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

Abstract

Solvent effects on the nitrogen shieldings of 3-methylsydnone (1), a zwitterionic 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 contributions from three interactions, those due to solvent polarity, hydrogen bonding from solvent to solute, involving the lone pairs of the exocyclic oxygen atom in 1, and those involving basic centers in the solvent and the positively charged heteroaromatic ring in the vicinity of N-3 which bears a formal positive net charge. Ab initio DFT-GIAO magnetic shielding calculations employing B3PW91 functionals, a 6-311CCG ŁŁ basis set and geometries optimized using the same set are reported for 1 and some related molecules which include all of the existing oxazoles and oxadiazoles. The experimental range of nitrogen shielding considered is about 177 ppm and shows an excellent linear correlation with the calculated results. The least-squares standard deviation is only 1.4% of the observed shielding range. The effects on the nitrogen shielding of 1 caused by hydrogen bonding and the calculations indicate that the exocyclic oxygen atom in 1 is the primary acceptor of hydrogen bonds from solvent molecules. The calculations fully corroborate recent assignment for the 17 O NMR shieldings of 1.

(1Z,3Z)-1,4-Di(pyridin-2-yl)buta-1,3-diene-2,3-diol: The Planar Highly Conjugated Symmetrical Enediol with Multiple Intramolecular Hydrogen Bonds

“…It is known, however, that the strong interaction of the unshared electron pair of the nitrogen atom with the solvent is responsible for these differences. 25 On the other hand, experimental and calculated 1 H and especially 13 C NMR chemical shifts correlate very well with each other (correlation coefficients are usually higher than 0.99). It is noteworthy that the calculated chemical shifts are only slightly dependent on conformation of the tautomeric form.…”

“…Experimental and calculated 15 N NMR chemical shifts differ significantly from each other. It is known, however, that the strong interaction of the unshared electron pair of the nitrogen atom with the solvent is responsible for these differences . On the other hand, experimental and calculated 1 H and especially 13 C NMR chemical shifts correlate very well with each other (correlation coefficients are usually higher than 0.99).…”

Theoretical gas to liquid shift of 15N isotropic nuclear magnetic shielding in nitromethane using ab initio molecular dynamics and GIAO/GIPAW calculations

“…[5][6][7] Because of this, it is necessary to model the liquid phase and to perform magnetic shielding calculations on top of these models. The linear correlation between theoretical magnetic shieldings and experimental chemical shifts of all these substances gives the theoretical magnetic shielding of the reference compound by taking the intercept, considering that the slope is not far from 1.…”

“…However, this means that this correlation has to be performed each time the computational setting is modified. [5][6][7] Because of this, it is necessary to model the liquid phase and to perform magnetic shielding calculations on top of these models. In this article, several different strategies are proposed to determine accurate values of 15 N magnetic shielding of nitromethane in its liquid phase.…”