Water-generated dangling linkers in a metal-organic framework

Domain: biology
Published: 2026-03-11
Editorial outcome: minor_revision

Abstract
Metal-Organic Frameworks (MOFs) have attracted widespread attention for their applications in water-related contexts. A comprehensive understanding of the molecular-level interactions between water and MOFs is crucial for guiding molecular design and optimizing water-related applications. Water can act as a passive guest, interacting weakly with open metal sites or polar linkers without altering the framework, or as a reactive species that cleaves the dative bonds between inorganic clusters and organic linkers, leading to irreversible degradation. In this work, we uncover a significant impact of water on the metal-linker linkage in UiO-66, a prototype MOFs which is considered highly stable with water. The adsorption of water molecules in UiO-66 results in the displacement of firmly attached carboxylate groups of the linker, thereby transforming them into dangling carboxylate groups. These dangling groups are stabilized by water molecules and μ3-OH through hydrogen bonding. Remarkably, this structural transformation is reversible upon water removal. These findings were elucidated through the integration of multidimensional solid-state NMR, cutting-edge dynamic nuclear polarization (DNP) techniques, and computational calculations. By challenging conventional wisdom, our research has introduced a reversible molecular structure evolution scenario, redefining the understanding of water-MOSF interactions.

Introduction
Metal-organic frameworks (MOFs) have recently emerged as highly effective water sorbents1,2, unlocking a multitude of applications such as atmospheric water harvesting3–5, adsorptive heat transformation6–8, autonomous indoor humidity control9–11. Additionally, MOFs serve versatile roles as drug delivery carriers12, energy devices13 and sensors14 within aqueous environments. The success of above applications prompts a thorough investigation into the molecular-level interactions between water and MOFs.

Researchers have employed various techniques, including solid-state NMR15–21, which has proven especially useful in studying MOFs, as well as diffraction22,23, Raman24, infra-red25,26 to explore the water structure within MOFs. Notably, Yaghi group and co-authors have made significant progress by employing single X-ray diffraction to decipher the detailed water-filling mechanism, revealing step-by-step water-binding structures27. Water can behave very passively, interacting weakly with open metal sites23,26 or polar organic linkers27 within MOFs without causing significant structural changes. However, under certain conditions, it can also become highly reactive, cleaving the dative bonds between the inorganic building units and the organic linkers and often leading to irreversible framework degradation28.

In this study, we uncovered the active role of water, demonstrating its ability to induce dangling linkers within a zirconium-based MOF, UiO-66 (Zr6O4(OH)4(BDC)6, BDC = 1,4-benzenedicarboxylate)29. UiO-66 has been widely employed in diverse water-related applications, including water treatment30,31, water splitting32, and molecular delivery12,33/separation34 in aqueous solutions. Up to now, UiO-66 has been regarded as a stable material in aqueous environments, supported by water adsorption isotherm22,35 and powder X-ray diffraction measurements34. However, recent studies by the Pourpoint group have indicated the vulnerability of Zr-O bonds, suggesting that UiO-66 may exhibit lower stability than previously assumed36. Our research further demonstrated that water could break the linker-metal bonds in UiO-66 under ambient conditions, leading to the formation of dangling linkers. Moreover, water molecules not only bonded to the metal sites but also played a crucial role in stabilizing these dangling linkers via hydrogen bonding. This discovery of water-induced dangling linkers was revealed through multidimensional solid-state NMR, dynamic nuclear polarization (DNP) techniques, and computational calculations.

Results
Ideal UiO-66 sample in this study was synthesized using well-established methods37. When the MOF adsorbs a certain amount of water, it becomes crucial to measure the water content within the MOFs. Solid-state 1H NMR is a powerful technique for this purpose and has been utilized to quantify the water content in porous materials38–40. Two approaches were employed for water adsorption: either by exposing the MOF sample to ambient air for several days or by artificially adding water into the MOF powders directly. We examined 1H magic angle spinning (MAS) spectra for samples with varying water loading amounts (Figure 1a). As expected, the spectra feature mainly signals18 from aromatic protons of the BDC2- linker (~8 ppm), μ3-OH protons from the Zr-O cluster (0-3 ppm), and signals from adsorbed water. Interestingly, chemical shift corresponding to adsorbed H2O varies from 4 to 6 ppm depending on the amount of H2O adsorbed.

To ensure quantitative results, a recycle delay of 2-3 seconds (slightly longer than 5× the spin-lattice relaxation time, T1, Table S1) was used. Spectral deconvolution (see example from Figure S1) facilitated the naming of samples as “x H2O /u.c.” where x = the number of H2O molecules in the MOF unit cell Zr6O4(OH)4(BDC)6. The “fresh” sample refers to the synthesized sample activated under 120 °C. The “dried” sample is obtained by heating the “320 H2O /u.c.” sample overnight to almost eliminate all adsorbed water. As evident from the spectra, in both the “fresh” and “dried” samples, the water content in the pores is negligible. We conducted powder X-ray diffraction for all the samples (Figure S2), and they displayed similar patterns, indicating that the samples retained long-range order. This observation is consistent with past research where UiO-66 was considered highly stable under water environments34.

Nevertheless, such conclusion is contradicted by the 13C NMR spectra acquired through {1H}-13C cross-polarization (CP). As seen in Figure 1c, for the fresh UiO-66 sample, there are three characteristic peaks. The peak at 172 ppm corresponds to the carboxylate group (labeled as C1), while peaks C2 and C3 correspond to the -C and -CH carbons from the benzene ring. When there are few waters in one unit cell (26 H2O/u.c.), the 13C spectrum looks similar to the fresh sample. However, with an increase in water content up to 160 per unit cell, all peaks broaden, a new peak (labeled as C4) emerges to the left of C1, and C3 exhibits two components. These observations collectively suggest alterations in ligand dynamics, along with structural changes within the ligand itself. This pattern persists when the water content is doubled (320 H2O/u.c.). Upon the removal of water from the cages, the structural changes revert, indicating a reversible transition between short-range order and disorder in the structure during water adsorption. Notably, this reversible transition remains stable even under prolonged aqueous conditions for up to one year (Figure S3).

