MOLECULAR-STRUCTURES AND CONFORMATIONS OF 1,1,3,3-TETRACHLOROPROPENE, CIS-1,2,3,3-TETRACHLOROPROPENE AND TRANS-1,2,3,3-TETRACHLOROPROPENE, 1,1,2,3,3-PENTACHLOROPROPENE, AND 1,1,2-TRICHLORO-3,3-DIFLUOROPROPENE ACCORDING TO GAS-PHASE ELECTRON-DIFFRACTION

Four chloro-substituted propenes have been investigated at about 330 K by gas-phase electron diffraction, and the ED data for 1,1,2-trichloro-3,3-difluoropropene have been reanalyzed. For 1,1,3,3-tetrachloropropene only one conformer was observed in which the carbon-hydrogen bond eclipse the carbon-carbon double bond. This is also the predominating form of trans-1,2,3,3-tetrachloropropene and 1,1,2,3,3-pentachloropropene, but for these two compounds small amounts of a conformer with the C3-H bond anti to C=C are probably also present in the ps phase. For cis-1,2,3,3-tetrachloropropene we found substantial fractions of two conformers, namely, 41 (12)% of a form with C3-H syn to C1=C2 (H-C-C=C torsional angle phi = 0-degrees) and 59 (12)% of a form with C3-H gauche to C1-C2 (phi = 133 (4)-degrees). On the assumption that DELTAS-degrees equals R ln 2, this corresponds to an energy difference DELTAE-degrees = E-degrees(gauche) - E-degrees(syn) = 0.9 (7) kJ mol-1. Reanalyzing the ED data from an earlier investigation for 1,1,2-trichloro-3,3-difluoropropene, we have found the molecules to be a mixture of two conformers, both with C(s) symmetry. The majority (82 (8)%) of the molecules have a syn conformation with a C=C-C-H torsional angle of phi1 = 0-degrees, while the rest (18 (8)%) have an anti conformation with phi2 = 180-degrees.