The analysis of the n.m.r,
spectra of 2,2?-, 3,3?-, and 4,4?-bipyridyl and three dimethyl-2,2?-bipyridyls
is reported and the factors determining the relative chemical shifts of the
ring protons and methyl groups in several solvents are discussed. The diamagnetic
anisotropy of the neighbouring ring and electrostatic field effect of the
nitrogen lone pair electrons are shown to be of roughly equal importance for
derivatives of 2,2?-bipyridyl except in hydrogen bonding solvents. Attenuation
of the electrostatic field effect in polar, and particularly in hydrogen
bonding solvents, is established for 4- picoline, and for the bipyridyls, and
this effect is responsible for striking changes in the spectrum of 2,2?-bipyridyl
in hydrogen bonding solvents. An approximate interplanar angle of 58� is
derived for 3,3?- dimethyl-2,2?-bipyridyl, and 2,2?-bipyridyl and its 4,4?- and
5,5?- dimethyl derivatives appear to be trans coplanar in all solvents. 3,3?-
Bipyridyl and 4,4?-bipyridyl are probably highly twisted in all solvents, or
alternatively, behave as essentially free rotors. The predicted conformations
are in good agreement with the electronic spectral data.