Solvent effects on the nitrogen NMR shieldings of 3-methylsydnone and ab initio calculations of the shieldings including related oxazole and oxadiazole systems

Solvent effects on the nitrogen shieldings of 3-methylsydnone (1), a zwitte rionic isomer of the hypothetical 5-methoxy-1,2,3-oxadiazole, are found to be significantly different from those observed for oxazoles and oxadiazoles . A detailed analysis of the solvent-induced variations reveals contributio ns from three interactions, those due to solvent polarity, hydrogen bonding from solvent to solute, involving the lone pairs of the exocyclic oxygen a tom in 1, and those involving basic centers in the solvent and the positive ly charged heteroaromatic ring in the vicinity of N-3 which bears a formal positive net charge. Ab initio DFT-GIAO magnetic shielding calculations emp loying B3PW91 functionals, a 6-311++G** basis set and geometries optimized using the same set are reported for 1 and some related molecules which incl ude all of the existing oxazoles and oxadiazoles. The experimental range of nitrogen shielding considered is about 177 ppm and shows an excellent line ar correlation with the calculated results. The least-squares standard devi ation is only 1.4% of the observed shielding range. The effects on the nitr ogen shielding of 1 caused by hydrogen bonding and the calculations indicat e that the exocyclic oxygen atom in 1 is the primary acceptor of hydrogen b onds from solvent molecules. The calculations fully corroborate recent assi gnment for the O-17 NMR shieldings of 1. Copyright (C) 2000 John Wiley & So ns, Ltd.