Traps two-electron

The concept of color centers has been extended to surfaces to explain a number of puzzling aspects of surface reactivity. For example, in oxides such as MgO an anion vacancy carries two effective charges, V(2. These vacancies can trap two electrons to form an F center or one electron to form an F+ center. When the vacancy is located at a surface, the centers are given a subscript s, that is, Fs+ represents a single electron trapped at an anion vacancy on an MgO surface. As the trapping energy for the electrons in such centers is weak, they are available to enhance surface reactions. 

Regarding centers that trap two electrons, these have been analyzed by Neumark (1973) on the basis of He type wave functions and, more recently, by Jaros (1978) and by Riddoch and Jaros (1980). One conclusion from this work is that quantitative values of Auger cross sections depend strongly on the band structure, with the transition matrix element (MA) given by Neumark (1973) as... 

Any material which can form a color center contains two types of precursors as shown in Figure 2a. The hole center precursor is an atom, ion, molecule, impurity, or other defect which contains two paired electrons, one of which can be ejected by irradiation, leaving behind a hole center (Fig. 2b). The electron center precursor is an atom, ion, etc, which can produce an electron center by trapping the electron ejected from the hole center precursor. A hole and an electron center are thus formed simultaneously. Either or both can be the color center. Almost all materials have hole center precursors. If there is no electron center precursor, however, the displaced electron returns to its original place and the material remains unchanged. 

Rossi and Bunnett64 studied the chemical reductive cleavage of diphenyl sulfoxide, diphenyl sulfone and methyl phenyl sulfone under the action of potassium metal in liquid ammonia in the presence of acetone. The enolate ion is used to trap phenyl radicals formed eventually during the process, in order to determine whether one or two electrons are required for the mechanism of cleavage (Scheme 7). In all the runs, phenyl anion is... 

The work of Averim and Likharev had suggested considering two-junction systems trapping the electron inside it, based on the ideas emerging from the theory. 

Fds with conventional [Fe2-S2] clusters can undergo a one-electron transfer to a deeply valence-trapped FemFen species. For proteins of known structure (and presumably others) one iron atom is closer to the surface (by about 0.5 nm) and it has been established that the added electron resides on that atom. No instances are known where an [Fe2-S2] centre acts as a physiological two-electron donor or acceptor. In addition to the conventional [Fe2-S2] ferredoxins, the electron-transfer chains of mitochondria and photosynthetic bacteria contain Rieske proteins which have a cluster with the composition [(Cys.S)2FeS2Fe(N.His)2], in which the two imidazole groups are bound to the same iron atom (Figure 2.9). This atom is the site... 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

He is found in natural gas deposits principally because alpha particles are produced during natural radioactive decay processes. These alpha particles are 4 He nuclei they obtain two electrons from the surrounding material to become helium atoms. This gaseous helium then accumulates with the natural gas trapped beneath the earth. Although other noble gases are produced by radioactive decay—notably 40 Ar—they are not produced in the large quantities that helium is. 

One of the first notions of EGA-catalyzed reactions was the rationalization [8, 14] of the unexpected outcome of anodic oxidation of methyl arenes, (1), in MeGN containing various amounts of water. Preferentially A-benzyl acetamides, (3), rather than the benzyl alcohols, (2), were formed [15, 16] (with increasing amounts of water, increasing amounts of aldehyde was formed as a side product [16]). Since water is a more powerful nucleophile than MeCN, it is reasonable to believe that the carbocation formed by overall two-electron oxidation and deprotonation is initially trapped by water. However, the process is reversible in the presence of a strong EGA (protons liberated from the oxidized substrate), and the carbocation is eventually trapped by the excess MeCN, Scheme 1. 

An unusual type of cationic chemisorption occurs in the C(9 and Cimpurity levels. In this case an electron is lost from an impurity level for each foreign atom adsorbed, and two electrons are trapped in the C(P level. Now the wave functions for these electrons are small on the foreign atom, which exists therefore on the surface as the cation. This chemisorption is depletive. If the C<9 level lies between the bottom of the conduction band and the impurity levels, the chemisorption is still cationic, but the electrons in the impurity levels play no part in the process, and only one electron is trapped in the C(P level in the neighborhood of the first crystal atom. This chemisorption is cumulative. 

