Nitrogen chemical shifts temperature dependence

The (CH3)4N ion, another popular standard, shows a small but significant dependence of its nitrogen chemical shift on the concentration and on the gegenion involved (Table VII). In some papers (34, 36) 12 m aqueous (CH3)4NC1 has been reported as a standard, but since the solubility of the salt in water even at elevated temperatures is ca. 6 m, the standard was probably a saturated solution. Its shift relative to neat CH3N02 (36) (+336-7 ppm) is in very good agreement with that for the saturated solution reported in Table VII. 

When a proton is directly bonded to a strongly electronegative atom such as oxygen or nitrogen its chemical shift is critically dependent on the nature of the solvent, temperature, concentration, and whether acidic or basic impurities are present. The usual variations in chemical shift for such protons are so large (up to 5 ppm for alcohols) that no very useful correlations exist. 

The 15N CP/MAS NMR spectra of 15N doubly-labelled (Na from (15N) aniline, Nb from Na15N02), 3-methyl-l-phenylpyrazole-4,5-dione 4-phenylhydrazone (40), l-phenylazo-2-naphthol (35, R = H), 2-hydroxy-5- erf-butylazobenzene (29) and 4-hydroxyazobenzene (32) have been recorded and the temperature dependence of <5(15N) (Table 20) was followed105. For compound 35 (R = H), representing a mixture of the azo and hydrazone forms, the hydrazone content was calculated from the 15N chemical shifts of both nitrogen atoms at various temperatures (Table 21). The dependence of In K (K = [hydrazone form]/[azo... 

A second compound that has been used is Ytterbium tetramethyl-1,4,7,10-tetraazacyclododecane-l,4,7,10-tetraacetic acid (DOTMA) with octa-coordinate bonding of the ligand to Yb via four nitrogen and four oxygen atoms. The twelve equivalent protons of the methyl groups have a resonance at -18 ppm relative to water, and show a temperature dependence of -0.04 0.01 ppmK. The chemical shift difference between the ac and axl proton resonances in the ligand was measured in human serum and found to be linear over a range of 308-318 K with a sensitivity of -0.41 ppmK. ... 

The chemical shifts of protons bonded to nitrogen and oxygen are temperature- and concentration-dependent. 

There is still no unique standard for the calibration of15N NMR spectra. Six solid standards as well as liquid ammonia are frequently used as external references for the calibration of nitrogen shieldings in the solid state. Although chemical shifts in solids do not depend on the temperature and concentration (as the chemical shifts of liquid standards) there are discrepancies in the literature data on the exact chemical shifts for particular standards. In order to avoid errors created by conversion of literature data from different chemical shift scales, precise resonance positions of chemical shift standards are required. Since the difference in the resonance frequency between proton decoupled spectra... 

R-N-H O.S. O ppm Hydrogens attached to a nitrogen have a variable chemical shift depending on the temperature, acidity, amount of hydrogen bonding, and solvent. 

R(CO)-N-H 5.0-9.0 ppm Hydrogens attached to an amide nitrogen are variable in chemical shift, the value being dependent on the temperature, concentration, and solvent. 

The observable chosen in this experimental study to reveal the details of the motion is the chemical shift anisotropy. This parameter is the dominant feature of the broadband Nitrogen-15 NMR spectrum of solid nitrocellulose, and very recent theoretical advances have made its temperature dependence interpretable in terms of the slow motion associated with this polymer at environmental temperatures. 

TABLE 4. Temperature dependence of the Nitrogen-15 chemical shift anisotropy 6 = 27T7 Ho I a-i -ct2 I and the values of the orientational Brownian motion correlation times for NC, calculated as explained in the text. 

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