14N and 15N NMR

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. 

Table 2 gives the approximate range of nitrogen chemical shifts in the organic compounds with which this book deals. The 14N and 15N NMR spectra of amino- and nitro-aliphatic compounds have been reviewed1,2,15 and the effects of the alkyl group on the 15N chemical shifts have been investigated. 

The results of theoretical methods are surveyed, followed by data on molecular dimensions obtained from X-ray diffraction or microwave spectroscopy. The results of NMR spectroscopy, including 1H, 13C, 14N, and 15N NMR, are then surveyed. This is followed by a discussion of UV, visible, IR, and photoelectron spectroscopy and mass spectrometry. Each of the spectroscopic sections deals with both the parent rings and the effects of substituents. 

Table 17 14N and 15N NMR chemical shifts of representative azines and their salts... 

C, 14N, and 15N NMR spectroscopy was used for the examination of nitroimi-dazole derivatives (Table 3.10) and their cations and anions (Table 3.15) [337,338], As seen from Table 3.10, the chemical shifts of the pyridine nitrogen atom of nitroimidazoles lie in the -130 ppm range, whereas those of the pyrrole nitrogen are located in the -208 to -219 ppm range. This is in higher field compared with the corresponding nitropyrazoles. 

Nitro-l,2,3-triazole was synthesized from gem-dinitro compounds and studied by H, 13C, 14N, and 15N NMR spectroscopy [549] (Table 3.22). 

There has been considerable confusion as far as the calibration of nitrogen NMR spectra is concerned. This situation has not improved very much since the appearance of a comprehensive discussion on the subject, (Id) possibly it has grown worse. Modern spectroscopic techniques, in both 14N and 15N NMR, allow the determination of the positions of nitrogen resonance signals with a precision of the order of 0-1 ppm, but this does not mean that the accuracy of chemical shift determinations is of the same order. Very often careless use of reference compounds may make a precision of 0-1 ppm in signal position quite meaningless from the point of view of differences in actual screening constants. 

All of the common heteroatoms possess at least one naturally occurring isotope with a magnetic moment (Table 12). The electric quadrupole of 14N, 170 and 33S broadens the NMR signals so that line widths may be 50-1000 Hz or even wider. To some extent this problem is offset by the more extensive chemical shifts that are observed. The low natural abundances and/or sensitivities have necessitated the use of accumulation techniques for all of these heteroatoms. 14N and 15N chemical shifts are identical. 

Three 4-substituted tetrazole 1-oxides 560 (Scheme 171) derived from the parent structure 559 (Scheme 170) have been described. H NMR, C NMR, 14N NMR, and 15N NMR data have been obtained (1989BAU1488). Line broadening of NMR signals provided information about the rate constant of the equilibrium between the A/-oxides 554 and 559 observed at elevated temperatures. 

Nitrogen, on the other hand, has two magnetically active isotopes, 14N and 15N. Because nitrogen compounds are so important in organic chemistry and its applied fields of natural products, pharmacology, and biochemistry, both of these isotopes have been the subject of intensive NMR investigation. 

In addition to many structure determinations by X-ray diffraction, nmr studies using 14N and 15N have allowed the comparison of NR with other N-bonded ligands like NO, NNR, etc.140... 

During the past four years a considerable body of published work has appeared relating to nitrogen NMR. Significant developments have occurred in all the areas of investigation on which we reported previously, (la, 2a) These developments are due to improvements in experimental techniques and instrumentation as well as to an increased awareness of the utility of both 14N and 15N nuclei in NMR spectroscopy. 

Since then, a variety of nitrosonium salts have been isolated. The important ones are with the following counterions BF4 , PF6 , FSO , HS04, BCI4, and SbCl6. The ion has been characterized by 15N NMR, IR, and X-ray analysis.245,448,514,531,532 A detailed NMR study by Mason and Christe have showed448 that conditions (solvent, counterion, temperature) have minor effects on observed 14N NMR shifts (814N 372.5-376.8). 

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