THE EXISTENCE OF 2ND, 3RD AND 4TH ROW ANOMERIC INTERACTIONS - AN AB-INITIO STUDY OF THE TORSIONAL BEHAVIOR IN HSCH2SH, HSECH2SEH, HSCH2SEH,HTECH2TEH, HSCH2TEH AND HSECH2TEH

The torsional behaviour in the title compounds has been probed by ab i nitio calculations of the gauche, gauche (g,g), gauche,anti (g,a), ant i,gauche (a,g) and anti,anti (a,a) conformations at the MINI-1, 3-21G () and 3-21G* levels. In the case of HSCH2SH, results at the 6-31G* l evel have also been obtained. Although the trends in relative energies and geometrical changes computed with the 3-21G and MINI-1* basis se ts are similar for the different conformations, the C-X and C-Y bond l engths are overestimated in the latter case. For the conformations of HSCH2SH, good correspondence is obtained for the relative energies and geometrical parameters with the 3-21G and 6-31G* basis sets. The exi stence of putative anomeric interactions in the HXCH2YH molecules has been evaluated by examination of the relative energies and geometrical variations of the different conformers and also by the group separati on energies obtained from the isodesmic reaction HXCH2YH + CH4 --> CH3 XH + CH3YH. In general, the a,a conformers either relax without barrie r to another conformer or are not true minima. The stabilization of th e g,g relative to the a,g conformation (the anomeric effect) decreases for the heavier chalcogens and the trend even reverses for HTeCH2YH ( Y = Te, S, Se). The group separation energies (3-21G()) including zer o-point energy corrections indicate anomeric stabilizations of about 1 kcal mol-1 for the most stable conformers (g,g) of XCH2YH (X, Y = S, Se). The g,g and a,g forms are stabilized to the same extent in HSeCH2 SeH vis-a-vis the isodesmic reaction. For HTeCH2TeH, a stabilization e nergy of only 0.17 kcal mol-1 is observed for the most stable conforme r (a,g). We conclude that at the 3.21G level of computation even anom eric interactions with tellurium are small but nonetheless present. We propose also that with the heavier chalcogens the energy difference b etween the a,g and a,a conformations may provide a better measure of a nomeric stabilization in diheterosubstituted methanes.