%0 Journal Article %J Journal of Magnetic Resonance %D 2003 %T Solvent polarity and hydrogen-bonding effects on the nitrogen NMR shieldings of N-nitrosamines and DFT calculations of the shieldings of C-, N-, and O-nitroso systems %A Michal Witanowski %A Zenobia Biedrzycka %A Wanda Sicinska %A Zbigniew Grabowski %K Algorithms %K Binding Sites %K Carbon %K Computer Simulation %K Hydrogen Bonding %K Macromolecular Substances %K Magnetic Resonance Spectroscopy %K Models, Molecular %K Molecular Conformation %K Molecular Structure %K Nitrogen %K Nitrogen Isotopes %K Nitrosamines %K Nitroso Compounds %K Oxygen %K solutions %K Solvents %X High-precision nitrogen NMR shieldings, bulk susceptibility corrected, are reported for dimethyl-N-nitrosamine (I) and diethyl-N-nitrosamine (II) in a variety of solvents which represent a wide range of solvent properties from the point of view of polarity as well as hydrogen bond donor and acceptor strength. The observed range of solvent-induced nitrogen shielding variations of (I) and (II) is significant for the amino-type nitrogens, up to about 16 ppm, and originates essentially from the deshielding effect of the increasing polarity of solvent. On the other side, the nitroso nitrogen shieldings reveal an even stronger response to solvent effects, within about 20 ppm, but in this case the increasing polarity and hydrogen bond donor strength of solvent produce enhanced shielding. DFT quantum-mechanical calculations using the GIAO/B3PW91/6-311++G** approach and geometry optimizations employing the same basis set and hybrid density functionals show an excellent correlation with the experimental data on C-, N-, and O-nitroso moieties and reproduce not only major changes but also most of the subtle variations in the experimental nitrogen shieldings of the nitroso systems as a whole. A combination of the calculations involving the corresponding N and O-protonated species and the trends observed in the solvent-induced nitrogen shielding variations shows clearly that the prime acceptor site for hydrogen bonding is the nitroso oxygen atom. %B Journal of Magnetic Resonance %V 164 %P 212-9 %8 2003 Oct %G eng %N 2 %0 Journal Article %J Magnetic Resonance in Chemistry %D 1997 %T Solvent-Induced Effects on the Nitrogen NMR Shieldings of Some Nitrosobenzene Systems %A Michal Witanowski %A Zenobia Biedrzycka %A Wanda Sicinska %A Graham A. Webb %K 14N %K calculations %K Nitroso Compounds %K NMR %K Solvent effects %X High-precision14N NMR measurements of solvent-induced shielding variations are reported for some nitrosobenzene systems. These variations are shown to result from a combination of three major factors, solvent to solute hydrogen bonding where the solute nitrogen lone pair electrons are involved, solvent polarity and interactions between the electron-deficient benzene ring of the nitrosobenzenes and basic centres in the solvent molecules. The last of these three factors produces nitrogen deshielding of the nitroso group, and in the present work this interaction was found to be the largest of its type so far observed. Consequently, this implies that, in nitroso aromatic compounds, the benzene ring shows a remarkable deficit of electronic charge. The former two factors produce an increase in nitroso nitrogen shielding, thus indicating a strong electron-withdrawing effect of the nitroso group which is consistent with previous observations. INDO/S parameterized molecular orbital calculations of solute nitrogen shieldings, incorporating the Solvaton model of non-specific solute–solvent interactions, predict that the nitrogen shielding will increase as the polarity of the medium increases. This is in very good agreement with the observation that the nitroso nitrogen shielding analysis yields a large and positive value for thesterm which describes the influence of solvent polarity/polarizability on the shielding variation as a function of solvent. For the 0.2 M solutions studied at 35°C, a significant amount of the dimeric isodioxy form is only observed foro-nitrosotoluene. © 1997 by John Wiley & Sons, Ltd %B Magnetic Resonance in Chemistry %I John Wiley & Sons, Ltd. %V 35 %P 262–266 %G eng %U http://dx.doi.org/10.1002/(SICI)1097-458X(199704)35:4<262::AID-OMR96>3.0.CO;2-5 %R 10.1002/(SICI)1097-458X(199704)35:4<262::AID-OMR96>3.0.CO;2-5