Isotope effects with heavy elements

An isotopolog of the analyte that has been labeled with heavy element isotopes (usually C, or 0) is the preferred choice as SIS since kinetic isotope effects (text box in Chapter 2) and back exchange of the isotopic labels with natural abundance atoms in the solvent are negligible, and co-elution of analyte and SIS is essentially guaranteed. Labeling by substitution of H with H (D) is often fit... 

The effects of a rather distinct deformed shell at = 152 were clearly seen as early as 1954 in the alpha-decay energies of isotopes of californium, einsteinium, and fermium. In fact, a number of authors have suggested that the entire transuranium region is stabilized by shell effects with an influence that increases markedly with atomic number. Thus the effects of shell substmcture lead to an increase in spontaneous fission half-Hves of up to about 15 orders of magnitude for the heavy transuranium elements, the heaviest of which would otherwise have half-Hves of the order of those for a compound nucleus (lO " s or less) and not of milliseconds or longer, as found experimentally. This gives hope for the synthesis and identification of several elements beyond the present heaviest (element 109) and suggest that the peninsula of nuclei with measurable half-Hves may extend up to the island of stabiHty at Z = 114 andA = 184. 

The replacement of light isotope with heavy isotope in activated state also lowers the energy. Let the lowering in energy in activation state be represented as AZ o H has been observed that AE is less than AE0 and, therefore, ratio of k/kj > 1. When the bond involving the isotope element in activated complex is completely broken AE and k/k will be maximum. However, when the bonds in activated complex is stronger than in the initial molecule, i.e. AE > A/i o, the value of ktk will be less than unity. This is called reverse isotope effect. 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

The y-process involves the photodisintegration of heavy elements. Obviously, this process is not very efficient, as the effective seed nuclei are the primordial heavy-element constituents of the star. Most of the heavy p-elements with atomic mass number greater than 100 that are produced in massive stars (see Figure 5) are formed by this mechanism. It is primarily for this reason that these proton-rich heavy isotopes are rare in nature. 

The relative importance of non-zero-point contributions to heavy atom effects also makes exact comparisons of results with different isotopes of the same element difficult. There is no counterpart of the Swain-Schaad equation (equation 1.15) for isotopes of hydrogen, although for isotopes of carbon, the intuitive expectation that effects would be around double effects was confirmed eqn (3.15) held for a series of effects, with 1.6[Pg.106]

Obtaining both an isotope effect, k /k, and an element effect, k fk, are the experimental evidence that the C-H and C-X bonds are breaking in the transition structure. Since measurement of heavy atom isotope effects requires special instrumentation, the element effect has taken the place of heavy atom isotope effects in most investigations. The element effect was first proposed by Bunnett in a 1957 paper dealing with the nucleophilic substitution reactions of activated aromatic compounds [27], and later applied to dehydrohalogenation mechanisms by Bartsch and Burmett [28]. The lack of any incorporation of deuterium prior to elimination has also been used as experimental evidence favoring the concerted mechanism [29]. The stereochemistry should be a trans-elimination. 

Abstract Shell effects on nuclear stability have created an island of relative stability for nuclides near A = 230-240 and Z = 90-92. Three nuclides, Th, and have half-Uves long enough for significant amounts to have survived since the heavy elements in the Earth s crust were created. When one of these nuclides decays, it starts a journey that ends with an isotope of lead (Z = 82, A 208). The predominant steps in this journey are a and P decays, so that each of the long-lived parents heads a distinct chain. Each chain, as well as a fourth one that is extinct, is described. 

O Figure 21.11 of Chap. 21, Superheavy Elements, gives a clearer picture of the nuclides beyond A 200. There is an abrupt absence of nuclides with moderate much less long half-lives between ° Pb and Th. This is due to shell effects that are not included in the semiempirical equation. Of course shell effects are crucial for stabilizing the several islands of stability among heavy elements, which include the parents of the natural decay series as well as surprisingly stable isotopes of elements well beyond uranium. 

Optical spectroscopy is being considered for various nobelium isotopes (Backe et al. 2007). Here, investigations of the relativistic effects of the iimer electrons of nuclei with large Z, which shift the optical levels and ionization potentials are of interest. Relativistic effects are already observed in gold as nomelativistic gold would be white in color. For heavy elements with large... 

It is interesting to note that these data (amply confirmed with many additional experiments on uranium and other systems) show that the temperature dependence and the odd-even mass ratios of isotope-exchange fractionation factors of uranium and other metals differ significantly from the predictions of the well-established theory of isotope effects based on vibrational properties (Bigeleisen and Mayer 1947). That observation, one of theoretical importance, has been shown to be a consequence of the large nuclear isotope field shifts found in the heavy elements. The field shifts introduce new mechanisms for understanding the... 

The toxic effects of the intake of uranium are based purely on its characteristics as a heavy element. There are no reports of radiation-induced effects in humans from the inhalation or ingestion of uranium. Inferential material on radiation effects has been gathered through animal experiments and comparison with human exposures to radium isotopes. Extensive reviews of the uranium literature concerning experimental studies in animals and humans are available [3,9,10]. 

Isotopes with 1 = 0 have no hyperfine structure, but in transitions between energy levels in a mixture of I = 0 isotopes of the same element, a line structure may still be obtained. This effect is called the isotopic shift. It has two origins and a distinction is made between the mass effect and the volume effect. The mass effect can be divided up into the normal and the specific mass effects. The normal mass effect is due to the movement of the nucleus, which is due to the fact that it is not infinitely heavy. For hydrogenic systems it is possible to take this into account by using the reduced mass fi instead of m... 

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