Reaction probabilities and cross-sections in the reaction of isotopically pure hydrogen atoms and propane

Abstract

The reactions of photochemically generated hydrogen atoms of selected energies with propane have been investigated. It has been possible for the first time to study the isotopically pure system and to monitor simultaneously abstraction from both primary and secondary C—H bonds in an alkane: H + C₃H₈→ H₂+ 1-C₃H₇→ H₂+ 2-C₃H₇. The phenomenological laboratory threshold energies were found to be 46 ± 3 and 30 ± 5 kJ mol^–1 for reaction at the primary and secondary sites, respectively. At energies well above the thresholds reaction probabilities per C—H bond were greater at the secondary than at the primary position, and also greater than for the abstraction of D atoms from alkanes in isotopically mixed systems. Taking the reaction probabilities measured here, together with data from xenon-moderated studies, and calculating H atom collision densities in Xe at the initial energies of this work, it was possible to estimate reaction cross-sections in H + C₃H₈. The cross-section rises from the threshold at 27 ± 5 kJ mol^–1 for secondary and 40 ± 3 kJ mol^–1 for primary abstraction to values of (1.8 ± 0.8)× 10^–3 nm² and (1.3 ± 0.5)× 10^–3 nm² per secondary and primary C—H bond, respectively. These values are higher than found for abstraction of D atoms but still very small compared with the total collisional cross-section.