Based on this new finding, revealing the emerging disordered structure, and understanding the role of H2O in the UiO-66 become quite important. 1H-13C two-dimensional (2D) heteronuclear correlation (HETCOR) method was employed on the 160 H2O/u.c. sample, targeting the local environment surrounding the newly identified C4 and its interaction with H2O molecules (Figure 2b, Figure S4). Under a mixing time of 5 ms, the HETCOR spectrum reveals contribution from several 1H peaks, including two distinct H2O environments from bulk H2O (~4.7 ppm) and hydrogen-bonded surface water (~6.2 ppm), as well as aromatic 1H from the linkers at ~8.0 ppm. In addition, one can also observe several 1H resonances in the 0-3 ppm range, which are tentatively assigned to the μ3-OH resonances. These resonances are clearly confirmed on the 1D 1H NMR spectrum of the 160 H2O/u.c. sample recorded at higher MAS frequencies (Figure S5), which provides improved spectral resolution. It is interesting to note that the 0-3 ppm resonances, assigned to μ3-OH species, vary with the amount of water content, but that the fresh and dried samples mostly show a single μ3-OH resonance at ~ 2 ppm (Figure 1b). In absence of H2O molecules in the pore, all 3- OH are equivalent (four by metal cluster) and thus resonate at the same frequency (2 ppm). The changes in the μ₃-OH resonances suggest the formation of detached linkers, likely due to the displacement of carboxylate linkers from the metal clusters, which can influence the chemical shift of the remaining μ₃-OH groups.

In addition, the HETCOR spectrum (Figure 2b) reveals that all linker carbon atoms, including C4, exhibit correlation peaks with bulk H2O, hydrogen-bonded surface H2O or aromatic protons. Finally, it is interesting to note the absence of cross-peaks between C4 and isolated μ3-OH resonances (in the 0-3 ppm range) at 5 ms HETCOR spectrum. This indicates that water molecules cluster around linkers and that the C4 site is probably associated with hydrogen-bonded water. Interestingly, the chemical shift of aromatic proton near C4 (7.6 ppm) is slightly lower than that of the aromatic proton on an intact linker (8.0 ppm), a phenomenon explained by DFT calculations below.

To investigate this further, 2D ¹H-¹H double-quantum single-quantum (DQ-SQ) NMR experiment41 with short-recoupling times (57 μs and 171 μs) were performed on the 160 H₂O/u.c. sample (Figure 2c, S6, and S7). These experiments reveals three distinct μ3-OH species (labeled in brown), appearing in the 0-3 ppm range, that are not in close spatial proximity to each other. It also demonstrates spatial correlations between all original μ3-OH groups and aromatic protons (labeled in blue, ~8 ppm). In addition, a cross-peak is detected between one of the μ3-OH species and a resonance at approximately 6 ppm. This resonance is tentatively assigned to a μ3-OH group as well (labeled in orange) associated with the C4 site, as further supported by the DFT calculations described below. We note this μ3-OH species in orange resonates similarly to hydrogen-bonded water. We propose that the spatial correlations observed in the dotted orange region result from the proximity among aromatic protons, the μ3-OH group close to the detached linker, and hydrogen-bonded water, as well as their respective self-correlations. We also note that this interpretation is consistent with the HETCOR data since a spatial correlation between the C4 site and a resonance at ~6 ppm is also present in Figure 2b, consistent with the presence of a μ3-OH specie.

All together, these experimental data suggest that the C4 resonance arises from a detached linker coordinating with both water and a μ3-OH moiety. In order to prove this, and to explore their relative distributions within the MOF structure, 13C-13C through-space correlation experiments would be highly effective. However, the practicality of such 2D experiments faces hindrance due to the low natural abundance of 13C (1.1%), which makes finding an NMR active spin-pair two orders of magnitude lower. To overcome this sensitivity limitation, state-of-the-art DNP techniques42–49 were utilized. The DNP efficiency (εDNP) was evaluated by mixing the MOF with a glass-forming matrix d8-glycerol: D2O: H2O (6:3:1 by volume) containing the polarizing agent AMUPol,50 resulting in an enhancement factor of 60, when comparing 13C CP spectra with and without microwave irradiation (Figure 2d)51. The complete 13C spectrum, including signals from the glycerol, is presented in Figure S8. Since the distance between two nitroxide centers of AMUPol50 (~12 Å) exceeds the dimensions of the UiO-66 octahedral (~9 Å diameter) and tetrahedral cages (~7 Å diameter), AMUPol cannot penetrate the UiO-66 microcrystals. Nevertheless, hyperpolarization is effectively transferred from AMUPol to the protons within the MOFs via 1H-1H spin diffusion.

The significant enhancement renders natural-abundance 13C-13C 2D DQ-SQ experiments feasible, allowing for the acquisition of a good signal-to-noise spectrum within 15 hours. Notably, under a short recoupling time of 2.4 ms using the S3 dipolar recoupling sequence52,53, correlation peaks indicate short-range C-C proximity within 2-3 Å (Figures 2d, S9a). The short-range correlation primarily reveals intramolecular peaks for the BDC4- ligand, denoted as C1-C2, C1-C3, C2-C3, etc. (Figure S9b). Intriguingly, strong correlations C4-C2, C4-C3 also emerge, indicating that the C4 peak originates from the linker carboxylate carbon. This implies the existence of two distinct environments for the BDC4- linker carboxylate group: one where the group remains firmly bounded to metal-oxide sites, referred as the intact linker; and another where it may detach from these sites, due to the presence of water clusters, referred as the dangling linker (as illustrated in Figure 2a). The concept of dangling linker was proposed during the post-synthetic linker exchange in UiO-66, primarily based on computational insights54, and has not yet been confirmed by experimental data. With a longer recoupling time of 7.2 ms, C1-C4 correlations are also observed. Extending the recoupling time further, up to 30 ms (Figure S10), reveals C-C proximities up to 8 Å (as simulated in Figure S9a). Importantly, no self-correlated peaks from dangling carboxylate groups emerge, indicating that the dangling linkers in UiO-66 are primarily disconnected on only one side rather than both sides. This also suggests these dangling linkers are likely homogeneously distributed throughout the framework, rather than clustered in localized regions.

