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 CS 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.