Previous theoretical and experimental investigations of the trans-stilbene isomerization reaction in the excited S1 state indicated that the gas phase thermal rate at room temperature is much smaller than the thermal rate in the liquid phase. This was based on the observations that: (a) A combination of measured energy-dependent rates and RRKM calculations led to an isolated molecule thermal rate at T=300 K of 2×109 s−1; (b) An experiment of Balk and Fleming [J. Phys. Chem. 90, 3975 (1986)] in which stilbene vapor at 300 K excited at the S0 to S1 zero point to zero point electronic transition energy (000), gave a lifetime in the excited state of ∼780 ps. The liquid state lifetime in ethane is ∼30 ps. In this paper we present theoretical computations of the rate in the gas and liquid phases, based on a new potential model of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. We find that: (a) RRKM rates are in agreement with measured energy-dependent rates; (b) The thermal rate derived from the new RRKM rates is the same as the thermal rate in liquid ethane; (c) The laser excitation experiment of Balk and Fleming leads to laser cooling of the excited state suggesting that their measured lifetime is longer than the lifetime in the liquid. The surrounding liquid heats up the molecule on a time scale which is faster than the isomerization lifetime. Experiments are suggested to verify this interpretation.

A theoretical investigation of the experimental measurements of the isomerization rate of trans-stilbene in liquids is presented. Monte Carlo and molecular dynamics simulations of the reaction indicate that the predominant solvent effect is in raising the isomerization barrier in the potential of mean force as the solvent density is increased. Dynamic friction effects are small. Good agreement is obtained between the numerical and experimental rates.

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Variational transition state theory (VTST) is applied for the first time to a chemical reaction in a liquid. The theory provides accurate estimates of reaction rates and leads to well defined microscopic friction functions. The structure of the optimized planar dividing surface provides insight into the range of solute–solvent interactions for which there is an appreciable effect on the reaction dynamics. The VTST method also allows for separation of the frictional effects of solvent translation, rotation, and stretch modes. The numerical cost is less than an analogous molecular dynamics reactive flux computation and the insight gained is greater.

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