  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 paper revealed an
influence of hydrophobic sidechain 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.
