Solvent effects in nuclear magnetic resonance spectroscopy. Part XII. Correlations between benzene-induced solvent shifts and structure in pyridines, quinolines, pyrroles, and indoles

Abstract

The solvent shifts (Δ=δ_{carbon tetrachloride}–δ_benzene p.p.m.) of proton resonances in pyridines, quinolines, pyrroles, and indoles are in accord with the orientation of benzene solvent molecules by the solute dipoles. The shifts are so characteristic in sign and magnitude as to be useful in structure elucidation. The introduction of methyl groups into the unsubstituted heterocycles in general causes an algebraic decrease in the Δ values relative to those observed in the parent heterocycle. The algebraic decreases of Δ values caused by a methyl substituent are approximately additive as evidenced by the solvent shifts observed in di- and tri-methylpyridines.