Solvent effects in the N.M.R. spectra of aza-aromatics

Solvent effects on proton chemical shifts in a number of substituted pyridines have been investigated. The systems chosen were asymmetric 3 and 4 spin systems, which were analysed by the ‘Effective Larmor Frequency Method’ of Pople and Schaefer. The chemical shifts were then investigated in a number of solvents ranging from the non-polar n-hexane to the highly polar water. The shifts could be qualitatively interpreted on the basis of Buckingham's theory. The rather interesting but anomalous shifts obtained on dilution with water have been explained on the basis of the highly ordered water structure around the solute molecule.