Rotational isomerism and crystal structures of 2,2′-diphenyl-2,2′-bi-1,3-dithianyl and 2,2′-diphenyl-2,2′-bi-1,3-dioxolanyl

Dipole moments of the compounds 2,2'-diphenyl-2,2'-bi-1,3-dithianyl 1 and 2,2'-diphenyl-2,2'-bi-1,3-dioxolanyl 2 in carbon tetrachloride and benzene have been measured over a range of temperatures. The crystal and molecular structures of the compounds were determined by single-crystal X-ray diffraction methods. Analyses of the crystal structures and relative permittivity data show that the compounds exist in the trans conformation in the solid state and as rotameric mixtures in solution with a predominantly high trans population of 85% and 80% respectively at 25 degrees C. The experimentally derived values of the energy difference between the gauche and trans rotamers and the gauche/trans population quotients were compared with the values predicted by molecular orbital calculations.