14N NMR spectroscopy

Some illustrative examples of the application of 14N NMR spectroscopy in sulfur-nitrogen chemistry include (a) studies of the (NSC1)3<->3NSC1 equilibrium in solution28 and (b) identification of the S-N species present in solutions of sulfur in liquid ammonia.29... 

Witanowski has shown that 14N NMR spectroscopy can be used for the location of an N-oxide position in 1,2,4-triazines (80OMR( 14)305). 

N-NMR spectroscopy can be useful for ammine and amine complexes, but 14N is a quadrupolar nucleus, and quadrupolar relaxation is dominant when the environment of 14N has a low symmetry. This can lead to very broad lines and a consequent reduction in sensitivity. On the other hand, short relaxation times also have the advantage of allowing rapid pulsing so that a large number of transients can be acquired. Thus it is possible to follow reactions of cisplatin in blood plasma and cell-culture media at milli-molar drug concentrations and to detect ammine release [6],... 

C and 14N NMR spectroscopy was successfully used for the investigations of some 4-nitro- and 3-nitrofuroxan isomers [521, 522],... 

Nitrogen chemical shifts in 14N NMR spectroscopy have been used to estimate the equilibrium compositions of a series of mercaptopyridines in solution. Thus, ca. 95% of the thione form was estimated for 2-mercaptopyridine in acetone and methanol and for 4-mercaptopyridine in methanol and acetone/DMSO (790MR379). 15N NMR shielding measurements provided the quantitatively reliable estimates of tautomeric equilibria for 3-methoxycarbonyl-2-mercaptopyridine and 3-mercaptopyridine. The equilibrium of 3-methoxycarbonyl-2-mercaptopyridine is shifted predominantly to the thione tautomer (95%), whereas 3-mercaptopyridine exists in the thiol form (at least 94%) (85MRC790). By 13C NMR spectroscopy, 3-nitropyridine-4-thiol exists in the thiol form (86H(24)1301). Another NMR criterion, used for estimation of the tautomeric equilibrium in mercaptopyridines, is based on 3/(ch) values for the CN-CH, C N = CH, and C = N-CFI structural elements (83OMR20). 

The spectroscopic (UV, IR, NMR) studies of 2-aminopyridine, 2-(phenylami-no)pyridine and 5-nitro-2-(phenylamino)pyridine indicated the presence of only amino tautomer in a solution. The tautomeric equilibrium constants, calculated from the acidity of the fixed derivatives, were found to be 2 x 105 for 2-(phenylamino)pyridine and 8 x 103 for 5-nitro-2-(phenylamino)pyridine, indicating the shift of the equilibrium toward the imino tautomer on introduction of a nitro group (76BCJ2770, 80BCJ717). The molar fractions of 2-aminopyridine and 4-aminopyridine in acetone were estimated as 92+6% and 94+6%, respectively, by 14N NMR spectroscopy... 

Analysis of nitronates by 14N and 15N NMR spectroscopy has an auxiliary character (see Table 3.12). The 14N NMR signals are often broadened and, hence, are difficult to observe and are poorly informative, although magnitudes of their chemical shifts could in principle help in distinguishing between covalent nitronates and salts. It is difficult to observe 15N NMR signals in natural-abundance NMR spectra of nitronates, while an introduction of a label is an expensive procedure. 

NMR spectroscopy (a commercial unit shown in Fig. 1.49) uses the fact that some atomic nuclei have a magnetic moment, e. g. very distinct in a proton, the nucleus of hydrogen, but also inl3C, 3IP, 14N, and 33S. In an external magnetic field the energy levels split, as described in quantum mechanics. The size and extend of the split is given by Eq. (9)... 

In contrast to ESR spectroscopy, which can only be used to study species with unpaired electrons, NMR spectroscopy is applicable to the investigation of all polymer samples. Nuclei with non-zero total nuclear spin (e.g., 1H, l3C, 19F, 14N) will have a magnetic moment which will interact with an external magnetic field resulting in quantized energy levels. Transitions between these energy levels form the basis of NMR spectroscopy. 1H and 13C... 

Spin-spin (transversal) relaxation (relaxation time T2) is the second mechanism which is related to molecular mobility. It influences the half-height line widths w [tv = lj(n T2) in the frequency-domain NMR spectrum and is of some relevance in the NMR spectroscopy of quadrupolar nuclei such as, 2H, 14N, 170, 33S and others. In the context of this section, T2 is of very limited relevance. 

It is inappropriate here to discuss details of nmr spectroscopy. However, the full possibilities for structural studies arise from the use of probes that bind to the molecule under study and perturb the nmr spectrum. These probes are generally paramagnetic species, in particular the lanthanide cations. In the nmr spectrum separate signals arise from each nucleus in the molecule, provided that the nucleus possesses a nonzero nuclear spin (e.g., H, 13C, 14N). The extent of the spectral perturbations of a given signal depends on the relative geometries of the paramagnetic species and the nucleus in question. Thus structural parameters can be obtained, in principle, for most atoms (nuclei) in a protein molecule.5... 

Carbazole, 2-hydroxy-reactions with citral, 4, 235 Carbazole, 2-hydroxy-9-methyl-synthesis, 4, 294 Carbazole, N-hydroxymethyl-as metabolite of carbazole, 1, 230 Carbazole, N-isopropyl-PE spectroscopy, 4, 190 Carbazole, N-methyl-14N NMR, 4, 175 X-ray spectroscopy, 4, 163 Carbazole, 1-nitro-synthesis, 4, 282 Carbazole, tetrahydro-dehydrogenation, 4, 282, 312 synthesis, 4, 107, 337, 353 Carbazole, 1,2,3,4-tetrahydro-reduction, 4, 255, 256 synthesis, 4, 312, 325, 352 Carbazole, 1,2,3,4-tetrahy dro-1 -oxo-synthesis, 4, 337 Carbazole, 9-trifluoroacetyl-synthesis, 4, 218 Carbazole, vinyl-polymers, 1, 275, 301 Carbazole, 9-vinyl-copolymer... 

An improved method developed by Minkwitz et al.972 allowed to obtain other salts [Eq. (4.227)]. The structure of the cation was characterized by vibrational spectroscopy, 14N NMR (a single peak at 814N -282 from CH3N02), and by ab initio theory. 

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