The existence of second, third and fourth row anomeric interactions. An ab initio study of the torsional behaviour in HSCH 2SH, HSeCH 2SeH, HSCH 2SeH, HTeCH 2TeH, HSCH 2TeH and HSeCH 2TeH

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The torsional behaviour in the title compounds has been probed by ab initio calculations of the gauche, gauche (g,g), gauche.anti (g,a), anti.gauche (a,g) and anti,anti (a,a) conformations at the MINI-1 ∗, 3-21G( ∗) and 3-21G ∗ levels. In the case of HSCH 2SH, results at the 6-31G ∗ level have also been obtained. Although the trends in relative energies and geometrical changes computed with the 3-21G ∗ and MINI-1 ∗ basis sets are similar for the different conformations, the C-X and C-Y bond lengths are overestimated in the latter case. For the conformations of HSCH 2SH, good correspondence is obtained for the relative energies and geometrical parameters with the 3-21G ∗ and 6-31G ∗ basis sets. The existence of putative anomeric interactions in the HXCH 2YH molecules has been evaluated by examination of the relative energies and geometrical variations of the different conformers and also by the group separation energies obtained from the isodesmic reaction HXCH 2YH + CH 4 → CH 3XH + CH 3YH. In general, the a,a conformers either relax without barrier to another conformer or are not true minima. The stabilization of the g,g relative to the a,g conformation (the anomeric effect) decreases for the heavier chalcogens and the trend even reverses for HTeCH 2YH (Y = Te, S, Se). The group separation energies (3-21 G( ∗)) including zero-point energy corrections indicate anomeric stabilizations of about 1 kcal mol −1 for the most stable conformers (g,g) of XCH 2YH (X, Y = S, Se). The g,g and a,g forms are stabilized to the same extent in HSeCH 2SeH vis-a-vis the isodesmic reaction. For HTeCH 2TeH, a stabilization energy of only 0.17 kcal mol −1 is observed for the most stable conformer (a,g). We conclude that at the 3.21G ∗ level of computation even anomeric interactions with tellurium are small but nonetheless present. We propose also that with the heavier chalcogens the energy difference between the a,g and a,a conformations may provide a better measure of anomeric stabilization in diheterosubstituted methanes.