TY - JOUR

T1 - Cost-Effective Implementation of Multiconformer Transition State Theory for Alkoxy Radical Unimolecular Reactions

AU - Zhao, Qian

AU - Moller, Kristian H.

AU - Chen, Jing

AU - Kjaergaard, Henrik G.

PY - 2022

Y1 - 2022

N2 - Alkoxy radicals are important intermediates in the gas-phase oxidation of volatile organic compounds (VOCs) determining the nature of the first-generation products. An accurate description of their chemistry under atmospheric conditions is essential for understanding the atmospheric oxidation of VOCs. Unfortunately, experimental measurements of the rate coefficients of unimolecular alkoxy radical reactions are scarce, especially for larger systems. As has previously been done for peroxy radical hydrogen shift reactions, we present a cost-effective approach to the practical implementation of multiconformer transition state theory (MC-TST) for alkoxy radical unimolecular (H-shift and decomposition) reactions. Specifically, we test the optimal approach for the conformational sampling as well as the best value for a cutoff of high-energy conformers. In order to obtain accurate rate coefficients at a reduced computational cost, an energy cutoff is employed to reduce the required number of high-level calculations. The rate coefficients obtained with the developed theoretical approach are compared to available experimental rate coefficients for both 1,5 H-shifts and decomposition reactions. For all but one of the reactions tested, the calculated MC-TST rate coefficients agree with experimental results to within a factor of 7. The discrepancy for the final reaction is about a factor of 15, but part of the discrepancy is caused by pressure effects, which are not included in MC-TST. Thus, for the fastest alkoxy reactions, deviation from the highpressure limit even at 1 bar should be considered.

AB - Alkoxy radicals are important intermediates in the gas-phase oxidation of volatile organic compounds (VOCs) determining the nature of the first-generation products. An accurate description of their chemistry under atmospheric conditions is essential for understanding the atmospheric oxidation of VOCs. Unfortunately, experimental measurements of the rate coefficients of unimolecular alkoxy radical reactions are scarce, especially for larger systems. As has previously been done for peroxy radical hydrogen shift reactions, we present a cost-effective approach to the practical implementation of multiconformer transition state theory (MC-TST) for alkoxy radical unimolecular (H-shift and decomposition) reactions. Specifically, we test the optimal approach for the conformational sampling as well as the best value for a cutoff of high-energy conformers. In order to obtain accurate rate coefficients at a reduced computational cost, an energy cutoff is employed to reduce the required number of high-level calculations. The rate coefficients obtained with the developed theoretical approach are compared to available experimental rate coefficients for both 1,5 H-shifts and decomposition reactions. For all but one of the reactions tested, the calculated MC-TST rate coefficients agree with experimental results to within a factor of 7. The discrepancy for the final reaction is about a factor of 15, but part of the discrepancy is caused by pressure effects, which are not included in MC-TST. Thus, for the fastest alkoxy reactions, deviation from the highpressure limit even at 1 bar should be considered.

KW - MOLECULAR-ORBITAL METHODS

KW - GAUSSIAN-BASIS SETS

KW - RATE CONSTANTS

KW - ATMOSPHERIC CHEMISTRY

KW - SEMIEMPIRICAL METHODS

KW - INITIATED OXIDATION

KW - 2-BUTOXY RADICALS

KW - ORGANIC-COMPOUNDS

KW - MASTER EQUATION

KW - PHOTO-OXIDATION

U2 - 10.1021/acs.jpca.2c04328

DO - 10.1021/acs.jpca.2c04328

M3 - Journal article

C2 - 36053271

VL - 126

SP - 6483−6494

JO - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory

JF - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory

SN - 1089-5639

IS - 37

ER -