STEREOELECTRONIC EFFECTS IN PENTAOXYSULFURANES - PUTATIVE INTERMEDIATES IN SULFURYL-GROUP TRANSFER

Ab initio calcluations have been performed on rotamers of the trigonal bipyramidal species (HO)(3)SO2- and CH3O(HO)(2)SO2- at the HF/3-21+G(*) and HF/6-31+G*//3-21+G(*) levels. Significant stereoelectronic effects on energy and bond length are observed, correlated with rotation about the equatorial S-O(H) bond in the nonmethylated rotamers, in simile with those observed in phosphoranes. Three approaches are taken to decompose individual contributions to these effects. (1) rigid rotor conformational energy surface scans with Mulliken population analysis; (2) comparison of torsional profile periodicity with that calculated for HOSO3+; (3) full geometry optimization and natural bond order (NBO) analysis of charge transfer (CT) interactions for each rotamer. NBO analysis indicates that the combination of(a) internal hydrogen bonding and (b) the consequent increase in geminal sigma-->sigma* CT interactions, contributes significantly to changes in geometry and stabilization of the rotamer with the equatorial OH bond in the apical plane. This analysis is confirmed by observation of the loss of stabilization in the methylated congener. In neither system does any evidence exist for the n-->sigma* interaction proposed to dominate in phosphoranes.