An application of the reaction class transition state theory to the kinetics of hydrogen abstraction reactions of hydrogen with methyl esters at the methoxy group

Abstract

The hydrogen abstraction reactions of methyl esters from methyl formate (MF) to methyl decanoate (MD) by hydrogen (H) atom at the methoxy group have been investigated on the basis of isodesmic reactions within the framework of reaction class transition state theory (RC-TST). The rate constants for the four reactions of H with MF, methyl acetate (MA), methyl propanoate (MP), and methyl butanoate (MB) are used as the benchmarking values, and are calculated via the transition state theory (TST) with the Eckart method to account for tunneling effect by employing potential energy surface information computed at the CCSD(T)/CBS//BH&HLYP/6-31G(d,p) level. The calculated rate constants of reactions H with MF and MA are in excellent agreement with experimental results. For the target reactions of H with MA, MP and MB, it is demonstrated that the calculated energy barriers and rate constants at the BH&HLYP/6-31G(d,p) level with a simple correction scheme based on isodesmic reactions within the framework of RC-TST can approach the accuracy of the CCSD(T)/CBS level. Based on the correction scheme, reaction enthalpies, energy barriers and rate constants for large reaction systems including the reactions of H with methyl valerate (MV), methyl hexanoate (MHex), methyl heptanoate (MHep), methyl octanoate (MO), methyl nonanoate (MN) and MD at the methoxy group are computed and fitted into the modified Arrhenius expression in the temperature range 300–2500K for chemical kinetic modeling studies. Further, it is found that the reaction rate constants of H with large methyl ester molecules (from MA to MD) is nearly identical and is slightly larger than that of H with MF at the methoxy group.