After meticulously gathering structural insights from the previous experiments, it becomes apparent that dangling carboxylate groups within the MOF are enveloped by water molecules. This rearrangement results in the generation of higher chemical shifts compared to firmly attached carboxylate group. To further explore our experimental findings and discern the possible configurations of these dangling carboxylate groups, we conducted quantum mechanical calculations to compare the energy of formation (ΔE) for various configurations originating from the attached carboxylate group. The reaction we considered is:
UiO66+n∙H2O → UiO66∙nH2O (n = 1 or 2) ∆E
Where n represents the number of water molecules directly interacting with the carboxylate groups, resulting in various dangling linker configurations with the remaining post-reaction small molecules (i.e. H2O or OH−) occupying the uncoordinated metal sites. Two cases were considered here: (A) the dangling linker points toward the closest μ3-OH, and (B) the dangling linker points away from the closest μ3-OH. Based on charge balance, several dangling linker configurations can be conceived for both cases (Figures 3a, 3b, S11, S12): (I) COO−+2H2O; (II) COOH+OH−+void; (III) COO− + H2O; (IV) COOH+OH−+H2O; and (V) COOH+OH−. Here, configurations with a positive ΔE are considered unlikely to exist, while those with a more negative ΔE are presumed to be more stable. Our investigations revealed that only the (I) COO−+2H2O, and (IV) COOH+OH−+H2O configurations are stable and could potentially exist in both case (A) and case (B) (Figures 3a, 3b, S11, S12). Among these, the (I) COO−+2H2O configuration, in which the dangling linker points toward the μ3-OH group (Case A), exhibits the greatest stability with an energy of ∆E = −62.7 kJ/mol.

To further examine these configurations, we performed 13C chemical shift calculations, confirming that for both case (A) and case (B), only the (I) COO−+2H2O combination aligns with our experimental data. This configuration exhibits a ~5 ppm higher chemical shift for dangling carboxylate group (peak h) compared to the intact carboxylate group (peak a) (Figure 3c). Conversely, for the (IV) COOH+OH−+H2O and all other combinations (II, III, V), the dangling carboxylate group’s chemical shift is lower or comparable to that of the intact group (Figure S11, S12). Besides, the absence of a highly deshielded COOH 1H peak (~ 9-12 ppm) (Figure 1b, S5), along with the absence of signals corresponding to the C4 and COOH protons in 1H-13C 2D experiments (Figure 2b), enabled us to rule out the possibility of the (IV) COOH+OH⁻+H₂O configuration. It is worth noting that the concentration of dangling carboxylate groups within the entire MOF is notably high (30%, as depicted in Figure S13), making it improbable for the dissolved products of acidic COOH and basic OH⁻ to exist at such high concentrations.

Lastly, the calculated 1H chemical shifts for the labeled μ3-OH groups (highlighted in orange in Figure 3a) in configurations (A-I) and (B-I) were compared. In configuration (A-I), the μ₃-OH group forms a hydrogen bond with the dangling linker, which strengthens its deshielding and shifts its resonance to a higher frequency of 5.9 ppm. This observation supports the absence of a HETCOR correlation peak between the C4 carbon and protons in the 0-3 ppm range in Figure 2b, and aligns with the appearance of a μ3-OH resonance at 6 ppm in the 1H-1H 2D NMR spectrum (Figure 2c). In contrast, configuration (B-I) shows a typical μ3-OH chemical shift of 1.8 ppm. In this case, the distance between the dangling carboxylate carbon (C4) and the nearest μ3-OH proton is approximately 4.6 Å, a HETCOR correlation would still be expected, since similar distances between μ3-OH protons and closet intact linker C3 carbons produce observable correlations (Figure S14, Figure 2b). The correlation between C4 and μ3-OH at 1.8 ppm is however absent in the HETCOR. These findings strongly support (A-I) COO−+2H2O configuration, in which the dangling linker pointing toward the μ3-OH group, as the most plausible structure. The calculated average aromatic 1H chemical shift of the dangling linker (8.40 ppm) is slightly lower than that of the aromatic 1H in the intact linker (8.47 ppm) (Figure S15). This difference explains the slightly reduced 13C-1H correlation on proton dimension of the dangling linker in the HETCOR spectrum (Figure 2b).

Upon closer examination of the solved structure A-I in Figure 3a, The two water molecules occupy the uncoordinated Zr sites and co-stabilize the dangling COO⁻ group together with the neighboring μ3-OH group through multiple hydrogen bonds, consistent with previous proposition by molecular dynamics calculations55. The water molecules function as intermediates in the metal–linker coordination, exhibiting behavior analogous to the hemilability concept previously described for STAM-17-OEt28. Such interactions not only stabilize the dangling carboxylate group but also contribute to the overall structural integrity and properties of the MOF. This finding is further supported by FT-IR experiments, which reveal a more disordered structure and changes in the out-of-plane stretching vibration56 of the carboxylate group upon water adsorption in UiO-66 (Figure S16).

Our investigation has revealed the significant impact of water molecules on the structural evolution of UiO-66 MOFs. Through meticulous experimentation and analysis, we have observed water induces structural changes, leading to the dissociation of carboxylate groups and the formation of water-stabilized dangling linkers within the framework. These findings provide valuable insights into the complex interplay between water molecules and MOF structures, shedding light on the mechanisms governing their behavior and stability. The reversible structural changes suggest that these MOFs can be effectively used for water adsorption and desorption while maintaining structural integrity. This deeper understanding opens up exciting possibilities for the design and optimization of MOFs tailored for a wide range of water-related applications.

Methods
Synthesis of ideal UiO-66
The ideal UiO-66 was synthesized following the Lillerud's recipe37. We sequentially added 3.781 g ZrCl4 (16.22 mmol), 2.865 ml 35 % HCl (32.45 mmol), and 5.391 g H2BDC(32.45 mmol) to a 150 ml Teflon liner containing 97.40 ml N,N’-dimethyl formamide (1258 mmol). After all the reagents dissolved, the liner was sealed in a stainless-steel autoclave and then placed in an oven at 220 °C for 24 hrs. The resulting ideal UiO-66 was washed three times with DMF, three times with methanol for a day each, and three times with deionized water for a day each. The washed products were separated from the solvent by centrifugation, dried under vacuum at 120 °C for 1 day, and grounded with a mortar and pestle. This sample is labeled as “fresh UiO-66”. The successful preparation of ideal UiO-66 was confirmed by powder X-ray diffraction and thermogravimetric analysis.

Water adsorption / desorption in UiO-66
Two approaches were used for water adsorption in fresh UiO-66: (1) exposing the UiO-66 sample to ambient air for several days, or (2) directly adding a known volume of water (e.g., 10 µL to 120 µL) to 50 mg of MOF powder using a pipette. To ensure uniform water adsorption, the mixture was gently ground. Despite the water adsorption, the UiO-66 samples retained a dry appearance. For water desorption, these samples were heated to 120 °C under vacuum overnight to eliminate almost all the adsorbed water, resulting in the “dried” UiO-66 sample.

Characterization Methods
PXRD were carried out on samples placed on a quartz holder using a Rigaku Ultimate-IV X-ray diffractometer operated at 40 kV/30 mA with Cu Kα line (λ = 1.5418 Å). Patterns were collected in reflectance Bragg-Brentano geometry in the 2θ range from 3 to 50°.

