1SN NMR chemical shift

The previously mentioned series of alkyl derivatives of tetramethyl quinolizine-l,2,3,4-tetracarboxylate, which is present as mixtures of the 4H- and 9aH- tautomers 17 and 18, was studied by 5N NMR, both experimentally and using ab initio calculations. The data are collected in Table 4. The ab initio 1SN NMR chemical shifts reproduce well the trends observed in the experimental data, but their values are ca. 72 ppm smaller than the experimental ones <2003JST719>. 

Wiench et al. investigated chemical shifts of a set of positional isomers of azaindolizines <1994MRG373>. In Table 3, 13C and 1SN NMR chemical shifts of two related ring systems, the unsubstituted [l,2,4]triazolo[l,5- ]-pyrimidine A and [l,2,4]triazolo[3,2- ]pyrimidine B, are compared. 

Thorough nuclear magnetic resonance (NMR) investigations of some tetrazolo[l,5-/7]pyridazines and tetrazolo[l,5- J-pyrazine-7-oxides have been carried out by Cmoch et al. <1999MRC493, 2003MRC693>. Comparison of 13C and 1SN NMR chemical shifts of the ring atoms are shown in Table 1. 

The same authors also investigated the NMR spectroscopic properties of 1- and 2-alkylated tetrazolo[l,5- ]-pyridazinium salts <2002MRC507> and concluded that the site of alkylation can be accurately determined from the 1SN NMR chemical shifts of these compounds. The obtained chemical shifts with these salts are summarized in Table 2. 

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>. 

The electron-withdrawing effect of pentazole is similar to a nitro group and the H NMR shifts of nitrophenyl and pentazolylphenyl compounds are similar <2002ZFA1933>. The 7i-electron densities calculated by the Hiickel molecular orbital (HMO) method show a linear correlation with 111, 13C, and 14N NMR chemical shifts in the azole series <1977IJB168>. The 1SN NMR chemical shifts of the following compounds have been reported cesium/tetramethyl ammonium (TMA) pentazolylphenolate (8 —81.1 (N-l), —29.7 (N-3/N-4), 1.9 (N-2/N-5)) <2002AGE3051> ... 

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>. 

A detailed analysis of the H, 13C, and 1SN NMR spectra of 1,3,4-thiadiazoles was reported in CHEC(1984) <1984CHEC(6)545> and summarized in CHEC-II(1996) <1996CHEC-II(4)379>. Representative chemical shifts are given in Figure 1. 

NMR shifts ( H, 13C, and 1SN) of 1-alkyl-, 2-alkyl-, and 3-aryltetrazolo[l,5- ]pyridinium salts have also been measured <1999JST119>. The data are compiled in Table 3. The 1SN shifts of these salts seemed of particular importance as they revealed quite big shielding changes for the nitrogen nuclei. These chemical shifts were also calculated by the ab initio GIAO-CHF method, and the result was found to be in fairly good agreement with the experimental values. 

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. 

Two 15N-enriched urea-formaldehyde resins with different crosslink density were studied by tfie solid state CP MAS 15N NMR. Despite at least six expected 15N chemical shifts arising from tertiary, secondary and primary amides in the different structural moieties, both resins exhibit only two major peaks. The lower field resonance is more pronounced in the highly cured resin, suggesting its origin in the tertiary amides. A DD experiment, which would confirm this assumption, does not result in clearly separated secondary and tertiary amides. Thus, from the analytical point of view, it seems that 13C NMR spectra are more useful than 15N NMR spectra, although 1SN resonance data provide a useful supplement 252). 

Along with solution-state NMR spectroscopy, analysis of the solid-state spectra of nitrogen heterocycles has been carried out to evaluate the various chemical shift tensors associated with the 1SN nucleus <1997JA9804>. Importantly, this showed the dominance of the tensor perpendicular to the plane of the aromatic system along with the key effect of protonation of the pyridine nitrogen. 

All of the heteroatoms possess at least one naturally occurring isotope with a magnetic moment (Table 15). The nuclei UN, 170 and 33S also possess an electric quadrupole moment which interacts with the electric field gradient at the nucleus, providing a very efficient mechanism for relaxing the nuclear spin. The consequence of this facilitation of relaxation is a broadening of the NMR signals so that line widths may be 50-1000 Hz or even wider. To some extent this problem is offset by the more extensive chemical shifts that are observed. The low natural abundances and/or sensitivities have necessitated the use of accumulation techniques for all of these heteroatoms. The relative availability of 170 and 1SN enriched... 

MULTINUCLEAR NMR OF AZO DYES AND THEIR METAL COMPLEXES 17 Omura et al.79 reported <5(1SN) chemical shifts in compounds 19 (Table 8). 

Calculated -rr-bond orders are summarized in Table 1. These calculations are supported by chemical evidence that the S—N bond is the one most easily cleaved. Attempts have been made to relate bond orders and electron densities to NMR coupling constants (74CJC833, 7700619) and 13C NMR <7500596, 7500677) and 1SN NMR <78JOC4693> chemical shifts, with limited success. 

With improvements in instrument sensitivity and the use of techniques such as enhancement by polarization transfer (INEPT), it can be expected that natural abundance 15N NMR spectra will become increasingly important in heterocyclic chemistry. The chemical shifts given in Table 10 illustrate the large dispersion available in 1SN NMR, without the line broadening associated with 14N NMR spectra. 

As illustrated in Fig. 4.4, nitrogen chemical shifts cover a range of about 1000 ppm and make 14N and 1SN very useful nuclides for distinguishing structural features. Both nuclides have very low inherent sensitivity, about 10-3 as great as that for H. 14N is over 99% naturally abundant, but it has large quadrupole moment, which often leads to rapid relaxation and very broad lines, as we shall see in Chapter 8. Nevertheless, in many compounds line widths are narrow enough to allow discrimination between chemically shifted environments. 15N has a spin of V2, hence no quadrupole moment, but its natural abundance of less than 0.4% makes direct observation difficult at natural abundance. However, isotopic enrichment and/or the use of indirect detection methods (discussed in Chapter 10) permits relatively facile study of 15N, particularly in two- and three-dimensional NMR. 

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