><sup>+</sup>/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.11</td><td>0.08</td><td>0.64</td></tr><tr><td>#BzTr<sup>+</sup>/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.17</td><td>0.04</td><td>0.15</td></tr><tr><td>#Others/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.13</td><td>0.09</td><td>0.10</td></tr></tbody></table>

As expected, the fragmentation of 2,3-diMeNp$^{•+}$ predominantly yields 2-NpCH$_2^+$ fragments, although a minor fraction of 1-NpCH$_2^+$ is also produced – a situation that is comparable to the case of 1,4-diMeNp$^{•+}$, but with the relative abundances of the NpCH$_2^+$ isomers inverted. A representative mechanism is shown in Figure 4. As illustrated, the loss of CH$_3$ is preceded by H migration, which generates an sp$^3$ carbon atom bonded to the CH$_3$ group (step (1)). This weakens the C–CH$_3$ bond, facilitating the subsequent loss of CH$_3$. In addition, a significant fraction of BzTr$^+$ is produced from 2,3- and 1,4-diMeNp$^{•+}$, which is not the case for 1,5-diMeNp$^{•+}$. This can be rationalized by the expansion into a 7-membered ring as shown in Figure S5 for 2,3-diMeNp$^{•+}$.
<figure><img src="image_5.png" /><figcaption>Figure 3: Snapshots from a 15.0 eV MD/DFTB simulation illustrating the dissociation of 1,5-diMeNp⁺ into 1-NpCH₂⁺.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 4: Snapshots from a 15.0 eV MD/DFTB simulation illustrating the dissociation of 2,3-diMeNp⁺ into 2-NpCH₂⁺.</figcaption></figure>

## 3.2 Reactions between C₁₁H₉⁺ and dimethylated naphthalenes

In previous work,³⁰ we observed the formation of C₁₂H₁₁⁺ at m/z 155 and an adduct C₂₂H₁₉⁺ at m/z 283 when C₁₁H₉⁺, produced from 1-MeNp, reacted with its neutral precursor. Here, we extend this investigation to the reactivity of C₁₁H₉⁺ species generated from diMeNp with their neutral precursors. In this case, the reaction products do not include any adduct but in addition to C₁₂H₁₁⁺ (eq 1), another product, C₁₃H₁₃⁺ at m/z 169 (eq 2) is observed. This product is particularly abundant in reactions involving 2,3-diMeNp (see Figure S2). The reaction channels observed at 300 K can therefore be summarized as:

$$ \underset{141}{\mathrm{C}_{11}\mathrm{H}_{9}^{+}} + \underset{156}{\mathrm{C}_{12}\mathrm{H}_{12}} \rightarrow \underset{155}{\mathrm{C}_{12}\mathrm{H}_{11}^{+}} + 142 \mathrm{u} \qquad (1) $$

$$ \rightarrow \underset{169}{\mathrm{C}_{13}\mathrm{H}_{13}^{+}} + 128 \mathrm{u}, \qquad (2) $$
with the neutral products at 142 u (reaction 1) and 128 u (reaction 2) likely corresponding to methyl-naphthalene and naphthalene.

Table 2 summarizes the relative populations of reactive and nonreactive channels under complete reaction conditions (see Section 2.1). The BR between the two observed products varies notably depending on the specific diMeNp isomer. For 1,4- and 1,5-diMeNp, $C_{12}H_{11}^+$ is the dominant product, whereas $C_{13}H_{13}^+$ predominates in the case of 2,3-diMeNp. These experimental findings can result from a combination of factors, including differences in the relative populations of the 1-NpCH$_2^+$ and 2-NpCH$_2^+$ isomers, as well as the reactivity of the neutral reactants, which may favor one reaction pathway over another. In all cases, the non-reactive fraction, with relative abundances of 11–18%, is attributed to $BzTr^+$ based on our previous study.³⁰ This trend is generally consistent with our MD simulations (Tables 1 and S2), although larger discrepancies are observed for 1,5-diMeNp compared to the other two precursors.

<table><caption>Table 2: Relative populations of both the reactive and nonreactive channels for reactions between $C_{11}H_9^+$ and their neutral precursors.</caption><thead><tr><th rowspan="2">Precursor/<br>Neutral reactant</th><th colspan="2">Reaction</th><th>No Reaction</th></tr><tr><th>C<sub>12</sub>H<sub>11</sub><sup>+</sup></th><th>C<sub>13</sub>H<sub>13</sub><sup>+</sup></th><th>C<sub>11</sub>H<sub>9</sub><sup>+</sup></th></tr></thead><tbody><tr><td>2,3-diMeNp</td><td>0.07</td><td>0.79</td><td>0.14</td></tr><tr><td>1,4-diMeNp</td><td>0.70</td><td>0.12</td><td>0.18</td></tr><tr><td>1,5-diMeNp</td><td>0.83</td><td>0.06</td><td>0.11</td></tr></tbody></table>

### 3.3 Photodissociation of reaction cationic products

We first focus on $C_{12}H_{11}^+$ species, which are produced when $C_{11}H_9^+$ ions react with the various diMeNp isomers, except for 2,3-diMeNp, where the relative abundance of $C_{12}H_{11}^+$ was too low. This set of experiments also includes the $C_{12}H_{11}^+$ product formed from 1-MeNp, prepared following the previously described procedure.³⁰

The photodissociation of all $C_{12}H_{11}^+$ species under visible irradiation from the Xe lamp follows the same pathway, as illustrated by the representative mass spectrum in Figure S3.
This involves the loss of either 2H or H₂, with the resulting C₁₂H₉⁺ cation further dissociating through hydrogen loss to yield C₁₂H₈⁺ at *m/z* 152 (eq 3).

