Fermi resonance in solvated H₃O⁺: a counter-intuitive trend confirmed via a joint experimental and theoretical investigation

Abstract

The spectral features of H3O+ between 3000 and 3800 cm-1 are known to be dominated by coupling between the fundamentals of stretching modes and the overtones of bending modes. A strong Fermi resonance (FR) pattern has been observed in Ar-tagged H3O+, and the sensitive dependence of the FR pattern on the number of Ar tags has been analyzed by Li et al. [J. Phys. Chem. A, 2015, 119(44), 10887]. Based on ab initio anharmonic calculations with MP2/aug-cc-pvDZ, Tan et al. investigated the influence of different types of rare gas and found a counter-intuitive trend that the strength of the coupling between the overtones of bending modes and the fundamentals of stretching modes decreases as the strength of solvation increases [Phys. Chem. Chem. Phys., 2016, 18(44), 30721]. In the present work, we combine both experimental and theoretical tools to gain a better understanding of the FR in H3O+. Experimentally, spectra of H3O+ with light and much more weakly-bound Ne tags were measured for the first time and spectra of Ar-tagged H3O+ were re-measured for comparison. Theoretically, we have implemented several computational schemes to improve both the accuracy and efficiency of the anharmonic treatments with higher-level ab initio methods (up to CCSD/aug-cc-pVTZ). With the good agreement between the experimental and theoretical spectra, we are confident about the prediction of the modulation of coupling strength by the solvation environments.

Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton‐Bound Dimers

“…It is obvious that it decreases from 136 cm −1 in TMA‐H + …Ar to 104 cm −1 in TMA‐H + …EtOH. A similar trend has been found in our previous work on H 3 O + solvated in rare gas; in those systems, the coupling strength between fundamental of the O‐H + stretching and overtone of the bending modes decreases as the strength of hydrogen bonding increases (which is correlated with the decrease of the frequency of the stretching mode) [26] . While this tendency is seen in both solvated H 3 O + and TMA‐H + …X, the coupling strength in TMA‐H + is two to three times greater than its counterparts in H 3 O + .…”

“…The current version of our ab initio anharmonic algorithm is based on the Discrete Variable Representation (DVR) method [23, 24] . Details of the implementation of our Gauss‐Hermite DVR programs have been described in the previous papers on the FR in micro‐solvated H 3 O + [25, 26] . In the previous works, we found that CCSD offers a substantial improvement over MP2 in predicting frequencies of proton stretching modes; we also found seven grid points for the bending and stretching modes of micro‐solvated H 3 O + are sufficient to reduce the uncertainty of eigenvalues to be less than 2 cm −1 .…”

Deconstructing Vibrational Motions on the Potential Energy Surfaces of Hydrogen-Bonded Complexes

“…A similar situation has been observed in the vibrational spectra of symmetric and antisymmetric free OH modes of H 3 O + ⋯N 2 and H 3 O + ⋯Ar. 34 To support our arguments, we simulated the rotational profiles of ν 3free and ν 3HB using PGOPHER 35 at different rotational temperatures. The resulting spectra are shown in Supporting Information Figure S9.…”

Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton‐Bound Dimers

“…[23,24] Details of the implementation of our Gauss-Hermite DVRprograms have been described in the previous papers on the FR in micro-solvated H 3 O + . [25,26] In the previous works,wefound that CCSD offers asubstantial improvement over MP2 in predicting frequencies of proton stretching modes;w ea lso found seven grid points for the bending and stretching modes of micro-solvated H 3 O + are sufficient to reduce the uncertainty of eigenvalues to be less than 2cm À1 . Thanks to the reduced dimensionality and rapid convergence of Gauss-Hermite quadrature (seven grid points for each mode), both the PES and dipole moment surface (DMS) of TMA-H + …X are computed directly at CCSD/aug-cc-pVDZ in the present case for better accuracy.…”

“…As imilar trend has been found in our previous work on H 3 O + solvated in rare gas;i n those systems,the coupling strength between fundamental of the O-H + stretching and overtone of the bending modes decreases as the strength of hydrogen bonding increases (which is correlated with the decrease of the frequency of the stretching mode). [26] While this tendencyi ss een in both solvated H 3 O + and TMA-H + …X, the coupling strength in TMA-H + is two to three times greater than its counterparts in…”