The Mechanism of the Gas Phase Photolysis of Acetone

A mechanism for the photolysis of acetone has been derived from experimental data in including studies on the photosensitized isomerization of olefines. Dissociation into methyl and acetyl radicals occurs from both the first excited singlet and triplet states; the rate constants for excitation by light of 3130 Å4 wavelength are 2.5 x 10¹⁶ exp (-16900/RT) and 2.5 x 10¹⁰ exp (-6400/RT) 1. mole^-1 s^-1 respectively. At low intensities of absorption, decomposition of acetyl radicals predominates over radical-radical interactions. The triplet state is extensively populated at moderate temperatures, the yield at 48 ^circC being 0.98. The low phosphorescence efficiency of acetone can be ascribed to intersystem crossing to the ground state for which the first order rate constant is 1.25 x 10^4% s^-1. The isomerization data requires the recognition of at least two vibrational levels of the acetone triplet, the rate of electronic energy transfer depending upon the excess energy possessed by the donor.