$$ \underset{155}{C_{12}H_{11}^{+}} \xrightarrow{\underset{h\nu}{-2H/H_2}} \underset{153}{C_{12}H_{9}^{+}} \xrightarrow{\underset{h\nu}{-H}} \underset{152}{C_{12}H_{8}^{+}} \quad (3) $$

Figure 5 shows the photofragmentation kinetics of C₁₂H₁₁⁺ species recorded under broad-band irradiation for $\lambda \gtrsim 400$ nm. Slight differences are observed in the decay of the parent ions. The C₁₂H₁₁⁺ species formed from 1-MeNp exhibit the fastest decay, whereas those formed from 1,5-diMeNp show a dissociation delay of approximately 1.5 s under the lower irradiation conditions. The dissociation of PAH cations under Xe lamp irradiation was described by Boissel et al.³³, who showed that irradiation builds an internal energy distribution, and dissociation is primarily driven by single-photon absorption by ions with the highest internal energy. Thus, a delay in dissociation can be interpreted as a lower absorption cross section in the visible range, and hence a reduced heating efficiency. This points to different isomers being involved with different absorption cross sections. A similar reasoning applies to the dissociation of the primary fragment C₁₂H₉⁺ at *m/z* 153 (Figure 6).
<figure><img src="image_7.png" /></figure>

Figure 5: Normalized intensity of C₁₂H₁₁⁺ as a function of irradiation time using a Xe arc lamp under two conditions. Left panel: CF at 400 nm + NDF. Right panel: CF at 400 nm alone. Solid lines represent exponential decay fits (Table S1). The C₁₂H₁₁⁺ ions were generated from different precursors (see text for further details).

<figure><img src="image_8.png" /></figure>

Figure 6: Normalized fragment intensities from the photodissociation kinetics of C₁₂H₁₁⁺ under irradiation with a Xe arc lamp. Left panel: CF at 400 nm + NDF. Right panel: CF at 400 nm alone.

The C₁₃H₁₃⁺ products were studied for the reaction of C₁₁H₉⁺ with 2,3-diMeNp. A representative mass spectrum after irradiation is shown in Figure S3, and the dissociation kinetics are presented in Figure S4. Detailed kinetics curves (Figure 7) reveal two primary fragments, C₁₂H₁₀⁺ and C₁₁H₉⁺, with branching ratios of around 60% and 40%, respectively, independent
of photon flux. The first dissociation channel involves a mass loss of 15 u, consistent with CH₃ elimination (Eq 4). The resulting C₁₂H₁₀⁺ then undergoes successive H losses (Figure 7) to form C₁₂H₈⁺. The second channel proceeds via a 28 u loss, yielding C₁₁H₉⁺ cations, which can further dissociate into C₉H₇⁺ and neutral C₂H₂ (Eq 5). The photofragmentation channels are summarized as follows:

$$ \underset{169}{C_{13}H_{13}^{+}} \xrightarrow{\substack{-15\,\text{u} \\ \text{h}\nu}} \underset{154}{C_{12}H_{10}^{+}} \xrightarrow{\substack{-\,\text{H} \\ \text{h}\nu}} \underset{153}{C_{12}H_{9}^{+}} \xrightarrow{\substack{-\,\text{H} \\ \text{h}\nu}} \underset{152}{C_{12}H_{8}^{+}} \quad (4) $$

$$ \underset{\text{h}\nu}{-28\,\text{u}} \rightarrow \underset{141}{C_{11}H_{9}^{+}} \xrightarrow{\substack{-C_{2}H_{2} \\ \text{h}\nu}} \underset{115}{C_{9}H_{7}^{+}} \quad (5) $$

<figure><img src="image_9.png" /><figcaption>Figure 7: Normalized fragment intensities from the photodissociation kinetics of C₁₃H₁₃⁺ species under two irradiation conditions. Left panel: CF at 400 nm + NDF. Right panel: CF at 400 nm alone.</figcaption></figure>

We further investigated the C₁₃H₁₃⁺ species using multiple-photon dissociation (MPD) spectroscopy (see Section 2.1). The recorded MPD spectrum is shown in Figure S6, displaying both the total dissociation yield and the contributions from the individual fragmentation channels. Overall, a broad feature is observed extending from 430 to 610 nm, with no significant dissociation observed at longer wavelengths.
Analysis of the secondary fragments (Figure S6) reveals three additional trends: (i) $C_{12}H_{10}^+$ shows negligible dissociation in the 450–550 nm range and absorbs at shorter and longer wavelengths; (ii) $C_{12}H_9^+$ does not absorb at wavelengths above ~570 nm; and (iii) $C_{11}H_9^+$ mainly dissociates in the 500–580 nm range, producing $C_9H_7^+$.

Finally, because $C_{12}H_8^+$ species was identified as a common final photofragment of both $C_{12}H_{11}^+$ and $C_{13}H_{13}^+$, we examined this species in more detail. To this end, we isolated the $C_{12}H_8^+$ cations produced via Eqs 3 and 4, using parent cations generated from 1-MeNp, 1,4-diMeNp, and 2,3-diMeNp. In all cases, $C_{12}H_8^+$ was found to be remarkably photostable compared with the other studied species, both under visible broadband irradiation and under narrowband irradiation at 266 nm. Moreover, $C_{12}H_8^+$ was observed to undergo charge-transfer reactions with 1-MeNp and with all investigated diMeNp isomers. Figure 8 presents two representative spectra illustrating both the photostability and the charge-transfer reactions of $C_{12}H_8^+$ produced from parent cations generated from 1-MeNp.

