Isotropic nitrogen chemical shift

The calculations reproduce quite well the isotropic shifts of both forms, but show larger discrepancies in the principal values of the chemical shifts of the N7-H tautomeric form. The lack of experimental values, for the principal components of the nitrogen chemical shifts in the N9-H tautomeric form, precludes a comparison of the calculated data with the experiment. 

Similar to the situation for 13C, isotropic 15N chemical shifts and the principal components of 15N chemical shift tensors have been used to study N-H- -0=C hydrogen bonds in peptides. It has been shown that isotropic 15N chemical shifts of proton donors (such as N-H) are displaced downfield by ca. 15 ppm, whereas those of proton acceptors are shifted upfield by ca. 20 ppm [110-112]. Amongst the CSA components, S33 (parallel to the C-N bond) has been shown to be most sensitive to the hydrogen bond strength, as reflected by the N- -O distance [113]. Detailed studies of the principal components and orientations of 15N chemical shift tensors for amide nitrogens in simple peptides have been reported recently [114]. This work confirmed that S33 and Siso are the 15N chemical shift parameters that are the most sensitive to details of the hydrogen bonding. It was also found that N-H... 

A further problem of N chemical shift calculations is the influence of electron correlation. In calculations on the Hartree-Fock level the error is typically 20 ppm for isotropic nitrogen values. The observed range of backbone nitrogen chemical shifts is also about 20 ppm, hence only calculations on the MP2 level or better are suitable for structure elucidation or it must be assumed that the correlation effects are constant within the pool of studied structures. 

This paper presents a study of the i5N and, 3C chemical shifts in both tautomeric forms of purine. All the principal values of the, 5N and, 3C chemical shifts tensors were measured in a sample in which only the N7-H tautomeric form was present. In another sample, containing both tautomeric forms, the principal values of the quaternary carbons, the isotropic shifts of the remaining carbons and isotropic shifts of the nitrogens could also be obtained for the N9-H tautomeric form. The experimetal procedures are described in detail elsewhere. 

Isotropic chemical shifts, obtained from solution studies, are useful indicators of the electronic environment around the nucleus, but they provide only a fraction of the available information. The shielding tensor components, which can be extracted from spinning sidebands in solid-state spectra, also contain useful information on bonding and structure, because they are sensitive to bond type. Techniques other than nitrogen NMR, such as spin-rotation 165) or relaxation 166) measurements, can be used to determine the nitrogen shielding tensor. 

Extremely high-resolution N NMR spectra of MgSiN2 have been obtained with linewidths of only 25 Hz for the 2 NSi2Mg2 environments (Figure 9.18) (Harris et al. 1992). These 2 sites have only minor crystallographic inequivalence, illustrating the sensitivity of the N isotropic chemical shift. The N NMR chemical shifts of a series of titanium carbonitrides decrease from 24 to — 15 ppm as the nitrogen content decreases from 100 to 35.4% (MacKenzie et al. 1995). The metallic character of these samples resulted in relaxation times somewhat reduced by comparison with other nitride ceramics. 

MAS spectra of enriched samples of SiBN3C reveal two signals with isotropic chemical shifts at 45 and 58 ppm, respectively. These shifts lie well in the region of the chemical shifts of hexagonal BN and Si3N4 (a- and / -), and thus are indicative for nitrogen in a trigonal planar coordination by boron and silicon [125]. 

Heteronuclear chemical shift correlation methods establish the direct link between protons and the respective, directly attached carbons (or nitrogens). In the case of methylenes with inequivalent (anisochronous) protons, the "multiplicity of the carbon in question is irrefutably obvious. For isotropic methylenes and other resonances, the multiplicity of the resonance (CH, CH2 or CH3) in question may be less obvious. Early work by Kessler and co-workers addressed this issue via the development of the DEPT-HMQC experiment. [120] Multiplicity editing is also available for experiments such as GHSQC. An extra pair of delays and pulses, with the flip angle of the proton pulse being adjustable, allow the acquisition of data in... 

M. Mirzaei, N.L. Hadipour, Study of hydrogen bonds in l-methyluracU by DFT calculations of oxygen, nitrogen, and hydrogen quadrupole coupling constants and isotropic chemical shifts, Chem. Phys. Lett. 438 (2007) 304. 

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