Fourier Transform Infrared (FT-IR) spectra were recorded on a Perkin-Elmer 100 FT-IR spectrometer, fitted with a liquid-nitrogen cooled mercury-cadmium-telluride (MCT) detector. Both the dried and wet UiO-66 samples were analyzed directly, without dilution or mixing with KBr.

Solid-state NMR and DNP-enhanced NMR
Room temperature 13C and 1H solid-state NMR experiments of fresh / dried and water adsorbed UiO-66 samples were performed on a Bruker Avance III HD 400 MHz NMR spectrometer (1H, 400.13 MHz; 13C, 100.61 MHz) using a 3.2 mm magic angle spinning (MAS) probe. 1H spectra were acquired under the spinning rate of 10-20 kHz, using a one-pulse sequence, with a recycle delay of 2-3 s, which is slightly 5 times longer than their spin-lattice relaxation times. 13C spectra were collected using either cross-polarization (CP) or direct polarization (DP) sequences under MAS of 15 kHz. The recycle delay for DP sequence was set to 200 s to ensure all the 13C signals were recovered to equilibrium, while the recycle delay of CP was set to 1.5 s. The 1H radio frequency (RF) field strength was 100 kHz and the 13C RF field strength was 83 kHz. Two-dimensional ¹H-¹H double quantum-single quantum (DQ-SQ) NMR experiments were conducted using a 1.3 mm MAS probe. The experiments employed a rotor-synchronized BABA (Back-to-Back) pulse sequence41, with the mixing time set to one rotor period. The magic angle spinning (MAS) rate was maintained at 35 kHz. The 1H and 13C signals were referenced to those of adamantane at 1.8 ppm (1H) and 38.5 ppm (13C methylene).

For DNP sample preparation, a mixture containing 30 mg of the fresh MOF sample and 45 μL of 10 mM AMUPol solution (comprising 60% d8-glycerol, 30% D2O, and 10% H2O by volume) was transferred into a 3.2 mm zirconia thin rotor. Proton polarization enhancement originating from electron polarization was subsequently transferred to 13C nuclei via a standard CP step. The efficiency of the DNP process was obtained by comparing 13C CP spectra with and without microwave irradiation.

For DNP-enhanced 13C-13C double quantum-single quantum (DQ-SQ) experiments were performed on a Bruker Avance III 400 MHz system equipped a low temperature (~100 K) double resonance 3.2 mm MAS probe. The 2D spectra were recorded at 100 K and a MAS rate of 13.2 kHz. Dipolar recoupling sequence S352,53 was used for DQ excitation and reconversion. 100 kHz RF-field strength was used for heteronuclear decoupling using SW-TPPM during indirect (t1) and direct (t2) detection periods, and continuous wave (CW) during S3 recoupling. A z-filter of 100 μs was inserted before acquisition. The experiments were performed with a recycle delay of 8 s. A total of 352 scans were recorded for a mixing time of 2.5 ms, and 160 scans were recorded for a recoupling time of 7.2 ms.

The double-quantum recoupling efficiency curves for the S3 and BABA recoupling sequences were simulated using the SIMPSON57 software. The S3 sequence was simulated under varying 13C-13C dipolar distances, while the BABA sequence was simulated under varying 1H-1H dipolar distances. The SIMPSON input files for these and related pulse sequences are maintained by Dr. Subhradip Paul and are available on GitHub at https://github.com/dnp-grenoble/simpson, with an archived version available on Zenodo (https://doi.org/10.5281/zenodo.17457240).

Calculation methods
Structural optimization and energy calculation. Our system consisted of the unit cell of UiO-66 (Zr) with the formula [Zr6O4(OH)4(BDC)6], where BDC represents 1,4-benzenedicarboxylate. The structure of intact UiO-66 was taken from ref.58.We studied both intact UiO-66 and UiO-66 with a dangling linker. We optimized their structures by periodic density functional theory (DFT) simulations using the CP2K package59. Perdew-Burke-Ernzerhof (PBE) exchange correlation functional60 was combined with DZVP-MOLOPT-SR-GTH basis set61,62. The Grimme’s DFT-D3 semi-empirical method63 was used to account for the van der Waals interactions. The plane-wave energy cutoff was 400 Ry. The integration over the irreducible Brillouin zone was computed over the Gamma point. Subsequently, the energies were computed by the PBE functional with pcseg-1 basis set64 for H, C, and O atoms, and TZVP-MOLOPT-SR-GTH basis set for Zr atoms. The Self-Consistent Continuum Solvation (SCCS) implicit solvent model65,66 was employed. All the CP2K input files are generated by Multiwfn package67.

NMR calculation. We used the Gaussian 16 software to compute 13C and 1H NMR. A part of the energy-optimized structures, the so-called cluster model, was used without periodic boundary conditions to reduce the computational cost. We terminated the surface by adding hydrogen atoms and included a few water molecules inside. The cluster formula is represented by Zr6O4(OH)4(HCOO)2·nH2O (n=1 or 2). The positions of the added hydrogen atoms were optimized while fixing the remaining part, using the b3lyp functional68 with 6-31G* for H, C, and O atoms, and LANL2DZ basis set69 for Zr atoms.

