ROLE OF POLARIZATION CAGING IN THE S(N)1 REACTION OF DIPHENYLMETHYL CHLORIDE - A PICOSECOND KINETIC-STUDY

Picosecond absorption spectroscopy is used to study the effect of solv ent upon the dynamics of the contact ion pair formed upon bond heterol ysis in diphenylmethyl chloride. The dynamics of the diphenylmethyl ca tion-chloride anion are probed in acetonitrile and propionitrile as a function of temperature. The Arrhenius parameters for the collapse of the contact ion pair to form the carbon-chlorine bond are A = 9.2 x 10 (11) s(-1) and E(a) = 3.2 kcal/mol for acetonitrile and A = 5.1 x 10(1 1) s(-1) and E(a) = 2.8 kcal/mol for propionitrile. Deviations from eq uilibrium solvation transition state theory are found, and it is concl uded that ion pair recombination occurs in the polarization caging reg ime.