Accurate ab initio calculations on the rate constants of the direct hydrogen abstraction reaction C₂H + H₂ → C₂H₂ + H

The direct hydrogen abstraction process for the reaction C₂H + H₂ →C₂H₂ + H is theoretically investigated at the UQCISD/6-311+G(d,p) and G2//UQCISD levels. The reaction rate constants over a wide range of temperatures from 20-5000 K are calculated over a wide range of temperatures from 20-5000 K using the canonical variational transition state theory along with the small curvature tunneling correction. At the G2// UQCISD level without and with zero-point vibrational correction, the obtained forward reaction barrier are 2.22 and 2.52 kcal/mol, respectively, which, for the first time, well match the value 2.0-2.5 kcal/mol expected by many groups. The obtained forward rate constants are also in good agreement with most of the available experimental results covering a wide temperature range. Furthermore, the calculated reverse rate constants agree well with two recent experimental estimates yet differ from the other experimental estimates and one ab initio calculation at lower level. Finally, our calculated results show that for the forward reaction, the variational effect should be considered at high temperatures whereas the tunneling correction is very important at the temperatures below 500 K and is much more remarkable below 150 K. For the reverse reaction within the considered temperature range 800-5000 K, the variational effect should also be included in the calculation of rate constants at high temperatures while the small-curvature tunneling correction is small.