PYRROLE AND N-METHYLPYRROLE AS MODELS FOR SOLVENT POLARITY AND SOLUTE-TO-SOLVENT HYDROGEN-BONDING EFFECTS ON NITROGEN NMR SHIELDING

Nitrogen NMR shieldings of pyrrole-type nitrogen atoms are shown to be quite sensitive to solvent effects and seem to provide a means of insight into solvent-induced electron charge redistribution in five-membered heteroaromatic systems. Significant deshielding effects on the nitrogen nucleus are observed upon the formation of NH-to-solvent hydrogen bonds and, independently, upon the increasing solvent polarity, and this behavior is reproduced by quantum-mechanical calculations. A clear distinction is found between pyrrole-type and pyridine-type nitrogens from the point of view of the directions of solvent effects on the respective nitrogen shieldings. N-Methylpyrrole nitrogen shieldings in solutions are shown to provide a probe for an empirical scale of solvent polarity. © 1993 by Academic Press, Inc.