15N NMR

Analogies for such a mechanism in diazotization are found in the nitrous acid-catalyzed nitration of A,A-dimethylaniline, mesitylene, 4-nitrophenol, and some related compounds, which were investigated by 15N NMR spectroscopy in Ridd s group (Ridd and Sandall, 1981 Ridd et al., 1992 Clemens et al., 1984a, 1984b, 1985 Johnston et al., 1991 review Ridd, 1991). Ridd and coworkers were able to demonstrate clearly that not only the nitration proper, but also the preceding C-nitrosation, is accompanied by a marked 15N nuclear polarization. This was at-... 

However, we have to criticize more specifically the paper by Lown et al. (1984), who characterized alkanediazonium ions, as well as (E)- and (Z)-alkanediazoate ions, by 15N NMR spectroscopy. They also report NMR data on the (E)- and (Z)-benzenediazohydroxides as reference compounds, describing the way they obtained these compounds in only three lines. Obviously the authors are not familiar with the work on the complex system of acid-base equilibria which led 30 years earlier to the conclusion that the maximum equilibrium concentration of benzenediazohydroxide is less than 1 % of the stoichiometric concentration in water (see Ch. 5). The method of Lown et al. consists in adding 10% (v/v) water to a mixture of benzenediazonium chloride and KOH in dimethylsulfoxide. In the opinion of the present author it is unlikely that this procedure yields the (Z)- and CE>benzenediazohydroxides. Such a claim needs more detailed experimental evidence. 

In conclusion, with regard to the structure of benzenediazonium compounds with electron donor substituents in the 2- or 4-position, the most recent experimental data, mainly X-ray analyses and 13C and 15N NMR data, are consistent with 4.4 as the dominant mesomeric structure of quinone diazides, as proposed by Lowe-Ma et al. (1988). For benzenediazonium salts with a tertiary amino group in the 4-position the data are consistent with the quinonoid structure 4.20 as the dominant mesomeric form. 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

Table 10. 15N-NMR Chemical Shifts (5) of Methyl-Substituted Tetrazoliums 130-132 ...

Yuan P, Marshall VP, Petzold GL, Poorman RA, Stockman BJ. Dynamics of stromelysin/inhibitor interactions studied by 15N NMR relaxation measurements Comparison of ligand binding to the SrS3 and S -Sy subsites. J Biomol NMR 1999 15 55-64. 

C (or 15N) NMR signals recorded by both CPMAS and DDMAS NMR, however, could be broadened or suppressed, when fluctuation motions with intermediate frequencies of 104-105 Hz interfere with... 

Figure 1 Schematic representation of the 13C (or 15N) spin-lattice relaxation times (7"i), spin-spin relaxation (T2), and H spin-lattice relaxation time in the rotating frame (Tlp) for the liquid-like and solid-like domains, as a function of the correlation times of local motions. 13C (or 15N) NMR signals from the solid-like domains undergoing incoherent fluctuation motions with the correlation times of 10 4-10 5 s (indicated by the grey colour) could be lost due to failure of attempted peak-narrowing due to interference of frequency with proton decoupling or magic angle spinning.

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

G. E. Martin, M. Solntseva and A. J. Williams, Applications of 15N NMR spectroscopy in alkaloid chemistry, in Modern Alkaloids Structure, Isolation, Synthesis and Biology, E. Fattorusso and O. Taglialatela-Scafati (eds.), Wiley, New York, 2008, pp. 409-476. 

Noteworthy NMR studies involving nuclei other than phosphorus have been carried out for some P-chloro-NHPs where the possible occurrence of spontaneous P-Cl bond dissociation was probed by II NMR titrations and 35C1 NMR [20], and for P-cyclopentadienyl derivatives where measurement of solid-state 13C CP-MAS NMR spectra allowed one to substantiate the preservation of the circumambulatory ring migration of cyclopentadienyl groups in the solid state [47], Several neutral and cationic derivatives have also been studied by 15N NMR [20, 53],... 

Table 4 Experimental and ab Zn/ffo-calculated 15N-NMR chemical shifts of the 4H-and 9aH-tautomers of a quinolizine system...

There are very few references to the 15N NMR of 1,2,3-thiadiazoles. The 1SN NMR spectra of 15 monosubstituted 1,2,3-thiadiazoles have been published <1993JHC301> and selected data are given in Table 6. 

When the N-3 atom is quaternized, as is the case of 4,5-diphenyl-3-trimethylsilylmethyl-l,2,3-thiadiazol-3-ium triflate, there is a large upfield shift for the N-3 atom of 160ppm, and a smaller shift is also observed for the N-2 atom of 25 ppm in the 15N NMR spectrum <1999J(P1)1415>. 

SN NMR has been used to study the mechanism of the photochemical reaction of 5-phenyl-l,2,4-thiadiazole (see Section 5.08.5.2). 5-Phenyl-l,2,4-thiadiazole-4-1SN and 3-phenyl-l,2,4-thiadiazole-2-1SN were synthesized. The 15N NMR chemical shifts reported for the 4-position derivative was +302.2 ppm (acetone- ) and for the 2-position derivative +258.4 ppm (CDCI3) relative to a reference of ammonia <2003JOC4855>. 

If benzamidine is replaced by benzamidoxime, then the 4-oxide derivative 107 is obtained as a minor product (8%) together with 4-chloro-l,2,3-dithiazol-5-one 108 (32%) and the 5-thione 109 (15%) (Equation 30). The formation of the 4-oxide, as opposed to the isomeric 2-oxide, was confirmed by mass spectral and 15N NMR data. The yield of the 4-oxide product could be improved by using an O-acyl benzamidoxime in place of benzamidoxime <1999J(P1)2243>. 

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