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Thermochemistry and Kinetic Analysis of the Unimolecular Oxiranyl Radical Dissociation Reaction: A Theoretical Study
Author(s) -
Wang Heng,
Bozzelli Joseph W.
Publication year - 2016
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201600152
Subject(s) - chemistry , thermochemistry , radical , intramolecular force , methyl radical , dissociation (chemistry) , standard enthalpy of formation , ketene , bond dissociation energy , transition state , enthalpy , bond cleavage , computational chemistry , elementary reaction , hydrogen atom , photochemistry , thermodynamics , alkyl , kinetics , medicinal chemistry , organic chemistry , physics , quantum mechanics , catalysis
Oxirane structures are important in organic synthesis, and they are important initial products in the oxidation reactions of alkyl radicals. The thermochemical properties (enthalpy of formation, entropy, and heat capacity) for the reaction steps of the unimolecular oxiranyl radical dissociation reaction are determined and compared with the available literature. The overall ring opening and subsequent steps involve four types of reactions: β‐scission ring opening, intramolecular hydrogen transfer, β‐scission hydrogen elimination, and β‐scission methyl radical elimination. The enthalpies of formation of the transition states are determined and evaluated using six popular Density Functional Theory (DFT) calculation methods (B3LYP, B2PLYP, M06, M06‐2X, ωB97X, ωB97XD), each combined with three different basis sets. The DFT enthalpy values are compared with five composite calculation methods (G3, G4, CBS‐QB3, CBS‐APNO, W1U), and by CCSD(T)/aug‐cc‐pVTZ. Kinetic parameters are determined versus pressure and temperature for the unimolecular dissociation pathways of an oxiranyl radical, which include the chemical activation reactions of the ring‐opened oxiranyl radical relative to the ring‐opening barrier. Multifrequency quantum Rice Ramsperger Kassel (QRRK) analysis is used to determine k (E) with master equation analysis for falloff. The major overall reaction pathway at lower combustion temperatures is oxiranyl radical dissociation to a methyl radical and carbon monoxide. Oxiranyl radical dissociation to a ketene and hydrogen atom is the key reaction path above 700 K.