Rate constants and kinetic isotope effects on the reaction of C2(X(1)Sigma(g)+) with CH4 and CD4.

Rate constants and kinetic isotope effect (KIE) for the reaction of singlet dicarbon, C(2)(X(1)Sigma(g)(+)), with CH(4) and CD(4) have been measured over the temperature range 294-376 K by using the pulsed laser photolysis/laser-induced fluorescence technique. C(2)(X(1)Sigma(g)(+)) were generated by multiphoton laser decomposition of C(2)Cl(4) at 248 nm and its decay trace was monitored on the (0,0) band of the Mulliken system at 231.2 nm. Measured rate constants showed slightly positive temperature dependence, whereas the KIE [= k(CH(4))/k(CD(4))] was almost independent of temperature and the value of which was 2.1 +/- 0.2 as a simple average of the values of KIE at different temperatures. Quantum chemical calculation with CASPT2 method indicated that the reaction proceeds via a direct hydrogen abstraction mechanism to form C(2)H and CH(3) radicals. Variational transition-state theory calculations were performed employing a dual-level method. Anharmonic effects along transitional modes were included in the calculation, and a comparison of the rate constants with and without anharmonic corrections demonstrated the importance of anharmonicity. The calculated rate constants and KIE showed good agreement with the experiments except for the temperature dependence of the KIE. A possible cause of the discrepancy was discussed in terms of the long-range interaction between the reactants.