Torsional barriers in biphenyl, 2,2′-bipyridine and 2-phenylpyridine

The rotational barriers for biphenyl, 2,2′-bipyridine and 2-phenylpyridine were calculated with HF, B3LYP, BPW91, and MP2 and double-zeta polarized basis sets. The methods differ significantly in the near-planar region, and are in close agreement otherwise, with DFT results similar to experiment over the whole range (for biphenyl). Steric repulsions, stabilizing C–H–N′ interactions, π-conjugation and electron correlation give rise to DFT-calculated dihedrals for biphenyl of about 40° and similar 0° and 90° barriers, dihedrals of 23° and an almost non-existent 0° barrier for 2-phenylpyridine and two minima for 2,2′-bipyridine, which is strongly stabilized by two C–H–N′ interactions in the coplanar conformation with anti-standing nitrogens.