  Gas phase thermal isomerization rates of trans-stilbene at 300 K obey the
Arrhenius law. In solution the rates increase ten-fold and depend only very
weakly on excitation wavelength or intramolecular temperature. Both
observations are difficult to reconcile within RRKM theory. Previously
discussed mechanisms, like restricted IVR, nonadiabaticity, solvent-dependent
barriers, or excitation-induced cooling, may explain the increase of the rates
in solution but not their dependence on the excitation wavelength. The latter
suggests that solvent collisions can directly promote the isomerization.