We first computed the isotropic magnetic shielding (𝜹𝒄𝒂𝒍𝒄.𝑿 ) of 13C and 1H in the optimized structures using the Gauge-Invariant Atomic Orbital (GIAO) scheme70–73. The calculations were carried out with the revTPSS functional74,75, employing pcsSseg-1 basis set76 for H, C and O atoms, while the LANL2TZ69,77, basis set was used for Zr atoms. The solvent model density (SMD) continuum solvation model was applied78. We then computed the chemical shift (𝜹𝒄𝒂𝒍𝒄.𝑿 ) through an equation of 𝜹𝒄𝒂𝒍𝒄.𝑿 = 𝝈𝒓𝒆𝒇.𝒄𝒂𝒍𝒄. −𝝈𝒄𝒂𝒍𝒄.𝑿 + 𝜹𝒆𝒙𝒑.𝒓𝒆𝒇., where “ref. calc.” and “exp.” represent reference and experiment, respectively. The 𝜹𝒄𝒂𝒍𝒄.𝒓𝒆𝒇. of 38.5 ppm on 13C was taken from the methylene of adamantane and the 𝜹𝒄𝒂𝒍𝒄.𝒓𝒆𝒇. of 1.8 ppm on 1H was taken from the adamantane protons.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, De Paëpe et al. elegantly demonstrate the formation of dangling linkers in the presence of high amount of water using solid-state NMR. By employing 1D and 2D DNP NMR experiments and calculations, they succefully localize the water in the system. This work provides new insights into intriguing materials. However, several concerns need to be addressed before the manuscript is ready for publication.\n1) My main concern is relative to the HETCOR experiment. It was performed with a long mixing time (5 ms). If the goal is to observe close proximity interactions, a shorter mixing time would be more appropriate. Additionallt, the C4-OH correlation task is not observed even though the proximity is confirmed by the calculations. Authors note line 94 : « it is interesting and also puzzling to note the absence of cross-peaks between C4 and OH resonances…. ». Could they elaborate on this? Is this due to mobility issues? If so, could a lower temperature experiment help detect this signal?\n2) The figures 2 for the 2D experiments are too small and should be enlarged for better clarity.\n3) Regarding the DS-SQ experiment, no correlation between H2O and OH is observed? Since both are Hydrogen-bonded, shouldn’t they correlate in the DQ-SQ? Did the authors try performing the experiment at lower temperature to reduce the mobility? Would increasing the spinning speed or using homonuclear decoupling improve the resolution of this experiment?\n4) I think that adding a table with experimental and calculated chemical shifts would be beneficial for the reader.\n5) In the Supporting information, Figure S7, adding a diagonal would likely enhance the clarity fir the reader.\n6) Line 126, « Figure 2c » must be replace by « Figure 2d »\n7) Could authors comment on the difference of temperature (mobility) that exists between the experimental and calculations datas? Could/should it play a role in the assignment of the experimental datas?", "reviewer_2": "Dear Dr. Davies,\nManuscript NCOMMS-25-51094 reports on an intriguing phenomenon that helps to understand the high stability of specific metal-organic frameworks (MOFs) against water attack. MOFs are an exciting class of porous materials that are discussed for a broad variety of applications. One aspect often preventing commercialisation is the limited stability of many MOFs when exposed to water or humid environments. Therefore, several strategies were developed to prevent water from cleaving the dative bonds between the inorganic building units (IBUs) and the organic linkers, mostly irreversibly. Approaches range from hydrophobisation of the inner surfaces to reduce the water uptake to shielding the IBUs by sterically demanding side chains attached to the linker molecules. A particularly intriguing concept was introduced by Morris et al. in 2018 (Nature Chem. 2018, 10, 1096). By providing sacrificial bonds, the integrity of the MOF topology is protected despite water molecules reacting with the frameworks. This strategy was referred to as the hemilability concept of MOFs, and it was proposed that it might be relevant for several framework types. However, so far, specific examples are rare. Manuscript NCOMMS-25-51094 now presents a study where one carboxylate group of the terephthalate linkers is cleaved but stabilised at the zirconium cluster close to the created open coordination site by multiple hydrogen bonding. The resulting open coordination site then takes up water molecules without destabilising the framework integrity. As both processes occur in the same cluster, the process is reversible solely depending on the water uptake.\nAlthough numerous studies focus on MOF water interactions, the reversibility of the reported observation is remarkable and warrants publication in Nature Communications. Nevertheless, the manuscript should be improved in several places before publication.\nThe authors seem to regard adsorbed water molecules solely as passive guest molecules. This is pointed out several times, e.g., in the abstract and introduction. It is, however, a view that is too narrow, as, in many cases, water is active, competing with the dative bonds of the linker molecules and the IBUs. In particular, the authors do not seem to be aware of the above-mentioned hemilability concept, which is based on similar mechanisms observed for this study. Thus, I recommend including the concept and broadening the focus of the discussion.\nOn page one, it is pointed out that “… water could break the linker-metal bonds in UiO-66 under neutral pH and ambient conditions.” I wonder what neutral pH means in this context. Water molecules are adsorbed in the samples, and the adsorbed aqueous phase will have different properties compared to bulk water. Strictly speaking, a pH is not defined for the adsorbed water molecules.\nOn page two, the quantity of adsorbed water molecules is defined as the uptake in the unit cell. Its content is given as Zr24O24(BDC)24, which is wrong. The unit cell content must be a multiple of UiO-66’s stoichiometry, which is Zr6O4(OH)4(BDC)24. To simplify matters, I would use the stoichiometric unit to reference the water uptake.\nThe 13C MAS NMR spectra for 160 H2O/u.c. and 320 H2O/u.c. (Fig. 1c) exhibit similar ratios for resonances 4 and 1, suggesting that the quantity of defect sites is independent of the water uptake in this region. Based on the DNP data, one-sided linker termination could even be quantified to 22%. I find it surprising that the defect site proportions reach a limit already at medium water uptakes, and I would like to know if the authors have an explanation for this observation. It might be worth measuring lower water uptakes as well, where the defect formation is not saturated yet. This would provide access to the underlying thermodynamic properties, offering more profound insight into this phenomenon.\nConcerning the 1H-1H DQ NMR spectra. Does the phrase mixing time refer to the DQ excitation time? An excitation time of 57 ms seems relatively short. Such spectra should predominantly sample short distances < 2 Å. Therefore, the three intense resonances for the µ3-OH groups are a bit surprising. Their average distance is probably larger than 4 Å. Spimpson simulations, such as those for the 13C-13C spin pairs, would be helpful, and additional information about the DQ experiment would be appreciated.