In this work, we describe mode coupling between the OH stretching mode and the lower frequency motions in hydrogen oxalate anion. Hydrogen oxalate is a hydrogen-bonded system with the proton bound between two oxygen atoms that can transfer on a non-linear path. We use molecular dynamics (MD), driven molecular dynamics (DMD), and vibrational perturbation theory (VPT2) to predict IR spectra and characterize fundamental vibrational transitions, overtones, and combination bands. The interaction potential has been calculated at the MP2/aug-cc-pVDZ level of theory. In MD and DMD simulations, the potential energy surface, gradients, dipoles, and dipole derivatives were evaluated as needed “on the fly.” The effect of the H/D isotopic substitution has been evaluated as well. The low-frequency bending modes play a crucial role in the proton transfer (PT) motion. In particular, the CO2 bending, CCO bending, and CC stretch vibrations indicate that a large amplitude motion implies a substantial shortening of the O-O separation distance that facilitates the hydrogen transfer, bringing it over the top of the barrier. OH stretch vibrations span over 2800 - 3200 cm-1 range. We found that the low-frequency fundamental vibrations at 433 and 823 cm-1 (CC stretch/CO2 bending) resulted in PT motion. Based on analysis of the atomic motion along the DMD trajectories propagated up to 10 ps, the spectral feature near 823 cm-1 is assigned to an overtone of two quanta of 433 cm-1 and a combination band consisting of 300 (CO2 rocking ) + 567 cm-1 (OCC bending) modes. We also characterized a very weak, unreported spectral feature near 1500 cm-1 that is most likely a combination band of 567 cm-1 and 982 cm-1 (H+ out of plane bending).

Theoretical investigation of mode-coupling in hydrogen oxalate using driven molecular dynamics method

ACS Spring Meeting 2021

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