Nitrogen chemical shifts screening constant

Electron attracting substituents in the nearest vicinity to the nitro group tend to increase the screening constant for the latter. Nitrogen chemical shifts differentiate clearly nitroalkanes> nitroaromatics, Af-nitro and 0-nitro groups. They are also helpful in distinguishing between the true nitro structure and that of an aci-nitro compound ... 

In order to provide a means for the precise recalculation of nitrogen chemical shifts reported since 1972, it is necessary to have accurate values of the differences in the screening constants between neat CH3N02 and the large number of reference compounds which have so far been used. Table VII shows the results of precise, 4N measurements (61) which have been carried out in concentric spherical sample and reference containers in order to eliminate bulk susceptibility effects on the shifts. Since the technique adopted (61, 63) involves the accumulation of a large number of individually calibrated spectra with the subsequent use of a full-lineshape analysis by the differential saturation method, (63) the resulting random errors comprise those from minor temperature variations, phase drifts, frequency instability, sweep nonlinearity, etc. so that systematic errors should be insignificant as compared with random errors. 

Nitrogen chemical shifts constitute a formidable means of differentiation between the various isomeric or tautomeric structures which are abundant in both organic and inorganic compounds of nitrogen, since relative differences in nitrogen screening constants are usually large. Sometimes they may reach several hundred ppm. 

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. 

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