Accurate ab Initio Calculations on the Rate Constants of the Direct Hydrogen Abstraction Reaction C₂H + H₂ → C₂H₂ + H

Abstract

The direct hydrogen abstraction process for the reaction C 2 H + H 2 f C 2 H 2 + 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.

A modified potential energy surface for the C2H+H2↔C2H2+H reaction and a theoretical study on its rate constants

“…The C-H bonds that will be formed are 51% and 49% longer than the equilibrium bond lengths of the ab initio and the MWB PES cases, respectively. This shows that the transition state is reactant-like, that is, the reaction has an ÔearlierÕ small barrier height and large exothermic energy, which are consistent with the conclusions of [18,20,21]. The calculated harmonic vibrational frequencies and zero point energies (ZPEs) of the stationary points, together with the corresponding experimental data [31], are presented in Table 2.…”

“…The linear transition state was located and the barrier height was found to be 4.0 kcal/mol. According to the recent conclusions [5,7,13,[19][20][21], lower barrier heights of 2.0-2.5 kcal/mol may reproduce the experimental rate constants better over a broad range of temperatures. Based on the properties of the saddle point reported by Harding et al [18], Wang and Bowman [23] have constructed the first global PES of the C 2 H + H 2 system and accomplished a quantum dynamics calculation.…”

“…Experimentally, the rate constants have been measured with various methods over wide temperature range of 178-4650 K [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Theoretically, the ab initio calculations have been carried out by a few groups [18][19][20][21][22]. Among them, Harding et al [18] had presented for the first time the ab initio determination of the rate constant of the forward reaction calculated at the POL-CI/DZP level.…”

“…First, ab initio calculation at a higher level than those reported in [18,20,21] will be done for getting more reliable information of the stationary points and the minimum energy path (MEP). Second, on the basis of the stationary point properties of the ab initio data as well as the experimental results, the barrier height of the WB PES is reduced to a reasonable value, and then the MWB PES will be obtained.…”

The scavenger effect – Does it work?

“…In reference [3] threshold quadrupole mass spectrometry was applied to solve the problem of ambivalent mass spectra, and some of the information can be successfully applied to our experiments. In [9] it is shown that e.g. gas phase reaction rates with NH 3 for high sticking species such as C 2 H are high.…”