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 h ave been measured over a range of temperatures. The crystal and molecular s tructures of the compounds were determined by single-crystal X-ray diffract ion methods. Analyses of the crystal structures and relative permittivity d ata show that the compounds exist in the trans conformation in the solid st ate and as rotameric mixtures in solution with a predominantly high trans p opulation of 85% and 80% respectively at 25 degrees C. The experimentally d erived values of the energy difference between the gauche and trans rotamer s and the gauche/trans population quotients were compared with the values p redicted by molecular orbital calculations.