Molecular Modeling and Structure-Spectrum Correlations of Aqueous Proton Vibrational Response

<strong>School of Physical Sciences</strong> <strong>Molecular Modeling and Structure-Spectrum Correlations of Aqueous Proton Vibrational Response</strong> <strong>Rajib Biswas</strong>&nbsp; (Chicago University, Chicago) Date:&nbsp;<strong>October 26, 2016</strong> <strong>Abstract:&nbsp;</strong>Infrared spectroscopy of the water O—H stretch has been widely used to probe both the local hydrogen-bonding structure and dynamics of aqueous systems. Although of significant interest, the IR spectroscopy of excess protons in water remains difficult to assign as a result of extensive and strong intermolecular interactions in hydrated proton complexes. As an alternate approach, we develop a mixed quantum-classical model for the vibrational spectroscopy of the excess proton in isotopically dilute water that draws on frozen proton-water clusters taken from reactive molecular dynamics trajectories of the latest generation multi-state empirical valence bond proton model (MS-EVB 3.2). A semi-empirical single oscillator spectroscopic map for the instantaneous transition frequency and transition dipole moment is constructed using potential energy surfaces (PES) for the O—H stretch coordinate of the excess proton using electronic structure calculations. Calculated spectra are compared with experimental double-difference FTIR spectra of dilute H+ to demonstrate the validity of the map. The model is also used to decompose IR spectra into contributions from different aqueous proton configurations. We find that the O—H transition frequency continuously decreases as the oxygen-oxygen length for a special pair proton decreases, shifting from Eigen to Zundel-like configurations. The same shift is accompanied by a shift of the flanking water stretches of the Zundel complex to higher frequency than the hydronium O—H vibrations. 1. R. Biswas, W. Carpenter, G. A. Voth and A. Tokmakoff, J. Chem. Phys. 145, 154504 (2016) 2. R. Biswas, Y. L. Tse, A. Tokmakoff and G. A. Voth, J. Phys. Chem. B 120, 1793, (2016) 3. R. Biswas, W. Carpenter, G. A. Voth and A. Tokmakoff, J. Chem. Phys. (submitted)