Nuclei natural abundance

Although continuous wave NMR is sufficient for naturally abundant nuclei with strong magnetic moments such as hydrogen, fluorine and phosphorous, the study of low abundance nuclei and/or weak magnetic moments such as carbon 13 or silicon 29 requires pulse NMR. 

I and the measured line intensities are fitted to an exponential expression. S (-e) =A + B exp(-i / T ). The inversion-recovery experiments are often perfonned for multiline spectra of low-natural abundance nuclei,... 

Dipolar interactions with protons are usually very dominating, whereas homonuclear interactions of low natural abundant nuclei, like Si and can usually be neglected due to strong intemuclear distance dependence. 

REDOR technique has been applied to natural abundance nuclei coupled to a singly labeled nucleus to determine the interatomic... 

For low naturally abundant nuclei, such as the nucleus, resonance lines reflecting the chemical shift anisotropy (CSA) can be observed for solid organic materials by eliminating the effects of local field with use of the so-called dipolar decoupling method. However, the CSA resonance lines are usually broad and, therefore, the respective CSA lines are superposed with each other for polymers composed of different C species. To separately measure these CSA lines, the following different methods were proposed ... 

Nuclei can be further classified as to their natural abundance nuclei with 99-f% natural abundance, e. g., H, and are referred to as being abundant, while nuclei with low natural abundances, e. g. and N, are termed dilute or... 

C spectra and those of other low natural abundance nuclei are not normally recorded under quantitative conditions. For structural identifica-... 

The interatomic distances in crystalline specimen of C, N doubly labelled simple peptides, N-acetyl-Pro-Gly-Phe, evaluated from REDOR data were compared with those from X-ray diffraction studies and found to justify such a novel approach. The REDOR-derived conformation of this peptide was P turn type I, consistent with the X-ray diffraction study. The maximum deviations of the distances determined by NMR and X-ray diffraction is 0.08 A despite the complete neglect of the dipolar interactions with the labelled nuclei of neighbouring molecules and natural abundance nuclei. The precision and accuracy given by REDOR experiments are of the order of 0.05 A. Distinction between the two types of P turn forms, including the P turn type II, found in the monocllnlc crystal of this peptide (whose interatomic distances are different by about 0.57 A) is made possible only by very accurate REDOR measurement. 

Nucleus NMR activity Natural abundance (%) Nucleus NMR activity Natural abundance (%)... 

Fluorine-19 is a highly NMR-sensitive and naturally abundant nucleus. An important advantage of F MRS is the fact that it allows direct observation of fluorinated compounds and their metabolites in the human body without background signal from tissue. In vivo F MR spectroscopy has been used to detect noninvasively anticancer and psychoactive drugs and other fluorinated compounds and to study their metabolism. It has also been used to monitor the effects of anaesthetics and to measure intracellular pH and calcium levels. 

Carbon-13 nmr. Carbon-13 [14762-74-4] nmr (1,2,11) has been available routinely since the invention of the pulsed ft/nmr spectrometer in the early 1970s. The difficulties of studying carbon by nmr methods is that the most abundant isotope, has a spin, /, of 0, and thus cannot be observed by nmr. However, has 7 = 1/2 and spin properties similar to H. The natural abundance of is only 1.1% of the total carbon the magnetogyric ratio of is 0.25 that of H. Together, these effects make the nucleus ca 1/5700 times as sensitive as H. The interpretation of experiments involves measurements of chemical shifts, integrations, andy-coupling information however, these last two are harder to determine accurately and are less important to identification of connectivity than in H nmr. 

Other Nuclei. Although most nmr experiments continue to involve H, or both, many other nuclei may also be utilized Several factors, including the value of I for the nucleus, the magnitude of the quadmpolar moment, the natural abundance and magnetogyric ratio of the isotope, or the possibihty of preparing enriched samples, need to be considered. The product of the isotopic parameters can be compared to the corresponding value for providing a measure of relative sensitivity or receptivity. Table 1 summarizes these factors for a number of isotopes. More complete information may... 

Although the natural abundance of nitrogen-15 [14390-96-6] leads to lower sensitivity than for carbon-13, this nucleus has attracted considerable interest in the area of polypeptide and protein stmcture deterrnination. Uniform enrichment of is achieved by growing protein synthesi2ing cells in media where is the only nitrogen source. reverse shift correlation via double quantum coherence permits the... 

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. 

The most convenient technique used to study organotin(IV) derivatives in solution and in solid state is Sn NMR spectroscopy. The Sn nucleus has a spin of 1 /2 and a natural abundance of 8.7% looking only at the isotopic abundance, it is about 25.5 times more sensitive than The isotope Sn is slightly less sensitive (natural abundance 7.7%) but it has not been used as much. Both nuclei have negative gyromagnetic ratios, and, as a consequence, the nuclear Overhauser enhancements are negative. Some examples of the applications of this method are mentioned later, in different sections. 

Figure 2-19 shows the mass spectrum of the element neon. The three peaks in the mass spectrum come from three different isotopes of neon, and the peak heights are proportional to the natural abundances of these isotopes. The most abundant isotope of neon has a mass number of 20, with 10 protons and 10 neutrons in its nucleus, whereas its two minor isotopes have 11 and 12 neutrons. Example illustrates how to read and interpret a mass spectmm. 

Nucleus Natural abundance (%) Relative sensitivity Ease of use... 

The series of molecules which has guided us through this book so far was chosen for a good reason it allowed us to discuss in detail the most important nuclei, the proton and carbon-13, while demonstrating the effect of a very important heteronucleus , phosphorus-31, on the spectra of the two key nuclei. In addition, we could discuss the NMR investigation of this heteronucleus, which exists in 100% natural abundance and has a spin of Vi> and in contrast of oxygen-17, a low-abundance nucleus with a spin greater than Vi. 

Fluorine-19, like phosphorus-31, is a spin-Vi nucleus with 100% natural abundance. The signals it produces are almost as strong as those of the proton, and the resonance frequency at a given field is also relatively close to that of the proton. Although for many years it was in fact necessary to have a special probehead for fluorine-19, those days have gone and fluorine has become a completely normal nucleus. 

As far as NMR is concerned, the spin-Vi nucleus silicon-29 has a natural abundance of 4.7%. The chemical shift range is around 600 ppm, and the shift of TMS is the zero-point. 

Hydrogen is an example of an abundant nucleus. That is, there is a high concentration of nuclei with a nuclear isotope of high natural abundance (1H, I = 99.8%) in the sample. In this... 

Because of the low natural abundance of 13C nuclei (1.1%), practically all observed product molecules contain only one 13C nucleus. Accordingly, the polarization signals in the 13C NMR spectrum clearly do not originate from one and... 

The most widely used nucleus in biological NMR spectroscopic studies is the 31P nucleus. The reason is that its natural abundance is 100%, and its... 

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