L. Stefaniak

Nitrogen NMR Spectroscopy

Publisher Summary This chapter focuses on Nitrogen nuclear magnetic resonance (NMR) spectroscopy. Significant developments are due to improvements in experimental techniques and instrumentation as well as due to an increased awareness of the utility of both Nitrogen-14 and Nitrogen-15 nuclei in NMR spectroscopy. In NMR spectroscopy, the main interest is in relative screening constants––that is, chemical shifts––both for different nuclei in the same molecule and for nuclei in different polyatomic molecules rather than in absolute screening constants. A theoretical approach that gives gauge-dependent results leads to difficulties when relative screenings for various nuclei are to be compared. A Nitrogen-14 NMR study of order fluctuations in the isotropic phase of liquid crystals has been reported. The experimental data for the isotropic phases of p-azoxyanisole and of diethylazoxy benzoate are accounted for in terms of short-range order fluctuations of the nematic and of the smectic types, respectively.

Nitrogen-14 nuclear magnetic resonance of azoles and their benzo-derivatives

Abstract The 14 N NMR spectra of some azoles, diazoles, triazoles, tetrazoles, oxazoles, thiazoles, and some bicyclic benzo-derivatives show a linear relationship between the chemical shifts and the SCF-PPP-MO π-charge densities. The resonances of the N-methyl nitrogen atoms are very characteristic of the type of ring system and may be used for distinguishing between tautomeric forms. The effects of tautomeric equilibria on the appearance of the 14 N spectra are considered. The average excitation energy approximation of the theory of chemical shifts is found to be reliable for the 14 N shifts of pyrrole-type nitrogen atoms in azole ring systems, even if the effect of polarisation in the σ-bonds is ignored.

External neat nitromethane scale for nitrogen chemical shifts

Abstract Precise NMR measurements of 14N screening constants relative to those of neat nitromethane as external standard in concentric spherical sample containers show that external and internal referencing systems which have so far been used are not suitable for nitrogen shift calibration with an accuracy of the order of 0.1 ppm. Accurate data are reported for recalculating nitrogen shifts referred to various external and internal standards. Neat nitromethane is recommended as a primary reference substance. The role of solvent and concentration effects on nitrogen chemical shifts is indicated for a few selected molecules.

14N and 13C NMR of tautomeric systems of hydroxy-pyridines

Abstract 14N NMR is shown to be an efficient method of investigation of the tautomeric equilibria in hydroxy-and related substituted pyridines. Large relative 14N chemical shifts within tautomeric pairs estimated from N-Me and O-Me derivatives make possible quantitative determinations of the equilibria. Qualitative conclusions may be drawn from 14N line widths as well as from 13C and proton chemical shifts.

Nitrogen-14 nuclear magnetic resonance of some six-membered aromatic heterocycles

Abstract The 14N NMR chemical shifts of some aromatic heterocycles including pyridine, 1,2-diazine. 1,3-diazine, 1,4-diazine, and 1,3,5-triazine systems in mono-, bi-, and tricyclic structures (azabenzene, azanaphthalene, and azaphenanthrene type) are shown to depend almost linearly on the π-charge density at the N atom obtained from P-P-P calculations. This dependence is considered in terms of various approximations within the general theory of chemical shifts. A similar correlation is observed for the pyridine ring containing various electron-donating mono-substituents. The 14N NMR shifts show that the three isomeric aminopyridines exist as such in acetone, whereas two of the corresponding hydroxy derivatives have essentially the pyridine-type structure. Careful re-examination of the δN scale of 14N chemical shifts shows that the zero chemical shift difference between the two primary internal standards, nitromethane and the nitrate ion, is correct as previously indicated rather than the difference of 3 ppm reported by other authors.

SYNTHESIS AND STRUCTURE DETERMINATION OF SOME OXADIAZOLE-2-THIONE AND TRIAZOLE-3-THIONE GALACTOSIDES

The syntheses of 5-pyridyl-3((β-D-galactopyranosyl)-1,3,4-oxadiazole-2-thiones 3a–3c and 5-pyridyl-2((β-D-galactopyranosyl)-4-benzyl-1,2,4-triazole-3-thiones 6a–6c are reported. The existence of N-galactosides – not S-galactosides – was proven by IR and 15N NMR spectroscopy. The structures of the final products and the intermediates were elucidated by IR, 1H, 13C and 15N NMR spectroscopy and mass spectrometry. †Deceased.

13 C NMR differentiation of diastereoisomeric complexes of cis-decalin with β-cyclodextrin

13 C NMR spectra show chiral discrimination between two invertomers of cis-decalin complexed with β-cyclodextrin.

The tetrazole–azide tautomerism of some nitrotetrazolo [1,5-a]pyridines studied by NMR, IR spectroscopy and molecular orbital calculations

Abstract The 6-nitro- and 8-nitro- groups in the tetrazolo[1,5- a ]pyridine molecule exhibit completely different influences on the tetrazole–azide equilibrium. Introduction of the methyl-, nitro-, azido groups or a bromine atom in positions 5, 6, 7 or 8 of the nitrotetrazolopyridine produce changes in the equilibrium constants. Based on the IR spectra taken in the solid state, the tetrazole structure was assigned for almost all the compounds studied. Only one of them, 2,6-diazido-3-nitropyridine, exists in the diazido-form in the solid. The 1 H, 13 C, 15 N, and 17 O NMR spectral parameters (coupling constants, chemical shifts) as well as ab initio molecular orbital calculations were used to describe the tetrazole–azide tautomerism in solutions. The differences in the NMR parameters between the neutral compound (6,8-dinitrotetrazolo[1,5- a ]pyridine) and its σ-adducts are also included as data for distinguishing between both molecules.

Nitrogen Chemical Shifts in Organic Compounds

The range of nitrogen chemical shifts in organic compounds is about 800 p.p.m. and may be even larger for paramagnetic complexes. However, the usefulness of the nuclear magnetic resonance spectra of any nucleus for structural investigations depends very much not only on the magnitude of the chemical shifts but also on the possibility of finding reasonable correlations of the shifts with molecular structure. Another important factor is the possible differentiation of closely related structures, such as those of isomeric compounds or molecules occurring in various chemical equilibria. Fortunately, a great deal has already been done by means of systematic investigation of organic compounds by nitrogen nuclear magnetic resonance, and rather simple empirical relationships between nitrogen chemical shift and molecular environment have been established for many important classes of such compounds. Theoretical explanations of observed chemical shifts usually provide a tool for probing deeply into the electronic structure of organic molecules.

NMR Studies of the Equilibria Produced by 6‐ and 8‐Substituted Tetrazolo[1,5‐a]pyridines

1H, 13C and 15N NMR data are reported for nine tetrazoles. Five of these compounds are found to exhibit valence tautomeric equilibrium between the tetrazole and azide forms. The position of this equilibrium at 298 K is found to be dependent on the solvent and the position and nature of the substituent. More polar solvents favour the tetrazole form. Protonation studies on the two forms using TFA as a solvent are reported. The favoured site of protonation is found to be N‐4 for the azide form and N‐1 for the tetrazole form.