Semiclassical Transition State Theory (SCTST) Rate Coefficients for the Unimolecular Decomposition of the Ethoxy (CH3CH2O) Radical

Thanh Lam Nguyen; Gregory H Jones; Bryan Changala; Binod Raj Giri; John R Barker; John F Stanton

doi:10.1021/acs.jpca.4c05775

Semiclassical Transition State Theory (SCTST) Rate Coefficients for the Unimolecular Decomposition of the Ethoxy (CH₃CH₂O) Radical

J Phys Chem A. 2024 Nov 12. doi: 10.1021/acs.jpca.4c05775. Online ahead of print.

Authors

Thanh Lam Nguyen¹, Gregory H Jones¹, Bryan Changala², Binod Raj Giri³, John R Barker⁴, John F Stanton¹

Affiliations

¹ Quantum Theory Project, Departments of Chemistry and Physics, University of Florida, Gainesville, Florida 32611, United States.
² Center for Astrophysics, Harvard & Smithsonian, Cambridge, Massachusetts 02138, United States.
³ Institute for Thermodynamics and Thermal Process Engineering, Brandenburg University of Technology, Cottbus-Senftenberg, 03044 Cottbus, Germany.
⁴ Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143, United States.

PMID: 39531279
DOI: 10.1021/acs.jpca.4c05775

Abstract

The thermal unimolecular decay of ethoxy is important in high-temperature combustion environments where the ethoxy radical is a key reactive intermediate. Two dissociation pathways of ethoxy, including the β-C-C scission to yield CH₃ + CH₂O and the H-elimination to make H + CH₃CHO, were characterized using a high-level coupled-cluster-based composite quantum chemical method (mHEAT-345(Q_Λ)). The former route is found to be dominant while the latter is insignificant, in agreement with previous experimental and theoretical studies. Thermal rate coefficients are calculated for P = 0.001-658 atm (of air) and T = 300-2500 K using semiclassical transition state theory (SCTST) in combination with a pragmatic two-dimensional E,J-resolved master equation (2DME). The effects of tunneling and anharmonicity on the calculated rate constants are also examined. The tunneling factor is found to be inversely dependent on pressure, contrary to previous observations of pressure-dependent tunneling in entrance channels.