<figure><img src="image_10.png" /><figcaption>Figure 8: Top panel: mass spectrum recorded after broadband irradiation (10 s, $\lambda \gtrsim 400$ nm) of the $C_{12}H_8^+$ species. Bottom panel: mass spectrum recorded after reactions (charge transfer) between $C_{12}H_8^+$ and neutral 1-MeNp. In both cases, the $C_{12}H_8^+$ species were generated from the 1-MeNp precursor.</figcaption></figure>
# 4 Discussion

The results presented in Section 3 establish $C_{11}H_9^+$ as a key intermediate in the photodissociation and subsequent reactivity of methylated naphthalenes. Its prominence as a common photofragment of both diMeNp and MeNp, together with its ability to react with these species, leads to the formation of two primary reaction products, $C_{12}H_{11}^+$ and $C_{13}H_{13}^+$. Photophysical characterization of these two products further reveals distinct fragmentation pathways.

In the following sections, we examine how our experimental findings data support the formation of acenaphthene derivatives and can be used to propose chemical scenarios that could account for the production of these species.

## 4.1 Evidence for acenaphthene derivatives

Among the candidate structures for the $C_{12}H_{11}^+$ products, our results point to protonated acenaphthene, $[ACN+H^+]$ (Figure 9). This assignment is supported by dissociation via $2H/H_2$ loss, consistent with the results reported by Szczepanski et al.⁴¹, who showed that $[ACN+H^+]$ dissociates primarily through the ejection of two mass units, attributed to $H_2$ loss from the pentagonal ring.

Regarding the $C_{13}H_{13}^+$ products, our experimental results provide evidence for assigning its two primary fragments, $C_{12}H_{10}^+$ and $C_{11}H_9^+$. Based on our MPD spectrum (see Figure S6 and Section 3.3), we attribute $C_{12}H_{10}^+$ to the acenaphthene radical cation, $ACN^+$. This assignment is supported by previous studies of the acenaphthene radical cation ($ACN^+)^{42-45}$ and by our computed vertical excitation energies (see Table S3). Moreover, the observed sequential loss of hydrogen atoms aligns with earlier reports on the photodissociation of $ACN^+$.⁴⁵ In contrast, the second primary fragment, $C_{11}H_9^+$, dissociates predominantly within the spectral range corresponding to the lowest-energy absorption band of $2-NpCH_2^+$.³⁰,⁴⁰

Additional experimental results support the formation of acenaphthene derivatives. We
<figure><img src="image_11.png" /><figcaption>Figure 9: Molecular structures of the most stable isomers of protonated acenaphthene ([ACN+H⁺]), acenaphthene radical cation (ACN•⁺), and acenaphthylene radical cation (ACYN•⁺).</figcaption></figure>

found that the C₁₂H₈⁺ species – produced as the common final fragment from the dissociation of both C₁₂H₁₁⁺ (eq 3) and C₁₂H₁₀⁺ (eq 4) – undergoes charge-transfer reactions with 1-MeNp as well as with 1,4- and 2,3-diMeNp. This behavior is consistent with C₁₂H₈⁺ corresponding to the acenaphthylene radical cation (ACYN•⁺), as the ionization potential of ACYN is higher than that of all compounds with which charge-transfer was observed.⁴⁶ Furthermore, ACYN•⁺ is known to be photostable under broadband Xe-lamp irradiation,⁴⁷ in agreement with our observations.

In summary, reactions between C₁₁H₉⁺ and methylated naphthalenes yield two species associated with acenaphthene derivatives: one likely formed directly (C₁₂H₁₁⁺) and one produced through subsequent photoprocessing of the C₁₃H₁₃⁺ products (C₁₂H₁₀⁺). Upon irradiation, both species ultimately converge to the stable C₁₂H₈⁺, which is most consistent with the acenaphthylene cation.
## 4.2 Formation of acenaphthene derivatives: building chemical scenarios

A detailed exploration of the underlying mechanisms in the reactions between $C_{11}H_9^+$ and methylated naphthalenes is beyond the scope of this work. In the following, we consider a scenario, leading to the formation of a side chain at position 1 or 2 of the naphthalene core, depending whether the 1- or 2-$NpCH_2^+$ isomer participates in the reaction. This chain growth mechanism is inspired from Martínez et al.⁴⁸ who suggested a growth pathway for alkanes based on the sequential addition of $CH_2$ groups on the surface of carbonaceous nanoparticles. We support our discussion by calculations of electronic structure and relative energy for selected structures that are possible reaction products, and for their photofragments.

