Stabilized electrons

A part of the chemical consequences of the cyclic orbital interactions in the cyclic conjngation is well known as the Hueckel rule for aromaticity and the Woodward-Hoffmann rule for the stereoselection of organic reactions [14]. In this section, we describe the basis for the rnles very briefly and other rules derived from or related to the orbital phase theory. The rules include kinetic stability (electron-donating and accepting abilities) of cyclic conjugate molecules (Sect. 2.2.2) and discontinnity of cyclic conjngation or inapplicability of the Hueckel rule to a certain class of conjngate molecnles (Sect. 2.2.3). Further applications are described in Sect. 4. 

Table 6.4 gives the radiation yields of stabilized electrons in some selected glasses at 77 K with about 15% uncertainty. A detailed discussion of these yields has been given by Kevan (1974). Hase et al. (1972a) observed et spectrum in ethanol at 4 K with a peak at 1500 nm on quick warming to 77 K, the spectrum relaxed with the same peak in the visible as that of es in liquid ethanol. Hase et al. (1972b) also observed et spectrum at 4 K in 3MP with no clear maximum. On quick warming to 77 K, the spectrum relaxed with a clear maximum at -1700 nm. 

TABLE 6.4 Radiation Yields (G Values) of Stabilized Electrons in Organic Glasses Under /-Irradiation at 77 K... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

T.W. Hamann, N.S. Lewis, Control of the stability, electron transfer kinetics, and pH-dependent energetics of Si/H2 0 interface through methyl termination of Si(III) surfaces,/. Phys. Chem. B 110 (2006) 22291-22294. 

The effective mass of the electrons changes due to lattice strain, alloy additions, radiation damage, phase transformation, and phase content, directly relates to the ability to use electronic property measurements to assess microstructure phase stability. Electronic properties, such as thermoelectric power coefficients, resistivity and induced resistivity measurements, have a demonstrated correlation to solute and phase content, potential phase transformations, as well as residual strain. 

Hard carbon nucleophiles that do not contain stabilizing electron-withdrawing substituents have been shown to react with allylic carbonates only recently in the... 

A summary is given of the structures for the known homopolyatomlc anions of the representative elements from groups IV and V and for a variety of heteropolyatomlc examples. Also considered are factors Important In their stabilization, electronic requirements, Isoelectronlc analogs, and new results for Sby3, Sb 2- Pb2Sb2 and the unusual i[KSng ]. The contributions of Ralph Rudolph to the study of these anions In solution are noted. 

Bunz et al. pointed out that it would be of interest to develop materials that combine the stability, electron affinity, and high emissive quantum yield of PPEs with the excellent hole injection capabilities of poly(p-phenylene vinylene)s (PPVs) [48]. In line with this notion,recent synthetic activities have focused on the engineering of the band gap, conduction band, and valence band of PAEs with the objective to render these materials more useful for practical applications that exploit their electrically (semi)conducting nature. Examples of materials that emerged from these efforts are discussed in detail in other portions of this volume (in particular the chapters by Bunz, Klemm, and Yamamoto). They include, among others, poly(heteroarylene ethynylenes) such... 

As already mentioned, CAAC ligands can stabilize electron-deficient metal centers such as in cationic gold complexes. Complex 29 catalyzes a very unique reaction of enamines with acetylene, which produces a cumulene and an imine... 

Polypyridyl ligands are able to stabilize electron-rich metal centers as has been shown for the d -systems Ru and Os. This suggests this class of ligands also to be appropriate for rhenium(I) compounds, and this will be discussed more in detail in Section 5.3.2.7.2, and rhenium(II) complexes which represent intermediates in the syntheses of the Re species or are accessed by oxidation of Re. ... 

By definition, a generalized anomeric effect is observed at carbon of an XCY system when a molecule preferentially adopts a conformation that optimizes a secondary, stabilizing electronic interaction involving overlap between the lone pair on one heteroatom with the a orbital of the bond between the central carbon atom and the second heteroatom . Figure 5a illustrates that in XNY systems, as with anomeric carbon centres, two anomeric interactions are possible and involve either an ny-CT x ° nx-o NY overlap where nx and ny represent the p-type lone pairs on X and Y and NX and NY represent the N—X and N—Y a orbitals. In either case, the result is a net stabilization of the lone pair of electrons (Figure 5b). Except where the nitrogen is symmetrically substituted, one of these interactions will be strongest. 

