Photochemical alpha-cleavage of ketones: revisiting acetone

The photochemical alpha-cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets. The fluorescence decay time of the isolated molecule spans a range of more than six orders of magnitude, from similar to10(-6) s near the origin of the S-0-S-1 transition to less than 10(-12) s at about 20 kcal mol(-1) excess energy. In contrast, the decay time of the excited singlet (S-1, (1)npi*) in the bulk is around 10(-9) s and independent of excitation wavelength. Initial excitation to the (1)npi* state is followed by internal conversion (IC) to the ground state and intersystem crossing to the lowest-lying triplet. The rate constants of these processes are comparable to the radiative decay rate constant for excess energy up to 7 kcal mol(-1) above the origin of the S-0 - S-1 transition. Beyond that energy, the triplet state becomes dissociative and the ISC rate becomes much larger than other processes depleting S1. The primary reaction on the triplet surface is a barrier-controlled alpha-cleavage to form the triplet radical pair CH3. + CH3CO.. Direct reaction from the S-1 is negligible, and the non-quenchable reaction ( by triplet quenchers) observed in the bulk gas phase is due to hot triplet molecules that dissociate on the timescale of 10(-12) s or less. The singlet-state decay time measured in the bulk(similar to1-2 ns) arises from collision-induced processes that populate low-lying levels of S-1. The analysis is aided by detailed state-resolved studies on related molecules ( in particular formaldehyde and acetaldehyde) whose photophysics and photochemistry parallel those of acetone.