\nTo compare the DNP-enhanced NMR spectroscopic experiments to the conventional ones, the water uptake should be similar. I could not find information on how many H2O, D2O, and glycerol molecules are adsorbed in the UiO-66 samples before the samples are cooled down for the measurements. Is the uptake of the solvent mixture similar enough to the 160/u.c. sample? Please elaborate on this aspect, and also whether you expect an isotope effect for the one-sided linker displacement. For sure, D2O will preferentially adsorb to the created open coordination sites. Please also add some information about how far the polarisation can be transferred from the MOF surfaces, with the radicals, into the UiO-66 crystallites, to make sure that chemical variations in surface-near regions do not influence the quantification.", "reviewer_3": "The results presented by Fu et al. examine the influence of water on the Metal-Ligand interaction within UIO-66, assessing the bonding and de-bonding nature of the carboxylic acid and how this ligand can interact with water molecule(s). They demonstrate new findings using a combination of carefully prepared materials, solid-state NMR, DNP NMR and calculations. Overall the work may be of interest to Nature Communications, however some further details are needed to ensure reproducibility to support their conclusions. Below, a few major and minor comments are provided for the authors to consider.\n1. FTIR method requires details on sample preparation.\n2. SSNMR methods for MOFs is cited, however, I would recommend the authors add 2 papers from Yining Huang - who has done Zr NMR on UIO and has also demonstrated extensive dynamic effects using 13C and 17O in other systems which are related to the impact SSNMR has had in this field.\n3. Can the authors describe the different types of water and u-OH species, it is unclear how different chemical shifts are provided for these functional groups. Perhaps the DFT can provide some guidance to these differences.\n4. The authors should not use the terms upfield and downfield in the 1H NMR section; perhaps higher/lower frequencies or shielded/deshielded terminology is more appropriate.\n5. American vs British spelling - pick one.\n6. 13C-13C should be superscripted, while Figure 3a, The should be the\n7. The T1 values measured for the 1H signals should be provided in the SI (for table for example)…stating these are 5x or more in the text is insufficient as the authors make this claim a few times.\n8. References to the NMR section should be added such as the references used for 1H/13C, BABA sequence.\n9. Does the glycerol/D2o/H2O influence the MOF structure/water interactions the authors report…the solution contains 40% water - does this impact the ligand - water / ligand-metal interaction?\n10. Full experimental conditions are needed for the 13C-13C DQ-SQ DNP MAS NMR results, including recycle delay, transients, pulse powers (which pulse sequence), etc.\n11. The SIMPSON input file should be provided in the SI with the pulse program implemented.\n12. Figure S10 states DNP-enhanced but the experimental/test indicates this was a bloch pulse experiment using SSNMR with a recycle delay of 250s. I suspect this is incorrect, as direct 13C DNP NMR would likely not be effective using Amupol under these sample conditions, while the nuclear T1 for 13C would be much longer than 250s at 100 K."}, "author_response": "Reviewer #1 (Remarks to the Author):\nIn this manuscript,DePaëpeet al. elegantlydemonstrate the formation of dangling linkers\nin the presence of high amount of water using solid-state NMR. By employing 1D and 2D\nDNP NMR experiments and calculations, they succefully localize the water in the system.\nThis work provides new insights into intriguing materials. However, several concerns need to\nbe addressed before the manuscript is ready for publication.\n1) My main concern is relative to the HETCOR experiment. It was performed with a long mixing\ntime (5 ms). If the goal is to observe close proximity interactions, a shorter mixing time would\nbe more appropriate. Additionally, the C4-OH correlation task is not observed even though\nthe proximity is confirmed by the calculations. Authors note line 94 : « it is interesting and also\npuzzling to note the absence of cross-peaks between C4 and OH resonances…. ». Could they\nelaborate on this? Is this due to mobility issues? If so, could a lower temperature experiment\nhelpdetect this signal?\nAnswer: Thank you for your valuable comment. We have also acquired a HETCOR spectrum\nwith a shorter mixing time of 1 ms (see Figure S4), which shows similar results to those\nobtained at 5 ms. For the main manuscript, we have retained the 5 ms spectrum due to its\nhigher sensitivity, while the 1 ms spectrum has been added to the Supporting Information for\ncomparison.\nAlso, we realize that the wording we use was not ideal. There is nothing “puzzling” in our\nresults since we are able to fully explain the absence of cross peak between C4 and OH\nresonances between 1 to 3 ppm (see point 3 below for more details). Thus, we have corrected\nthe text and improve the wording to avoid confusion.\n2) The figures 2 for the 2D experiments are too small and should be enlarged for better clarity.\nAnswer: Thank you for your comment. 2D experiments including 1H-1H 2D experiment and\n13C-1H HETCOR experiment in Figure 2 have been enlarged to improve clarity, as suggested.\n3) Regarding the DS-SQ experiment, no correlation between H2O and OH is observed? Since\nboth are Hydrogen-bonded, shouldn’t they correlate in the DQ-SQ? Did the authors try\nperforming the experiment at lower temperature to reduce the mobility? Would increasing\nthe spinning speed or using homonucleardecoupling improve the resolution of this\nexperiment?\nAnswer: Thank you for your comment. In fact, the H₂O molecules engaged in hydrogen\nbonding do show correlations with the μ₃-OH groups in Figure 2c. In the figure, the green\nsignal (around 6 ppm) corresponds to H₂O involved in hydrogen bonding, while the orange\nsignal represents neighboring μ₃-OH groups, which also shift to approximately 6 ppm due to\nhydrogen bonding with dangling linkers, as illustrated in the A-I structure of Figure 3a. This\ninteraction contributes to stabilizing the formation of the dangling linker. Because these\nsignals have very similar chemical shifts, their correlations overlap in the spectrum. To improve\nclarity, we have enlarged Figure 2c so that these correlations are more distinctly visible.\n4) I think that adding a table with experimental and calculated chemical shifts would be\nbeneficial for the reader.\nAnswer: Thank you for your suggestion. We have added two tables, S2 and S3, in the\nSupporting Information, which compare the experimental and calculated chemical shifts for\ncarbons and protons.\n5) In the Supporting information, Figure S7, adding a diagonal would likely enhance the\nclarity fir the reader.\nAnswer: Thank you for your comment. A diagonal line has been added to previous Figure S7\n(now Figure S10) to improve clarity.\n6) Line 126, « Figure 2c » must be replace by « Figure 2d »\nAnswer: Thank you for pointing this out. We have corrected this typo.\n7) Could authors comment on the difference of temperature (mobility) that exists between\nthe experimental and calculations data? Could/should it play a role in the assignment of the\nexperimental data?\nAnswer: DNP-enhanced spectra were obtained at low temperature (~100 K) and show a\nconsistent chemical shift for C4 compared with the experimental spectra recorded at 298 K.\nMoreover, both the DNP-enhanced spectra at 100 K and the DFT calculations at 0 K yield\nconsistent relative chemical shifts between C4 and C1. Therefore, we believe that temperature\nand molecular mobility do not substantially influence our conclusions.