Solvent influence on the conformational equilibrium of 2,2′-bipyridine

The relative energies and dipolar properties of two conformers of 2,2′-bipyridine (1) were calculated using AM1 and ab initio methods. Dipole moments of 1 and pyridine in many organic solvents were determined. On this basis, the Gibbs energy ΔG(s) for the conformational s-trans⇌s-cis equilibrium of 1 was estimated. The solvent effect on ΔG(s) is discussed quantitatively using the reaction-field model for the continuum dielectric medium, in the framework of which both the dipolar and quadrupolar terms are considered. This simple model works very well when the adopted radius of the solvent cavity is that obtained from the apparent molar volume of 2,2′-bipyridine in solutions. It is also shown that the mean spherical approximation (MSA) model predicts a weaker dependence of ΔG(s) on the solvent's relative permittivity than the continuum model.