Isotopic modification

Terbium is reasonably stable in air. It is a silver-gray metal, and is malleable, ductile, and soft enough to be cut with a knife. Two crystal modifications exist, with a transformation temperature of 1289oC. Twenty one isotopes with atomic masses ranging from 145 to 165 are recognized. The oxide is a chocolate or dark maroon color. 

A unique problem arises when reducing the fissile isotope The amount of that can be reduced is limited by its critical mass. In these cases, where the charge must be kept relatively small, calcium becomes the preferred reductant, and iodine is often used as a reaction booster. This method was introduced by Baker in 1946 (54). Researchers at Los Alamos National Laboratory have recently introduced a laser-initiated modification to this reduction process that offers several advantages (55). A carbon dioxide laser is used to initiate the reaction between UF and calcium metal. This new method does not requite induction heating in a closed bomb, nor does it utilize iodine as a booster. This promising technology has been demonstrated on a 200 g scale. 

For shorter penetration the use of p emitting isotopes is widely made applying much the same absorption law for p radiation as for y rays, with a minor modification. An alternative procedure used by Kingety and Paladino for the study of diffusion in AI2O3 employs two right cylinders, one only containing the radioactive Al isotopic species which emits relatively low energy ( ,nax = 0.511 MeV) p particles. 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

A recent modification of this technique utilizes A,A-d2-propylamine as the solvent for the lithium reduction, thereby eliminating the inconveniences associated with the preparation and handling of liquid deuterioammonia. Under these conditions the reaction can be carried out at room temperature and less overreduction of the carbonyl group is observed. For example, the reduction of A" -3-keto steroids (159) under these conditions, followed by back exchange in protic media, leads to the corresponding 5a-di-3-ketones (160) which exhibit good isotopic purity. ... 

V.A. Sobyanin, V.I. Sobolev, V.D. Belyaev, O.A. Mar ina, A.K. Demin, and A.S. Lipilin, On the origin of the Non-Faradaic electrochemical modification of catalytic activity (NEMCA) phenomena. Oxygen isotope exchange on Pt electrode in cell with solid oxide electrolyte, Catal. Lett. 18, 153-164 (1993). 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

The gas-phase structure of 1,3-dithietane 1-oxide (189) has been determined from its microwave spectrum and the spectra of eight isotopic modifications . The ring is puckered, the angle between the two CSC planes being 39.3° with the oxygen equatorial. 

Ellerbe, P, Meiselman S, Sniegoski LT, Welch MJ, and White E V (1989) Determination of serum cholesterol by a modification of the isotope dilution mass spectrometric definitive method. Anal Chem 61 1718-1723. 

Reagan MK, Volpe AM, Cashman KV (1992) and Th-series chronology of phonolite fractionation at Mount Erebus, Antartica. Geochim Cosmochim Acta 56 1401-1407 Reagan MK, Morris JD, Herrstrom EA, Murrell MT. (1994) Uranium series and beryllium isotope evidence for an extended history of subduction modification of the mantle below Nicaragua. Geochim Cosmochim Acta 58 4199-4212... 

A common theme is the existence of modified (enantioselective) sites and unmodified (racemic) sites. For the case of the tartaric acid modified Ni, it is postulated that the tartaric acid is adsorbed on the surface and stereodirects (through hydrogen bonding) adsorption of the incoming 3-ketoesters.18 19 Support for this comes from an isotope effect from deuterium labeling.23 Increased enantioselectivities resulting from co-modification with NaBr is believed to result from poisoning the racemic sites.24 A similar technique in... 

GC-C-IRMS instrumentation enables the compound-specific isotope analysis of individual organic compounds, for example, n-alkanes, fatty acids, sterols and amino acids, extracted and purified from bulk organic materials. The principle caveat of compound-specific work is the requirement for chemical modification, or derivatisation, of compounds containing polar functional groups primarily to enhance their volatility prior to introduction to the GC-C-IRMS instrument. Figure 14.7 summarises the most commonly employed procedures for derivatisation of polar, nonvolatile compounds for compound-specific stable isotope analysis using GC-C-IRMS. 

---