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.