WAVELENGTH EFFECTS IN THE PHOTOLYSIS OF KETONES - INTRAMOLECULAR HYDROGEN ABSTRACTION OF O-METHYL-DIBENZYL KETONES VS ALPHA-CLEAVAGE IN MICELLAR SOLUTION
En. Step et Nj. Turro, WAVELENGTH EFFECTS IN THE PHOTOLYSIS OF KETONES - INTRAMOLECULAR HYDROGEN ABSTRACTION OF O-METHYL-DIBENZYL KETONES VS ALPHA-CLEAVAGE IN MICELLAR SOLUTION, Journal of photochemistry and photobiology. A, Chemistry, 79(3), 1994, pp. 173-179
The chemical yields of indanols, the products of intramolecular delta-
hydrogen abstraction produced in the photolysis of o-methyldibenzyl ke
tone (o-MeDBK) and o, o'-dimethyldibenzyl ketone (o, o'-Me2DBK) in SDS
micelles are sensitive to the wavelength of excitation: the yields of
indanols are about two times smaller for excitation at 254 nm than at
ca. 320 nm. At the same time the magnitude of the secondary cage effe
ct, arising from coupling of geminate benzyl radical pairs is increase
d slightly for excitation at 254 nm relative to 300 nm. In contrast to
the results for micellar solution, in hexane no wavelength effect on
the yields of indanols is observed. A kinetic analysis of the reaction
scheme shows that the decrease in the yield of indanol in micelles fo
r shorter wavelength excitation is caused mainly by a decrease in the
recombination probability P(r) of the primary phenacyl/benzyl radical
pair resulting from alpha-cleavage. This decrease in probability for r
ecombination of the primary geminate pair causes an increase in the fo
rmation of the secondary geminate benzyl/benzyl pair and a higher rela
tive yield of geminate cage recombination. Intramolecular hydrogen abs
traction as well as further cyclization of the biradical to indanol ar
e not wavelength dependent processes. A Salem correlation diagram for
hydrogen abstraction provides theoretical support for the proposed mec
hanism.