Figure 10 reports plausible structures for the $C_{12}H_{11}^+$ reaction products and their photofragments. Selected $C_{12}H_{11}^+$ structures include both the side-chain isomer ($^3$A1) and the ring-closure isomer ($^1$B1), the latter undergoing an additional hydrogen migration to yield $[ACN+H^+](^1C1)$. Whether this H migration takes place during the reaction or is triggered by photon irradiation remains uncertain. Given the triplet ground state of $CH_2$, the side-chain configuration A1 in Figure 10 should also lie on the triplet surface. Ring closure (formation of B1) is possible only when the $CH_2$-bearing chain adopts a geometry compatible with intersystem crossing (ISC) to a singlet state. An alternative pathway to ring closure could involve H migration toward the terminal $CH_2$ group, leading to the formation of an ethylidene function (=CH-CH$_3$), and this isomeric form would be stabilized by mesomerism as in 1- and 2-$NpCH_2^+$ (Figure S7). The formation of this chain would quench the reactivity and open a dissociation channel through CH$_3$ loss. Our experiments show no evidence of CH$_3$ loss or any other C-loss channel in the dissociation of *m/z* 155. In addition, MD simulations of the two $C_{12}H_{11}^+$ isomers (*m/z* 155) featuring a =CH-CH$_3$ chain at positions 1 and 2 (see Section S6 in the SI) show no significant formation of $[ACN+H^+]$, in contrast with the experimental identification of $C_{12}H_{11}^+$ as $[ACN+H^+]$. All our results therefore point to ring closure as the mechanism leading to the formation of *m/z* 155. Since ring closure is not expected for
methylated naphthalenes, to [ACN+H⁺]. In contrast, C₁₃H₁₃⁺ (m/z 169) can be attributed to reactions involving 2-NpCH₂⁺. In this case, we also find clear experimental evidence for cyclization leading to the formation of ACN•⁺ and ultimately to ACYN•⁺ upon photoprocessing. These results indicate that some molecular configurations favor ISC to a singlet state. For 1-NpCH₂⁺, our experiments suggest, though do not definitively prove, that this process can occur without photon irradiation. In contrast, for 2-NpCH₂⁺, photon absorption induces isomerization and may enhance ISC efficiency for specific geometries.

<figure><img src="image_12.png" /><figcaption>Figure 10: Proposed structures involved in the reactions of 1-NpCH₂⁺ with either neutral MeNp or diMeNp and subsequent photodissociation. The structures within black dashed boxes correspond to species experimentally identified, while those in red boxes indicate the final photoproducts. All reactive intermediates were locally optimized at the DFT level. Relative energy values, expressed in eV, are given with respect to the CH₂ + 1-NpCH₂⁺ asymptote. Values for high-spin (resp. low-spin) states are shown in blue (resp. black), with the spin state indicated in superscript.</figcaption></figure>

# 5 Implications

The chemistry discussed in this work centers in the reactivity of the C₁₁H₉⁺ ion, a common fragment produced during the dissociative ionization (or sequential ionization and dissociation) of methyl- and dimethyl-naphthalene. Species in which one hydrogen atom of a PAH
<figure><img src="image_13.png" /><figcaption>Figure 11: Proposed structures involved in the reactions of 2-NpCH₂⁺ with 2,3-diMeNp and subsequent photodissociation. Structures within black dashed boxes correspond to experimentally identified species, while those in red boxes indicate the final photoproducts. All reactive intermediates were locally optimized at the DFT level. Relative energy values, expressed in eV, are given with respect to the CH₂ + 2-NpCH₂CH₂⁺ asymptote. Values for high-spin (resp. low-spin) states are shown in blue (resp. black), with the spin state indicated in superscript.</figcaption></figure>

is replaced by a methylene cation (CH₂⁺) from a family of carbocations that are expected to be highly reactive.⁵¹ In addition, they are resonance-stabilized through charge delocalization over the aromatic π-system. Their study is of interest not only for fundamental gas-phase chemistry, but also for understanding processes in astrochemical environments such as cold interstellar clouds and Titan's ionosphere.

From a fundamental perspective, this work provides new insight into using chemical reactivity to distinguish between C₁₁H₉⁺ isomers. Early studies on the benzylium/tropylium (C₇H₇⁺) system showed that benzylium reacts with toluene to form C₈H₉⁺,⁵²,⁵³ via a proposed mechanism involving CH₂⁺ transfer from the ion to the neutral molecule without H scrambling (see Scheme II in Shen & Dunbar⁵²). We propose here an alternative mechanism involving chain growth through CH₂ addition, as hypothesized in Section 4.2. If this
mechanism also applies to benzylium, it would lead to the formation of the 2-phenylethyl cation ($C_6H_5-CH_2-CH_2^+$). Furthermore, we show that this reaction scheme makes it possible to distinguish the 1-NpCH$_2^+$ and 2-NpCH$_2^+$ isomers based on their respective reaction products.

Two isomers of cyanoacenaphthylene (1- and 5-$C_{12}H_7CN$) were recently detected in TMC-1.⁴ Interestingly, the derived column density of this species is 1.7 times higher than that of the smaller cyanonaphthalene isomers (1- and 2-$C_{10}H_7CN$).¹ Phenalene ($C_{13}H_{10}$) has also been detected in TMC-1.⁵ The authors of that study proposed that phenalene is most likely formed through the reaction of acenaphthylene (ACYN) with $CH_3^+$, producing $C_{13}H_{11}^+$ via radiative association, followed by dissociative recombination to yield $C_{13}H_{10}$. However, no pathway was proposed for the formation of ACYN itself, highlighting missing reactions in current chemical networks that could enable efficient PAH growth in cold dark clouds.⁴

The (photo)chemistry presented here reveals an efficient route to acenaphthene (protonated and radical cation) through ion-molecule reactions, which are expected to proceed efficiently at low temperatures. Although the experiments were conducted at 300 K rather than under cryogenic conditions, the isolation conditions ensure that only radiatively stabilized processes occur, mirroring those relevant in astrophysical environments. The key step involves the reactivity of $C_{11}H_9^+$, a typical fragment arising from dissociative ionization of methyl- and dimethyl-naphthalene. Our results show that side-chain growth can be initiated by the $CH_2$ unit on NpCH$_2^+$, followed by ring closure. Chain growth via $CH_2$ transfer is mediated by complex formation with methylated naphthalenes. Acenaphthene derivatives form when 1-NpCH$_2^+$ participates, though 2-NpCH$_2^+$ can also yield such structures through consecutive ring-closure and ring-opening steps (Figure 11). Further dissociation toward $C_{12}H_8$ requires an additional energy input, plausibly supplied by mild UV photons or electron recombination.

