Nitrogen and Oxygen NMR Spectroscopy

Some other data on, 5N chemical shifts, referred to internal MeN02, are recorded in Table 25. Comparison of pyrrole with imidazole demonstrates that the deshielding of the pyrrole-type nitrogen is about 15 ppm. Increased nitrogen shielding is observed for 2,1,3-benzothiadiazole ( + 49 ppm) and the parent 1,2,5-thiadiazole ( + 34 ppm). In contrast, the corresponding oxadiazoles were deshielded, as in 2,1,3-benzooxadiazole ( — 36 ppm) and 1,2,5-oxadiazole (— 34 ppm). 

There is a growing interest in the use of 15N NMR spectroscopy for elucidation of various structural problems of azole chemistry, especially tautomerism. For example, the mole fractions of the prototropic tautomers were obtained from the 15N chemical shifts of the NH tautomers and the corresponding N-methyl derivatives. By this method, the average mole fraction for the 2-NH tautomer of benzotriazole is 0.02 in both CDC13 and DMSO, and that of 1,2,3-triazole is 0.34 in CDC13 and 0.55 in DMSO (82JOC5132). 

Low temperatures studies, for instance in THF at 175K, can elucidate annular tautomerism (92JCS(P2)1737) a large collection of 15N chemical shifts of NH-pyrazoles with tautomerism frozen has been published (94MRC699). The tautomerism is also frozen in the solid state. Thus two 15N shifts at 150 ppm (NH) and 222 ppm ( = N-) are observed for solid imidazole the average is close to the single signal at 186 ppm found in solution. 

The discovery, by the combined use of crystallography and CPMAS NMR, of the dynamic properties of 3,5-dimethylpyrazole (an H-bonded trimer, cf. 122) in the solid state opened a new field of research (85JA5290,92JA9657). Other cyclic structures, dimers like (121) and tetramers like (123) were discovered (94JHC695). 15N CPMAS (with labeled derivatives) proved to be superior to 13C CPMAS. 

Some other examples of application of 15N NMR spectroscopy include establishing the protonation site of A2-pyrazolines (87MI301-01) and assigning the structures to isomeric N-7 and N-9 substituted purines (86T5073). 

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