Deuterium Kinetic Isotope Effect and Temperature Dependence in the Reactions of CH[²Π] with Methane and Acetylene

Abstract

The rate coefficients of the reactions of CH and CD with CH4, CD4, C2H2, and C2D2 have been measured in the temperature range 290 < T < 700 K. All the rate constants show a slightly negative temperature dependence. They can be fitted in the observed temperature range by the following expressions: kCH+CH4 = (6.7 ± 0.3)(T/293 K)-(0.4±0.1) × 10-11 cm3 molecule-1 s-1; kCH+CD4 = (4.2 ± 0.2)(T/293 K)-(0.3±0.1) × 10-11 cm3 molecule-1 s-1; kCD+CH4 = (5.7 ± 0.2)(T/293 K)-(0.32±0.04) × 10-11 cm3 molecule-1 s-1; kCD+CD4 = (3.6 ± 0.1)(T/293 K)-(0.27±0.08) × 10-11 cm3 molecule-1 s-1; kCH+C2H2 = (3.2 ±0.2)(T/293 K)-(0.2±0.1) × 10-10 cm3 molecule-1 s-1; kCD+C2H2 = (2.8 ± 0.1)(T/293 K)-(0.13±0.06) × 10-10 cm3 molecule-1 s-1; kCH+C2D2 = (3.2 ± 0.2)(T/293 K)-(0.2±0.1) × 10-10 cm3 molecule-1 s-1; and kCD+C2D2 = (2.8 ± 0.2)(T/293 K)-(0.2±0.1) × 10-10 cm3 molecule-1 s-1, where all error estimates are ±2σ and represent the precision of the fit. The observed temperature and pressure dependences are in accordance with a barrierless addition of the methylidyne radical to methane or acetylene with rapid decomposition of the adduct. The deuterium isotope effects on the reactions are not well-described by loose-transition state models and may reflect changes in the vibrational frequencies in the transition state for initial complex formation.