Jochims et al. reported a critical energy of 7.8 eV for the survival of ACYN*⁺ in astrophysical environments, compared to only 3.9 eV for methyl-naphthalene cations.²⁸ More recent
experiments using electrostatic storage rings have enabled measurements of improved astro-
physical relevance by probing the competition between radiative cooling and dissociation of
isolated ions over long timescales.<sup>54,55</sup> Using the cryogenic DESIREE setup, it was recently
determined that ACYN<sup>•+</sup> remains stable up to vibrational energies of approximately 7.6 eV,<sup>56</sup>
a value comparable to that of the anthracene cation (C<sub>14</sub>H<sub>10</sub>)<sup>54</sup> and significantly higher than
the 5.8 eV derived for naphthalene cations.<sup>57</sup> ACYN<sup>•+</sup> was identified as the main fragment
ion produced in the dissociative ionization of C<sub>14</sub>H<sub>10</sub> (anthracene and phenanthrene),<sup>58</sup> which
indicates that it is also the principal fragment of C<sub>14</sub>H<sub>10</sub> dissociation. However, any environ-
ment capable of photodissociating C<sub>14</sub>H<sub>10</sub> would also lead to the dissociation of ACYN<sup>•+</sup>,
making it unlikely that this pathway accounts for the cyanoacenaphthylene observed in
TMC-1.

The chemistry explored in our study may also influence hydrocarbon chemistry in dense
and FUV-irradiated regions, such as the Orion Bar. These photodissociation regions (PDRs)
contain dense and warm gas (T<sub>gas</sub> = 300-600 K) and exhibit active chemistry of small
hydrocarbon radicals, such as C<sub>2</sub>H and CH<sub>2</sub>.<sup>59</sup> Models predict abundances of these small
hydrocarbons at their peak of a few 10<sup>-7</sup> relative to H. Assuming that 10% of carbon is
locked in PAHs with a C/H ratio of 1.4 × 10<sup>-4</sup>, the PAH abundance would be 1.4 × 10<sup>-6</sup> if
all PAHs are similar in size to naphthalene, or 2.8 × 10<sup>-7</sup> for PAHs with an average size of
50 C atoms. Under these conditions, highly reactive radicals could readily react with PAHs,
potentially leading to aliphatic chain growth at the periphery of the aromatic systems and
releasing species such as C<sub>2</sub>H<sub>4</sub> into the gas phase.

To date, astrochemical models predict only a minor contribution of PAHs to the forma-
tion of small hydrocarbons, primarily C<sub>2</sub>H<sub>2</sub> produced via PAH photodissociation by FUV
photons.<sup>59,60</sup> Our results suggest that including the chemistry of PAHs at the hydrocarbon
peak may enhance the production of hydrocarbons containing two or more carbon atoms.
Further constraints on this chemistry would require investigations of hydrocarbons containing
three or more carbon atoms, as previously proposed.<sup>59</sup> Finally, it is noteworthy that even
in TMC-1, the chemistry leading to the formation of carbon chains and aromatic species appears to be interconnected.¹²,⁶¹

$C_{11}H_9^+$ is among the abundant large cations observed by *itin* situ mass spectrometry in Titan's ionosphere⁶²,⁶³ and could therefore play a key role in PAH chemistry there.⁶⁴ Understanding this chemistry is important, as it contributes to the formation of tholins, which are responsible for Titan's atmospheric hazes.⁶⁵ Westlake et al.⁶⁶ showed that Cassini mission data on large organic ions are consistent with ion-molecule reactions, highlighting the formation of $C_6H_7^+$ as a precursor of benzene. They also emphasized the need to investigate ionic chemistry for larger species, particularly those containing 8-13 carbon atoms.⁶⁶,⁶⁷ Our results on the reactivity of $C_{11}H_9^+$ are directly relevant to Titan's ionosphere, indicating an active photochemistry that promotes the growth of carbon chains and PAHs, including species containing pentagonal rings.

## 6 Conclusion

Using the multiplex capabilities of PIRENEA, we studied the reactivity of $C_{11}H_9^+$ with di-MeNp under isolated conditions relevant to gas-phase chemistry in space.⁶⁸ The $C_{11}H_9^+$ ion at *m/z* 141 was consistently found to be the sole fragment produced in the photodissociation of diMeNp cations, highlighting its central role in the chemistry described here. This behavior was observed for all investigated diMeNp isomers, indicating that the positions of the methyl groups have little influence on the fragmentation process. A similar pattern was observed for MeNp cations, where $C_{11}H_9^+$ again appeared as the only detected fragment.³⁰,³⁹ Together, these results demonstrate the favored formation of $C_{11}H_9^+$ irrespective of whether the naphthalene core bears one or two methyl groups – a finding further supported by molecular dynamics simulations.

