Abstract
A Reaction Class Transition State Theory (RC-TST) is applied to calculate thermal rate constants for hydrogen abstraction by OOH radical from alkanes in the temperature range of 300–2500 K. The rate constants for the reference reaction C2H6 + ∙OOH → ∙C2H5 + H2O2, is obtained with the Canonical Variational Transition State Theory (CVT) augmented with the Small Curvature Tunneling (SCT) correction. The necessary parameters were obtained from M06-2X/aug-cc-pVTZ data for a training set of 24 reactions. Depending on the approximation employed, only the reaction energy or no additional parameters are needed to predict the RC-TST rates for other class representatives. Although each of the reactions can in principle be investigated at higher levels of theory, the approach provides a nearly equally reliable rate constant at a fraction of the cost needed for larger and higher level calculations. The systematic error is smaller than 50% in comparison with high level computations. Satisfactory agreement with literature data, augmented by the lack of necessity of tedious and time consuming transition state calculations, facilitated the seamless application of the proposed methodology to the Automated Reaction Mechanism Generators (ARMGs) programs.
Highlights
Since the Reaction Class Transition State Theory (RC-TST) rates of any representative of the title reaction family may be considered as an extrapolation of the reference process, exhaustive knowledge with the best accuracy possible is of crucial importance
Methane is known to possess unusual stability due to its high symmetry and lack of the C-C bond. This results in possible problems with the extrapolation of its rate to the other class representatives. This expectation was confirmed by our calculations, as well as by results previously reported in Aguilera-Iparraguirre et al (2008), where the barrier for CH4 + ·OOH → ·CH3 + H2O2 reaction is higher by about 4.5-5 kcal/mol than that for reaction with C2H6 (R1) for all theory levels employed
The C2H6 + ·OOH → ·C2H5 + H2O2 (R1) reaction was chosen as the reference, as discussed further later in this article
Summary
The crucial initialization step of the combustion of hydrocarbons is the H abstraction (Walker and Morley, 1997; Handford-Styring and Walker, 2002; Scott and Walker, 2002; Aguilera-Iparraguirre et al, 2008; Battin-Leclerc et al, 2013; Shi, 2018; Curran, 2019; Hashemi et al, 2019). Because the title reaction family stands as a significant channel of decay of the OOH radicals, its accurate kinetic parameters are needed to quantify the initial stages of combustion, and to properly predict the fate of the peroxy compound species. Since the combustion models are intended for a wide range of possible fuels, a number of rates for each reaction class are needed This is a challenging task, especially if the model is to be created with automated reaction mechanism generators (ARMG’s). An RC-TST framework was employed to derive the kinetic parameters necessary for the estimation of the rate constants of any reaction belonging to the alkane + ·OOH → alkyl radical + H2O2 reaction family. To further clarify this way of coding, a schematic representation of example p, s and t type H abstractions (namely processes R11, R13, and R12) are available in the Supporting Information (Supplementary Figure S1)
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