Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: A Computational Approach

The current computational study investigates the changes in solvation dynamics of water when introducing hydrophobic side chains to the molecular probe N-methyl-6-oxyquinolinium betaine. High-precision transient fluorescence and absorption measurements published in the companion article (10.1021/acs.jpcb.7b05031) revealed an influence of hydrophobic side chain alterations on the observed solvation dynamics of a chromophore in water. As the influence of shape, size, and structure of chromophores on the time-dependent Stokes shift was so far thought to play a role only in slowly rotating solvents compared to the solute or if the hydrogen bonding ability of the solute changes, this finding is quite unexpected. Analysis of the time-dependent Stokes shift obtained from nonequilibrium simulations corroborates experimental retardation factors and activation energies, and indicates that solute motion, namely vibration, is mainly responsible for the observed retardation of solvation dynamics. The faster dynamics around the smaller chromophore is in fact achieved by some normal modes located at the pyridinium part of the chromophore. Rotation also contributes to a very small extent to hydration dynamics, but for small and large derivatives alike. Local residence times furthermore reveal slight retardations in the first solvent shell around the chromophores. The current picture of the solute acting as a passive molecular probe therefore needs to be revised even for solvents like water.