\nReviewer #2 (Remarks to the Author):\nDear Dr. Davies,\nManuscript NCOMMS-25-51094 reports on an intriguing phenomenon that helps to\nunderstand the high stability of specific metal-organic frameworks (MOFs) against water\nattack. MOFs are an exciting class of porous materials that are discussed for a broad variety of\napplications. One aspect often preventing commercialisation is the limited stability of many\nMOFs when exposed to water or humid environments. Therefore, several strategies\nweredeveloped to prevent water from cleaving the dative bonds between the inorganic\nbuilding units (IBUs) and the organic linkers, mostly irreversibly. Approaches range from\nhydrophobisation of the inner surfaces to reduce the water uptake to shielding the IBUs by\nstericallydemanding sidechains attached to the linker molecules. A particularly intriguing\nconcept was introduced by Morris et al. in 2018 (Nature Chem. 2018, 10, 1096). By providing\nsacrificial bonds, the integrity of the MOF topology is protecteddespite water molecules\nreacting with the frameworks. This strategy was referred to as the hemilability concept of\nMOFs, and it was proposed that it might be relevant for several framework types. However,\nso far, specific examples are rare. Manuscript NCOMMS-25-51094 now presents a study where\none carboxylate group of the terephthalate linkers is cleaved but stabilised at the zirconium\ncluster close to the created open coordination site by multiple hydrogen bonding. The\nresulting open coordination site then takes up water molecules withoutdestabilising the\nframework integrity. As both processes occur in the same cluster, the process is reversible\nsolelydepending on the water uptake.\nAlthough numerous studies focus on MOF water interactions, the reversibility of the reported\nobservation is remarkable and warrants publication in Nature Communications. Nevertheless,\nthe manuscript should be improved in several places before publication.\nThe authors seem to regard adsorbed water molecules solely as passive guest molecules. This\nis pointed out several times, e.g., in the abstract and introduction. It is, however, a view that\nis too narrow, as, in many cases, water is active, competing with the dative bonds of the linker\nmolecules and the IBUs. In particular, the authors do not seem to be aware of the above-\nmentioned hemilability concept, which is based on similar mechanisms observed for this study.\nThus, I recommend including the concept and broadening the focus of the discussion.\nAnswer: Thank you for your insightful comment. We have revised the abstract, introduction\nand discussion part to incorporate the concept of hemilability and broaden the discussion on\nthe role of water, in line with your suggestion.\nOn page one, it is pointed out that “… water could break the linker-metal bonds in UiO-66\nunder neutral pH and ambient conditions.” I wonder what neutral pH means in this context.\nWater molecules are adsorbed in the samples, and the adsorbed aqueous phase will have\ndifferent properties compared to bulk water. Strictly speaking, a pH is notdefined for the\nadsorbed water molecules.\nAnswer: Thank you for pointing this out. The sentence has been revised to: “Water molecules\ncould break the linker–metal bonds in UiO-66 under ambient conditions.”\nOn page two, the quantity of adsorbed water molecules isdefined as the uptake in the unit\ncell. Its content is given as Zr24O24(BDC)24, which is wrong. The unit cell content must be a\nmultiple of UiO-66’s stoichiometry, which is Zr6O4(OH)4(BDC)24. To simplify matters, I would\nuse the stoichiometric unit to reference the water uptake.\nAnswer: Thank you for pointing this out. We initially used the unit cell formula\nZr24O24(BDC)24 to represent the activated UiO-66 structure. According to the literature (W.\nZhou et al., J. Am. Chem. Soc., 2013, 135, 10525–10532), upon full activation at high\ntemperature (~250 °C) under vacuum, each Zr6O4(OH)4 cluster loses two H2O molecules,\nresulting in a Zr6O6 node. However, since a significant number of μ-OH groups remain in the\nstructure, we chose to describe the unit cell as Zr24O16(OH)16(BDC)24 in the text. We\nappreciate your careful observation and clarification.\nThe 13C MAS NMR spectra for 160 H2O/u.c. and 320 H2O/u.c. (Fig. 1c) exhibit similar ratios\nfor resonances 4 and 1, suggesting that the quantity ofdefect sites is independent of the water\nuptake in this region. Based on the DNP data, one-sided linker termination could even be\nquantified to 22%. I find it surprising that thedefect site proportions reach a limit already at\nmedium water uptakes, and I would like to know if the authors have an explanation for this\nobservation. It might be worth measuring lower water uptakes as well, where thedefect\nformation is not saturated yet. This would provideaccess to the underlying thermodynamic\nproperties, offering more profound insight into this phenomenon.\nAnswer: Thank you for your comment. We attribute the plateau in defect site proportion at\nmedium water uptakes to the fact that the initial water molecules predominantly interact with\nthe MOF pore walls. As a result, the intensity of the dangling linker-related carboxylate groups\n(C4) in the 160 H₂O/u.c. and 320 H₂O/u.c. samples, relative to the original carboxylate signals\n(C1) in the CP experiments, remains nearly constant. While CP is not strictly quantitative, the\nsame contact time was used for both samples, and since both the C4 and C1 signals arise from\ncarboxylates, the peak intensity ratio provides a reliable basis for comparison.\nAfter the MOF pores are saturated with water, additional molecules are likely confined within\nthe pores and do not interact directly with the ligands or pore walls. We agree that studying\nlow-defect structures would be valuable; however, the low defect concentration makes it\ndifficult to obtain a clear signal, even with DNP.\nConcerning the 1H-1H DQ NMR spectra. Does the phrase mixing time refer to the DQ\nexcitation time? An excitation time of 57 us seems relatively short. Such spectra should\npredominantly sample short distances < 2 Å. Therefore, the three intense resonances for the\nµ3-OH groups are a bit surprising. Their average distance is probably larger than 4 Å.\nSpimpson simulations, such as those for the 13C-13C spin pairs, would be helpful, and\nadditional information about the DQ experiment would be appreciated.\nAnswer: We thank the reviewer for raising this point regarding the short mixing time. The 57\nμs reported in the manuscript corresponds to the total mixing (recoupling) time, which\nincludes both the excitation and reconversion periods of the DQ experiment. We intentionally\nchose a short mixing time to minimize relayed transfers and ensure that the observed\ncorrelations primarily reflect direct, short-range 1H–1H interactions.\nAs shown in our accompanying SIMPSON simulations (Figure S7, shown below), the transfer\nbetween aromatic protons separated by ~2.5 Å is predicted to be about two orders of\nmagnitude stronger than that between μ₃–OH protons (~5.6 Å apart). This trend is consistent\nwith the experimental DQ–SQ spectrum in Figure 2c, where the autocorrelation peak of the\naromatic protons is significantly more intense than that of the μ₃–OH group.\nFor completeness, we also performed measurements with a longer recoupling time of 171 μs\n(Figure S6, shown below). The resulting spectra were essentially identical to those obtained\nwith the 57 μs mixing time, confirming that longer mixing does not introduce additional\ncorrelations under our conditions. However, the 171 μs experiment exhibited lower overall\nsensitivity despite the same number of scans (160), so we chose to present the 57 μs data in\nthe main text and include the 171 μs spectrum in the Supporting Information for comparison.