Stereoelectronic Effects in Sulfate Diesters and Sulfuric Acid

An X-ray crystallographic investigation of two 2, 2-dioxo-l, 3, 2-dioxathianes has been undertaken, together with a comparative study of 1, 1-dioxothiane, in a search for evidence of stereoelectronic effects in sulfate diesters. However, the differences between the axial and equatorial S=O bond lengths observed are so small that they cannot be detected by the X-ray experiment. Ab initio calculations on sulfuric acid and dimethyl sulfate showed that the sulfur-oxygen bond overlap populations, electron density at the sulfuryl oxygen atoms, and conformational energy were dependent on the S-OH(Me) torsion angle. These effects are interpreted in terms of stereoelectronic interactions between σ and π nonbonding electron pairs on divalent oxygen and the antibonding orbitals of sulfur-oxygen bonds. If an anomeric effect is defined simply as any n→σ* orbital mixing interaction, this stereoelectronic effect must be viewed, at least in part, as anomeric. The stereoelectronic effect operating in H₂SO₄ and sulfate diesters may weaken S-O single bonds and influence overall charge distribution. Stereoelectronic effects in the chair conformation of six-membered cyclic sulfates result in the axial sulfuryl bond being longer than the equatorial. The bond length difference is such that it cannot be detected with confidence in the X-ray structure determinations presented, but it could account for the observed differences in the vibrational spectra of isotopically labeled sulfates.