Molecular modeling and assignment of IR spectra of the hydrated excess proton in isotopically dilute water.

Infrared (IR) 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 for the O-H stretch coordinate of the excess proton using electronic structure calculations. Calculated spectra are compared with experimental spectra of dilute H+ in D2O obtained from double-difference FTIR 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.