Unimolecular reactions in the gas and liquid phases: A possible resolution to the puzzles of the trans-stilbene isomerization

Previous theoretical and experimental investigations of the trans-stilbene isomerization reaction in the excited $S_1$ 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 $\text{T=300\hspace{0.17em}}\mathrm{K}$ of ${\text{2×10}}^9\hspace{0.17em}{\mathrm{s}}^{\mathrm{-1}};$ (b) An experiment of Balk and Fleming [J. Phys. Chem. 90, 3975 (1986)] in which stilbene vapor at 300 K excited at the $S_0$ to $S_1$ zero point to zero point electronic transition energy ${\text{(0}}_0^0\mathrm{),}$ gave a lifetime in the excited state of $\text{∼780\hspace{0.17em}}\mathrm{ps}.$ The liquid state lifetime in ethane is $\text{∼30\hspace{0.17em}}\mathrm{ps}.$ 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.