$C_{11}H_9^+$ cations react with neutral MeNp and diMeNp to form two primary products at *m/z* 155 and *m/z* 169, with relative abundances that depend on the precursor. The corre-
sponding reaction products, $C_{12}H_{11}^+$ and $C_{13}H_{13}^+$, exhibit distinct photodissociation pathways: $C_{12}H_{11}^+$ predominantly undergoes sequential hydrogen loss, whereas $C_{13}H_{13}^+$ follows two competing channels, yielding either $C_{12}H_{10}^+$ or regenerating $C_{11}H_9^+$. In both cases, $C_{12}H_8^+$ emerges as a stable terminal photofragment upon irradiation with visible light from the Xe lamp.

These reactivity studies show that the three long-lived $C_{11}H_9^+$ isomers – 1-NpCH$_2^+$, 2-NpCH$_2^+$, and BzTr$^+$ – previously identified from 1-MeNp and 2-MeNp,$^{30}$ are also present when diMeNp serves as the precursor. They further confirm that BzTr$^+$ is non-reactive, in agreement with previous studies on benzylium/tropylium cations ($C_7H_7^+$$^{52,53,69}$) and on the $C_{11}H_9^+$ isomers.$^{30,39}$ In addition, we show that the 1-NpCH$_2^+$ and 2-NpCH$_2^+$ isomers can be distinguished based on their reaction products.

The reactivity of the $C_7H_7^+$ benzylium cation with toluene produces $C_8H_9^+$, which is generally interpreted as arising from the formal transfer of a cationic methylene unit.$^{52,53,69}$ In contrast, our results for the $NpCH_2^+$ reaction products are consistent with a different mechanism, involving chain growth through successive CH$_2$ additions, as previously proposed for alkane growth on the surfaces of carbonaceous nanoparticles.$^{48}$ The resulting chains are terminated by a reactive carbocation located on the methylene group, which can subsequently promote ring closure.

The proposed reaction pathways still require further investigation to fully validate each step. Nevertheless, our experimental results, supported by DFT calculations, demonstrate that this chemistry provides a viable mechanism for the formation of acenaphthene and its derivatives in TMC-1, for which no formation pathway has been identified to date.$^4$ Moreover, due to its special stability, $NpCH_2^+$ could serve as a CH$_2$ chain initiator,$^{70}$ promoting the growth of hydrocarbons and more complex organic molecules in space. These considerations are consistent with the idea that similar chemical processes contribute to the formation of linear carbon chains and aromatic rings in cold molecular clouds.$^{12}$

Our study highlights the central role of $C_{11}H_9^+$ as both a common photofragment and a reactive intermediate that drives the formation of acenaphthene derivatives and related
species. This does not, however, rule out alternative growth pathways involving the reactivity
of naphthalene with small hydrocarbon radicals and their ions, which would require dedicated
investigations. We are entering an exciting era in which the carbon chemistry underlying
chemical complexity in star- and planet-forming regions can be characterized in ever greater
detail using powerful astronomical instruments. The discovery of PAHs in cold clouds with
radio telescopes, together with the recent detection of CH<sub>3</sub><sup>+</sup> by JWST,<sup>71</sup> poses new challenges
for chemical modeling and opens perspectives for laboratory astrophysics in the coming
years. These experiments must nevertheless contend with the unusual conditions prevailing in
space, namely extremely low temperatures and pressures. In particular, identifying radiative
association channels experimentally is far from straightforward,<sup>72-76</sup> and this is precisely
where ICR setups such as the one used here can provide definitive answers.

Acknowledgments

This work was supported by the Agence Nationale pour la Recherche, ANR grant No ANR-
21-CE30- 0010, SynPAHcool. The running costs for cryogenic fluids, required for the super-
conducting magnet of the PIRENEA setup, were also supported by recurrent funding for
the Nanograin platform at IRAP, as well as by the Thematic Action "Physique et Chimie du
Milieu Interstellaire" (PCMI) of the INSU Programme National "Astro", with contributions
from CNRS Physique, CNRS Chimie, CEA, and CNES. The authors gratefully acknowledge
Loïc Noguès, David Murat and Odile Coeur-Joly for their sustained technical support of the
PIRENEA setup over many years. AS thanks the computing mesocenter CALMIP ("CAL-
cul en MIdi Pyrénées", UAR 3667 of CNRS) for generous allocation of computer resources
(project p17002).
# Associated content

## Data Availability Statement

The dataset associated with this work can be found under

## Supporting Information

The Supporting Information is available free of charge at

Figure S1: mass spectra illustrating the production of $C_{11}H_9^+$ from 1,4-diMeNp; Figure S2: mass spectra for reactions between $C_{11}H_9^+$ and diMeNp; Figure S3: Mass spectra showing the protofragments of $C_{13}H_{13}^+$ and $C_{12}H_{11}^+$ ions; Figure S4: Photofragmentation kinetic curves of $C_{13}H_{13}^+$; Figure S5: snapshot for the dissociation of 2,3-diMeNp$^+$ resulting in 1-NpCH$_2^+$; Figure S6: MPD spectrum of $C_{13}H_{13}^+$ species and their photofragments; Figure S7: molecular structure of the two $C_{12}H_{11}^+$ isomers bearing a CHCH$_3$ chain.

