Nitrogen-15 chemical shifts

As is the case for substituent effects on proton and carbon, but not phosphorous NMR chemical shifts, nitrogen chemical shifts exhibit deshielding when in the vicinity of substituents that are more electronegative than carbon (Scheme 2.11). Presumably, although there are little data to support this assertion, the effect diminishes rapidly with distance of the substituent from the nitrogen, as is the case for proton and carbon chemical shifts. However, in aromatic heterocycles, such as pyridine, the effect of the substituent is transmitted through the n system, as indicated by the significant influence of para substituents of pyridines (Scheme 2.12).10... 

Table 7. Cadmium-113 Chemical Shifts—Nitrogen Ligands...

NMR Similarly carbons that are bonded to nitrogen are more shielded than those bonded to oxygen as revealed by comparing the chemical shifts of methylamme and methanol... 

Table 7.64 Nitrogen-15 Chemical Shifts in Monosubstituted Pyridine 7.115...

TABLE 7.63 Nitrogen -15 (or Nitrogen-14) Chemical Shifts Continued)... 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an oletinic proton. Thus the olefin stmcture (1) for PX is also supported by nmr. 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

Some nitrogen chemical shifts relating to azines are given in Table 8. More detailed information can be found in the review of Witanowski et al. The following trends and observations are noteworthy ... 

Table 8 Nitrogen Chemical Shifts of Typical Azines and their Derivatives ...

When the lone electron pair is protonated, the nitrogen chemical shift moves by ca. 100 p.p.m, to higher field. Large upheld shifts are also found when a compound exists in a tautomeric form with a proton on the nitrogen. The nitrogen NMR spectrum is often of considerable value in studies of tautomerism of this type. 

Chemical shifts of pyridine and the diazines have been measured as a function of pH in aqueous solution and generally protonation at nitrogen results in deshielding of the carbon resonances by up to 10 p.p.m. (73T1145). The pH dependence follows classic titration curves whose inflexions yield pK values in good agreement with those obtained by other methods. 

For the NH azoles (Table 3), the two tautomeric forms are usually rapidly equilibrating on the NMR timescale (except for triazole in HMPT). The iV-methyl azoles (Table 4) are fixed chemical shifts are shifted downfield by adjacent nitrogen atoms, but more by a pyridine-like nitrogen than by a pyrrole-like iV-methyl group. 

Table 14 C NMR Chemical Shifts for Nitrogenous Azoles (a) NH Derivatives...

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