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

The rate coefficients of the reactions of CH and CD with CH 4, CD 4, C 2 H 2, and C 2 D 2 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: k CH+CH 4 = (6.7 ± 0.3)( T /293 K) -(0.4±0.1) × 10 -11 cm 3 molecule -1 s -1; k CH+CD 4 = (4.2 ± 0.2)( T /293 K) -(0.3±0.1) × 10 -11 cm 3 molecule -1 s -1; k CD+CH 4 = (5.7 ± 0.2)( T /293 K) -(0.32±0.04) × 10 -11 cm 3 molecule -1 s -1; k CD+CD 4 = (3.6 ± 0.1)( T /293 K) -(0.27±0.08) × 10 -11 cm 3 molecule -1 s -1; k CH+C 2 H 2 = (3.2 ±0.2)( T /293 K) -(0.2±0.1) × 10 -10 cm 3 molecule -1 s -1; k CD+C 2 H 2 = (2.8 ± 0.1)( T /293 K) -(0.13±0.06) × 10 -10 cm 3 molecule -1 s -1; k CH+C 2 D 2 = (3.2 ± 0.2)( T /293 K) -(0.2±0.1) × 10 -10 cm 3 molecule -1 s -1; and k CD+C 2 D 2 = (2.8 ± 0.2)( T /293 K) -(0.2±0.1) × 10 -10 cm 3 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.