Electronic Effects in the IR Spectrum of Water under Confinement

We compare calculations of infrared (IR) spectra of water confined between nonpolar surfaces, obtained by molecular dynamics simulations with forces either computed using density functional theory or modeled by empirical potentials. Our study allows for the identification of important electronic effects, contributing to IR signals, that are not included in simulations based on empirical force fields, and cannot be extracted from the analysis of vibrational density of states. These effects originate from electronic charge fluctuations involving both surface and water molecules in close proximity of the interface. The implications of our findings for the interpretation of experimental data are discussed.