Gaucheltrans equilibria of 2,2′-bi-1,3-dioxolanyl, 2,2′-dimethyl-2,2′-bi-1,3-dioxolanyl, 2,2′-bi-1,3-dithiolanyl and 2,2′-dimethyl-2,2′-bi-1,3-dithiolanyl in different media -: theory and experiment

The dipole moments of 2,2'-bi-1,3-dioxolanyl (1), 2,2'-dimethyl-2,2'-bi-1,3-dioxolanyI (2), 2,2'-bi-1,3-dithiolanyl (3) and 2,2'-dimethyl-2,2'-bi-1,3-dithiolanyl (4) in carbon tetrachloride and benzene have been measured over a range of temperatures. Analysis of the relative permittivity data in carbon tetrachloride show that at 25degreesC, 1, 2 and 3 favour the trans form. However, replacement of the hydrogens on C-2 of the dithiolane ring with methyl groups completely reverses the trans gauche equilibrium such that 4 exists in 70% gauche conformation. In benzene, all four compounds were found to exist predominantly in the trans form. The experimentally derived values of the energy difference between the gauche and trans conformers and the gauche/trans population quotients were compared with values predicted by molecular orbital calculations. Theory predicts that 1, 2 and 4 prefer a gauche conformer in the gas phase due to favourable CH...X (X = O and S) interactions. Surprisingly, we found that a solvent reaction field has a larger stabilization effect on the less polar trans form. For compounds 1 and 2, the solvent effect is sufficiently large that the gauche/trans equilibrium is reversed on going from the gas phase to a polar dielectric medium. The crystal and molecular structures of 2 and 4 were determined by single-crystal X-ray diffraction methods. In the solid state, compound 2 exists in the trans conformation whilst 4 favours the gauche form.