Deuterium kinetic isotope effects on the gas-phase reactions of C2H with H2(D2) and CH4(CD4)

Abstract

Kinetics of the ethynyl (C₂H) radical reactions with H₂, D₂, CH₄ and CD₄ was studied over the temperature range of 295–396 K by a pulsed laser photolysis/chemiluminescence technique. The C₂H radicals were generated by ArF excimer-laser photolysis of C₂H₂ or CF₃C₂H and were monitored by the chemiluminescence of CH(A²Δ) produced by their reaction with O₂ or O(³P). The measured absolute rate constants for H₂ and CH₄ agreed well with the available literature data. The primary kinetic isotope effects (KIEs) were determined to be k_H2/k_D2 = 2.48 ± 0.14 and k_CH4/k_CD4 = 2.45 ± 0.16 at room temperature. Both of the KIEs increased as the temperature was lowered. The KIEs were analyzed by using the variational transition state theory with semiclassical small-curvature tunneling corrections. With anharmonic corrections on the loose transitional vibrational modes of the transition states, the theoretical predictions satisfactorily reproduced the experimental KIEs for both C₂H + H₂(D₂) and C₂H + CH₄(CD₄) reactions.