Vibrational Spectroscopy and Proton Transfer Dynamics in Protonated Oxalate

The dynamics and infrared spectroscopic signatures of proton transfer in protonated oxalate (p-Oxa) are studied using classical and quantum dynamics. The intermolecular interactions are described by a force field suitable to follow proton transfer. This allows to carry out multiple extended classical molecular dynamics (MD) and ring polymer MD simulations from which the infrared spectrum is determined. Simulations at 600 K sample the quantum mechanical ground state probability distribution and best reproduce the experimentally observed maximum absorption wavelength and part of the line shape. Comparison with the experimentally measured spectrum provides an estimate for the barrier height for proton transfer which can not be determined directly from experiment. A barrier of 4.2 kcal/mol is found to best reproduce the position and width of the infrared absorption of the transferring proton in p-Oxa and also leads to an infrared (IR) spectrum in good agreement with experiment for the deuterated species d-Oxa. A novel means to capture the two resonance forms of oxalate depending on the localization of the excess proton on either CO moiety is found to yield improved results for the spectroscopy in the framework region between 1000 and 2000 cm^-1.