Irradiation ionic crystals

A second example of the use of ionic chiral auxiliaries for asymmetric synthesis is found in the work of Chong et al. on the cis.trans photoisomerization of certain cyclopropane derivatives [33]. Based on the report by Zimmerman and Flechtner [34] that achiral tmns,trans-2,3-diphenyl-l-benzoylcyclopropane (35a, Scheme 7) undergoes very efficient (0=0.94) photoisomerization in solution to afford the racemic cis,trans isomer 36a, the correspondingp-carboxylic acid 35b was synthesized and treated with a variety of optically pure amines to give salts of general structure 35c (CA=chiral auxiliary). Irradiation of crystals of these salts followed by diazomethane workup yielded methyl ester 36d, which was analyzed by chiral HPLC for enantiomeric excess. The results are summarized in Table 3. 

Irradiation of all kinds of solids (metals, semiconductors, insulators) is known to produce pairs of the point Frenkel defects - vacancies, v, and interstitial atoms, i, which are most often spatially well-correlated [1-9]. In many ionic crystals these Frenkel defects form the so-called F and H centres (anion vacancy with trapped electron and interstitial halide atom X° forming the chemical bonding in a form of quasimolecule X2 with some of the nearest regular anions, X-) - Fig. 3.1. In metals the analog of the latter is called the dumbbell interstitial. 

The temporal evolution of spatial correlations of both similar and dissimilar particles for d = 1 is shown in Fig. 6.15 (a) and (b) for both the symmetric, Da = Dft, and asymmetric, Da = 0 cases. What is striking, first of all, is rapid growth of the non-Poisson density fluctuations of similar particles e.g., for Dt/r = 104 the probability density to find a pair of close (r ro) A (or B) particles, XA(ro,t), by a factor of 7 exceeds that for a random distribution. This property could be used as a good aggregation criterion in the study of reactions between actual defects in solids, e.g., in ionic crystals, where concentrations of monomer, dimer and tetramer F centres (1 to 3 electrons trapped by anion vacancies which are 1 to 3nn, respectively) could be easily measured by means of the optical absorption [22], Namely in this manner non-Poissonian clustering of F centres was observed in KC1 crystals X-irradiated for a very long time at 4 K [23],... 

Irradiation of ionic crystals results in atomic and electronic dislocations. The trapping of displaced electrons by anion vacancies results in the absorption of visible and near ultraviolet light, which give these crystals their characteristic colors. These pseudoatomic electrons and their vacancies are called color centers. 

The exposure of colored ionic crystals to visible or ultraviolet light causes the annealing of trapped electrons and results in bleaching of the colorations induced by irradiation. In some cases in which the crystal remains uncolored upon irradiation, thermoluminescence is observed in the annealing process. 

These and numerous other experiments prove that in metals the implanted atoms change their position and their surroundings between liquid He temperature and RT and that a considerable reordering of the lattice takes place even at low temperatures. The low-temperature recovery of ion-bombarded compounds is unfortunately completely unknown. Very few experiments at liquid-N temperatures indicate a strong temperature dependence too. A recovery of the majority of the point defects and centers below RT was found experimentally only for ionic crystals irradiated with electrons ... 

It seems best to consider the data on metals first, because radiation damage to them by heavy particles is probably better understood than that to oxides and other ionic crystals. Furthermore, the acquisition of direct evidence about surfaces by electron microscopy is farther advanced for metals than for other types of solid. Table VII gives the results of irradiation experiments on the surface area of some metals. 

The enhanced diffusion demonstrated for metals is probably generally operative and has indeed been directly observed in an ionic crystal 133). A crystal of sodium nitrate, on which etch pits visible in a microscope had been produced, was exposed to ca. 8 x lO i ev/gm of y-rays, after which the etch pits were no longer visible, having presumably been filled by diffusion under the irradiation. It is somewhat surprising to find a detectable effect at so low a dose, but it may be that the etch pits, which are probably at the ends of dislocations, are particularly well situated for localization of the enhanced diffusion. 

Fig. 1 a Crystal structure of the carboxylate anion portion of the (R)-(+)-l-phenylethyl-amine salt of keto-acid 37a before irradiation, and b after 70% conversion to the corresponding cyclobutanol (ionic chiral auxiliary not shown)... 

In addition, the glass matrix has an essential merit in comparison with the solvent which crystallizes at low temperature. For example, Smith et al. irradiated several olefins at 77° K and examined their ESR spectra, and they found that the electrons were trapped in the frozen state of glass but never in the crystalline state (9). This is also the case with 3-methylpentane (70), and other compounds such as alcohols and ethers. This fact may imply that the radiation-formed ionic intermediates are much more stable in the glass matrix than in the crystalline matrix, though the reason has not yet been confirmed. 

The conservation of energy and momentum is the fundamental requirement which determines the behavior of the SE s in metals, semiconductors, and ionic compounds irradiated by particles. Although we shall not deal with the basic physics of elementary collision processes in our context of chemical kinetics, let us briefly summarize some important results of collision dynamics which we need for the further discussion. If a particle of mass mP and (kinetic) energy EP collides with a SE of mass ms in a crystal, the fraction of EP which is transferred in this collision process to the SE is given by... 

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