Rate constants of hydrogen abstraction by methyl radical from n-butanol and a comparison of CanTherm, MultiWell and Variflex

Rate constants of hydrogen abstraction by the ĊH3 radical from n-butanol have been calculated based on coupled-cluster in the complete basis set limit, CCSD(T)/CBS, electronic energies. Accurate rate constants have been calculated using MultiWell and Variflex for all possible abstraction channels employing conventional transition state theory and the rigid-rotor harmonic oscillator approximation for all but the torsional degrees of freedom. Geometries, frequencies, and hindrance potentials were obtained at the B3LYP/6-311G(2d,d,p) level. In addition, the same procedures have been used but at a lower energetic level, ROCBS-QB3, with three kinetic programs, namely, CanTherm, MultiWell and Variflex. Computed reaction rate constants show very good agreement among all three programs for the α, β, γ and alcoholic H-atom abstraction channels while larger discrepancies have been found for the δ abstraction channel. Variflex predicts the fastest rate constants for the δ-channel, and its ratio over the lowest rate constants, predicted by MultiWell, is 2.5 at 500 K and increases to 3.0 at 2000 K. Branching ratio analysis shows that α and γ channels are favoured while the contribution from the β, δ and alcoholic abstraction channels are minor. Both MultiWell and Variflex predict that the α-channel is favoured over the γ-channel, while CanTherm predicts the γ-channel as the most dominant.