A conformational analysis of the six 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 inter‐conversion to planar structures, thereby preventing conjugation between the p‐electrons of the sulfur atom and the π 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.