N Chemical shifts

Before considering the whole of shifts, we shall recall here some of the problems connected with the accuracy of the reported values. Indeed the intrinsic accuracy of the published values depends 

Connected to this problem of accuracy of the N shifts, it is useful to be aware of the possibility of rather high isotope effects when 

To conclude, it can be said that the shifts are measured with a high degree of precision but that the comparison of the literature data is frequently only accurate within a 1 ppm range. Moreover, when the reported values concern molecules dissolved in different sol- 

Frequently the N chemical shifts (Table 2.3) of molecular fragments and functional groups containing nitrogen complement their H and C shifts. The ammonia scale of shifts used in 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

The reversed polarity of the double bond is induced by a n electron-accepting substituent A (A = C=0, C=N, NO2) the carbon and proton in the p-position are deshielded (-A/effect, larger shifts). These substituents have analogous effects on the C atoms of aromatic and heteroaromatic rings. An electron donor D (see above) attached to the benzene ring deshields the (substituted) a-C atom (-/ effect). In contrast, in the ortho and para positions (or comparable positions in heteroaromatic rings) it causes a shielding +M effect, smaller H and C shifts), whereas the meta positions remain almost unaffected. 

An electron-accepting substituent A (see above) induces the reverse deshielding in ortho and para positions (-M effect, larger //and shifts ), again with no significant effect on meta positions. 

Azo compounds Diazo-, diazonium Pyridine-N Imino-N Pyrrole-N Cyanides Isocyanides Guanidines Sulfonamides Thioamides Amides Thioureas, ureas j Enamines Anilines Amines 

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Interpolation methods based on N chemical shifts require the use of the general equations.Those reported in the previous edition (76AHCSl,p. 29, see also 82JOC5132) have been slightly modified for the present purpose. We call / x the observed average property, and the property of the individual tautomers (A or B), / ma and / mb a corresponding property that can be measured (in a model compound or in the solid state) or calculated theoretically, and P and / b the correction factors defined as P = -... 

Px). In some instances, it is assumed that P = Pb, but this is not necessarily true. The quality of the interpolation depends on the ratio (Pmb Pma)/( )- In the case of N chemical shifts (spectral range —900 ppm), MB ma is larger than in the case of H (spectral range —12 ppm) or even chemical shifts (spectral range —230 ppm). However, the same variation in range occurs for P and it is by no means evident that the ratio is more favorable in the case of N NMR. 

Papers describing the use of N chemical shifts for the study of tautomerism, classified according to the type of tautomerism, are summarized in Table IX. 

For convenience, all three compounds are numbered as shown for compounds 2 and 3. Table III. N-Chemical Shifts for Triazolopyridines 1, 2, and 3 ... 

Figure 6.11 A 3D heteronuclear HMQC-COSY spectrum of a tripepiide. The o),-axis represents N chemical shifts, whereas the (Or and 88, copyright (1988), with permission from Academic Pre.ss, Inc.)...

Nitrogen chemical shifts of organic compounds are reviewed in Chapter 5 of Reference 4. The range of N chemical shifts is about 600 ppm wide. If extreme values for metal complexes are included, it extends to over 1400 ppm. Nitromethane is the recommended reference (it can be added in a sealed-off capillary). Values are also frequently quoted with respect to a saturated aqueous solution of ammonium chloride or ammonium nitrate. The following N shift values can be used to convert the 5-values ... 

TABLE 3. N chemical shifts of examples of the more important classes of compounds (5-values... 

Selected H, and N chemical shifts of oximes 65-67 are compared to those of similar compounds 63, 64 and 68-72 in Table 12. 

The N chemical shifts and /( N— H) coupling constants of 1 -amino- and 2-aminobenzotriazole in DMSO solution have been reported and the N assignments are shown in Figure 9. There is a correlation between the chemical shifts of A -aminoazoles and A -methylazoles <90JCS(P2)237>. 

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>. 

Figure 11 The N chemical shifts of 5,5-dialkyldihydro-4//-l,2,3-triazole-4-ones.

Some representative C and N chemical shifts for ring carbon and nitrogen atoms are collected in Table 3. The N chemical shift in (32) suggests a strongly shielded N atom and implies a... 

No data on N chemical shifts have yet been reported for any class of l,4-(oxa/thia)-2-azoles. 

While the neutral 1,2,3,4-oxatriazoles (1) still await synthesis, some of their properties have been predicted by theoretical calculations. AMI calculations combined with a principal component analysis loading data from other related heteroaromatics have been used to estimate geometric characteristics, aromaticity, energy of formation, and N chemical shifts <90JPR885>. The oxatriazoles (1) and (7) and the 1,2,3,5-thiatriazoles, which also have not been prepared, are calculated to be in the group with the lowest classical and magnetic aromaticity. 

N chemical shifts (relative to MeN02, 50 MHz) deduced from N- H HMBC... 

Effects of Nitrogen and Sulfur Lone Pairs on H, 13C and N Chemical Shifts... 

Iron species Solvent 15 N chemical shift Reference... 

K pulse in the middle of the A period. However, the N 180° pulse in the middle to the later t2 period will eventually refocus the N chemical shifts and JNH, so we can concentrate on the effect of JNC during the A period. As we have seen previously, evolution of N magnetization for a period 1/2JNC develops N magnetization that is antiphase in C.Thus, the initial term HzNy becomes HZNXCZ, and the first C 90° pulse converts this to DQC and ZQC, as we have seen for HMQC ... 

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