Natural Abundance of Important Isotopes

Natural Abundance of Important Isotopes Rules for Determination of Molecular Formula Neutral Moieties Ejected from Substituted Benzene Ring Compounds Order of Fragmentation Initiated by the Presence of a Substituent on a Benzene Ring... 

Atoms of elements are composed of isotopes. The ratio of natural abundance of the isotopes is characteristic of an element and is important in analysis. A mass spectrometer is normally the best general instrument for measuring isotope ratios. 

Fourier Transform NMR is very important for 13C NMR where the signals are very weak owing to the low natural abundance of 13C isotopes. Here computer accumulation of the responses, obtained after each RF pulse, gives spectra of sufficient quality in a relatively short time. The resolution of the NMR measurements is such that even very small changes in the position of the resonance line caused by the environment of the nuclei (electron density) can be determined with great accuracy. So a wealth of information on chemical and physical structure can be obtained from NMR. 

Table 1 lists some of the important properties of several commonly observed nuclides in the study of pharmaceuticals. Notice that some elements such as hydrogen, have several magnetically active isotopes with very different properties. Interestingly, has the highest sensitivity to detection of any nucleus, but its use is limited by the added complexity of working with a radioactive isotope. The absolute sensitivity listed in the table takes into account the natural abundance of the isotope. Sensitivity can be improved in some studies by the chemical incorporation of magnetically active isotopes such as and... 

All of the atoms making up the zeolite lattice [those enumerated in the bracketed part of Eq. (1) j have NMR-active isotopes and thus can be investigated by solid-state NMR. The important nuclei and their natural abundances are Si (4.7%), Al (100%), and O (0.04%). Because of the low natural abundance of O, isotopic enrichment is usually necessary. The 100% abundant P nucleus is also present in the aluminophosphate systems discussed above. In addition, H (100%) may be present in any of these framework structures in OH groups at defect sites. We will discuss the information regarding zeolite structure available through study of these nuclei and the interactions of their spins with local electric and magnetic fields. 

Mercury (Hg) occurs in nature as a mixture of seven stable isotopes the average atomic mass of the blend is 200.6. The atomic masses and natural abundances of the isotopes are 196.0 (0.15 %), 198.0 (10.1 %), 199.0 (17 %), 200.0 (23.1 %), 201.0 (13.2 %), 202.0 (29.65 %) and 204.0 (6.8 %). Two radioactive isotopes, and ° Hg, have been widely used in toxicological studies, radiometric analysis, and also in checks of yield of analytical procedures. The fact that mercury is an isotope mixture may be of importance in analytical work using mass spectrometry, since there are no reference samples with well-defined isotope composition. 

Mass spectrometry was originally developed to identify and analyze quantitatively the natural abundances of stable isotopes. The determination of isotopic abundances is still important today, but for different reasons. Isotopic labeling of molecules is quite important in studies of chemical mechanisms and kinetics, both in the organic and biochemical areas. Prior to studies of this kind, the extent of labeling must be determined, and mass spectrometry is usually the method of choice. For example. 

In the structure elucidation of unknown compounds, NMR is routinely used in conjunction with NMR spectroscopy. An important two-dimensional technique is inverse H, C-correlation to assign direct correlations between adjacent protons and carbon atoms. Due to the low natural abundance of the isotope of 1.1%, several hours of accumulation and injected sample amounts in the low pg range in the 1.5 pL detection volume are required for successful 2D NMR experiments. 

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. 

Uranium is the fuel of nuclear reactors. The most important of its minerals is pitchblende, U02 (Fig. 17.28), much of which is obtained from strip mines in New Mexico and Wyoming. Uranium is refined to reduce the ore to the metal and to enrich it that is, to increase the abundance of a specific isotope—in this case, uranium-235. The natural abundance of uranium-235 is about 0.7% for use in a nuclear reactor, this fraction must be increased to about 3%. 

Turning now to lithium, we have two nuclides available for NMR measurements Li and Li. Both are quadrupolar nuclei with spin quantum number / of 1 and 3/2, respectively. The natural abundance of Li (92.6%) provides enough NMR sensitivity for direct measurements, but also Li (7.4%) can easily be observed without enrichment. However, isotopic enrichment poses no practical problem and is advantageous if sensitivity is important, as for measurements of spin-spin coupling constants in solution and of quadrupole coupling constants in the sohd state. 

The isotope N, with a natural abundance of 99.9%, has nuclear spin 7 = 1 and gives broad signals which are of little use for structural determinations. The N nucleus, with I = 1/2, is therefore preferred. However, the low natural abundance of about 0.4% and the extremely low relative sensitivity (Table 1) make measurements so difficult that N NMR spectroscopy was slow to become an accepted analytical tool. A further peculiarity is the negative magnetogyric ratio since, in proton decoupled spectra, the nuclear Overhauser effect can strongly reduce the signal intensity. DEPT and INEPT pulse techniques are therefore particularly important for N NMR spectroscopy. 

To obtain molecular dimensions the moments of inertia of isotopic species must be determined. These (most conveniently) are sufficient in their natural abundance. Of particular importance here is 13C, but other atoms such as 2H, lsO and 15N require isotopic enrichment. Application of Kraitchman s equations (53MI20400) then yields atomic coordinates to a few thousandths of an Angstrom and bond angles to an accuracy of better than... 

Isotope Enrichment. - The natural abundance of the magnetic nucleus under study is vitally important to overall sensitivity. Natural-abundance-2H n.m.r., for example, is roughly a million times less sensitive than 1H n.m.r. This factor must therefore be carefully considered before embarking on an n.m.r. study of an insensitive nucleus. One means to improve the sensitivity of low-abundance nuclei is to perform isotope enrichment. This is frequently done for nuclei such as13C,15N, and 2H it remains, however, a fairly expensive option. 

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