Oxygen natural abundance

Oxygen is the most abundant element on earth The earths crust is rich in carbonate and sili cate rocks the oceans are almost entirely water and oxygen constitutes almost one fifth of the air we breathe Carbon ranks only fourteenth among the elements in natural abundance but trails only hydro gen and oxygen in its abundance in the human body It IS the chemical properties of carbon that make it uniquely suitable as the raw material forthe building blocks of life Let s find out more about those chemi cal properties... 

Interestingly, the oxygen-17 chemical shifts for the thiirane oxide (16a) and thiirane dioxide (17a) were found to be 71 and 111 ppm (downfield from natural-abundance 170 in HzO), respectively. The oxygen-17 shift reveals that this oxygen is the most highly shielded oxygen atom so far reported80,70. 

Yakir, D. 1992 Variations in the natural abundance of oxygen-18 and deuterium in plant carbohydrates. Plant, Cell, and Environment 15 1005-1020. 

The experiments we have so far described have been used to study nuclei with spin I = Vi ( ll, 13C, 31P). Our model compounds 1 and 2 contain two further atoms (oxygen and chlorine), which have no NMR-active isotope with spin Vi. Oxygen does however have an NMR-active isotope with spin I = 5/2 but very low natural abundance (0.037%) this is 170. Chlorine has two NMR-active isotopes 35C1 (I = 3/2,75.53%) and 37C1 (I = 3/2,24.47%). 

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. 

Formally oxygen is also an A + 2 element. However, the natural abundance of the lsO isotope is only 0.2% of that of the main isotope ieO. That is why reliable calculation of the number of oxygen atoms based on the intensity of the isotopic peaks is hardly possible. Nevertheless, it is quite possible to estimate this number. Thus, if the intensity of the M + 2 peak in the mass spectrum of a sample with a low number of carbon atoms is higher than 0.5% relative to M+, this compound may contain one or more oxygen atoms. 

Water is a mixture of varying isotopic composition (Franks, 2000). In addition to the two most common isotopes, 160 and there are two stable oxygen isotopes (170, lsO), one stable hydrogen isotope (2H, deuterium), and one radioactive hydrogen isotope (3H, tritium, half-life = 12.6 years). Water also contains low concentrations of hydronium (H30+) and hydroxide ions (OH-) and their isotopic variants. In total, water consists of more than 33 chemical variants of HOH however, these variants occur in relatively minor amounts (Fennema, 1996). Table II gives the natural abundance isotopic composition of the four major water species. 

The elements whose isotopes are routinely measured with gas inlet mass spectrometers are carbon (12C and 13C, but not 14C), oxygen (160, 170, l80), hydrogen ( H, 2H, but not 3H), nitrogen (14N and 1SN) and sulphur (32S, 33S, 34). Stable isotopes of H, C, N, O, and S occur naturally throughout atmosphere, hydrosphere, lithosphere, and biosphere. They are atoms of the same elements with a different mass. Each element has a dominant light isotope with the nominal atomic weight (I2C, 160,14N, 32S, and H) and one or two heavy isotopes (l3C, nO, 180, 15N, 33S, 34S, and, 2H) with a natural abundance of a few percent or less Table 1). 

Materials NaGeX zeolite was kindly supplied by Dr. G. Poncelet (Universite Catholique de Louvain) and the mixed tin-antimony oxide catalysts (SnSbO) by I.C.I. Ltd. The H-Z is the acidified form of commercially available Norton mordenite. The ZSM-5 and ZSM-11 zeolites were synthesized following the patent literature (15,16). 1-Butene (Prochem) was a natural abundance compound, while methanol (95 % l C, British Oxygen Corporation (B.O.C.)), ethanol (95 % C, B.O.C.) and ethylene ( 90 % C, Prochem) were JC-enriched compounds. For the latter a 30 % v/v dilution was realized prior to adsorption. 

It is possible to obtain NMR spectra of nearly all elements, although not always from observing the isotope with the highest natural abundance, as can be seen from the examples of carbon and oxygen. For reasons connected with the historical development of NMR spectroscopy, nuclei of all species other than H are referred to as heteronuclei. 

We see from Table 1 that the only observable nuclide for oxygen, 0, has a very low natural abundance, even in comparison with those of popular nuclides like (1.108%) and N (031%). Moreover, its quadrupole moment prevents any practical utilization of polarization transfer techniques like INEPT or DEPT, now widely used in and N NMR spectroscopies. A range of chemical shifts much wider than those of and N is an important point in favour of utilization of 0. All these properties did not prevent important applications of O NMR spectroscopy in organic chemistry, even from the times of continuous wave NMR spectroscopy. Interesting examples of such pioneering works can be found both at natural abundance as well as with enriched samples . However, also in the case of O NMR spectroscopy, FT NMR proved to be decisive for its development. 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

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