Photochemical α-cleavage of ketones: Revisiting acetone

Abstract

The photochemical α-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 ∼10−6 s near the origin of the S0–S1 transition to less than 10−12 s at about 20 kcal mol−1 excess energy. In contrast, the decay time of the excited singlet (S1,1nπ*) in the bulk is around 10−9 s and independent of excitation wavelength. Initial excitation to the 1nπ* 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 S0–S1 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 α-cleavage to form the triplet radical pair CH3˙+ CH3CO˙. Direct reaction from the S1 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 (∼1–2 ns) arises from collision-induced processes that populate low-lying levels of S1. 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. © 2004 The Royal Society of Chemistry and Owner Societies.