MOLECULAR-STRUCTURES AND CONFORMATIONAL COMPOSITIONS OF 1,2-DICHLORO-1,1,2,3,3,3-HEXAFLUOROPROPANE AND 1,2-DIBROMO-1,1,2,3,3,3-HEXAFLUOROPROPANE, AS DETERMINED BY GAS-PHASE ELECTRON-DIFFRACTION AND AB-INITIO CALCULATIONS

The molecular structures and conformational compositions of 1,2-dichlo ro-1.1.2,3,3,3-hexafluoropropane (CFP) and 1,2-dibromo-1.1,2,3.3,3-hex afluoropropane (BFP) have been studied by gas-phase electron diffracti on at 23 and 25-degrees-C, respectively. Both compounds form a mixture of two conformers, a more stable anti form (with the C1-X bond anti t o the C2 C3 bond) and a less stable gauche+ form. For CFP and BFP the mole fractions of the anti forms, with uncertainties estimated at 2sig ma were found to be 0.67(13) and 0.82(11), respectively, corresponding to energy differences of DELTAE(CFP) = 0.53 and DELTAE(BFP) = 1.0(5) kcal mol-1. To obtain a more detailed investigation of the conformatio nal geometries, results from ab initio molecular orbital calculations have been included in the analysis.