Natural abundance samples

The possibility offered by new instruments to obtain N NMR spectra using natural abundance samples has made " N NMR spectroscopy a method which holds no interest for the organic chemist, since the chemical shifts are identical and the signal resolution incomparably better with the N nucleus (/ = ) than with " N (/ = 1). H- N coupling constants could be obtained from natural abundance samples by N NMR and more accurately from N-labelled compounds by H NMR. Labelled compounds are necessary to measure the and N- N coupling constants. 

Some time later the feasibility of obtaining 170 spectra for natural abundance samples at relatively low concentration was demonstrated55,56. This was an important step because intermolecular interactions can markedly affect chemical shifts and, if only electronic substituent effects are to examined, then all extraneous influences must be minimized. 

The two isotopes N (7 = 1) and N (7 = 1/2) both have only small values of S and thus belong to the class of insensitive nuclides. Although the electric quadrupole moment of N is relatively small and the signals are therefore not very greatly broadened, the overwhelming majority of nitrogen NMR studies are now performed on N using natural abundance samples, despite the difficulties involved. The spread of chemical shifts for... 

These limitations made the development of 13C NMR spectroscopy lag significantly behind the development of 111 NMR. In the earliest work the relatively weak sensitivity of 13C was a major stumbling block and compounds specifically labeled with 13C had to be prepared in order to obtain usable spectra. Today it is possible to obtain excellent 13C spectra on natural abundance samples of <25 mg in less than 30 min. The hardware and software advances which have enabled such progress to be made lie in three areas ... 

The study of nucleic acid bases by NMR has been reported in a number of monographs (/), but very little data is available on the, 3C and, 5N NMR chemical shift tensors in these compounds. The low sensitivity of NMR spectroscopy and the long relaxation times exhibited by many of these compounds have posed the main impediments for these studies. The use of sample doping with free radical relaxation reagents, to reduce the relaxation times facilitating 2D multiple pulse experiment (2, 3), enables one to measure and analyze the principal values of the chemical shift tensors in natural abundance samples. In previous papers from this laboratory we have presented, 5N NMR chemical shift principal values for adenine, guanine, cytosine, thymine and uracil (4, 5). 

Table 23. 15N chemical shift assignment for polymorphs B and C of 38 (natural abundance samples). Chemical shifts are in ppm relative to the signal for nitromethane... 

N NMR spectra using natural abundance samples has made N NMR spectroscopy a method which holds no interest for the organic chemist, since the chemical shifts are identical and the signal resolution incompar-... 

Bax, A. Griffey R.H. Hawkins, B.L. Sensitivity-enhanced correlation of N and H chemical shifts in natural-abundance samples via multiple quantum coherence. J. Am. Chem. Soc. 1983, 105, 7188-7190. 

In many ways the information available from NMR studies on these two isotopes is complementary. Although 14N is by far the more common, in natural abundance, it has an electric quadrupole moment while the less abundant 15N nucleus does not. Thus the l4N nucleus has been more frequently used in chemical shift studies on natural abundance samples as well as in quadrupolar relaxation investigations. The larger magnetogyric ratio of the l5N nucleus, together with its sharper lines, renders it more suitable for studies involving spin-spin coupling constants in addition to those on the less efficient nuclear relaxation processes. 

Fig. 22. Ge linewidth of natural abundance and enriched germanium metal, (a) Natural abundance sample, (b) klc enriched sample. Reproduced, with permission, from Takeuchi et Copyright 2001, Chemical Society of Japan.

Highly efficient spin diffusion can be achieved if the chemical-shift difference is scaled below the linewidth due to the homonuclear dipolar couplings of the carbons alone. For uniformly C-labeled samples, this is a few kilohertz and for natural-abundance samples typically in the order of 15 Hz. Assuming a chemical-shift difference of 10 kHz (100 ppm at 10 Tesla), this would (in natural abundance) lead to the requirement of a spin-lock field with ft>rf/(27r) = 3.3 MHz to achieve the necessary scaling (see Equation... 

Hatfield et al. [20] prepared N-labeled nylon 6 to get the spectrum with a reasonable signal-to-noise ratio. This leads to a high quality discussion on the structure of nylon 6 compared with a natural abundance sample. The CP/MAS NMR spectra of nylon 6 samples, which are predominantly amorphous (A), high in a-form crystallinity (B) and high in y-form crystal-... 

The improved sensitivity of 15N enriched nylon-12 samples allows the clear identification of a shoulder appearing at 122.8 ppm (see Fig. 6C ). Although the exact identity is unclear, this peak is assigned on the basis of spin-lattice relaxation data to material epitaxially crystallized on the surface of 7 crystallites or to an ordered interphase region. The spectra of quenched samples (Fig. 6B, 6B ) are very similar in shape. However, the broader base of natural abundance sample indicates that the residual monomer (lauryllactam) may be acting as a plasticizer and increasing the size of conformational mobility of the amorphous region. The residual monomer is probably responsible for the difference in the relative intensities between natural abundance and 15N enriched samples (Fig. 6A, 6A ) [50]. 

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