Ab initio calculations and mass spectrometric determination of the gas-phase proton affinities of 4,4′-disubstituted 2,2′-bipyridines

The gas-phase proton affinities of 4,4'-di(R)-2,2'-bipyridines (R: H, Br, Cl, NO2, Me) were determined by mass spectrometric measurements and by ab initio calculations at the HF/6-31G** and MP2/6-31G** levels of theory. The energy barriers for rotation about the central C-C bond were also studied computationally. Two minima were found for both unprotonated and protonated species, the global minima being at the trans planar and cis planar conformations, respectively. Local minima for the unprotonated compounds were at the cis nonplanar conformation and for the protonated compounds at the trans nonplanar. Two different proton affinity values were calculated for each compound by employing different conformations for the protonated species. The computational values were in good agreement with the experimental proton affinities. Substituents affect the proton affinity according to their ability to withdraw or to donate electrons, halogen and nitro-substituted bipyridines having a lower proton affinity and methyl-substituted bipyridine having a higher proton affinity than 2,2'-bipyridine itself.