Theoretical investigation of the structures of H₂SO₄, (CH₃O)₂SO₂ and CH₃OSO₃

Abstract

The energies of the various conformers formed by rotating around the S---O bond in H₂SO₄, (CH₃O)₂SO₂ and CH₃OSO₃- have been determined by ab initio molecular orbital methods using minimal (STO-3G) and extended (4-31G) basis sets. The (90°, 90°) conformation in H₂SO₄, has been found to be the most stable, in agreement with the microwave spectral data for gaseous sulfuric acid. Calculations on dimethyl sulfate indicate that geometry optimization can dramatically alter the relative energies of the conformational isomers. Thus, after RO---S---OR bond angle optimization, the energy difference between the (75°, 75°), (75°, 180°) and (180°, 180°) conformations is only about 0.21 kcal mol^-1 suggesting that all three rotational isomers are likely to occur. This explains the detection of different conformers of dimethyl sulfate by different experimental techniques. The barrier to rotation around the S---O bond in the CH₃OSO₃- system is calculated to be very small, ∼1.2 kcal mol^-1. A comparison of these species with related anomeric systems is presented and the nature of the interactions of polar bonds in determining the preferred conformation is discussed.