Environment-specific spectroscopic maps for water: Decoding the vibrational signature of non-hydrogen-bonded OH groups

The quantum/classical mixed approach to vibrational spectroscopy of complex molecular systems provides an excellent tool for obtaining computational spectra with high fidelity and low cost. The vibrational spectroscopic maps prepared through quantum chemical calculations are the central part of the quantum/classical mixed approach, allowing for the construction of time-dependent Hamiltonians from trajectories of classical molecular dynamics simulations. In practice, the vibrational spectroscopic maps, which directly affect the fidelity of the computational spectra to the experimental data, still leave room for improvement in many cases. Here, we report a new technique to build the vibrational spectroscopic maps, based on the concept of environment-specific mapping. We apply this technique to bulk liquid water where hydrogen-bonded (HB) and non-hydrogen-bonded (NHB) OH groups are separately mapped. Armed with these new environment-specific maps in the quantum/classical mixed approach, we successfully reproduce the weak shoulder band in the Raman spectrum around 3650 cm-1, which is the vibrational signature of the NHB OH groups. The present computational study elucidates how the NHB OH groups contribute to the shoulder band from a dynamical perspective, revealing that ultrafast chemical exchange between NHB and HB is encoded in the steady-state Raman spectrum.