Regularities of the conformations on a single bond; sulfonic esters and sulfonyl derivatives

Abstract

The so-called gauche rule preferring the less symmetrical sc conformations on sp³–sp³ bonds was revised by theoretical calculations of five sulfonyl derivatives: C₆H₅SO₂OCH₃, CH₃SO₂OC₆H₅, ClSO₂OC₆H₅, C₆H₅SO₂CH₂Cl and C₆H₅SO₂SO₂C₆H₅. At the same time the limiting possibilities of common theoretical models were tested in predicting exact geometry and minute energy differences between rotamers. With a larger basis set, for instance at the level B3LYP/6-311+G(3df,3pd)//B3LYP/6-311+G(3df,3pd) or MP2/6-311+G(2df,2pd)//MP2/6-311+G(2df,2pd) different methods yielded concordant values of the dihedral angle τ characterizing the conformation. However, the predicted values of energy were not in sufficient agreement to give reliable population of rotamers. Probably the best estimate of this population can be obtained by combining the calculated angle τ with some experimental quantities, for instance with dipole moments. According to these results, the gauche rule is not a generally valid law. Preponderance of sc conformation in different classes of compounds may have different reasons. In our case, we obtained a qualitative explanation assuming that conformation is determined by destabilizing interaction of two polar bonds in the neighbouring sc position, while lone electron pairs have no observable effect.