1SN-NMR spectroscopy

There are several examples of alkyl halides reacting with 1,2,3-thiadiazoles at nitrogen to yield either salts or mesoionic compounds <1996CHEC-II(4)289>. Similarly, with Meerwein s reagent, several substituted thiadiazoles yielded various 2- and 3-methylated 1,2,3-thiadiazoles (Scheme 4 Table 8) <1993JHC301>. The isomer ratios were determined by integrating the methyl singlets in the H NMR spectra and the compounds were further studied by 1SN NMR spectroscopy (Section 5.07.3.4). 

In the case of the 7-hydroxy-substituted compounds 44 (Scheme 2), 54 different derivatives were investigated by 13C NMR spectroscopy and, in some cases, also by 1SN NMR spectroscopy <1995JST(335)273>. With the help of proton-coupled 13C NMR spectra, semi-selective INEPT (insensitive nuclei enhanced by polarization transfer) experiments, and heteronuclear multiple bond correlation (HMBC) two-dimensional 2D-NMR spectra, all shifts could be unequivocally assigned. While the C-7 shifts did not allow the existing tautomeric situation to be determined, a clear decision could be made by H NMR spectroscopy in this respect. The 1SN NMR spectra revealed an equilibrium between the N(4)H and N(3)H tautomeric forms, which is fast on the NMR timescale. 

SN NMR spectroscopy is still underexploited in structural analysis of nitrogen-containing heterocycles. It can however be a powerful tool, for instance in tautomerism studies <2002JP2126>. Some 1SN NMR investigations of lanthanide induced shift have been performed on bicycles 50-52, showing that the complexing site of these 4,5-dihydro-l/f-l,2,3-triazoles is located on N-3. 

Systematic NMR studies of a set of heterocycles containing guanidine and thiourea structural moiety have been published by an English team <1995MRC389>. In the frame of these investigations, some imidazo- and thiazolo-[l,2,4]triazinones having the general structure 50 have been analyzed by 13C and 1SN NMR spectroscopy. The chemical shifts of some derivatives are compiled in Table 3. 

Smernik, R. J., and Baldock, J. A. (2005). Does solid-state 1SN NMR spectroscopy detect all soil organic matter nitrogen Biogeochem. 75, 507-528. 

Knicker, H., and Skjemstad, J. O. (2000). Nature of organic carbon and nitrogen in physically protected organic matter of some Australian soils as revealed by solid-state BC and 1SN NMR spectroscopy. Aust. J. Soil Res. 38,113-127. 

Achtnich, C., Fernandes, E., Bollag, J. M., Knackmuss, H. J., and Lenke, H. (1999). Covalent binding of reduced metabolites of TNT to soil organic matter during a bioremediation process analyzed by 1SN NMR spectroscopy. Environ. Sci. Technol. 33,4448-4456. 

The 13C NMR shift of the tetrazole C(5) atom is particularly sensitive to the substitution pattern of the ring and allows discrimination between isomers. Two crystal forms of the angiotensin II antagonist irbesartan, which is a tetrazole derivative, have been identified as desmotropes using solid-state 13C (and 1SN) NMR spectroscopy. Substantial differences between the spectra of these forms were observed and one crystal form was deduced to be the 1H-... 

The Hantzsch pyridine synthesis affords 1,4-dihydropyridines 214, although spontaneous oxidation to pyridines often occurs. In its simplest form it involves the condensation of two molecules of a -keto ester with an aldehyde and ammonia (Scheme 119) . Compounds resulting from the condensation of ammonia with one of the carbonyl components can be used in the Hantzsch synthesis. Thus, -aminocrotonic ester 215 can replace the ammonia and one mole of acetoacetic ester in Scheme 119. The mechanism of the Hantzsch synthesis has been clarified by 13C and 1SN NMR spectroscopy <1987T5171>. 

The existence of tautomers and rotamers of dehydroacetic acid (239) and derived Schiff s bases has been investigated by H, 13C and 1SN NMR spectroscopy (82JCS(P2)513>. The tautomeric form (239) proposed earlier (61AK(17)523) was confirmed and its existence as the rotamer (240) was established by variable temperature studies. 

SN NMR spectroscopy has not been applied widely to 1,3,4-thiadiazoles due to the low natural abundance and low sensitivity of detection of 15N. Recent technological advances, however, have facilitated the recording of such spectra. 

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