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.