Quantum energy flow and trans-stilbene photoisomerization: An example of a non-RRKM reaction

We apply multireference ab initio quantum chemistry and microcanonical transition state (RRKM) theory with quantum energy flow corrections from local random matrix theory (LRMT) to determine the kinetics of trans-stilbene photoisomerization. With a single ab initio potential energy surface and no adjustable parameters, simultaneous agreement with experiment of the microcanonical isomerization rates for the d₀, d₂, d₁₀, and d₁₂ isotopomers is obtained. We are also be able to reproduce the pressure dependence of the thermal rate. Laser cooling effects on the isomerization rate are calculated and found to be quite small. The S₁/S₂ energy gap at the transition state is found to be quite large (0.86 eV), suggesting that nondiabatic effects are negligible. Using the ab initio results in a simple RRKM theory without corrections for finite quantum energy flow does not lead to agreement with experiment. We conclude that non-RRKM effects are essential to understand photoisomerization of trans-stilbene and that these can be predicted using LRMT.