The reaction of CN with CH₄ and CD₄: an experimental and theoretical study

The kinetics of the CN reaction with CH₄ and CD₄ were studied by laser induced fluorescence in the temperature range of 183 to 740 K. The measured rates may effectively be expressed, in units of cm³/s, by k_CH4 = 5.15 × 10^-16 T^1.53 exp(-504/T) and k_CD4 = 8.51 × 10^-19 T^2.38 exp(-403/T). Both reactions increased monotopically with temperature and exhibited a significant isotope effect. Variational TST calculations were performed for these reactions in which the energies of the transition state were calculated at several points along the reaction coordinate using the BAC-MP4 method. It was determined that as the temperature was increased, the position of the transition state moved towards smaller values of the NC:HCH₃(DCD₃) reaction coordinates for both CH₄ and CD₄ reactions. This affected both the zero-point energies and entropies of reactions. Good agreement was found between the calculated and experimental activation energies of the reactions.