Solvent Effects on the Nitrogen NMR Shieldings in Oxazole and Oxadiazole Systems

Abstract

High-precision14N NMR shieldings are reported for all five available oxazoles and oxadiazoles in a variety of solvents. Both solvent polarity and hydrogen-bond effects on the nitrogen nuclear shieldings of the solutes are significant and comparable in magnitude; both give rise to shielding increases. The increasing solvent polarity favors delocalization of electrons from oxygen atoms into the heteroaromatic rings with a concomitant electron charge accumulation on the nitrogen atoms concerned. This trend is parallel to those found for analogous diazole and triazole heteroaromatic systems. Solvent-to-solute hydrogen bonding 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 nitrogen atom but also those arising from competition between other acceptor sites in a given molecule; these include oxygen and other nitrogen atoms. TNDO/2-calculated nitrogen shieldings, with respect to neat nitromethane, give a very good linear correlation with experimental results when all of the nitrogen atoms in the molecules studied are taken into account. The present study of nitrogen shieldings provides a valuable insight into solvent polarity effects on electric charge distribution in nitrogenous heteroaromatic systems, as well as into possible competition between various hydrogen-bond acceptor centers with respect to hydrogen-bond donor solvents.