A gas phase electron diffraction study of the molecular structure of 1,1,2-trifluoroethane, assisted by calculation of the vibrational amplitudes for 1,2-difluoroethane from spectroscopic data

Abstract

The gas phase conformational mixture of the rotamers of 1,1,2-trifluoroethane has been subjected to an electron diffraction study at 265 K. The radial distribution curve has a peak at 3.5 Å attributable to an anti F … F distance, indicating a predominance of the conformer possessing this distance. Effective least-squares refinement of the geometry and the relative proportions of the conformers was achieved with vibrational amplitudes for both conformers fixed at values derived from spectroscopic data for 1,1,2,2-tetra- and 1,2-di-fluoroethane. The amplitudes for the latter molecule were calculated from an optimised force field which reproduced the observed wavenumbers for both of its conformers. For 1,1,2-trifluoroethane at 265 K, the conformer with the F … F anti distance was found to be present in 92% abundance, which corresponds to an energy difference between conformers of 900 cal mol −1. The geometrical parameters ( r a values) and e.s.d. are CC = 1.500 ± 0.005 Å, CH(mean) = 1.088 ± 0.011 Å, CF(CF 2H gp) = 1.353 ± 0.004 Å, CF(CH 2F gp) = 1.387 ± 0.008 Å, ∠CCF(mean) = 109.0 ± 0.5°, ∠CCH(mean) = 108.9 ± 1.4°, ∠FCF = 106.8 ± 0.7°. As in 1,2-difluoroethane ∠HCH ≈ 120°. The τ HCCF torsional angles of the predominant F … F anti conformer are 38, 47, 56 and 78°, τ HCCH = 66° and τ FCCF = 75°.