Photochemical α-cleavage of ketones: revisiting acetone

Abstract

The photochemical [small 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 approximately 10(-6) s near the origin of the S(0)-S(1) transition to less than 10(-12) s at about 20 kcal x mol(-1) excess energy. In contrast, the decay time of the excited singlet (S(1), (1)n pi) 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 x 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 S(1). The primary reaction on the triplet surface is a barrier-controlled alpha-cleavage to form the triplet radical pair CH(3)(*)+ CH(3)CO(*). 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 (approximately 1-2 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.

Exploring Multiple Potential Energy Surfaces: Photochemistry of Small Carbonyl Compounds

“…Photolysis of acetone is one of the most extensively studied photochemical reactions [118]. The CC bond dissociation, called "Norrish type I reaction" (CH 3 ) 2 CO → CH 3 CO + CH 3 , has been the main focus of many theoretical and experimental studies [119][120][121][122][123][124][125][126][127][128][129][130][131][132][133].…”

Photophysical effects on laser induced grating spectroscopy of toluene and acetone

“…Specifically it has been shown that, in the case of toluene as absorber, the signal contrast increases with total pressure whereas, in the case of acetone, the contrast decreases with increasing total pressure. This phenomenon has been explained qualitatively in terms of the model presented by Haas whereby the effective quench rate via rapid photodissociation of acetone following ISC for excitation above the triplet barrier is reduced by rapid, collision-induced internal relaxation to lower lying and longer-lived singlet and triplet states [17]. The present work has been restricted to excitation of toluene and acetone by radiation at 266 nm but future work could explore the variation in relative relaxation rates as a function of excitation energy relative to the lowest bound excited states of such hydrocarbons.…”

“…This is explained by the presence of a dissociative triplet state, with an associated barrier in the T 1 state approximately 94 kcal mol À1 above S 0 [17]. Dissociation via the triplet state results in rapid energy transfer to the bulk gas.…”

“…The following model is based on an analysis of the photophysics of excited acetone put forward by Haas [17]. Absorption in the case of most hydrocarbons such as toluene and acetone involves a transition from the ground singlet state S 0 to an electronically excited singlet state S 1 followed by relaxation processes that include internal conversion to S 0 , IC, internal vibrational relaxation, IVR, inter-system crossing, ISC, to the triplet state, T 1 , dissociation from singlet and triplet states, fluorescence from S 1 and phosphorescence from T 1 .…”

Nanosecond laser-induced shock propagation in and above organic liquid and solid targets

“…In fact, by some accounts [35], the photolysis of acetone is one of the most studied of all gas-phase reactions. For excitation at a wavelength of 266 nm, the photoreaction proceeds by a Norrish Type I process in which the alpha carbon undergoes bond scission as follows:…”