Detailed energy decomposition of the rotational barrier in 2,2′-bipyridine: a density functional study

Detailed analysis of the torsional profile for 2,2′-bipyridine with different model chemistries and basis sets shows a strong stabilization of the coplanar nitrogen anti conformation and a second minimum for the cis conformation with a dihedral of about 40°. The entropic contributions are unfavourable near the minima, contrary to the enthalpy curve. The electronic energy barrier is comprised of small geometry relaxation effects, and mostly dominated by contributions from exchange and electron correlation. The barrier for a dihedral φ of 0° is mostly due to steric repulsion of the hydrogens, and to a lesser extent due to Coulomb repulsion of the nitrogen lone pairs.