DUAL-LEVEL DIRECT DYNAMICS STUDIES ON THE HYDROGEN ABSTRACTION REACTION OF CF3CHFCF3 (HFC-227ea) WITH Cl ATOM

Abstract

The hydrogen abstraction reaction of CF3CHFCF3 (HFC-227ea) by Cl atom is studied theoretically over a temperature range of 50–2000 K. The reaction path information such as geometries, gradients, etc. are initially obtained at the BB1K/6-31+G (d,p) level. In order to improve the energies, high level single point energy (SPE) calculations are carried out at BB1K/MG3S on BB1K/6-31+G(d,p) geometries. The rate coefficients are obtained by dual-level direct dynamics approach with the interpolated SPE method at BB1K/MG3S//BB1K/6-31+G(d,p) level. Aforementioned rate coefficients are calculated based on canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The results show that for the title reaction, variational effect is small while the tunneling effects are significant at low temperature. In order to investigate the reliability of improved BB1K potential energy surface in predicting barrier height of the reaction, different quantum models built such as QCISD(T)/6-311++G(d,p)//MP2/6-31G(d,p) , QCISD(T)/aug-cc-pv-DZ//MP2/6-31G(d,p) , and MP4SDQ/aug-cc-pv-DZ//MP2/6-31G(d,p) are applied to calculate the rate coefficients of the reaction. The results show the rate coefficients calculated based on MP2 potential energy surface are underestimated as compared to the experimental value. Finally three models are applied to fit the calculated rate coefficients over the temperature range of 50–2000 K. It is shown that the new four-parameter model provided a better temperature dependence than the other expressions. It is expected that obtained theoretical results can be useful for modeling the kinetics nature of the reaction of HFC-227ea and Cl atom over temperature range where no experimental data is accessible.