A CONFORMATIONAL-ANALYSIS OF THE 6 ISOMERS OF THIOBISPYRIDINE

The conformational behaviour of the six isomers of thiobispyridine has been investigated using ab initio STO-3G*//rigid-roto, STO-3G*//STO-3G* and 6-31G**//STO-3G* molecular orbital models. The analysis reveals both the importance of optimising critical structure parameters and the basis set dependence of calculated rotational barrier heights. The most reliable model (6-31G**//STO-3G*) clearly indicates that the minimum energy conformers are not planar and that energy barriers between 30-100 kJ mol-1 restrict interconversion to planar structures, thereby preventing conjugation between the p-electrons of the sulfur atom and the pi-system of both pyridine rings. From the calculated barrier heights, two mechanisms can be employed to explain conformer interconversion about the C-S bond: a disrotatory one-ring flip or a conrotatory two-ring flip mechanism. Where comparisons can be made (eg. 2,2'-thiobispyridine), dipole moment calculations are shown to be in good agreement with experiment. Finally, of the six isomers, appropriately substituted 2,2', 2,3'- and 2,4'-thiobispyridines are most prone to a Smiles rearrangment.