Theoretical gas phase study of the gauche and trans conformers of 1-fluoro-2-haloethanes CH2F-CH2X (X=Cl, Br, I) by ab initio and density functional methods:: Absence of gauche effect

This is a systematic theoretical gas phase study of the gauche and trans conformers of 1-fluoro-2-haloethanes (FCH2CH2X, X=Cl, Br and 1). The methods used are second order Moller-Plesset theory (MP2) and density functional theory (DFT). The basis set used is 6-31 1++(d,p) for all atoms except that 3-21G and CRENBL ECP are used for iodine atom. The functional used for DFT method is B3LYP. G2/MP2 calculation has also been carried out using MP2 optimised structure. The results indicate that unlike 1,2-difluoroethane, there is the absence of gauche effect and thus there is more preference for the trans conformer. The preference for the more stable trans conformer increases with increasing atomic size of the substituted halogen atom. The same trend is observed for energy difference between the gauche and trans conformers. The 1-fluoro-2-haloethanes have also been subjected to vibrational analysis.