PHOTOISOMERIZATION OF TRANS-STILBENE IN COMPRESSED SOLVENTS - KRAMERS-TURNOVER AND SOLVENT-INDUCED BARRIER SHIFT

The photoisomerization rates of trans stilbene in n-pentane and n-hexane were measured over a wide pressure and temperature range. In this way, solvent effects in liquid phase reaction dynamics, caused by attractive solute solvent interactions, can be separated from purely frictional effects which result mainly from repulsive short range interactions. The observed change of the isothermal pressure dependence of the photoisomerization rate constant with temperature reveals a strong change of the activation energy with solvent density, caused by electrostatic solute-solvent interactions which lead to a substantial lowering of the reaction barrier in the S-1-state with increasing density. As observed earlier in supercritical ethane solution, also in n-pentane and n-hexane the multidimensionality of the barrier crossing process leads to a strong increase of the apparent imaginary barrier frequency with increasing temperature. The viscosity dependence of the isomerization rate coefficient can be represented by a combination of standard unimolecular rate theory and Kramers' frictional model over the entire pressure range investigated, if one accounts for the density dependence of the effective barrier height, and the temperature dependence of the imaginary barrier frequency for the photoisomerization in the S-1-state of trans-stilbene.