An Ab initio study of the stability of the symmetrical and unsymmetrical difluoroethylenes relative to ethylene and monofluoroethylene

Abstract

Ab initio calculations of the total molecular energy (E_T), charge population, and dipole moments for ethylene, monofluoroethylene, cis- and trans-1,2-difluoroethylene, and 1,1-difluoroethylene have been carried out using a (7,3) basis set with full geometry optimization, and (7,3), (7,3,1), and (9,5) basis sets using recent experimental geometries. We confirm the finding of Kollman, who used partial geometry optimization at the STO-3G level and earlier experimental geometries, that 1,1-difluoroethylene is more stable than either of the 1,2-isomers, in accord with the behaviour of 1,1- and 1,2-disubstituted ethylenes in general as noted by Epiotis et al. The stability of the difluoroethylenes is examined in terms of ΔE_T, and where possible the corresponding reaction heat (ΔH₀⁰)_zpe, for the disproportionation reaction difluoroethylene + ethylene → 2 monofluoroethylene. The closed shell molecular species CH₂CH₂ and CH₂CHF are thus utilized as a composite molecular energy baseline to assess destabilization or stabilization effects, in contrast to the procedure followed by Epiotis et al. and by Whangbo et al. which involves open shell molecular fragments. This alternative approach, using the disproportionation reaction, leads to the conclusion that the slightly greater stability of the cis-relative to the trans-isomer, (ΔH₀⁰)_zpe=+1.08 kcal mol^–1, is due to lesser destabilization in the cis-isomer, since ΔE_T for both disproportionation reactions is substantially negative lying in the range –6 to –8 kcal mol^–1.