The picosecond dynamics for the photoinduced homolysis and heterolysis of (4-methylphenyl)phenylmethyl chloride and bis(4-methylphenyl)methyl chloride in acetonitrile are examined. In less than 20 ps, both the geminate radical pair and contact ion pair are formed from the first excited singlet state. The geminate radical pair decays by either diffusional separation to free radicals or electron transfer back to the ground state surface which partitions between reactant and contact ion pair. The contact ion pair decays by diffusional separation to the solvent separated ion pair or by collapse to form the carbon-chlorine bond. The kinetics for these processes are employed in the development of the potential energy surfaces for the S N 1 reaction for diphenylmethyl chlorides in polar solvents.
The kinetics of both the ions and radicals formed upon photolysis of diphenylmethyl chloride, (4-methoxyphenyl)phenylmethyl chloride, and bis(4-methoxyphenyl)methyl chloride in acetonitrile and propionitrile are examined by picosecond pump-probe spectroscopy. Both radical pairs and ion pairs are formed directly from a common excited state. In addition, the geminate radical pair decays by electron transfer to form either the contact ion pair or a covalent bond, as well as undergoes diffusional separation to free radicals.
Picosecond absorption spectroscopy is used to study the dynamics of the collapse of the diphenylmethyl bromide contact ion pair to form the carbon-bromine bond. The dynamics of the diphenylmethyl cationbromide anion are probed as a function of temperature. The Arrhenius parameters for the collapse of the contact ion pair to form the carbon-bromine bond are A ) 4.9 × 10 11 s -1 and E a ) 3.0 kcal/mol. Deviations from equilibrium solvation transition state theory are found, and it is concluded that the collapse of the ion pair occurs in the polarization caging regime.
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