Reactions electron captures

The first two reactions are found for many of the heavy radioactive nucleides. Alpha emission has been observed also for a number of the neutron-rich nucleides in the rare-earth region. The third reaction, positron emission, occurs for most neutron-rich nucleides, many of which also decompose by electron capture (the fourth reaction). (Electron capture is classed as a spontaneous decomposition because the electrons are always available in the atom for capture it is the s electrons, principally l5, that are captured they are the only electrons with finite probability at the nucleus.) The last two reactions, proton and neutron emission, occur only rarely. 

In this group the most commonly used reaction is that of radiative neutron capture. Also here are to be found (n, 2 n) and (y, n) reactions, although very few studies have been done with these reactions, and isomeric transitions (although these may often be more profitably discussed along with electron capture reactions). 

A series of papers by Merz and Riedel describe work designed to compare radiochemical behaviour following n,y n,p E.C. and p decay. Gallium isotopes are produced in most of the cases studied, but isotopes of Sn, Pb, Ge and Sb were also involved. Unfortunately, the various chromatography fractions were not well identified, so that it is not easy to draw definite conclusions from this work. Nevertheless, several things do appear to be clear. Some interesting data are presented in Table 5, comparing the effects of electron capture, neutron capture, and the (n,p) reaction. 

C22-0099. Complete the following nuclear reactions (a) positron emission from Si (b) electron capture by... 

In this case, we use 6 as a small fraction since the actual number of defects is small in relation to the overall number of ions actually present. For the F-Center, the brackets enclose the complex consisting of an electron captured at an anion vacancy. Note that these equations encompass all of the mechanisms that we have postulated for each of the individual reactions. That is, we show the presence of vacancies in the Schottlqr case and interstitial cations for the Frenkel case involving either the cation or anion. The latter, involving an interstitlcd anion is called, by convention, the "Anti-Frenkel" case. The defect reaction involving the "F-Center" is also given. 

As a test case, we report in this paper the study of the -t- He collision. This work has been undertaken in connection with photon spectroscopy experiments regarding the electron capture for the reactions... 

Selenium(IV) reacts with substituted 1,2-diaminobenzene or 2,3-diamlnonaphthalene in acidic solution to form stable cyclic derivatives which can t>e extracted into an organic solvent and analyzed by gas or liquid chromatography [682,683]. With chloro-, bromo-, or nitro-substituents the plazselenols can be determined with an electron-capture detector at the low picogram level. Se(VI) does not form piazselenol derivatives so the reaction with diaminobenzene can be used to determine the concentration of Se(IV) and Se(VI). Selenium(VI) may be redu to Se(IV) with... 

Cyanide and thiocyanate anions in aqueous solution can be determined as cyanogen bromide after reaction with bromine [686]. The thiocyanate anion can be quantitatively determined in the presence of cyanide by adding an excess of formaldehyde solution to the sample, which converts the cyanide ion to the unreactive cyanohydrin. The detection limits for the cyanide and thiocyanate anions were less than 0.01 ppm with an electron-capture detector. Iodine in acid solution reacts with acetone to form monoiodoacetone, which can be detected at high sensitivity with an electron-capture detector [687]. The reaction is specific for iodine, iodide being determined after oxidation with iodate. The nitrate anion can be determined in aqueous solution after conversion to nitrobenzene by reaction with benzene in the presence of sulfuric acid [688,689]. The detection limit for the nitrate anion was less than 0.1 ppm. The nitrite anion can be determined after oxidation to nitrate with potassium permanganate. Nitrite can be determined directly by alkylation with an alkaline solution of pentafluorobenzyl bromide [690]. The yield of derivative was about 80t.with a detection limit of 0.46 ng in 0.1 ml of aqueous sample. Pentafluorobenzyl p-toluenesulfonate has been used to derivatize carboxylate and phenolate anions and to simultaneously derivatize bromide, iodide, cyanide, thiocyanate, nitrite, nitrate and sulfide in a two-phase system using tetrapentylammonium cWoride as a phase transfer catalyst [691]. Detection limits wer Hi the ppm range. 

It is a straightforward task to construct a rate model for the reaction scheme shown in Figure 9. Electron capture and its inverse (autoionization),... 

Figure 3 shows different forms of chemisorption for a C02 molecule. In the weak form of chemisorption the C02 molecule is bound to the surface by two valency bonds, as shown in Fig. 3a. This is an example of adsorption on a Mott exciton which is a pair of free valencies of opposite sign (i.e., an electron-hole pair). This may be either a free exciton wandering about the crystal or a virtual exciton generated in the very act of adsorption. As seen from Fig. 3a, in the case of the C02 molecule the weak form of chemisorption is a valency-saturated and electrically neutral form. As a result of electron capture, this form is transformed into a strong acceptor form shown in Fig. 3b, while as a result of hole capture it becomes a strong donor form shown in Fig. 3c. Both these forms are ion-radical ones. It should, however, be noted that the ion-radicals formed in these two cases are quite different and, having entered into a reaction, may cause it to proceed in different directions.

Some examples of specific electron-capture by various organo-metallic compounds in such solvents are summarised in reactions [5]-[10] [refs. (5)-(H)) respectively]. In some cases, the parent anion ( AB ) is detected by e.s.r. spectroscopy, but in others subsequent reactions have occurred. 

In reactions [5]-[8] pure electron addition occurs, but in reaction [9] addition and dissociative electron capture giving loss of MeO occur concurrently. Furthermore, CH3 radicals are also formed, together, presumably, with (Me0)2P02 this being an alternative dissociative route. Reaction [10] occurs in methanol, there being no clear sign of the parent anion, P(0Me)3 . This protonation step is also accompanied by dissociative electron capture to give P(0Me)2 radicals. 

Use of CD30D or methyl tetrahydrofuran solvents to encourage electron capture, resulted in a complex set of reactions for methyl cobalamine. Initial addition occurred into the w corrin orbital, but on annealing a cobalt centred radical was obtained, the e.s.r. spectrum of which was characteristic of an electron in a d z.y orbital (involving the corrin ring) rather than the expected d2z orbital. However, the final product was the normal Co species formed by loss of methyl. Formally, this requires loss of CH3 , but this step seems highly unlikely. Some form of assisted loss, such as protonation, seems probable. 

Only when the very contamination-sensitive electron-capture detector is used is it necessary to provide separate gas streams, one for the reaction and stripping part of the system, the other for the carrier gas stream of the column and detector. Otherwise, the same gas stream can be used to strip the hydrides from solution and carry them into the detector, which greatly simplifies the apparatus. This is of considerable significance, as each additional surface and joint in the apparatus increases the possibility of irreversible adsorption of the sensitive hydrides, and thus is a potential contributor to analytical error. The... 

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