CONFORMATIONAL-ANALYSIS .15. 1,2-DIBROMOTETRAFLUOROETHANE AND 1,2-DIIODOTETRAFLUOROETHANE - ELECTRON-DIFFRACTION INVESTIGATIONS OF THE MOLECULAR-STRUCTURES, COMPOSITIONS, AND ANTIGAUCHE ENERGY AND ENTROPY DIFFERENCES

The molecular structures and conformational compositions of 1,2-dibromotetrafluoroethane (DBTF) and 1,2-diiodotetrafluoroethane (DITF) have been investigated in the gas phase by electron diffraction each at three nozzle-tip temperatures: DBTF, 273, 388, and 673 K; DITF, 293, 393, and 473 K. Both molecules exist as a mixture of anti and gauche rotamers with the former the more stable. Distances (r(g)/angstrom) and angles (angle(alpha)/deg) for the lowest temperatures, with estimated 2-sigma uncertainties, are as follows: (DBTF) r(C-F) = 1.340 (3), r(C-C) = 1.559 (13), r(C-Br) = 1.930 (5), angle-CCF = 109.9 (4), angle-FCF = 108.4 (8), angle-CCBr = 110.5 (5), and angle-BrCCBr(G) = 67 (3); (DITF) r(C-F) = 1.334 (3), r(C-C) = 1.542 (13), r(C-I) = 2.146 (7), angle-CCF = 109.1 (6), angle-FCF = 108.2 (7), angle-CCI = 111.7 (6), and angle-ICCI(G) = 70 (3). The rotameric compositions of DBTF at 273, 388, and 673 K were found to be 30 (8), 39 (14), and 53 (18) % gauche, from which the energy and entropy differences are calculated to be E-degrees(G) - E-degrees(A) = 3.6 (11) kJ/mol and S-degrees(G) - S-degrees(A) + R 1n 2 = 6.0 (34) J/(mol.K). For DITF at 298, 393, and 473 K, the compositions are 19 (6), 24 (11), and 35 (13) % gauche from which the energy and entropy differences are E-degrees(G) - E-degrees(A) = 5.1 (15) kJ/mol and S-degrees(G) - S-degrees(A) + R 1n 2 = 5.0 (44) J/(mol.K). The structures and thermodynamic properties are discussed.