On the Nature of Solvent Effects in the Proton Resonance Spectra of Unsaturated Ring Compounds. II. Heterocyclic Compounds

Abstract

In unsaturated heterocyclic compounds, the relative chemical shifts of the protons bonded to the ring are strongly dependent on solvent and concentration. In 6-membered ring compounds, as for example 4-picoline, the β hydrogens show a pronounced shift to low field in acetone solutions and to high field in benzene solutions. This behavior is similar to the behavior found for p-substituted toluenes (preceding paper). In the 5-membered heterocyclic compounds, for example pyrrole, the behavior of α and β hydrogens is reversed, the proton signals of α hydrogens being displaced selectively in acetone and benzene. The results can be interpreted in terms of a preferential hydrogen bonding of the α protons with the donor solvent molecules.