Reaction of C2(a 3Πu) with methanol: Temperature dependence and deuterium isotope effect

Bimolecular rate constants for the gas-phase reactions of C2(a 3Πu) with a variety of methanol isotopomers including CH3OH (k1), CH3OD (k2), CD3OH (k3), and CD3OD (k4) have been measured over the temperature range of 293-673 K by means of pulsed laser photolysis/laser-induced fluorescence technique. The rate constants, in units of cm3 molecule-1 s-1, can be fitted by the normal Arrhenius expressions: k1(T)=(1.32±0.02)x10-11 exp[-(366.80±4.44)/T], k2(T)=(1.34±0.02)x10-11 exp[-(376.86±5.09)/T], k3(T)=(1.09±0.02)x10-11 exp[-(640.00±7.23)/T], and k4(T)=(1.12±0.01)x10-11 exp[-(666.37±4.63)/T], where all error estimates are ±2σ and represent the precision of the fit. The observed deuterium kinetic isotope effects, k1/k2 and k1/k3, along with the positive temperature dependences of k(T), allow us to reach a conclusion that the reaction of C2(a 3Πu) with methanol in 293-673 K proceeds via a site-specific hydrogen abstraction mechanism, that is, H-atom abstraction from the methyl site rather than from the hydroxyl site dominating reactivity.