Automatization of theoretical kinetic data generation for tabulated TS models building -Part III: C-H and C-C β-scissions in alkyl radicals
Résumé
Linear and branched alkanes are omnipresent in usual and alternative fuels, lubricants, and additives. The decomposition of such compounds in the atmosphere and their combustion process is driven by radical reactions, mainly isomerisations, β-scissions, and hydrogen abstractions. In a recent work [Destro et al., Proc. Combust. Inst. (2022) 611-620], the use of tabulated models of transition state (TMTS) to estimate H-shift reactions in linear and branched alkyl radicals was developed and proposed. This approach sought a compromise between the accuracy and the calculation costs by defining the electronic method and kinetic resolution with an acceptable uncertainty, while allowing for the investigation of reaction moiety representativity in large and branched structures (C 8 -C 12 ). In this current study, we expanded this approach for βscission reactions, forming hydrogen atoms and alkyl radicals, using transition state theory for the calculation of high-pressure limit rate coefficients. Different ab initio methods for the electronic structure calculations (CBS-QB3, G4, B2PLYP-D3) are compared with a high-level ANL-F12 scheme. It was found that the double hybrid DFT method B2PLYP-D3 with the TZ basis allows the calculation of rates with high accuracy, deviations of less than a factor of 1.9 for four reference β-scission reactions. The inclusion of different sizes of alkyl groups at different positions in the chain was tested. The size of the alkyl group was found to have a minor impact on the rate constants, with differences lower than the estimated uncertainties of the method. On the other hand, the presence of branching has an important effect on the kinetics, by increasing rate coefficients for β-scission of branched alkyls.
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