Gauche/Trans Equilibria of 2,2'-Bi-1,3-dioxanyl, 2,2'-Dimethyl-2,2'-bi-1,3-dioxanyl, 2,2'-Bi-1,3-dithianyl, and 2,2'-Dimethyl-2,2'-bi-1,3-dithianyl in Different Media. Theory and Experiment

摘要

The gauche/trans conformational equilibria of 2,2'-bi-1,3-dioxanyl (1), 2,2'-dimethyl-2,2'-bi-1,3-dioxanyl (2), 2,2'-bi-1,3-dithianyl (3), and 2,2'-dimethyl-2,2'-bi-1,3-dithianyl (4) dissolved in carbon tetrachloride and benzene are studied through dipole moment determination. Analyses of the relative permittivity data show that the dioxa system favors the trans form at 25 ℃. Replacement of the oxygen atoms in 1 with sulfur completely reverses the gauche/trans equilibrium in carbon tetrachloride such that 3 exists predominantly in the gauche form. However, 4 exists mainly in the trans conformation in carbon tetrachloride solution and both 3 and 4 favor the trans form in benzene. X-ray crystallographic determination revealed that 1-4 favored the trans conformation in the solid state. Ab initio and DFT calculations were performed to examine the structural features of 1-4 and study the effects of solvent on these molecules. The calculated gauche/trans equilibria of 3 in different media are in excellent accord with the experimental findings. The observed difference in conformational preference of 3 in CCl_4 and benzene can be explained in terms of the specific solvation effect of benzene. CH…X (X = O or S) are important to understand the structures and relative energies of these 1,3-diheteroane systems.