%0 Journal Article %J International Journal of Spectroscopy %D 1991 %T Dinitrogen as a possible internal standard for nitrogen nuclear magnetic resonance %A Michal Witanowski %A Wanda Sicinska %A S. Biernat %A Graham A. Webb %B International Journal of Spectroscopy %V 9 %P 55-59 %G eng %0 Journal Article %J Journal of Magnetic Resonance %D 1991 %T Solvent effects on nitrogen NMR shieldings in azines %A Michal Witanowski %A Wanda Sicinska %A S. Biernat %A Graham A. Webb %X The ranges of solvent effects on the nitrogen shielding are reported for pyridine, 1,2-1,3-, and 1,4-diazine, and 1,3,5-triazine. These ranges are both significant and variable from about 48 ppm for 1,2-diazine to about 1 I ppm for the triazine. The former is the largest solvent-induced range so far observed in nitrogen NMR. Contributions to the nitrogen shielding ranges from hydrogen bonding and solvent polarity effects are estimated. The former effect usually provides a dominant contribution and can be associated with the strength of the hydrogen bond formed from the solvent to the solute. The contribution to the change in nitrogen shielding from a change in solvent polarity is also significant but usually less than that due to hydrogen bonding. Both effects operate to produce changes in the nitrogen shielding in the same direction. The polarity effects are unusually large in the case of 1,2-diazine and this is corroborated by INDO/S-SOS shielding calculations using the solvaton model. %B Journal of Magnetic Resonance %V 91 %P 289 - 300 %G eng %U http://www.sciencedirect.com/science/article/pii/002223649190193W %R http://dx.doi.org/10.1016/0022-2364(91)90193-W %0 Journal Article %J Magnetic Resonance in Chemistry %D 1989 %T 14N NMR nuclear shielding and the electronic structure of dibenzo[1,3a,4,6a]tetrazapentalene %A Michal Witanowski %A J. Sitkowski %A Wanda Sicinska %A S. Biernat %A Graham A. Webb %K 14N NMR %K Charge separation %K N-Heterocycles %K Nuclear shielding calculations %X 14N NMR data and semiempirical molecular orbital calculations are reported for dibenzo-[1,3a,4,6a]tetrazapentalene (1). The 14N signals were assigned by means of the results of the molecular orbital calculations and measured relative signal widths. A comparison was made with results for some azolopyridines. A linear correlation was obtained between the nitrogen shieldings, expressed with respect to the bridging nitrogen atom, and the corresponding relative charge densities. Thus nitrogen nuclear shielding results provide a good estimation of electron charge distribution in these molecules. Compound 1 provides the first case in which the shielding of a pyridine-type nitrogen atom is greater than that of the indolizine-type nitrogen. %B Magnetic Resonance in Chemistry %I John Wiley & Sons, Ltd. %V 27 %P 88–91 %G eng %U http://dx.doi.org/10.1002/mrc.1260270116 %R 10.1002/mrc.1260270116 %0 Journal Article %J International Journal of Spectroscopy %D 1989 %T Assignments of nitrogen NMR shieldings in azine heterocycles by means of a self-adjusting linear system of incremens %A Michal Witanowski %A Wanda Sicinska %A S. Biernat %B International Journal of Spectroscopy %V 7 %P 315-324 %G eng %0 Journal Article %J Journal of Magnetic Resonance %D 1989 %T A NMR study of solute-solvent interactions as a function of the nitrogen shielding of pyridine N-oxide %A Michal Witanowski %A Wanda Sicinska %A S. Biernat %A Graham A. Webb %X The range of solvent effects on the nitrogen shielding of pyridine N-oxide is found to be comparable to that of pyridine, amounting to about 30 ppm. The largest contribution to this range arises from hydrogen bonding from the solvents to the oxygen atom of pyridine N-oxide. For the first time it is demonstrated that hydrogen bonding to a neighboring atom can have a comparable effect on nitrogen shielding as would direct hydrogen bonding to the nitrogen atom. Medium polarity effects on the nitrogen shielding of pyridine and its N-oxide are of the same sign and order of magnitude. This is corroborated by INDO/S-SOS shielding calculations using the solvaton model. These medium polarity effects amount to a nitrogen shielding variation not exceeding 6 ppm while those due to hydrogen bonding exceed 20 ppm. %B Journal of Magnetic Resonance %V 83 %P 351 - 357 %G eng %U http://www.sciencedirect.com/science/article/pii/0022236489901960 %R http://dx.doi.org/10.1016/0022-2364(89)90196-0 %0 Journal Article %J International Journal of Spectroscopy %D 1989 %T Solvent polarity and hydrogen bonding effects on the nitrogen NMR shielding of nitromethane %A Michal Witanowski %A Wanda Sicinska %A S. Biernat %B International Journal of Spectroscopy %V 7 %P 305-314 %G eng