Although Taube s pyrazine Ru"—Ru dimer was produced by the Ag oxidation of [(NHjljRu—NC4H4N—Ru(NH3)5] , attempts to prepare similar Ru"-Ru " complexes from [(NH3)5Ru(C5H4N)2Ru(NH3)5]" and [(NHjljRu—NC5H4C2H4C5H4N—Ru(NH3)5]" were unsuccessful. Cyclic voltammetric data indicated a two-electron oxidation to Ru" -Ru " dimers. In view of the identical ligands around each Ru atom, Mayoh and Day have questioned the localization of the Ru valencies in Taube s dimer into discrete Ru" and Ru " centres. However, a theoretical calculation of the conditions necessary for valence trapping in any mixed valence system, showed that the condition is indeed satisfied by the above Ru compound. Other workers have suggested that the available data on this complex could also be explained by a molecular orbital scheme in which the Ru ion and pyrazine-filled n (or k ) molecular orbitals are mixed, and the unpaired electron is mainly but un-symmetrically shared by the two cations. ... 

Further data from the polarography and cyclic voltammetry in dimethylformamide are given in Table 5.1 for a series of overall two-electron processes leading to cleavage of a benzyl-heteroatom bond. The first electron transfer step is of the dissociative electron transfer type leading to a benzyl radical. This radical is reduced firrther, at the working potential, to the benzyl carbanion. The carbanion fi om benzyl chlorides, esters, ethers, sulphides, sulphones and quaternary ammonium salts can be trapped by carbon dioxide to form phenylacetic acid [2]. Reac-... 

Recently, alkylation of alkyl aromatic hydrocarbons such as toluene, ethylbenzene, cumene, and xylenes with ethene, propene, and 1,2-diphenylethene was investigated by Kijenski et al. (245), who used superbasic K-MgO and K-AI2O3 catalysts at low temperature at atmospheric and elevated pressures. The reaction kinetics, EPR measurements of adsorbed intermediates, and the effects of poisoning determined by the radical trap TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxyl, free radical) led the authors to conclude that sites are the catalytically active centers. To demonstrate the importance of strong one-electron donor sites (F ) for the alkylation and the inactivity of strong two-electron donor centers, the ethylation of cumene, ethylbenzene, and toluene was carried out with MgO-10%NaOH. On this catalyst, strong basic two-electron donor sites (27 33) were found, along... 

The system (4- Bu-pyH)3[Ru(0)3Cl ]/NM0/PMS/CH2Cl3 catalysed the oxidation of primary alcohols to aldehydes and of secondary alcohols to ketones like TRAP (Tables 2.1 and 2.2), such oxidations did not attack double bonds. As stoich. trans-(PPh )2[Ru(0)2Cl ] -/CH3CN it is a two electron oxidant for alcohols [561]. For tran -[Ru(0)2Cy - in solution the effective oxidant or oxidant precursor is [Ru(0)2Cl3]", and this species is coordinatively unsaturated. That this is the case is suggested by the observation that addition of extra Cl" (as (PPh )Cl) to the green [Ru(0)2Cl3]" in solution (Eq. 1.4) generating the red franx-[Ru(0)3Cl ] ", a markedly less effective catalytic oxidant for alcohols than [Ru(0) Cl ]" [561]. 

A Rh-catalyzed Reformatsky reaction of chiral imine (24) led to the stereoselective preparation of the a,a-difluoro-jS-amino acid (25). 25 was converted to difluor-oalkene (26), and subsequently L-Val-i/r[(Z)CF=CH]Gly derivative (23) in greater than 82% for both steps. The samarium diiodide-mediated reductive transformation of the y,y-difluoro-a, S-enoates proceeded via successive two-electron transfers to form a dienolate species which upon kinetically controlled trapping with fert-BuOH formed 23 (Scheme 6). 

Electrons have not been detected by optical absorption in alkanes in which the mobility is greater than 10 cm /Vs. For example, Gillis et al. [82] report seeing no infrared absorption in pulse-irradiated liquid methane at 93 K. This is not surprising since the electron mobility in methane is 500 cm /Vs [81] and trapping does not occur. Geminately recombining electrons have, however, been detected by IR absorption in 2,2,4-trimethyl-pentane in a subpicosecond laser pulse experiment [83]. The drift mobility in this alkane is 6.5 cm /Vs, and the quasi-free mobility, as measured by the Hall mobility, is 22 cm /Vs (see Sec. 6). Thus the electron is trapped two-thirds of the time. 

---