\nTo compare the DNP-enhanced NMR spectroscopic experiments to the conventional ones, the\nwater uptake should be similar. I could not find information on how many H2O, D2O, and\nglycerol molecules are adsorbed in the UiO-66 samples before the samples are cooled down\nfor the measurements. Is the uptake of the solvent mixture similar enough to the 160/u.c.\nsample? Please elaborate on this aspect, and also whether you expect an isotope effect for\nthe one-sided linker displacement. For sure, D2O will preferentially adsorb to the created\nopen coordination sites. Please also add some information about how far the polarisation can\nbe transferred from the MOF surfaces, with the radicals, into the UiO-66 crystallites, to make\nsure that chemical variations in surface-near regions do not influence the quantification.\nAnswer:Thank you for your comments. For the DNP sample preparation, approximately 45 µL\nof the DNP juice (Glycerol/D₂O/H₂O = 6:3:1) was added to the dry UiO-66 sample (around 30\nmg). This corresponds to an estimated water content of about 55 H₂O molecules and 165 D O\n2\nper unit cell of Zr24O16(OH)16(BDC)24. This value lies between the medium and high-water\nuptake regimes (approximately 160 and 320 H₂O per unit cell, respectively), and is therefore\ncomparable to the 160 H O /u.c. reference sample.\nisotope effect on the one-sided linker displacement. Although D₂O may preferentially adsorb\nat open coordination sites, its overall interaction with the framework should be comparable\nto that of H₂O.\nHyperpolarization can be transferred from the biradicals into the UiO-66 crystallites via ¹H–¹H\nspin diffusion. Depending on the 1H T1 relaxation time inside the MOF, this 1H\nhyperpolarization can be transported over hundreds of nanometer length scales. Therefore,\npolarization can reach well into the interior of the MOF particles, which reduces the influence\nof surface-localized chemical variations on the NMR quantification. Moreover, all quantitative\ndata discussed in the main text were obtained from conventional NMR measurements,\nensuring reliable quantification independent of DNP effects.\nReviewer #3 (Remarks to the Author):\nThe results presented by Fu et al. examine the influence of water on the Metal-Ligand\ninteraction within UIO-66, assessing the bonding andde-bonding nature of the carboxylic acid\nand how this ligand can interact with water molecule(s). Theydemonstrate new findings using\na combination of carefully prepared materials, solid-state NMR, DNP NMR and calculations.\nOverall the work may be of interest to Nature Communications, however some furtherdetails\nare needed to ensure reproducibility to support their conclusions. Below, a few major and\nminor comments are provided for the authors to consider.\n1. FTIR method requiresdetails on sample preparation.\nAnswer: Thank you for your helpful suggestion. We have added details on the sample\npreparation procedure for the FT-IR method in SI.\n2. SSNMR methods for MOFs is cited, however, I would recommend the authors add 2 papers\nfrom Yining Huang - who has done Zr NMR on UIO and has alsodemonstrated extensive\ndynamic effects using 13C and 17O in other systems which are related to the impact SSNMR\nhas had in this field.\nAnswer: We have now cited the relevant papers from Prof. Huang as recommended.\n3. Can the authorsdescribe the different types of water and u-OH species, it is unclear how\ndifferent chemical shifts are provided for these functional groups. Perhaps the DFT can\nprovidesome guidance to these differences.\nAnswer: The DFT-calculated 1H chemical shifts for all μ₃-OH groups across the different\nconfigurations are provided in Table S3 of the Supporting Information. Additionally, Table S4\npresents the DFT-calculated 1H chemical shifts for water-related protons. Together, these data\nallow clear differentiation between the various μ-OH groups and water species in the system.\n4. The authors should not use the terms upfield and downfield in the 1H NMR section; perhaps\nhigher/lower frequencies or shielded/deshielded terminology is more appropriate.\nAnswer: We have revised the text accordingly, replacing the terms upfield and downfield with\nhigher/lower frequencies as appropriate.\n5. American vs British spelling - pick one.\nAnswer: We have revised the manuscript to use American spelling throughout.\n6. 13C-13C should be superscripted, while Figure 3a, The should be the\nAnswer: Thank you for pointing out. We have corrected the mistakes.\n7. The T1 values measured for the 1H signals should be provided in the SI (for table for\nexample)…stating these are 5x or more in the text is insufficient as the authors make this\nclaim a few times.\nAnswer: We have now included the measured 1H T1 values for all 1H signals across all\nsamples in Table S1 of the Supporting Information.\n8. References to the NMR section should be added such as the references used for 1H/13C,\nBABA sequence.\nAnswer: We have now included references to the NMR section in the text.\n9. Does the glycerol/D2O/H2O influence the MOF structure/water interactions the authors\nreport…the solution contains 40% water - does this impact the ligand - water / ligand-metal\ninteraction?\nAnswer: Thank you for raising this important point. Although the DNP juice contains only 40%\nwater, the observed 13C chemical shift for the C4 peak remains unchanged compared to\nmeasurements in pure water. This consistency suggests that the glycerol/D₂O/H₂O mixture\ndoes not significantly affect the ligand-water or ligand-metal interactions within the MOF\nstructure.\n10. Full experimental conditions are needed for the 13C-13C DQ-SQ DNP MAS NMR results,\nincluding recycledelay, transients, pulse powers (which pulse sequence), etc.\nAnswer: We have now included the full experimental conditions for the 13C-13C DQ-SQ DNP\nMAS NMR measurements in the SI SSNMR method section.\n11. The SIMPSON input file should be provided in the SI with the pulse program implemented.\nAnswer: Thank you for your suggestion. We have included a link to the SIMPSON input file in\nthe Supporting Information (SI). The SIMPSON input files for various pulse sequences are\nmaintained by Dr. Subhradip Paul and are available at the following GitHub repository:\nhttps://github.com/dnp-grenoble/simpson\n12. Figure S10 states DNP-enhanced but the experimental/test indicates this was a bloch pulse\nexperiment using SSNMR with a recycledelay of 250s. I suspect this is incorrect, as direct 13C\nDNP NMR would likely not be effective using Amupol under these sample conditions, while\nthe nuclear T1 for 13C would be much longer than 250s at 100 K.\nAnswer: Thank you for raising this important point. To ensure the quantitative accuracy of our\nexperiment, we synthesized a MOF sample with 13C-labeled carboxylate groups of linker. For\nthe obtaining wet UiO-66, the direct polarization experiments, we set the recycle delay to 200\ns, given a measured 13C T₁ of 38 s at room temperature. The resulting spectrum accurately\nreflects the expected intensity ratio of the two carboxylate groups. We have updated the\nresults in the text and SI Figure S13."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript is now ready for publication", "reviewer_2": "The authors have thoroughly revised the manuscript NCOMMS-25-51094, addressing the raised questions and concerns satisfactorily. I suggest publishing the revised manuscript without further corrections."}, "author_response": ""}]

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