The dynamics of O(3P) + deuterated hydrocarbons: influences on product rotation and fine-structure state partitioning

Abstract

The dynamics of the reactions O(³P) + CH₄/CD₄ and O(³P) + cyclo-C₆H₁₂/C₆D₁₂ have been investigated experimentally. Translationally hot O(³P) atoms were generated by laser photolysis of NO₂ at 266 nm. The rotational and fine-structure state distributions in the nascent product OH or OD radicals were determined by laser-induced fluorescence. The deuterium-labelled reactants have provided confirmation of the main features of the product-state partitioning observed previously for normal-hydrogen compounds. Only OH ν′ = 0 is detectable from CH₄. CD₄ also produces predominantly OD ν′ = 0, with ν′ = 1 being just detectable with an estimated branching ratio of ∼ 0.06. Both cyclo-C₆H₁₂ and C₆D₁₂ produce significant yields of ν′ = 0 and 1 products. Despite the opposite trend in exothermicities, the ν′ = 0 products are rotationally hotter from methane than from cyclohexane, indicative of a stereochemical effect. The absolute rotational energy release is in each case found to be essentially independent of hydrogen isotope. It is argued that this supports the accepted collinear abstraction mechanism for this class of reactions. The OH fine-structure state partitioning is not influenced by hydrogen isotope, nor by product vibrational level despite distinct differences in the associated rotational energy release. This appears consistent with a previously derived model involving selected non-adiabatic coupling of adiabatic electronic fine-structure surfaces.