Rates of Molecular Vibrational Energy Transfer in Organic Solutions

For condensed-phase reactions, commonly used kinetic models assume that energy exchange from and to solvent molecules is much faster than any reactive steps. However, it is becoming increasingly evident that this does not always hold true. In this work, we use molecular dynamics simulations to explore the time scale for energy transfer between solvent and solute in some typical organic solvents. As a reference, energy transfer between solvent molecules is also considered. The time scale is found to depend most strongly on the identity of the solvent. Energy transfer occurs fastest, with a time scale of roughly 10 ps, for ethanol, DMSO or THF, while it is noticeably slower in dichloromethane and especially supercritical argon, where a time scale well in excess of a hundred picoseconds is found. This suggests that the experimental search for nonthermal effects on selectivity and reactivity in organic chemistry should pay special attention to the choice of solvent, as the effects may occur more frequently in some solvents than in others.