Charge-transfer Coulomb repulsion

Finally, we must remember that just as a d-d spectrum is not properly described at the strong-field limit - that is, without recognition of interelectron repulsion and the Coulomb operator - neither is a full account of the energies or number of charge-transfer bands provided by the present discussion. Just as a configuration... 

The activation energy for the charge reduction reaction is due to two factors the bond stretching and distortions of the originally near linear complex, so as to achieve the internal proton transfer and the increase of energy due to the Coulombic repulsion between the two charged products, a repulsion that leads to a release of kinetic energy on their separation. 

The first question is whether the redox systems can be subjected to successive electron-transfer reactions in extended redox sequences. What one needs to know thereby are the number of charges that can be transferred and what is the Coulombic repulsion arising between the charged subunits. The experimental methods that have to be applied are obvious. Cyclic... 

In this context it is noteworthy to refer to the unsaturated analogue l,2-di(9-anthryl)ethene [32] (Weitzel and Mullen, 1990 Weitzel et al., 1990). Like [6] (Becker et al., 1991), compound [32] forms a stable dianion and tetra-anion upon reduction. In the cyclic voltammogram of [32], the first two electrons are transferred at nearly the same potential, pointing to an effective minimization of the Coulombic repulsion between the charged anthryl units (Bohnen et al, 1992). This situation, which again corresponds to that in [6], could imply a torsion about the central olefinic bond (Bock et al., 1989). 

Fig. 11.5. Diagram illustrating the components of an ESI source. A solution from a pump or the eluent from an HPLC is introduced through a narrow gage needle (approximately 150 pm i.d.). The voltage differential (4-5 kV) between the needle and the counter electrode causes the solution to form a fine spray of small charged droplets. At elevated flow rates (greater than a few pl/min up to 1 ml/min), the formation of droplets is assisted by a high velocity flow of N2 (pneumatically assisted ESI). Once formed, the droplets diminish in size due to evaporative processes and droplet fission resulting from coulombic repulsion (the so-called coulombic explosions ). The preformed ions in the droplets remain after complete evaporation of the solvent or are ejected from the droplet surface (ion evaporation) by the same forces of coulombic repulsion that cause droplet fission. The ions are transformed into the vacuum envelope of the instrument and to the mass analyzer(s) through the heated transfer tube, one or more skimmers and a series of lenses.

The selection of the solvent is based on the retention mechanism. The retention of analytes on stationary phase material is based on the physicochemical interactions. The molecular interactions in thin-layer chromatography have been extensively discussed, and are related to the solubility of solutes in the solvent. The solubility is explained as the sum of the London dispersion (van der Waals force for non-polar molecules), repulsion, Coulombic forces (compounds form a complex by ion-ion interaction, e.g. ionic crystals dissolve in solvents with a strong conductivity), dipole-dipole interactions, inductive effects, charge-transfer interactions, covalent bonding, hydrogen bonding, and ion-dipole interactions. The steric effect should be included in the above interactions in liquid chromatographic separation. 

The bonding of ions to metals is dominated by Coulomb attraction since there is a significant difference in electron affinity between the metals and ions. The bonding also involves a redistribution of charge through intermolecular charge transfer (between adsorbed ions and the surface) and intramolecular polarization (in ions and on the surface), which reduces the Pauli repulsion. 

Incorporation of an ionic component into a donor/acceptor molecule is a very effective way of suppressing electron back-transfer. One interesting example consists of the photo-oxidation of leuko crystal violet (LCV) to crystal violet (CV, the dye) by benzophe-none bearing a quaternary ammonium ion (Tazuke Kitamura 1984). In this case, the cation radical of LCV formed is repulsed by the ammonium positive charge. At the same time, the benzophenone anion radical remains stabilized by the attached cationic atmosphere (Scheme 5-16). As shown in the scheme, two favorable results are achieved the stabilization of an ion radical pair by counterion exchange and the charge separation by coulombic repulsion between the two positive charges. This leads to 100% efficiency of the photo-oxidation. With unsubstituted benzophenone itself, the efficiency does not exceed 20%. 

Ion radicals play a role as mediators in these two-electron transfers. Each one-electron step achieves a maximal rate, and both rate constants become close. Coulombic repulsion of positive (or negative) charges makes the double-charged ion formation difficult. Therefore, donors (or acceptors) are preferable for which some possibility exists to disperse the charge. Extension of the 77-system reduces intramolecular coulombic repulsion in the dianion state. Electron-donor (or electron-acceptor) substituents should be located at diametrically opposite sites of the molecule. Examples are ll,ll,12,12-tetracyano-9, 10-an-thraquinodimethane, TCNQ, DCNQI, and tetracyanobenzene. 

Transfer of Cl+ to the arene provides some relief of the Coulombic repulsion in the multiply charged, superelectrophilic system. Under the reaction conditions, it is not yet known to what extent the N-halosuccinimides are protonated in BF3-H2O, but this acid-catalyst has an estimated acidity around IIq —12. 

To summarize, we have proposed in this paper that the metallic or insulating nature of fullerides depends primarily on the parity of the number of electrons transferred to the C60 molecule. We attribute this to the influence of JTD. As they are more favorable for evenly charged C60, they tend to induce attractive correlations in odd-electron systems that promote the formation of pairs of electrons and help to overcome the strong Coulomb repulsion. This reasoning is based on the comparative behavior of systems with an odd or even number of electrons per C60. 

Secondary binding forces are mainly classified into Coulomb forces, hydro-gen-bonding forces, van der Waals forces, charge transfer forces, exchange repulsion and hydrophobic interactions (Table 1). Besides these forces, there are other interactions such as ion-dipole and solvophobic interactions. 

As mentioned at the beginning of Section III, the conducting properties of organic charge-transfer materials are critically dependent on (1) their stoichiometry (i.e., the number of donor to acceptor molecules), and (2) the degree of charge transfer p per acceptor molecule. In particular, as a result of inherently strong Coulomb repulsions, all 1 1 TCNQ salts with complete ionization, or p = 1, are insulators [45]. 

It has long been known that alkali metals Na, K, Rb, and Cs form with the acceptor TCNQ 1 1 segregated charge-transfer salts, with complete ionization p = 1. They have regular chains at high temperature but are insulators due to Coulomb repulsions as discussed in the preceding section. They are then primarily one-dimensional magnetic materials whose properties have been studied extensively in the past [69]. 

The shift of oscillator strength toward higher frequencies due to large on-site coulombic repulsion corresponds physically in the limit U 4t to charge-transfer transitions into states with doubly occupied sites. Using... 

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