Ab initio, DFT and transition state theory calculations on 1,2-HF, HCI and CIF elimination reactions from CH₂F-CH₂Cl

Abstract

This paper reports ab intio, DFT and transition state theory (TST) calculations on HF, HCl and ClF elimination reactions from CH₂Cl-CH₂F molecule. Both the ground state and the transition state for HX elimination reactions have been optimized at HF, MP2 and DFT calculations with 6-31G*, 6-31G** and 6-311++G** basis sets. In addition, CCSD(T) single point calculations were carried out with MP2/6-311++G** optimized geometry for more accurate determination of the energies of the minima and transition state, compared to the other methods employed here. Classical barriers are converted to Arrhenius activation energy by TST calculations for comparisons with experimental results. The pre-exponential factors, A, calculated at all levels of theory are significantly larger than the experimental values. For activation energy, E_a, DFT gives good results for HF elimination, within 4-8 kJ mol^-1 from experimental values. None of the methods employed, including CCSD(T), give comparable results for HCl elimination reactions. However, rate constants calculated by CCSD(T) method are in very good agreement with experiment for HCl elimination and they are in reasonable agreement for HF elimination reactions. Due to the strong correlation between A and E_a, the rate constants could be fit to a lower A and E_a (as given by experimental fitting, corresponding to a tight TS) or to larger A and E_a (as given by high level ab initio calculations, corresponding to a loose TS). The barrier for ClF elimination is determined to be 607 kJ mol^-1 at HF level and it is unlikely to be important for CH₂FCH₂Cl. Results for other CH₂X-CH₂Y (X,Y=F/Cl) are included for comparison.