A STUDY OF SOLVENT EFFECTS ON THE NMR SHIELDINGS OF THE 2 TYPES OF NITROGEN-ATOMS CONTAINED IN DIAZOLE RING-SYSTEMS

The diazole ring systems, N-methyl derivatives of pyrazole and imidazo le, show a clear differentiation between pyrrole and pyridine types of nitrogen atoms from the point of view of their solvent-induced nitrog en shielding variations. The largest effects on the nitrogen shielding s derive from hydrogen bonding from solvent to solute, which involve t he lone pair electrons at the pyridine-type nitrogen atoms of the solu te. These effects may be used to identify sites of hydrogen-bond accep ters in the solute molecules. Solvent-polarity effects on the nitrogen shieldings suggest a significant charge migration from pyrrole- to py ridine-type nitrogen atoms in such systems. Solvaton calculations ampl y support this proposal by correctly predicting the observed direction s and relative magnitudes of the nitrogen shielding variations as a fu nction of solvent polarity. (C) 1994 Academic Press, Inc.