Results from condensed phase ab initio molecular dynamics (AIMD) simulations suggest a proton transfer reaction is facilitated by "presolvation" in which the hydronium is transiently solvated by four water molecules, similar to the typical solvation structure of water, by accepting a weak hydrogen bond from the fourth water molecule. A new version 3.2 multistate empirical valence bond (MS-EVB 3.2) model for the hydrated excess proton incorporating this presolvation behavior is therefore developed. The classical MS-EVB simulations show similar structural properties as those of the previous model but with significantly improved diffusive behavior. The inclusion of nuclear quantum effects in the MS-EVB also provides an even better description of the proton diffusion rate. To quantify the influence of anharmonicity, a second model (aMS-EVB 3.2) is developed using the anharmonic aSPC/Fw water model, which provides similar structural properties but improved spectroscopic responses at high frequencies.

The local environmental sensitivity of infrared (IR) spectroscopy to a 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 the ultrafast evolution of different solvated proton-water structures makes the assignment of liquid-phase IR spectra a challenging task. In this work, we apply a normal mode analysis using density functional theory of thousands of proton-water clusters taken from reactive molecular dynamics 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 there is a wide distribution of vibrational frequencies spanning 1200-3000 cm for every local proton configuration, with the region 2000-2600 cm 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 the 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 HO2+moiety.

Liquid water's rich sub-picosecond vibrational dynamics arise from the interplay of different high- and low-frequency modes evolving in a strong yet fluctuating hydrogen bond network. Recent studies of the OH stretching excitations of HO indicate that they are delocalized over several molecules, raising questions about whether the bending vibrations are similarly delocalized. In this paper, we take advantage of an improved 50 fs time-resolution and broadband infrared (IR) spectroscopy to interrogate the 2D IR lineshape and spectral dynamics of the HOH bending vibration of liquid HO. Indications of strong bend-stretch coupling are observed in early time 2D IR spectra through a broad excited state absorption that extends from 1500 cm to beyond 1900 cm, which corresponds to transitions from the bend to the bend overtone and OH stretching band between 3150 and 3550 cm. Pump-probe measurements reveal a fast 180 fs vibrational relaxation time, which results in a hot-ground state spectrum that is the same as observed for water IR excitation at any other frequency. The fastest dynamical time scale is 80 fs for the polarization anisotropy decay, providing evidence for the delocalized or excitonic character of the bend. Normal mode analysis conducted on water clusters extracted from molecular dynamics simulations corroborate significant stretch-bend mixing and indicate delocalization of δ on 2-7 water molecules.