Two further mechanisms are known to trap electronic charge in thin films intermolecular and resonance stabilization. In resonance stabilization, electron attachment to a molecular center produces an anion in a vibrationally excited state that is then de-excited by energy exchange with neighboring molecules. When the initial anion ground state lies below the band edge or lowest conduction level of the dielectric, then the additional electron may become permanently trapped at the molecular site. In this case, a permanent anion is formed (e.g., the case of O2 [220]). Intermolecular stabilization refers... 

There are many excellent books and reviews on the structure and reactions of secondary radical ions generated in radiolytic and photolytic reactions. Common topics include the means and kinetics of radical ion production, techniques for matrix stabilization, electronic and atomic structure, ion-molecule reactions, structural rearrangements, etc. On the other hand, the studies of primary radical ions, viz. solvent radical ions, have not been reviewed in a systematic fashion. In this chapter, we attempt to close this gap. To this end, we will concentrate on a few better-characterized systems. (There have been many scattered pulse radiolysis studies of organic solvents most of these studies are inconclusive as to the nature of the primary species.)... 

A recent development in the synthesis of 3//-3-benzazepin-2-ones has been the photocyc-lization of A-(chloroacetyl)phenethylamines (Scheme 25). Ring closure is by homolysis of the alkyl halide followed by intramolecular coupling of the alkyl radical with an aromatic radical cation. Yields are good, especially with a stabilizing electron-donating group (MeO, NMe2) at the position meta to the ethylamino function (i.e. ortho or para to the site of cyclization). Isomeric benzazepinones are normally obtained (Scheme 25) with meta-substituted phenethylamines (80H(14)ll). 

As noted earlier, the parameter V = V + a contains two terms, one of which characterizes the capture of stabilized electrons et, and the second, that of thermalized electrons et , by the acceptor. Since the value of V = (4/3)7tRt = (nal/6) n3vet grows monotonously with time, the relative contribution of these terms depends on the moment of measuring the radiation yield. From the values of the parameters ve, and a found from the analysis of kinetic curves for etr decay in the presence of different acceptors in the water alkaline matrices (see Table 2) one can draw certain conclusions about the relationship between V" and a. In Table 5 the values of a found for a number of reactions of etr in water-alkaline glasses are compared with those of V calculated for different times,t, from the relationship... 

The resulting values of Rt characterizing the radius of capturing the stabilized electrons by acceptors via a tunneling mechanism are also summarized in Table 4. For the strongest acceptors Rt is seen to amount to several tens of angstroms. 

The Electron Excess Center. In their earlier paper Schulte-Frohlinde and Eiben (57) had assigned the line A to the O ion and the other line to the stabilized electron subsequently they have reversed this assignment, and are therefore in agreement with other authors. However, the line with g = 2.0006 has been interpreted in different ways, although all interpretations relate it to the radiation-produced electron. Thus Schulte-Frohlinde and Eiben (57, 58) consider the species responsible for this line to be a stabilized free electron, while Ershov et al. (16) and Henriksen (23) identify it with a solvated electron or a po-laron in the same sense as these two terms are used in the radiation chemistry of water and aqueous solutions. According to the above authors, this species is not found in pure ice because of Reaction 30, whereas in alkaline systems such a reaction should not occur. (Henriksen does not offer any explanation about the specific role of alkali hydroxide in stabilizing the solvated electron. ) Both of these hypotheses can be shown to be incorrect. Thus, if Reaction 30 occurred to any extent in pure ice, one should be able to detect H atoms in neutral ice with a yield of at least as high as the maximum yield of the solvated electrons, viz.. 

The increase in acidity by 25 orders of magnitude between sp3- and sp-hybridized carbon acids is similar to that found for the difference in acidity between an ammonium ion (sp3 hybridization) and a protonated nitrile (sp hybridization). It is clear that the hybridization of the orbital they occupy can play a major role in stabilizing electron pairs and thus influencing the effective electronegativity of an atom. 

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