IR spectral assignments for the hydrated excess proton in liquid waterIR spectral assignments for the hydrated excess proton in liquid water

The local environmental sensitivity of infrared (IR) spectroscopy to hydrogen-bonding structure makes it a powerful tool for investigating the structure and dynamics of excess protons in water. Although of significant interest, the line broadening that results from ultrafast evolution of different solvated proton–water structures makes assignment of liquid-phase IR spectra a challenging task. In this work, we apply a normal mode analysis using density functional theory to thousands of proton–water clusters taken from reactive molecular dynamics (MD) trajectories of the latest generation multistate empirical valence bond proton model (MS-EVB 3.2). These calculations are used to obtain a vibrational density of states and IR spectral density, which are decomposed on the basis of solvated proton structure and the frequency dependent mode character. Decompositions are presented on the basis of the proton sharing parameter δ often used to distinguish Eigen and Zundel species, the stretch and bend character of the modes, the mode delocalization, and the vibrational mode symmetry. We find that there is a wide distribution of vibrational frequencies spanning 1200-3000 cm-1 for every local proton configuration, with the region
2000-2600 cm-1 being mostly governed by the distorted Eigen-like configuration. We find a continuous red shift of the special-pair O…H+…O stretching frequency and an increase in the flanking water bending intensity with decreasing δ. Also, we find that the flanking water stretch mode of Zundel-like species is strongly mixed with the flanking water bend, and the special pair proton oscillation band is strongly coupled with the bend modes of the central H5O2 moiety. An update on the status of this collaborative work, as well as its relationship (including to our earlier simulation work) to more recent experimental results will be given [Dahms, et al., Science, 357, 491 (2017)].