N.M.R. spectra and stereochemistry of the bipyridyls. I. Unsubstituted bridged biquaternary 2,2'-bipyridyls

Abstract

ABMX analysis of the N.M.R. spectra of six bridged biquaternary salts of 2,2?-bipyridyl is reported. The difference in chemical shift between H3(3?) and H5(5?) varies with the length of the bridging group, and the results are discussed in relation to the probable conformation of the salts. The several factors determining the difference in chemical shift are discussed, and it is concluded that the diamagnetic anisotropy of the neighbouring pyridinium ring is the major contributor, with further significant contributions from the asymmetric distribution of positive charge in the pyridinium ring. Interplanar angles are calculated and compared with values estimated for the singly bridged biphenyls; evidence for mobile and frozen conformations is presented.