Accurate ab initio calculations on the rate constants of the direct hydrogen abstraction reaction C2H+H-2 -> C2H2+H

The direct hydrogen abstraction process for the reaction C2H + H-2 --> C2H2 + H is theoretically investigated at the UQCISD/6-311+G(d,p) and G2//UQCIS D levels. The reaction rate constants over a wide range of temperatures fro m 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 sma ll curvature tunneling correction. At the G2//UQCISD level without and with zero-point vibrational correction, the obtained forward reaction barrier a re 2.22 and 2.52 kcal/mol, respectively, which, for the first time, well ma tch the value 2.0-2.5 kcal/mol expected by many groups. The obtained forwar d rate constants are also in good agreement with most of the available expe rimental results covering a wide temperature range. Furthermore, the calcul ated reverse rate constants agree well with two recent experimental estimat es yet differ from the other experimental estimates and one ab initio calcu lation at lower level. Finally, our calculated results show that for the fo rward reaction, the variational effect should be considered at high tempera tures whereas the tunneling correction is very important at the temperature s below 500 K and is much more remarkable below 150 K, For the reverse reac tion within the considered temperature range 800-5000 R, the variational ef fect should also be included in the calculation of rate constants at high t emperatures while the small-curvature tunneling correction is small.