Effects of π-electron densities and solvent on proton resonances of alkyl-substituted pyridines and benzenes

Abstract

Proton chemical shifts in equimolar solutions of monoalkyl-substituted benzenes and 4-alkyl-substituted pyridines have been studied in various solvents. Shift variations have been correlated with the dielectric function (ε– 1)(ε+ 1) where ε is the dielectric constant of the solvent. Alkyl groups were found to have comparable substituent effects on proton resonances of the pyridine and phenyl rings. Salts of the pyridinium and N-alkylpyridinium ions were studied in deuterium oxide solutions. The ring-proton and alkyl-proton shifts for each salt and the parent pyridine have been used to deduce relative π-electron densities which have been compared with those calculated with a Hückel Molecular Orbital (HMO) programme. Additivity effects for proton, methyl, and ethyl substituents on the shifts for the ring protons are evaluated. The effects of solvent interactions upon shifts and line-widths for the salts are discussed.