Visualization of fast energy flow and solvent caging in unimolecular dynamics

Energy flow in solution between physically or chemically evolving solute molecules and the surrounding solvent significantly affects the nature of chemical dynamics in liquids. It determines the extent to which the statistical theory of reaction rates is valid; the transfer of energy between solute and solvent influences the ease with which the transition state evolves into the products--the process central to transition-state theory. But analysing the energy flow in liquid-phase dynamics is difficult because these systems are so complex, and the degrees of freedom are consequently so numerous. Here we present a way to address this challenge. We introduce an approach for visualizing the energy flow directly, and apply it to the isomerization of cyclohexane (between boat and chair conformations) in liquid carbon disulphide, a process for which detailed information about the molecular motions is available from molecular dynamics simulations. Our method reveals in pictorial form the formation and relaxation of a solvent cage, and shows that the relaxation has a strong effect on energy flow to and from the transition state on sub-picosecond timescales. We anticipate that this visualization approach will be generally useful for elucidating dynamical molecular processes in solution.