Spectral diffusion in glycerol-water mixtures from theoretical two dimensional infrared spectroscopy: varying effects...

We have investigated the dynamics of vibrational spectral diffusion, hydrogen bonds and orientational relaxation of water in glycerol-water mixtures of varying concentration. We have looked at how these water dynamical properties are affected by glycerol in different solvation environments through calculations of the linear and two dimensional infrared (2DIR) spectra, and various time correlation functions. We have focused on the low concentration regime with the glycerol mole fraction (x_GLC) going up to 0.12. It is found that the linear infrared spectra do not show any changes in the stretch frequencies of water in this concentration regime, since the OH groups of glycerol molecules provide a hydrogen bonding environment similar to that of water OH groups. However, the dynamics of spectral diffusion calculated from 2DIR spectra at the non-Condon level and the frequency time correlation function (FTCF) of water molecules that are present in the hydration shells of two or more glycerol molecules, referred to as the trapped water, show a noticeable change in the rate of slowing down beyond the glycerol mole fraction of x_GLC ≈ 0.075. A similar change in the dynamics is also observed for orientational relaxation of trapped water molecules at the same glycerol concentration, which we refer to as the cross-over concentration for these mixtures. The dynamics of bulk water and also of those in the hydration shell of a single glycerol molecule are found not to exhibit any such crossover with increase of glycerol concentration. The distinct dynamical behavior of trapped water with variation of glycerol concentration can be linked to the glycerol induced effects on the dynamics of water-water hydrogen bonds.