Nitrogen-15, chemical shift determination

Figure L NMR Chemical Shifts of Nitrogen Compounds Representing Condensation Products of Aniline with Carbonyl and Other Functioned Groups. Chemical Shifts determined in DMSO-d in this lab unless otherwise noted, (a) From references 37,46,47 and 48. (b) Determined in solid state, (c) Tentative assignment.

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. 

XV). However, some important information may be obtained from nitrogen NMR spectra concerning isomeric and tautomeric forms where amide-type structures are involved. It has been shown (101) that nitrogen chemical shifts are far superior to, 3C shifts in the determination of the tautomeric equilibria of pyridone, [13a]-hydroxypyridine, [13b], systems. This point is discussed further in Section V K. 

The solvent effects on the nitrogen shieldings of 3-methylsydnone 10 and the hypothetical 5-methoxy-l,2,3-oxadiazole 11 were calculated <2000MRC580, 2001AHC1>. For stmcture 10, there is excellent linear correlation between the calculated and the experimentally determined chemical shift values for N2 and N3. 

Structure elucidation of three related derivatives of ring system 2 ( temozolomide 9a, mitozolomide 9b, and the related acid derivative 9c) has been carried out by 13C and 15N NMR spectroscopy <1995J(P1)249> (Scheme 2). The 1SN NMR chemical shifts measured in dimethyl sulfoxide (DMSO) solutions are listed in Table 1. For compound 9a, Lowdin charges of the nitrogen atoms have also been calculated and found to have a linear relationship with the experimentally determined chemical shifts of these atoms. The NMR data of 9a have been correlated with those of a series of heterocycles of related structure by the same team <2002MRC300>. 

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