Theoretical studies of the influence of rotational isomerism on the basicity of 2,2′-bipyridine in the gas phase. Comparison with other biazines and bidentate nitrogen ligands with flexible conformation

Abstract

The influence of the rotation of the pyridyl ring around the central CC bond on the basicity (as PA) of 2,2′-bipyridine has been studied in the gas phase by a semiempirical method (AM1), and compared with that for 3,3′-, 4,4′-, 2,3′- and 2,4′-bipyridines. A chelation effect of the proton (called “internal solvation”) in the syn-2,2′-pyridylpyridinium cation has been observed. Rotation and chelation of the proton increase the PA of the aza group in syn-2,2′-bipyridine by about 8 kcal mol −1 in comparison with the anti form. An additional aza group in the meta or para position to those already present has no influence on the chelation of the proton in syn-2,2′-bipyridine. The exceptionally high basicity of syn-2,2′-bipyridine has been compared with that observed for other bidentate nitrogen ligands with flexible conformation.