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  1. Free, publicly-accessible full text available September 6, 2024
  2. The photodissociation dynamics of the dimethyl-substituted acetone oxide Criegee intermediate [(CH 3 ) 2 COO] is characterized following electronic excitation on the π*←π transition, which leads to O ( 1 D) + acetone [(CH 3 ) 2 CO, S0] products. The UV action spectrum of (CH 3 ) 2 COO recorded with O ( 1 D) detection under jet-cooled conditions is broad, unstructured, and essentially unchanged from the corresponding electronic absorption spectrum obtained using a UV-induced depletion method. This indicates that UV excitation of (CH 3 ) 2 COO leads predominantly to the O ( 1 D) product channel. A higher energy O ( 3 P) + (CH3)2CO (T1) product channel is not observed, although it is energetically accessible. This is attributed to the relatively weak absorption cross section at UV excitation energies above the threshold. In addition, complementary MS-CASPT2 trajectory surface-hopping (TSH) simulations indicate minimal population leading to the O ( 3 P) channel and non-unity overall probability for dissociation (within 100 fs). Velocity map imaging of the O ( 1 D) products is utilized to reveal the total kinetic energy release (TKER) distribution upon photodissociation of (CH 3 ) 2 COO at various UV excitation energies. Simulation of the TKER distributions is performed using a hybrid model that combines an impulsive model with a statistical component, the latter reflecting the longer-lived (> 100 fs) trajectories identified in the TSH calculations. The impulsive model accounts for vibrational activation of (CH 3 ) 2 CO arising from geometrical changes between the Criegee intermediate and the carbonyl product, indicating the importance of CO stretch, CCO bend, and CC stretch along with activation of hindered rotation and rock of the methyl groups in the (CH 3 ) 2 CO product. Detailed comparison is also made with the TKER distribution arising from photodissociation dynamics of CH 2 OO upon UV excitation. 
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  4. Hydroperoxyalkyl radicals (˙QOOH) are transient intermediates in the atmospheric oxidation of volatile organic compounds and combustion of hydrocarbon fuels in low temperature (<1000 K) environments. The carbon-centered ˙QOOH radicals are a critical juncture in the oxidation mechanism, but have generally eluded direct experimental observation of their structure, stability, and dissociation dynamics. Recently, this laboratory demonstrated that a prototypical ˙QOOH radical [˙CH 2 (CH 3 ) 2 COOH] can be synthesized by an alternative route, stabilized in a pulsed supersonic expansion, and characterized by its infrared (IR) spectroscopic signature and unimolecular dissociation rate to OH radical and cyclic ether products. The present study focuses on a partially deuterated ˙QOOD analog ˙CH 2 (CH 3 ) 2 COOD, generated in the laboratory by H-atom abstraction from partially deuterated tert -butyl hydroperoxide, (CH 3 ) 3 COOD. IR spectral features associated with jet-cooled and isolated ˙QOOD radicals are observed in the vicinity of the transition state (TS) barrier leading to OD radical and cyclic ether products. The overtone OD stretch (2 ν OD ) of ˙QOOD is identified by IR action spectroscopy with UV laser-induced fluorescence detection of OD products. Direct time-domain measurement of the unimolecular dissociation rate for ˙QOOD (2 ν OD ) extends prior rate measurements for ˙QOOH. Partial deuteration results in a small increase in the TS barrier predicted by high level electronic structure calculations due to changes in zero-point energies; the imaginary frequency is unchanged. Comparison of the unimolecular decay rates obtained experimentally with those predicted theoretically for both ˙QOOH and ˙QOOD confirm that unimolecular decay is enhanced by heavy-atom tunneling involving simultaneous O–O bond elongation and C–C–O angle contraction along the reaction pathway. 
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    Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory‐based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH2OO, CH3CHOO, (CH3)2COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene‐derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH2OO and (CH3)2COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss ratesviaunimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever‐growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing1ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo‐induced fission of the terminal O–O bond, and speculates about possible alternate decay processes that could occur following non‐adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI.

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