The molecular structures and conformational preferences of bis(dimethylstibyl)-oxane, -sulfane and -selane, E(SbMe(2))(2), E=O, S or Se, Me=CH3, by density functional theory calculations and gas electron diffraction

Density Functional Theory calculations on E(SbMe(2))(2), E = O, S or Se, Me = CH3, indicate that the equilibrium structures are syn-syn or near syn-syn conformers with over-all C-2 symmetry. The barriers restricting rotation about E Sb bonds are very low, the equilibrium values for the dihedral angles phi(Sb-E-Sb-1p) where Ip denotes the direction of the electron lone pair on one of the Sb atoms are probably less than 45 degrees. The calculations further indicate the existence of syn-anti conformers some 4 kJ mel(-1) above the more stable syn-syn. Gas electron diffraction data show that both conformers are present in gaseous O(SbMe(2))(2), while the presence of the syn-arnri conformer in gaseous Se(SbMe(2))(2) is uncertain; least-squares refinements yielded the mole fraction chi = 0.27(18). The Sb-O and Sb-Se bond distances are 197.6(14) and 255.1(5) pm respectively, the valence angles of the syn-syn conformers are <SbOSb = 122.3(16)degrees and <SbSeSb = 96.3(11)degrees. It is suggested that the wide <SbOSb angle is due to across-angle repulsion between the Sb atoms.