Dynamic nature of the transition state for the SN1 reaction mechanism of diphenylmethyl acetates

Picosecond absorption spectroscopy is employed in the study of the reaction dynamics for the contact ion pairs produced upon the photolysis of a series of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol. From the temperature dependence of the rate constants, the activation parameters associated with covalent bond formation and diffusional separation to the solvent-separated ion pair are obtained. The activation parameters for bond formation are examined within the context of the Hynes theory for solvent dynamical effects on the passage through the transition state; deviations from the transition-state theory are found to be large. Factors that control nucleophilicity are discussed. Finally the validity of applying the Marcus equation to the SN1 reaction mechanism is addressed.