Table S1: fitting parameters of the experimental photofragmentation kinetic curves; Table S2: Branching ratios for the dissociation of 1,4-, 1,5-, and 2,3-diMeNp cations, obtained from MD/DFTB simulations over a 1 ns timescale at two different internal energies; Table S3: Vertical electronic excitations energies of acenaphthene radical cation; Table S4: H-loss energies from the different isomers of acenaphthylene cation; Table S5: Branching ratios for the isomerization of the structures shown in Figure S7.
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Supplementary material for: Acenaphthene
Derivatives as Signatures of C<sub>11</sub>H<sub>9</sub><sup>+</sup> Reactivity
with Methylated Naphthalenes

Ana I. Lozano,† Anthony Bonnamy,† Aude Simon,‡ and Christine Joblin*,†

†Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse,
CNRS, CNES, 9 Avenue du Colonel Roche, F-31028 Toulouse, France

‡Laboratoire de Chimie et Physique Quantiques LCPQ/FeRMI, Université de Toulouse,
CNRS, 118 Route de Narbonne, 31062 Toulouse, France

E-mail: christine.joblin@cnrs.fr
* Figure S1: Mass spectra illustrating the production of $C_{11}H_9^+$ from 1,4-diMeNp.

* Figure S2: Mass spectra for reactions between $C_{11}H_9^+$ and diMeNp.

* Figure S3: Mass spectra showing the protofragments of $C_{13}H_{13}^+$ and $C_{12}H_{11}^+$ ions.

* Figure S4: Photofragmentation kinetic curves of $C_{13}H_{13}^+$.

* Table S1: Fitting parameters of the experimental photofragmentation kinetic curves.

* Table S2: Branching ratios for the dissociation of 1,4-, 1,5-, and 2,3-diMeNp cations, obtained from MD/DFTB simulations over a 1 ns timesale at two different internal energies.

* Figure S5: Snapshot for the dissociation of $2,3-diMeNp^+$ resulting in $1-NpCH_2^+$.

* Figure S6: MPD spectrum of $C_{13}H_{13}^+$ species and their photofragments.

* Table S3: Vertical electronic excitations energies of acenaphthene radical cation.

* Table S4: H-loss energies from the different isomers of acenaphthylene cation.

* Figure S7: Molecular structure of the two $C_{12}H_{11}^+$ isomers bearing a CHCH$_3$ chain.

* Table S5: Branching ratios for the isomerization of the structures shown in Figure S7.
# S1 Mass Spectra

## S1.1 m/z 141 formation from 1,4-dimethylnaphthalene

<figure><img src="image_14.png" /><figcaption>Figure S1: Mass spectra illustrating the procedure used to produce the C₁₁H₉⁺ from 1,4-diMeNp. The steps include: 1- irradiation of the neutral precursor with the 266 nm laser during 2 s, 2- isolation of C₁₂H₁₂⁺, and 3- irradiation with the Xe lamp (CF at 610 nm) during 10 s.</figcaption></figure>
S1.2 Ion-molecule reaction experiments

<figure><img src="image_15.png" /><figcaption>Figure S2: Mass spectra for reactions between C₁₁H₉⁺ and different isomers of diMeNp. The precision of the ICR mass spectrometer allows us to confirm the identification of the two reaction products with derived masses at m/z 155.084 and 169.099.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure S3: Mass spectra illustrating the photofragments of C₁₃H₁₃⁺ (top) and C₁₂H₁₁⁺ (bottom).</figcaption></figure>
S2 Photoprocessing Kinetic Curves

<figure><img src="image_17.png" /><figcaption>Figure S4: Normalized intensity of C₁₃H₁₃⁺ species as a function of the irradiation time with the Xe arc lamp using a color filter (CF) at 400 nm and with or without and additional neutral density filter (NDF) of 37% transmission. The solid lines represent fits based on an exponential decay model (Table S1).</figcaption></figure>
<table><caption>Table S1: *k* values derived from the kinetic curves shown in Figures 2, 5, and S4, using an exponential decay model. When two *k* values are reported, the data were fitted with a two-exponential decay model, and the relative populations associated with each *k* value are given in parentheses.</caption><thead><tr><th>Species</th><th>Precursor</th><th>CF 400 nm + TF (37%)<br/>k (s<sup>-1</sup>) (A)</th><th>CF 400 nm<br/>k (s<sup>-1</sup>) (A)</th></tr></thead><tbody><tr><td rowspan="4">C<sub>11</sub>H<sub>9</sub><sup>+</sup><br/>(m/z 141)</td><td rowspan="2">2,3-diMeNp</td><td>0.116 (56 %)</td><td>0.86 (70 %)</td></tr><tr><td>0.00465 (44 %)</td><td>0.0394 (30 %)</td></tr><tr><td rowspan="2">1,4-diMeNp</td><td></td><td>0.721 (65 %)</td></tr><tr><td></td><td>0.0358 (35 %)</td></tr><tr><td rowspan="2">1,5-diMeNp</td><td></td><td>0.654 (63 %)</td></tr><tr><td></td><td>0.0342 (37 %)</td></tr><tr><td rowspan="3">C<sub>12</sub>H<sub>11</sub><sup>+</sup><br/>(m/z 155)</td><td>1-MeNp</td><td>0.134</td><td>0.992</td></tr><tr><td>1,4-diMeNp</td><td>0.105</td><td>0.741</td></tr><tr><td>1,5-diMeNp</td><td>0.096</td><td>0.75</td></tr><tr><td>C<sub>13</sub>H<sub>13</sub><sup>+</sup><br/>(m/z 169)</td><td>2,3-diMeNp</td><td>0.167</td><td>1.39</td></tr></tbody></table>

