Unveiling the oxygen addition kinetics of 1-Hydroxy-2-Hexyl and 2-Hydroxy-1-Hexyl at low temperature

Abstract

Hydroxyalkyl radicals formed from addition of OH to alkenes play a crucial role in the oxidation of alkenes. The oxygen addition kinetics of 1‑hydroxy-2-hexyl and 2‑hydroxy-1-hexyl at low temperature has been investigated by using quantum chemical calculations coupled with the RRKM/master equation simulations. The main reaction pathways that occur on the potential energy surfaces of HOC6H12 + O2 were studied at the CCSD(T)/CBS(cc-pV[T,Q]Z)//M06–2X/jun-cc-pVTZ level. Rate coefficients of key reactions have been determined at the high-pressure limit and the falloff region. The isomerizations of hydroxyhexylperoxy to hydroperoxy-hydroxyalkyl are found to be of great importance. The formed hydroperoxy-hydroxyalkyl radicals can either dissociate rapidly into aldehydes and ketones, or be accumulated to participate in the second oxygen addition. The formation of formaldehyde, n-pentanal and OH via the Waddington mechanism is among the most important product channels. H2O elimination pathways are identified for the most stable HOQOOH intermediates, with the radical site at α-carbon to the hydroxy group. The present study recognizes those hydroperoxy-hydroxyalkyl radicals that are able to undergo the second oxygen addition. Kinetic analysis reveals that the facile formation via 1,5 or 1,6 H-transfer and lack of fast decomposition pathways combine to enable the accumulation of particular hydroperoxy-hydroxyalkyl radicals, facilitating their involvement in subsequent chain-branching chemistry. This study further illuminates the intricate understanding of the low temperature chemistry of larger alkenes.