Theoretical spectroscopy of isotopically dilute water and hydrophobicity

Hydrophobic solutes significantly alter the water hydrogen bond network. The local alteration of solvation structures gets reflected in the vibrational spectroscopic signal. Although it is possible to detect this microscopic feature by modern infrared spectroscopy, bulk phase spectra often come with a formidable challenge of establishing the connection of experimental spectra to molecular structures. Theoretical spectroscopy can serve as a more powerful tool where spectroscopic data cannot provide the microscopic picture. In the present work, we build a theoretical spectroscopic map based on a hybrid quantum-classical molecular simulation approach using a methane-water system. The single oscillator O-H stretch frequency is well correlated with a collective variable solvation energy. We construct the spectroscopic maps for fundamental transition frequencies and also the transition dipoles. A bimodal frequency distribution with a blue-shifted population of transition frequency illustrates the presence of gas like water molecules in the hydration shell of methane. This observation is further complemented by a shell-wise decomposition of the O-H stretch frequencies. We observe a significant increase in the ordering of the first solvation water molecules, except those which are directly facing the methane molecule. This is manifested in the redshift of the observed transition frequencies. Temperature dependent simulations depict that the water molecules facing the methane molecule behave similarly to the high temperature water, and a few of the first shell water molecules behave more like cold water.