Rates of Molecular Vibrational Energy Transfer in Organic Solutions

Abstract

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.

The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating

“…On the other hand, the γ parameter (or, conversely, τ ss ) for a given combination of solute and solvent is variable, yet there is no apparent single method to establish an exact microscopic dependence. For some large organic molecules the transfer to solute time is estimated to be of the order of 10 ps 8,36. Taking all this into account, it could very well be that the reported variation of S* signal lifetimes from picoseconds19 to tens of picoseconds67 can be due to the latter process.…”

“…Therefore we consider the uniform heating experiments only as complimentary means to constrain η and, subsequently, γ parameters. It would essentially be intriguing to obtain direct estimates of γ from non-equilibrium MD simulations 36…”

“…Thus, the energy of a hot ground state of the carotenoid bound to OCP could be exploited to break the hydrogen bonds and initiate the OCP photocycle. Moreover, the role of solute-to-solvent energy transfer and local heating effects in the chemical reactivity and selectivity has been brought into question,36 as it is becoming evident that in some cases the inclusion of non-instantaneous thermalization might be necessary to adequately capture the chemistry of reactive intermediates 5,6…”

“…Lastly, there have been several analyses of relaxation mechanisms in truly large molecules, such as porphyrins32 or hemes in proteins,13,33 however, a universal theory for IVR and VC in intermediate and large molecules in liquids is still lacking 1. An appealing alternative is the non-equilibrium molecular dynamics (MD) approach 34–36. It offers a route that avoids complicated and possibly excessive parametrization of specific rates.…”

Re‐Evaluating the Transition State for Reactions in Solution

“…The equilibrium condition is usually satisfied for most gas‐phase bimolecular reactions and for reactions in the liquid phase, because energy exchange between solutes and solvent is usually rapid enough to maintain the equilibrium . However, there are cases where equilibrium is not maintained, even in solution . In addition, for unimolecular reactions of intermediates with low barriers to product formation, it is commonly the case that most trajectories coming from the reactant state will have enough energy in a product‐forming reaction coordinate to cross the second barrier as soon as they reach it …”