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The acetylperoxy + HO 2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O 2 (reaction (1a)) competing with singlet pathways leading to acetic acid + O 3 (reaction (1b)) and acetoxy + OH + O 2 (reaction (1c)). A recent experimental study has reported branching fractions for these three pathways ( α 1a , α 1b , and α 1c ) from 229 K to 294 K. We constructed a theoretical model for predicting α 1a , α 1b , and α 1c using quantum chemical and Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) simulations. Our main quantum chemical method was Weizmann-1 Brueckner Doubles (W1BD) theory; we combined W1BD and equation-of-motion spin-flip coupled cluster (SF) theory to treat open-shell singlet structures. Using RRKM/ME simulations that included all conformers of acetylperoxy–HO 2 pre-reactive complexes led to a 298 K triplet rate constant, k 1a = 5.11 × 10 −12 cm 3 per molecule per s, and values of α 1a in excellent agreement with experiment. Increasing the energies of all singlet structures by 0.9 kcal mol −1 led to a combined singlet rate constant, k 1b+1c = 1.20 × 10 −11 cm 3 per molecule per s, in good agreement with experiment. However, our predicted variations in α 1b and α 1c with temperature are not nearly as large as those measured, perhaps due to the inadequacy of SF theory in treating the transition structures controlling acetic acid + O 3 formation vs. acetoxy + OH + O 2 formation.
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Semiclassical transition state theory/master equation kinetics of HO + CO: Performance evaluation
Abstract Previously, master equation (ME) simulations using semiclassical transition state theory (SCTST) and high‐accuracy extrapolated ab initio thermochemistry (HEAT) predicted rate constants in excellent agreement with published experimental data over a wide range of pressure and temperatures ≳250 K, but the agreement was not as good at lower temperatures. Possible reasons for this reduced performance are investigated by (a) critically evaluating the published experimental data and by investigating; (b) three distinct ME treatments of angular momentum, including one that is exact at the zero‐ and infinite‐pressure limits; (c) a hindered‐rotor model for HOCO that implicitly includes the cis‐ and trans‐conformers; (d) possible empirical adjustments of the thermochemistry; (e) possible empirical adjustments to an imaginary frequency controlling tunneling; (f) including or neglecting the prereaction complex PRC1; and (g) its possible bimolecular reactions. Improvements include better approximations to factors in SCTST and using the Hill and van Vleck treatment of angular momentum coupling. Evaluation of literature data does not reveal any specific shortcomings, but the stated uncertainties may be underestimated. All ME treatments give excellent fits to experimental data atT≥ 250 K, but the discrepancy atT < 250 K persists. Note that each ME model requires individual empirical energy transfer parameters. Thermochemical adjustments were unable to match the experimental H/D kinetic isotope effects. Adjusting an imaginary frequency can achieve good fits, but the adjustments are unacceptably large. Whether PRC1 and its possible bimolecular reactions are included had little effect. We conclude that none of the adjustments is an improvement over the unadjusted theory. Note that only one set of experimental data exists in the regime of the discrepancy with theory, and data for DO + CO are scanty.
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- Award ID(s):
- 1664325
- PAR ID:
- 10455277
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- International Journal of Chemical Kinetics
- Volume:
- 52
- Issue:
- 12
- ISSN:
- 0538-8066
- Page Range / eLocation ID:
- p. 1022-1045
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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