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Photochemical α-cleavage of ketones: revisiting acetone

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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.

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  1. Department of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904, Israel

    Yehuda Haas

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Haas, Y. Photochemical α-cleavage of ketones: revisiting acetone. Photochem Photobiol Sci 3, 6–16 (2004). https://doi.org/10.1039/b307997j

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