Studies Using Lithium Isotopes

The cell diagram for a double-cell used for the study of lithium-isotope-exchange reactions is as follows... 

The prospect of separating lithium isotopes on a large scale by the extraction of lithium from aqueous solutions is not very promising. In the system we have studied, reflux could be accomplished by an acid-base mechanism however, because of the small separation factor, an extremely large reflux ratio would be required. This would necessitate a very large plant using enormous quantities of acid and base, and the cost would be excessive. 

Lithium metabolism and transport cannot be studied directly, because the lack of useful radioisotopes has limited the metabolic information available. Lithium has five isotopes, three of which have extremely short half lives (0.8,0.2, 10 s). Lithium occurs naturally as a mixture of the two stable isotopes Li (95.58%) and Li (7.42%), which may be determined using Atomic Absorption Spectroscopy, Nuclear Magnetic Resonance Spectroscopy, or Neutron Activation analysis. Under normal circumstances it is impossible to identify isotopes by using AAS, because the spectral resolution of the spectrometer is inadequate. We have previously reported the use of ISAAS in the determination of lithium pharmacokinetics. Briefly, the shift in the spectrum from Li to Li is 0.015 nm which is identical to the separation of the two lines of the spectrum. Thus, the spectrum of natural lithium is a triplet. By measuring the light absorbed from hollow cathode lamps of each lithium isotope, a series of calibration curves is constructed, and the proportion of each isotope in the sample is determined by solution of the appropriate exponential equation. By using a dual-channel atomic absorption spectrometer, the two isotopes may be determined simultaneously. - ... 

Since the solutions studied contain only minor concentrations of accompanying substances, it is possible to determine lithium from microlitre samples at levels of 10" to 10" g/1 without any pretreatment. The time required for one analysis is about 20 to 30 min. In general, it is found that 1 pg of lithium present on the emitter is sufllcient for a determination. This means that lithium concentrations as low as SO ng/1 can be determined in microliter samples using stable isotope dilution and FDMS. 

In theory, diffusion coefficients can be measured for any ion. In practice, however, most studies of ionic diffusion in glasses have been restricted to highly mobile ions which have a convenient radioactive isotope for use in tracer measurements. As a result, a majority of the data for ionic diffusion deals with sodium, with lesser amounts of data for potassium, rubidium, and cesium. Studies of lithium are very limited due to the lack of a radioactive isotope of lithium, while studies of divalent and other, more highly charged, ions are restrieted by the very low mobilities of these ions as compared to those of the monovalent ions. 

Early studies showed that lithium is widely distributed in tissues following oral administration or by intraperitoneal or intravenous injection to experimental animals (for review, see Refs. 12 and 17). Thellier et al., in a series of elegant neutron activation experiments using the isotope Li, provided visual localization of lithium in the whole body [18] and in different areas of the brain [8]. Of the soft tissues the pituitary, the thymus, and, most particularly, the thyroid glands have more than the average accumulation of lithium. Bone also appears to accumulate lithium. 

Since the late 1990s there has been a growing literature on lithium isotopes, as indicated in Table 1.14. Both 8 Li and S Li have been reported, since a standard nomenclature has not yet been established. This is quite unusual in isotopic work, and makes the values of different investigators difficult to compare. Nevertheless, the studies have been quite useful in determining the origin of some brines, the amount of lithium adsorbed on clays or other rocks, and on various other physical or chemical changes. 

In this study, benzaldehyde and benzaldehyde-methyllithium adduct were fully optimized at HF/6-31G and their vibrational frequencies were calculated. The authors used MeLi instead of lithium pinacolone enolate, since it was assumed that the equilibrium IBs are not much different for the MeLi addition and lithium enolate addition. Dehalogena-tion and enone-isomerization probe experiments detected no evidence of a single electron transfer to occur during the course of the reaction. The primary carbonyl carbon kinetic isotope effects and chemical probe experiments led them to conclude that the reaction of lithium pinacolone enolate with benzaldehyde proceeds via a polar mechanism. 

In 1998, Hasanayn and Streitwieser reported the kinetics and isotope effects of the Aldol-Tishchenko reaction . They studied the reaction between lithium enolates of isobu-tyrophenone and two molecule of beuzaldehyde, which results iu the formation of a 1,3-diol monoester after protonation (Figure 28). They analyzed several aspects of this mechanism experimentally. Ab initio molecular orbital calculatious ou models are used to study the equilibrium and transition state structures. The spectroscopic properties of the lithium enolate of p-(phenylsulfonyl) isobutyrophenone (LiSIBP) have allowed kinetic study of the reaction. The computed equilibrium and transition state structures for the compounds in the sequence of reactions in Figure 28 are given along with the computed reaction barriers and energy in Figure 29 and Table 6. 

In the solid state NMR study, uncomplexed phenyllithium, assumed to be a tetramer, as well as the TMEDA complexed dimer and the PMDTA complexed monomer were investigated. Both Li and Li isotopes were used in the preparations. The C spectra of the complexes are presented in Figure 12. It is evident that the substitution of Li with Li has profound effects on the Unewidths, especially of the ipso-carbon at ca 180 ppm in the aggregated uncomplexed system (Figure 12a and 12b, respectively). This is in accordance with the previously mentioned study of methyllithium. However, even the other positions are affected by the dipolar couplings to the four quadrupolar lithium cations, but to a lesser extent due to the larger C-Li distances. 

Arnett and coworkers later examined the reaction of lithium pinacolone enoiate with substituted benzaldehydes in THE at 25 °C. The determination of the heat of reaction indicated that the Hammett p value for the process is 331. Although the aldol reaction was instantaneous in THF at 25 °C, the reaction with o- or p-methylbenzaldehyde could be followed using a rapid injection NMR method in methylcyclohexane solvent at —80 °C. Application of Eberson s criterion based on the Marcus equation, which relates the free energy of ET determined electrochemically and the free energy of activation determined by kinetics, revealed that the barriers for the ET mechanism should be unacceptably high. They concluded that the reaction proceeds via the polar mechanism . Consistent with the polar mechanism, cyclizable probe experiments were negative . The mechanistic discrepancy between the reactions of benzaldehyde and benzophenone was later solved by carbon kinetic isotope effect study vide infraf. ... 

Germanium metal is also used in specially prepared Ge single crystals for y-ray detectors (54). Both the older lithium-drifted detectors and the newer intrinsic detectors, which do not have to be stored in liquid nitrogen, do an excellent job of spectral analysis of y-radiation and are important analytical tools. Even more sensitive Ge detectors have been made using isotopically enriched Ge crystals. Most of these have been made from enriched 7<5Ge and have been used in neutrino studies (55—57). 

The benefits of initial product studies, followed by a more detailed examination using isotopic labelling, are well illustrated by the amination of halobenzenes, Scheme 2.7. The reaction of p-bromomethoxybenzene (15) with lithium diethylamide in ether gives a 1 1 mixture of m- and p-diethylamino-substituted products (16 and 17), with no trace of the o-isomer. One possible mechanism for these and many related reactions was that a normal direct displacement reaction to give 17 was in competition with an abnormal displacement,... 

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