CONFORMERS, ENERGETICS, AND BASICITY OF 2,2'-BIPYRIDINE

The proton affinities of the cis and trans conformers of 2,2'-bipyridi ne have been determined at the MP2/6-31G*//HF/6-31G** level of theory . The neutral molecule and its protonated cation are both shown to pos sess stable cis and trans conformers: the barriers for cis/trans inter conversion, and the roles that electrostatic interactions and pi-conju gation play in these barriers, are analyzed. In addition, the barriers to rotation through planar structures are reported. The structures, t he effect of electron correlation on the energetics, and the ground st ate charge distributions are discussed (i) with respect to biphenyl an d 2,2'-bipyrimidine and (ii) in the context of dinitrogen proton spong es. With regard to (i) and cis --> trans interconversion, it appears t hat biphenyl, 2,2'-bipyridine, and 2,2'-bipyrimidine have almost ident ical conformers and rotational energetics. With regard to (ii), these results indicate that SCF estimates of proton affinities and proton tr ansfer barriers in such compounds are overestimated (by 1-2% and simil ar to 100%, respectively), and that barriers to rotation are underesti mated by some 20-30%.