Abundant nucleus

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. 

Indirect detection Method for the observation of an insensitive nucleus (e.g., 13C) by the transfer of magnetisation from an abundant nucleus (e.g., 1H). This method of detection offers great improvements in the sensitivity of proton-carbon correlated techniques. 

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... 

This technique involves transfer of polarization from one NMR active nucleus to another [166-168]. Traditionally cross polarization (CP) was employed to transfer polarization from a more abundant nucleus (1) to a less abundant nucleus (S) for two reasons to enhance the signal intensity and to reduce the time needed to acquire spectrum of the less abundant nuclei [168]. Thus CP relies on the magnetization of I nuclei which is large compared to S nuclei. The short spin-lattice relaxation time of the most abundant nuclei (usually proton) compared to the long spin-lattice relaxation time of the less abundant nuclei, allows faster signal averaging (e.g., Si or C). CP is not quantitative as the intensity of S nuclei closer to 1 nuclei are selectively enhanced. Nowadays CP has been extended to other pairs of... 

Isotopes in low abundance have long spin-lattice relaxation times which give rise to poor signal-to-noise ratios. Sensitivity can be improved by using a technique known as cross polarization where a complex pulse sequence transfers polarization from an abundant nucleus to the dilute spin thereby enhancing the intensity of its signal. 

Dipolar interactions, responsible for the broadening of NMR signals, can, in principle, be eliminated in the case of an abundant nucleus such as the proton, but the small range of proton chemical shifts makes the use limited. In the case of dilute nuclei, spectra of fair resolution are readily obtained. The magnetic interaction for such a nucleus is given by... 

Nucleus Spin Abundance (%) Nucleus Spin Abundance (%)... 

The importance of intermolecular relaxation processes (Hertz, 1967) in the interpretation of proton-relaxation data necessitates a rather deep understanding of the mechanics of liquids. Thus the H nucleus, although most extensively used in work concerned with chemical shifts and spin coupling constants, was not really taken as a probe for the molecular dynamics of complex organic molecules. It is only very recently that there has been a significant increase in literature on 7", for protons in organic molecules (Hall and Preston, 1974) stimulated most likely by 13C relaxation work. The nC nucleus, however, developed to be the abundant nucleus of relaxation experiments for three main reasons ... 

From the isotopic decomposition of normal He one finds that the mass-4 isotope, 4He, is 99.986% of all helium. It is the second most abundant nucleus in the universe Modern observations of the interstellar gas reveal it to be 10.3 times less abundant than hydrogen. The elemental abundance is He = 2.72 x 109 per million silicon atoms in solar-system matter. 

Carbon is the fourth most abundant element in the universe. Its abundance in the Sun is about one-half that of oxygen, butreveals differing ratios to oxygen in other stars and in nebulae. The most abundant isotope of carbon, 12C, is the fourth most abundant nucleus in the universe. The two most abundant, 2H and 4He, are remnants of the Big Bang, whereas l60, the third most abundant, and 12C are created during the evolution of stars. Carbon ranks therefore as one of the great successes of stellar nucleosynthesis. The evolution of stars makes evident why this is so. From the isotopic decomposition of normal carbon one finds that the mass-12 isotope, 12C, is 98.9% of all C isotopes. 

Na is the 20th most abundant nucleus within the universe, roughly comparable to A1 or 4°Ca, and the 14th most abundant element, between Ca (13 th) and Ni (15 th). 

This is the ninth most abundant nucleus within the universe. 

This makes 28Si the eighth most abundant nucleus in the universe, between 2H and 24Mg. 

K is the 37th most abundant nucleus within the universe. This makes 39K a very abundant nucleus, comparable to chlorine or manganese. 

K is the 56th most abundant nucleus within the universe. Of the lighter stable isotopes that are synthesized in stars, only 40Ar and 3 S are less abundant On the other hand, 41K is more abundant than any stable isotope heavier than zinc. In that sense then, 41K may be thought of as among those few dividing the very abundant isotopes from the rare ones. 

V is a very rare isotope, especially for its mass range. It is the 118th most abundant nucleus in the universe, comparable only to heavy elements such as iodine or to the rare light isotopes such as 9Be. 

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