The role of solute-solvent interactions in the dynamics of unimolecular reactions in compressed solvents

The role of the solvent in the liquid phase dynamics of unimolecular processes involving small energy barriers is studied by ultrafast laser spectroscopy. Different regions of the interaction potential between the solvent environment and the reactant are characterized by investigating photoisomerization reactions of trans-stilbene and related compounds in liquid n-alkanes and n-alkanols over a wide pressure range. By varying the physical properties of the solvent environment in such a controlled way, aspects of the reactant-solvent interaction which dominate the different density regimes are identified. In particular, the kind of friction acting along the reaction path, the role of solvent-induced changes of the effective potential energy surface for reaction, and the coupling of dielectric solvent shell relaxation and motion along the reaction coordinate are addressed.