Experiments are reported which describe the trapping of the unstable fruns conformer of 1.2difluoroethane in argon matrices. Assignments of the infrared spectra of the guuche and rrans conformers are given. The latter is shown to have a 600 c-al/ma1 higher free enthalpy.
1,2difluoroethane (DFE) may exist in three rotational conformations: cis,gauche, and tram, depending on the value of the dihedraI angle. Based on steric considerations, the cis form is relatively unstable. However, serious disagreement exists in the literature regarding the importance of the tram and gauche rotamers. From IR and Raman spectra, Klaboe and Nielsen [l] concluded that the two forms have approximately the same energy in the gas phase, whereas in the liquid and solid states only the gauche conformer exists. In an electron diffraction study, van Schaick et al. [2] found that the gauche rotamer comprised more than 38 % of the vapor phase. This led to a lower limit for the energy difference,Et-Eg, of 1700 cal/mol. Butcher et al. studied the microwave spectrum, and from the observed rotational constants and an assumed molecular geometry, a dihedral angle of 73S4O was obtained for the gauche form. From the soltient dependence of the NMR coupling constants, Abraham and Kemp [4] obtained a rotamer energy difference, Et-Eg, of +0.6 kca/mol for the vapor, and +2.6 kcal/mol for the liquid. The potential function for the internal rotation of DFE was calculated by Radoin et al: [S] in an LCAO SCF framework utilizing an extended + 31 G basis set. A value of-l.04 kcal/&ol wasobtained for Et-Eg.
In'our work argon. matrix spectra of DFE at liquidhelium temperature were taken with a set-up allowing.. simultaneous deposition of molecular beams fr&n an '. '.
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'. by the @El computer.center, is gratefully aCkIlQW~edged. Trofeso; G. I;eroi kindly helped us with the manuscript.