Hydrogen abstraction from n-butanol by the methyl radical: high level ab initio study of abstraction pathways and the importance of low energy rotational conformers

Abstract

Hydrogen abstraction reactions by the methyl radical from n-butanol have been investigated at the ROCBS-QB3 level of theory. Reaction energies and product geometries for the most stable conformer of n-butanol (ROH) have been computed, the reaction energies order α < γ < β < δ < OH. The preference for n-butane to favour H-abstraction at C_β and C_γ while, in contrast, n-butanol favours radical reactions at the C_α carbon is rationalised. Transition state (TS) barriers and geometries for the most stable conformer of n-butanol are presented, and discussed with respect to the Hammond postulate. The reaction barriers order as α < OH < γ < β < δ. This relative ordering is not consistent with product radical stability, C–H bond dissociation energies or previous studies using ȮH and HȮ₂ radicals. We provide a molecular orbital based rationalisation for this ordering and answer two related questions: Why is the γ-channel more stable than the β-channel? Why do the two C_γ–H H-abstraction TS differ in energy? The method and basis set dependence of the TS barriers is investigated. The Boltzmann probability distribution for the n-butanol conformers suggests that low energy conformers are present in approximately equal proportions to the most stable conformer at combustion temperatures where ĊH₃ radicals are present. Thus, the relative significance of the various H-abstraction channels has been assessed for a selection of higher energy conformers (ROH'). Key results include finding that higher energy n-butanol conformers (E(ROH′) > E(ROH)) can generate lower energy product radicals, E(ṘOH′) < E(ṘOH). Moreover, higher energy conformers can also have a globally competitive TS energy for H-abstraction.