Conformational Stabilities From Variable Temperature Raman Spectra, r(0) Structural Parameters, And Assignments Of 1,2- Diphosphinoethane

The 1,2-disubstituted ethane (YCH2CH2X) molecules are of interest since most of them have two conformers trans (anti) and gauche, present in the fluid states and their relative stabilities are believed to depend on the electronegativity and size of the substituents. For example, 1, 2-difluoroethane has the gauche conformer more stable than the trans form by 283c47 cm(-1) (3.38c0.56 kJ mol(-1)) in the vapor state [1]. However, 1,2-dichloroethane [2] have the trans form as the more stable rotamer by 323c7 cm(-1) (3.87c0.09 kJ mol(-1)). One might expect the l-chloro-2-fluoroethane molecule to have the gauche conformer as the more stable form since the fluorine and chlorine atoms are highly electronegative and the chlorine atom has a van der Waal's radius nearly the same as that of the methyl group. When Y is a methyl group and X is a fluorine atom (1-fluoropropane) the gauche conformer is more stable [3] than the trans form by 104c6 cm(-1) (1.24c0.07 kJ mol(-1)). However, it has been reported that the l-chloro-2-fluoroethane molecule has the trans form as the more stable rotamer from a determination of the enthalpy from the variable temperature Raman spectra of the gas [4] as well as from a variable temperature study of rare gas solutions [2]. Thus, it is clear that there are additional factors than just size and electronegativity that determines which conformer is the most stable form.