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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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Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates
ABSTRACT 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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- Award ID(s):
- 2003422
- PAR ID:
- 10443521
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Photochemistry and Photobiology
- Volume:
- 99
- Issue:
- 1
- ISSN:
- 0031-8655
- Page Range / eLocation ID:
- p. 4-18
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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