Aromatic hydrocarbons electron affinities

Table 1. Values of electron affinity for few aromatic hydrocarbons andfor graphite. 

Rathore et al. (2006) studied the intramolecular single-electron transfer in anion-radicals formed from fluorenylidene derivatives. The derivatives used for the reduction were Me— Flu—CH2—Flu—CH2—Flu—CH2—Flu—Me and its deuterated analog, Me—Flu—CH2—(Flu-d8)— CH2—(Flu-dj)—CH2—Flu—Me. Each parent compound initially gave an anion-radical in which an unpaired electron was tunneled between the two internal Flu nuclei and then occured within the outer Flu nuclei. In the outer part, coordinative solvation of the anion-radical by HMPA proceeded much more effectively because of ready space accessibility. Such a solvation provides a driving force for electron tunneling. As the solution electron affinities of perdeuterated aromatic hydrocarbons are less than those of perprotiated hydrocarbons, the electron tunneling was found to be at least an order of magnitude faster only in the case of [Me—Flu—CH,—(Flu-do)—CH,— (Flu-d8)-CH2-Flu-Me]-. ... 

For halogenated aromatic hydrocarbons like polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs), and polychlorinated dibenzo-p-dioxins (PCDDs) the binding to the aryl hydrocarbon (Ah) receptor regulates their toxicity [89]. The Ah receptor controls the induction of one of the cytochrome P450 enzymes in the liver. Toxic responses such as thymic atrophy, iveight loss, immu-notoxicity and acute lethality are associated ivith the relative affinity of PCBs, PCDFs and PCDDs for the Ah receptor [89]. The quantitative structure-activity relationship (QSAR) models predicting the affinity of the halogenated aromatic hydrocarbons ivith the Ah receptor describe the electron acceptor capability as well as the hydrophobicity and polarizability of the chemicals [89[. 

Reactions (30) and (31) may give the same products. In (31) the polarization energy decreases the energy demand for temporal charge separation and it can be exothermic when B has a considerable electron affinity. For aromatic hydrocarbon quenchers (e.g., anthracene) such mechanism leads to dissipation of the excitation energy on the vibrational levels. When the quencher molecules contain Cl or Br atom in the intermediate step, Cl or Br elimination is expected, e.g., with benzyl chloride additive ... 

V. D. Parker [56] obtained in acetonitrile the oxidation and reduction potentials (EQx and ERea) of alternant aromatic hydrocarbons (AAH) by cyclic voltammetry and examined how those potentials are related to the ionization potential (IP) and the electron affinity (EA) of the compounds (Table 8.8). As expected, he found linear relations of unit slopes between E0x and IP and between ERed and EA. Moreover, he found that E0x and ERed of each AAH was symmetrical with respect to a common potential MAAH (-0.31 V vs SCE). The values of (E0x-MAAH) and (ERed Maa ) are correlated with the values of IP and EA, obtained in the vacuum, by E0x-Maah = IP- +AGsV+ and ERed-MAAII = liA-r/t-AG, respectively (Fig. 8.21). Here, is the work function of graphite and equal to 4.34 eV, and AGj v+ and AG v are the differences in solvation energies for the 0/+1 and 0/-1 couples of AAH. Experimentally, AG°V+ and AG°V were almost equal, not depending on the species of AAH, and were equal to -1.94 eV in AN. 

Tab. 8.8 Oxidation potentials, reduction potentials, ionization potentials and electron affinities of alternant aromatic hydrocarbons (AAHs) ...

In order to understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation-radicals and anion-radicals. One-electron addition to a molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to molecule s electron affinity. For instance, one-electron reduction of aromatic hydrocarbons can result in the energy revenue from 10 to 100 kJ mol-1 (Baizer Lund 1983). If a molecule detaches one electron, energy absorption mostly takes place. The needed amount of energy consumed is determined by molecule s ionization potential. In particular, ionization potentials of aromatic hydrocarbons vary from 700 to 1,000 kJ-mol 1 (Baizer Lund 1983). 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

Wilkinson and Schroeder (1979) have shown that the triplet states of aromatic hydrocarbons are quenched by quinones, the efficiency of quenching being related to the electron affinity of the quinone and the ionisation potential of the triplet hydrocarbon (Schroeder and Wilkinson, 1979). It was concluded that the quenching did not involve full electron transfer in nonpolar solvents. Photolysis experiments have shown that in propionitrile tetrachloro-benzo-l,4-quinone reacts with naphthalene to give radical ions (Gschwind and Haselbach, 1979). The naphthalene radical cation reacts with naphthalene to give a detectable intermediate. 

Either fusion with alkali metals or reaction with aUcali-metal complexes with aromatic hydrocarbons will break down most fluorocarbon systems, due to the high electron affinities of these systems. Such reactions form the basis of some methods of elemental analysis [13], the fluorine being estimated as hydrogen fluoride after ion exchange. Surface defluorination of PTFE occurs with alkali metals and using other techniques [14]. Per-fluorocycloalkanes give aromatic compounds by passage over hot iron and this provides a potential route to a variety of perfluoroaromatic systems (Chapter 9, Section IB). 

Evans et (d. (1962) and Evans and Evans (1963) have studied electron-transfer from aromatic radical-anions to both tetraphenylethylene and 1,1,3,3-tetraphenylbut-l-ene. When the electron affinity of the hydrocarbon corresponding to the donor is large (e.g. anthracene), transfer is not observed when it is small (e.g. naphthalene), transfer is rapid and in two intermediate cases (picene and chrysene) donation is rapid to tetraphenylethylene and slow to 1,1,3,3-tetraphenylbut-l-ene. 

As seen in Fig. 10, in aromatic hydrocarbons the Jt-electron-rich regions can interact with cations through so-called cation-7t interactions [40], In mixtures with ionic liquids, the aromatic molecules may be solvated in the nonpolar domains but they may have sufficient affinity for the cationic head-groups to be found among the polar domain as well. When looking at the structure of such mixtures, it is useful to study the local environments of the ions and of the aromatic compound. 

The reactions of organic molecules in solution are related to gas phase electron affinities and electronegativities. Anions are often intermediates in such reactions. The electron conduction of polymers is related to the electron affinities of the components. The theoretical calculations of electron affinities of aromatic hydrocarbons and the effect of substitution on electrons affinities and gas phase acidities are important to organic chemistry. Pseudo-two-dimensional Morse potentials have been used to represent the dissociation of organic molecules and their anions [18]. 

It is generally assumed that closed-shell molecules do not interact strongly with each other. However, as early as 1909, it was observed that new intense absorption bands were observed when I2 was dissolved in an aromatic hydrocarbon. By the mid-twentieth century the concept of nonbonded charge transfer complexes was postulated to explain the intense new absorption spectrum that arose when a closed-shell donor was added to a closed-shell acceptor. The theory of such complexes was formulated in terms of the electron affinity of the acceptor and the ionization potential of the donor and led to the development of new techniques for the determination of properties of such complexes. 

In the area of electron affinities of organic molecules, other electrochemical measurements were made and compared with half-wave reduction potentials. Quantum mechanical calculations for aromatic hydrocarbons were carried out using self-consistent field calculations. Many advances were made in the determination of the acidity of organic molecules. The effect of substitution and replacement on electron affinities and bond dissociation energies was recognized. This work is summarized in Chapters 10 and 12. A. S. Streitweiser provides an excellent review of the role of anions in organic chemistry up to 1960 [12]. 

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