N NMR Spectroscopy

Because of the low natural abundance, low sensitivity, relaxation problems, and negative NOE (y being rather small, and negative), working with N presents various choices how to optimize the relaxation and NOE, whether to use N-enrichment or sensitivity enhancement, and so on. 

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Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

The possibility offered by new instruments to obtain N NMR spectra using natural abundance samples has made " N NMR spectroscopy a method which holds no interest for the organic chemist, since the chemical shifts are identical and the signal resolution incomparably better with the N nucleus (/ = ) than with " N (/ = 1). H- N coupling constants could be obtained from natural abundance samples by N NMR and more accurately from N-labelled compounds by H NMR. Labelled compounds are necessary to measure the and N- N coupling constants. 

An elegant application of Se NMR spectroscopy, in conjunction with N NMR spectroscopy, involves the detection of the thermally unstable eight-membered rings (RSeN)4 (3.13) from the reactions of a mixture of seleninic anhydrides (RSeO)20 (R=Ph, Pr) and N-enriched... 

Organic thionylamines have planar, cis structures (9.9) in the solid state and in solution, as determined by X-ray crystallography and N NMR spectroscopy, respectively. The gas-phase structures of the parent compound HNSO and MeNSO have been determined by microwave spectroscopy. The S=N and S=0 double bond lengths are 1.51-1.52 and 1.45-1.47 A, respectively. The bond angle [Pg.168]

G. J. Martin, M. L. Martin and J.-P. Gouesnard, NMR Volume 18 N NMR Spectroscopy, Springer-Verlag, Berlin,... 

Later, more detailed studies of analogous pyrazine-fused derivatives 126 (X = N) using UV, C, and N NMR spectroscopies showed that the position of... 

Several papers have reported N2 bound to Ru or Os. Aqueous solution studies with the aim of preparing [Ru(H20)5(N2)] " by reaction between [Ru(H20)6] and N2 ( N labeled) have been described. The reaction was monitored by use of 0 and N NMR spectroscopies and evidence... 

C- and N-NMR spectroscopy has been used to gain information on the contributions of resonance forms of type 3 to the hybrid structure of the ground state of diazoazoles. The C chemical shifts of the ipso carbon of the diazo group in some diazoazoles are reported in Table I where those of diazomethane and of two diazocyclopentadienes are compared. 

The technique of X-ray crystallography has been, and will remain, indispensable for the determination of the unusual structures of S—N compounds. A more recent development is the application of N NMR spectroscopy in S—N chemistry. Despite the necessity to employ N-enriched materials for these studies, the judicious application of this technique in both structural determinations and in monitoring the progress of reactions will undoubtedly accelerate the progress of the subject. The advent of MCD spectroscopy and the use of the perimeter model have also enhanced our understanding of the electronic structures of cyclic S—N molecules. Rapid advances in this area are to be expected. 

The isotope N, with a natural abundance of 99.9%, has nuclear spin 7 = 1 and gives broad signals which are of little use for structural determinations. The N nucleus, with I = 1/2, is therefore preferred. However, the low natural abundance of about 0.4% and the extremely low relative sensitivity (Table 1) make measurements so difficult that N NMR spectroscopy was slow to become an accepted analytical tool. A further peculiarity is the negative magnetogyric ratio since, in proton decoupled spectra, the nuclear Overhauser effect can strongly reduce the signal intensity. DEPT and INEPT pulse techniques are therefore particularly important for N NMR spectroscopy. 

N NMR spectroscopy has been described in some detail in CHEC-I <84CHEC-I(5)682> however, no N NMR spectra of 1,2,3-triazoles were reported before 1982 <84CHEC-i(5)684>. Since that time, N NMR spectroscopy has been widely applied to triazole and benzotriazole compounds. 

A number of 5,5-dialkyldihydro-4.ff-l,2,3-triazole-4-ones and 5-amino-4,4-diphenyl-4//-1,2,3-triazoles have been studied by N NMR spectroscopy. Their N chemical shifts (corrected to the external standard of ammonia) are listed in Figure 11 <93CB103>. The N NMR spectra of triazole and benzotriazole boron derivatives have been reported <89IC4022>. 4-(l-Azido-l-methylethyl)-lH-1,2,3-triazole shows N chemical shifts (D20/pyridine, MeN02 external standard) 3 — 284.6 (Na), - 161.5 (Ny), d - 135.8 (N/9), d-9lA,d- 79.2, and d - 60.4 ppm <89CB9l 1>. 

There are several examples of alkyl halides reacting with 1,2,3-thiadiazoles at nitrogen to yield salts or mesoionic compounds <84CHEC-l (6)447 >. Similarly, with Meerwein s reagent several substituted thiadiazoles yielded various 2- and 3-methylated 1,2,3-thiadiazoles (Table 9) <93JHC30l>. The isomer ratios were determined by integrating the methyl singlets in the H NMR spectra and the compounds were further studied by N NMR spectroscopy (Section 4.07.3.4). 

Proton, C, and N NMR spectroscopy has been employed to demonstrate the existence of ring-chain tautomerism between protonated forms of thiosemicarbazones and protonated 2-aryl-5-amino-2,3-dihydro-1,3,4-thiadiazoles <92KGS1689>. The use of NMR in various solvents, indicates that the isoxazoline (28) contains between 24% and 34% of the thiadiazoline (29) <85KGS1001>. Tautomerism was also discussed in Sections 4.10.2 <90BCJ2991> and 4.10.3.3 <83HCA1755>. 

N NMR spectroscopy, 32 364-365 heterometallic cluster synthesis by nitrosylation of performed heterometallic aggregates, 32 350-351 from sulfur-rich metal complexes, 32 352-353... 

Iron-sulfur-nitrosyl complexes (continued) from tetranuclear precursors, 32 343 tellurium analogs, 32 350 tetrairon complexes, N NMR spectroscopy, 32 365 x-ray crystallography [Fe(NO)(S2CNR2)2 and related clusters, 32 359-361... 

We see from Table 1 that the only observable nuclide for oxygen, 0, has a very low natural abundance, even in comparison with those of popular nuclides like (1.108%) and N (031%). Moreover, its quadrupole moment prevents any practical utilization of polarization transfer techniques like INEPT or DEPT, now widely used in and N NMR spectroscopies. A range of chemical shifts much wider than those of and N is an important point in favour of utilization of 0. All these properties did not prevent important applications of O NMR spectroscopy in organic chemistry, even from the times of continuous wave NMR spectroscopy. Interesting examples of such pioneering works can be found both at natural abundance as well as with enriched samples . However, also in the case of O NMR spectroscopy, FT NMR proved to be decisive for its development. 

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