2-phenylpyridine: To twist or not to twist?

Density functional theory methods were used to investigate the structures associated with 2-phenylpyridine, ppy, and several of its electronic states. The structure of ppy has the aromatic rings twisted with respect to one another by similar to 21 degrees, which is about half the value found for biphenyl. In comparison with ppy, both the isoelectronic cation, ppyH(+), and anion, ppy(-), have larger twist angles. The extent of twisting is governed by the delicate balance between pi conjugation and repulsive orbital/steric interactions, and the magnitudes of these interactions were investigated by examining the torsional energy barriers for all three molecular species. In contrast, every one of the investigated open-shell structures-ppy(center dot+), ppy(center dot-), ppy*, ppyH(+)*, and ppy(-)*-has coplanar aromatic rings, that is, no twist angle. Frontier molecular orbital analyses reveal that the pi-type bonding between the bridging carbons becomes dominant over any repulsive orbital and steric interactions, thereby leading to coplanar rings. Also, the energetics associated with ppy and its various electronic states were investigated and reported.