PRESSURE-DEPENDENCE OF SOLVENT-INDUCED BARRIER SHIFTS IN THE PHOTOISOMERIZATION OF TRANS-STILBENE

The photoisomerization of trans-stilbene in liquid n-hexane solution was studied over wide pressure and temperature ranges. Measurements of the pressure and temperature dependence of the rate, as in earlier studies from our laboratory, allow for the separation of specific solute-solvent interactions from purely frictional effects in the liquid phase reaction dynamics. The viscosity dependences along isotherms of the photoisomerization rate reveal strong changes of the activation energy with solvent density. From the analysis of the isotherms we conclude that specific solute-solvent interactions lead to a substantial lowering of the reaction barrier in the S1 state with increasing density. As observed earlier ethane as a solvent, also in n-hexane manifestations of the multidimensionality of the barrier crossing process are found. The viscosity as well as the temperature dependence of the isomerization rate coefficient over the entire range investigated can surprisingly well be represented by a combination of standard unimolecular rate theory with Kramers frictional model,