Rate constants for hydrogen-abstraction by [formula omitted] from n-butanol

Abstract

Rate constants of hydrogen-atom abstraction from n-butanol by the OH radical have been calculated using variational transition state theory and conventional transition state theory (the latter includes Eckart tunneling corrections) as implemented in Variflex. A stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel has been recognized in part of the abstracting processes. A two-transition-state model is used in calculating the individual rate constant for the channels with a barrier-less process in the entrance channel. Variable reaction coordinate transition state theory (VRC-TST) has been used to calculate the rate constants of the barrier-less process. Computed total and individual rate constants, based on a G3 potential energy surface, in the temperature range of 500–2000 K for n - butanol + O ˙ H are reported as follows (cm 3 mol −1 s −1): k t = ( 4.03 ± 3.33 ) × 10 1 × T 3.57 ± 0.10 exp [ ( 2128 ± 98 ) / T ] k α = ( 4.56 ± 2.65 ) × 10 3 × T 2.81 ± 0.07 exp [ ( 1855 ± 69 ) / T ] k β = ( 8.29 ± 8.94 ) × 10 - 1 × T 3.74 ± 0.13 exp [ ( 2050 ± 128 ) / T ] k γ = ( 5.09 ± 3.66 ) × 10 1 × T 3.35 ± 0.09 exp [ ( 2193 ± 85 ) / T ] k δ = ( 2.15 ± 0.93 ) × 10 4 × T 2.77 ± 0.05 exp [ ( − 93.3 ± 51.6 ) / T ] k oh = ( 5.88 ± 1.85 ) × 10 2 × T 2.82 ± 0.04 exp [ ( 294.2 ± 37.5 ) / T ] The computed individual and total rate constants based on the CCSD(T) potential energy surface over the same temperature range can be similarly expressed as: k t = ( 1.74 ± 2.20 ) × 10 1 × T 3.69 ± 0.16 exp [ ( 1703 ± 150 ) / T ] k α = ( 1.26 ± 1.49 ) × 10 3 × T 2.97 ± 0.15 exp [ ( 1339 ± 141 ) / T ] k β = ( 6.80 ± 7.27 ) × 10 - 1 × T 3.77 ± 0.13 exp [ ( 1848 ± 127 ) / T ] k γ = ( 1.19 ± 1.18 ) × T 3.79 ± 0.12 exp [ ( 2293 ± 118 ) / T ] k δ = ( 1.75 ± 0.77 ) × 10 4 × T 2.79 ± 0.06 exp [ ( − 220 ± 52 ) / T ] k oh = ( 5.22 ± 1.77 ) × 10 2 × T 2.84 ± 0.04 exp [ ( 317 ± 40 ) / T ] The computed total rate constants straddle the very recent shock tube measurements of the total reaction rate at 1017–1221 K, with the G3 results in particularly good agreement with experiment. Abstraction from the terminal methyl group is insignificant for temperatures below 500 K but becomes increasingly dominant at higher temperatures. Abstraction from the alcoholic group is never important.