## S3 MD simulations for dimethylnaphthalene cations

<table><caption>Table S2: Branching ratios for the dissociation of 1,4-, 1,5-, and 2,3-diMeNp cations, obtained from MD/DFTB simulations over a 1 ns timescale at two different internal energies. The values were averaged over 180 simulations with randomly assigned velocity distributions. Note: # denotes the number of occurrences.</caption><thead><tr><th rowspan="2">Internal energies (eV)</th><th colspan="3">15.0 (1 ns)</th><th colspan="3">16.5 (1 ns)</th></tr><tr><th>1,4-diMeNp</th><th>1,5-diMeNp</th><th>2,3-diMeNp</th><th>1,4-diMeNp</th><th>1,5-diMeNp</th><th>2,3-diMeNp</th></tr></thead><tbody><tr><td>#1-NpCH<sub>2</sub><sup>+</sup>/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.50</td><td>0.85</td><td>0.13</td><td>0.62</td><td>0.73</td><td>0.17</td></tr><tr><td>#2-NpCH<sub>2</sub><sup>+</sup>/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.13</td><td>0.03</td><td>60</td><td>0.09</td><td>0.08</td><td>0.50</td></tr><tr><td>#BzTr<sup>+</sup>/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.27</td><td>0.03</td><td>0.20</td><td>0.13</td><td>0.03</td><td>0.05</td></tr><tr><td>#Others/#C<sub>11</sub>H<sub>9</sub><sup>+</sup></td><td>0.10</td><td>0.09</td><td>0.07</td><td>0.16</td><td>0.15</td><td>0.26</td></tr></tbody></table>
<figure><img src="image_18.png" /><figcaption>Figure S5: Snapshots from a 15.0 eV MD/DFTB simulation illustrating the dissociation of 2,3-diMeNp•+ into 1-NpCH₂⁺.</figcaption></figure>

The following elementary steps are observed : (1) H migration, (2) CH₂ migration to
form two rings [6+3], (3) Formation of a 7-member ring, (4) and (5): H migration, (6)
Ring shrinking [6+3] (reverse of (3)) and opening: this step can lead to both isomers (1- or
2-NpCH₂⁺), (7) CH₃ loss.
## S4 Spectroscopy

<figure><img src="image_19.png" /><figcaption>Figure S6: MPD spectrum of C₁₃H₁₃⁺ species and their photofragments after 10 s of irradiation with the OPO laser at 5.5 mJ.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Table S3: Vertical electronic excitations of acenaphthene radical cation: symmetries, energies with respect to that of the ground state (ΔE) and oscillator strengths (f) corresponding to the first three electronic excited states of acenaphthene radical cation. The time-dependent DFT calculations were performed at the PBE0/cc-pVTZ level using the B3LYP/6-31G(d,p) optimised geometry of the ground electronic state.</figcaption></figure>

m/z 154

<table><thead><tr><th>Excited state</th><th>ΔE (eV)</th><th>λ (nm)</th><th>f</th></tr></thead><tbody><tr><td>1<sup>2</sup>B<sub>2</sub></td><td>1.41</td><td>877</td><td>0</td></tr><tr><td>2<sup>2</sup>B<sub>2</sub></td><td>2.13</td><td>582</td><td>0.0488</td></tr><tr><td>1<sup>2</sup>A<sub>1</sub></td><td>3.03</td><td>410</td><td>0.0044</td></tr></tbody></table>
## S5 H-loss Energy Calculations

Table S4: H-loss energies (in eV) from the different isomers of protonated acenaphthylene (ACYN+H⁺) determined at the B3LYP/6-31G(d,p) (DFT column) or CCSD(T)/cc-pvtz//B3LYP/6-31G(d,p) (CCSD(T) column) levels of theory. Zero-point energies computed at the B3LYP/6-31G(d,p) level of theory were taken into account. The yellow labels indicate the position of the proton.

<figure><img src="image_21.png" /></figure>

## S6 MD simulations for C₁₂H₁₁⁺

<figure><img src="image_22.png" /></figure>

Figure S7: Molecular structures of the two C₁₂H₁₁⁺ isomers bearing a CHCH₃ chain at positions 1 (1-EtNp⁺) and 2 (1-EtNp⁺) considered for the MD simulations.
<table><caption>Table S5: Branching ratios for the isomerization of 1-EtNp⁺ and 2-EtNp⁺ (Figure S7), obtained from MD/DFTB simulations over a 100 and 500 ps timescale at 13 eV internal energy. The values were averaged over 180 simulations with randomly assigned velocity distributions. [ACN + H⁺] refers to protonated acenaphthene, where the position of the extra H is indicated by the accompanying number.</caption><thead><tr><th rowspan="2">Time</th><th colspan="2">100 ps</th><th colspan="2">500 ps</th></tr><tr><th>1-EtNp<sup>+</sup></th><th>2-EtNp<sup>+</sup></th><th>1-EtNp<sup>+</sup></th><th>2-EtNp<sup>+</sup></th></tr></thead><tbody><tr><td>Parent</td><td></td><td></td><td></td><td></td></tr><tr><td>1-EtNp<sup>+</sup></td><td>0.40</td><td>0</td><td>0.15</td><td>0.03</td></tr><tr><td>2-EtNp<sup>+</sup></td><td>0.02</td><td>0.34</td><td>0.08</td><td>0.23</td></tr><tr><td>[ACN+H<sup>+</sup>] position 5</td><td>0.09</td><td>0</td><td>0.09</td><td>0.01</td></tr><tr><td>[ACN+H<sup>+</sup>] position 3</td><td>0.02</td><td>0</td><td>0.09</td><td>0.02</td></tr><tr><td>[ACN+H<sup>+</sup>] position 4</td><td>0.02</td><td>0</td><td>0.02</td><td>0</td></tr><tr><td>Others</td><td>0.48</td><td>0.66</td><td>0.58</td><td>0.73</td></tr></tbody></table>