Electron affinity relationship

The electron affinity relationship (Table 12.25) does not uniquely apply to radiation sensitization but also, for example, to toxicity (Olive 1980), and a warning was expressed against drawing a firm conclusion from this relationship as to the cause of the effect (Wardman and Clarke 1985). 

In fact, with a few exceptions, neither 02 nor nitro compounds react with free radicals by ET (as would be required for an electron affinity relationship in its proper sense) but rather undergo addition reactions. The ensuing radicals show different decay routes as is discussed in Chapter 6.3. When DNA is irradiated in deoxygenated aqueous solution in the presence of 14C-labeled mitronidazole, some of the sensitizer remains attached to DNA (Willson et al. 1974). Misonidazole and practically all other nitroimidazoles have a lower reduction potential than 02, and much higher concentrations have to administered to achieve the same effect (Whillans and Hunt 1982). 

Table 12.25. Some electron affinity relationships with nitro compounds compiled by Durand and Olive (1981) ...

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophilicity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, of dianhydrides (12). These were independendy deterrnined by polarography. Stmctures and electron affinities of various dianhydrides are shown in Table 1. 

Several facts are apparent when the data shown in Table 1.1 are considered. In order to see some of the specific results more clearly, Figure 1.10 has been prepared to show how the electron affinity varies with position in the periodic table (and therefore orbital population). From studying Figure 1.10 and the data shown in Table 1.1, the following relationships emerge ... 

When parameters of the Pariser-Parr-Pople configuration interaction molecular orbital (PPP-CI MO) method were modified so as to reproduce the Aol)s values for l,3-di(5-aryl-l,3,4-oxadiazol-2-yl)benzenes 16 and 17, the calculated HOMO and LUMO energy levels corresponded with the experimental ionization potential and electron affinity values. The relationships between the electrical properties and molecular structures for the dyes were investigated. The absorption maximum wavelengths for amorphous films were found to be nearly equal to those for solution samples <1997PCA2350>. 

Quantitative structure-physical property relationships (QSPR). There are two types of physical properties we must consider ground state properties and properties which depend on the difference in energy between the ground state and an excited state. Examples of the former are bond lengths, bond angles and dipole moments. The latter include infrared, ultraviolet, nuclear magnetic resonance and other types of spectra, ionization potentials and electron affinities. 

Periodic relationships including, for example, atomic radii, ionization energies, electron affinities, oxidation states... 

In the gas phase, homolytic bond dissociation enthalpies (D//) relate the thermochemical properties of molecules to those of radicals while ionization potentials (IP) and electron affinities (EA) tie the thermochemistry of neutral species to those of their corresponding ions. For example, Scheme 2.1 represents the relationships between RsSiH and its related radicals, ions, and radical ions. This representation does not define thermodynamic cycles (the H fragment is not explicitly considered) but it is rather a thermochemical mnemonic that affords a simple way of establishing the experimental data required to obtain a chosen thermochemical property. 

Figure 3.6 shows the various relationships between the energy levels of solids and liquids. In electrolytes three energy levels exist, Ep, redox, Eox and Ered- The energy levels of a redox couple in an electrolyte is controlled by the ionization energy of the reduced species Ered, and the electron affinity of the oxidized species Eox in solution in their most probable state of solvation due to varying interaction with the surrounding electrolyte, a considerable... 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

The transition states of free radical reactions generally show evidence of polar character wherein electron transfer to or from the radical has occurred (20). Thus, the electron affinity or ionization potential of the radical involved should affect the reaction. The much higher electron affinity (16) of ROo than CH3 radicals no doubt alters the transition state so that the reactivities toward it show less selectivity. The results of Szwarc and Binks (22) center around the fact that only carbon radicals were used for the correlation, and thus the electron affinity does not vary sufficiently to show in the correlation any deviation from the expected reactivity-selectivity relationship. 

We reported the relationships between the NMR chemical shifts and the rate constants of acylation (k) as well as such electronic-property-related parameters as ionization potential (IP), electronic affinity (EA), and molecular orbital energy for a series of aromatic diamines and aromatic dianhydrides. " The usefulness of... 

Relation between the LUMO and the half-wave potential of the first reduction wave When an organic compound, Q, is reduced, it accepts an electron from the electrode to its lowest unoccupied molecular orbital (LUMO). Here, the energy of the LUMO of Q corresponds to its electron affinity (EA). If the energies of LUMO ( lu) for a series of analogous compounds are obtained by the molecular orbital method, there should be a relationship ... 

Fig. 8.21 Relationship between the oxidation potential (E0x)> the reduction potential (ERed), the ionization potential (IP), the electron affinity (EA), and the solvation energies (AG°V+,...

A compound that is transparent within a spectral domain when in its isolated state can sometimes absorb when in the presence of a species with which it can interact through a donor-acceptor relationship (D-A). This phenomenon is related to the passage of an electron from a bonding orbital of the donor (which becomes a radical cation) to an unoccupied orbital of the acceptor (which becomes a radical anion), which has a close energy level (Fig. 11.6). The position of the absorption band in the spectrum is a function of the ionisation potential of the donor and the electron affinity of the acceptor. The value of e for these transitions is usually large. 

This second approach has the added benefit or calling attention to the very dose relationship between electron affinity and ionization potential. In fact, when the ionization energies and electron affinities of atoms are plotted, a smooth curve results and the function may be described rather accurately by the quadratic formula 30... 

The relationship between the Mulliken definition and that of Jaffe can be shown quite amply. Taking Eq. 5.62 and substituting q +1, we know that the energy, E, of the system will be that of the +1 cation, or the first ionization energy. Likewise for <7=1. the energy will be the negative29 of the electron affinity, so ... 

From what you know of the relationship between ionization energies, electron affinities, and electronegativities, would you expect the addition of some d character to a hybrid to raise or lower the electronegativity for example, will sulfur be more electronegative when hybridized spi or jp [Pg.649]

Figure 1. Energy level diagram for a photoelectrochemical cell illustrating the relationship between the electron affinity (EA) and the flatband potential (V/t). Energy levels are shown for zero external bias (Ec = 4.75 eV).

What is the relationship between the electron affinity of a monocation such as Na+ and the ionization energy of the neutral atom ... 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

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