Michal Witanowski

Solvent effects on nitrogen NMR shieldings in azines

Abstract 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.

Nitrogen-14 nuclear magnetic resonance—V1–4 : Nitriles and isonitriles

Abstract The N14 chemical shifts for R-CN are shown to move to lower fields on increasing the electro-negativity of R, while an opposite effect is observed for R-NC. The relative chemical shifts between the resonances of nitriles, isonitriles and the cyanide ion are reasonably accounted for by the changes in the bond order and charge density matrix elements without necessity of making any assumptions on the value of the mean excitation energy. The effects of diamagnetic shielding do not seem to be important for the changes in the N14 chemical shifts.

Isomeric nitropropenes and their nuclear magnetic resonance spectra

Abstract The NMR spectra of 3-nitropropene, 1-nitropropene and an equilibrium mixture of the two have been analysed. 1-Nitropropene obtained by dehydration of 2-nitro-1-propanol as well as by isomerization of 3-nitropropene is shown to be largely the trans isomer. Base-catalysed isomerization of 3-nitropropene leads to an equilibrium of a 3:1 molar ratio of the latter with 1-nitropropene. Correlation between charge distribution in 1-nitropropene (calculated by LCAO method) and the chemical shifts in its NMR spectrum is indicated.

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 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 6–311++G** 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 17O NMR shieldings of 1. Copyright

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

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.

Solvent-Induced Effects on the Nitrogen NMR Shieldings of Some Nitrosobenzene Systems

High‐precision 14N 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 the s term 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 for o‐nitrosotoluene.

Solvent Effects on Nitrogen NMR Shieldings of 1,2,4-Triazine

Abstract Nitrogen NMR shieldings of 1,2,4-triazine are shown, as an example, to be capable of providing deep insight into solvent-induced, site-oriented electric charge redistributions and solvent-solute hydrogen bonding effects in an unsymmetrical molecule. A sharp contrast is observed in the solvent effects on the nitrogen atoms in positions 1 and 2 with respect to that at position 4. The former pair of atoms exhibits a remarkable affinity to hydrogenbond donor solvents. Their electron densities appear to be significantly dependent upon the solvent polarity. In contrast N4 appears to be relatively uninfluenced by solvent effects.

A NMR study of solute-solvent interactions as a function of the nitrogen shielding of pyridine N-oxide

Abstract 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.

Nitrogen NMR shieldings of nitroalkanes as a structural and conformational probe

Nitrogen NMR shieldings (Δσ=‐δ on the frequency scale of chemical shifts), measured by high‐precision 14N NMR and bulk susceptibility corrected, of 20 nitroalkanes in dilute solutions in cyclohexane and in acetone are reported, including vic‐dinitro, gem‐dinitro and trinitro structures and tetranitromethane. The data obtained for cyclohexane solutions yield an excellent linear correlation with theoretical shieldings calculated for the corresponding isolated molecules by the semi‐empirical TNDO/2 method. The NO2 nitrogen shieldings of nitroalkanes are shown to depend linearly on the net charges of the corresponding nitro groups, each taken as a whole, but there is no correlation with the atomic charges of the nitrogen atoms concerned. Thus, the nitrogen shieldings seem to provide a measure of electron‐acceptor or electron‐donor strength of the substituted alkyl system bound directly to the nitro group. Solvent polarity effects on the shieldings are discussed. The nitro group nitrogen shieldings are shown to be generally sensitive to conformational effects, particularly in sterically hindered molecules, and to provide insight into conformational or rotamer equilibria in nitroalkane systems. Ab initio calculations of the shieldings of some simple nitroalkanes, using the Hartree–Fock/GIAO approach, are reported. They indicate that the semi‐empirical method employed, combined with the experimental data, yields results which are comparable to those obtained by the ab initio method, as far as relative shieldings are concerned. ©1998 John Wiley & Sons, Ltd.

Solvent and protonation effects on nitrogen NMR shieldings of isoamides (α-aminoethers)

High-precision solvent-induced 14N NMR shieldings are presented for some isoamides and their N-protonated forms. The results indicate that the major source of solute-solvent interactions, giving rise to nitrogen shielding variations, is solvent-tosolute hydrogen bonding where the lone pair electrons of the imino nitrogen act as the hydrogen-bond acceptor centre. This results in a remarkable shielding increase for the imino nitrogen atom. Another interaction which could be important, when NH moieties are present, is solute-to-solvent hydrogen bonding where the NH croup is engaged as a donor. In the present study this effect is found to be a modest one and produces nitrogen deshielding. The influence of non-specific interactions, arising from solvent polarity effects, is rather feeble compared with that for analogous imine systems. The direction and magnitude of this effect is correctly reproduced by molecular orbital calculations incorporating the Solvaton model for non-specific solutesolvent interactions. Protonation effects on the nitrogen shieldings of the isoamides are substantial, producing an increase in shielding of about 80 ppm. This is in accord with that observed for solvent-to-solute hydrogen bonding.