MECHANISM OF FORMATION OF O-METHYLBENZYL RADICAL BY PHOTODISSOCIATION OF O-XYLENE IN SOLUTION

Photodissociation of o-xylene in room temperature n-heptane solution has been studied by means of two-pulse laser-induced fluorescence and transient absorption spectroscopy. Excitation of o-xylene at 266 nm into the S-1 state causes the molecule to undergo carbon-hydrogen bond homolysis in its methyl group, resulting in formation of the o-methylbenzyl radical. The fluorescence of the o-methylbenzyl radical has been observed around 500 nm with a lifetime of 4.1 +/- 1.0 ns, when it has been excited with a 308-nm pulse after the photolysis pulse. The absorption of the o-methylbenzyl radical has been obtained with maxima at 309 and 320 nm. The formation rate constant of the o-methylbenzyl radical, (3.1 +/- 0.4) X 10(7) s(-1), agrees with the decay rate constant of the fluorescence of o-xylene, (2.7 +/- 0.3) x 10(7) s(-1). It isconcluded that excitation with one photon at 266 nm followed by vibrational relaxation populates the thermal equilibrium S-1 state of o-xylene, from which predissociation occurs.