SOLVENT EFFECTS ON THE NITROGEN NMR SHIELDINGS IN OXAZOLE AND OXADIAZOLE SYSTEMS

High-precision N-14 NMR shieldings are reported for all five available oxazoles and oxadiazoles in a variety of solvents. Both solvent polar ity and hydrogen-bond effects on the nitrogen nuclear shieldings of th e solutes are significant and comparable in magnitude; both give rise to shielding increases. The increasing solvent polarity favors delocal ization of electrons from oxygen atoms into the heteroaromatic rings w ith a concomitant electron charge accumulation on the nitrogen atoms c oncerned. This trend is parallel to those found for analogous diazole and triazole heteroaromatic systems. Solvent-to-solute hydrogen bondin g causes an increase in nitrogen shielding when the nitrogen atoms in question are involved in the hydrogen bonding as acceptor sites. This reflects not only local effects of hydrogen bonding to a particular ni trogen atom but also those arising from competition between other acce ptor sites in a given molecule; these include oxygen and other nitroge n atoms. TNDO/2-calculated nitrogen shieldings, with respect to neat n itromethane, give a very good linear correlation with experimental res ults when all of the nitrogen atoms in the molecules studied are taken into account, The present study of nitrogen shieldings provides a val uable insight into solvent polarity effects on electric charge distrib ution in nitrogenous heteroaromatic systems, as well as into possible competition between various hydrogen-bond acceptor centers with respec t to hydrogen-bond donor solvents. (C) 1996 Academic Press, Inc.