Aromatic hydrocarbons, half-wave potentials

Thermodynamic reduction potentials of numerous aromatics were first measured by Hoijtink and van Schooten in 96% aqueous dioxane, using polarography [15, 16]. These fundamental works were decisive tests of the HMO theory, showing that the polarographic half-wave potentials vary linearly with the HMO energies of the lowest unoccupied molecular orbitals (LUMO) of the hydrocarbons [1]. Hoijtink etal. had already noticed that most aromatics can be further reduced to their respective dianions [17]. They proposed a... 

In Fig. 8.15, the values of hvCT are plotted against the half-wave potentials of the acceptors measured in acetonitrile. A near-linear relation is observed. Peover also obtained, by an electron capture method, the values of EA of aromatic hydrocarbons in the gas phase and confirmed a near-linear relation with the reduction potentials in the solution phase. 

It is certain that, in the first reduction step in aprotic solvents, an electron is accepted by the LUMO of the organic compound. However, it was fortunate that this conclusion was deduced from studies that either ignored the influence of solvation energies or used the results in different solvents. Recently, Shalev and Evans [55] estimated the values of AG V(Q/Q ) for 22 substituted nitrobenzenes and nine quinones from the half-wave potentials measured by cyclic voltammetry. For quinones and some substituted nitrobenzenes, the values of AG V(Q/Q ) in a given solvent were almost independent of the EA values. Similar results had been observed for other aromatic hydrocarbons in AN (Section 8.3.2) [56]. If AG V(Q/ Q ) does not vary with EA, there should be a linear relation of unit slope between El/2 and EA. Shalev and Evans [55], moreover, obtained a near-linear relation between AG V(Q/Q ) and EA for some other substituted nitrobenzenes. Here again, the Ey2-EA relation should be linear, although the slope deviates from unity.8)... 

The most common electrochemical effects exerted in bulk solution are related to association (solvation, ion-pairing, complex formation, etc.) with the electroactive substance or electrochemically generated intermediates [4,19]. The importance of solvation can be gauged by comparing calculated and measured values of the parameter AE1/2 (defined as the difference, in volts between the half-wave potentials of the first and second polarographic waves) exhibited by polycyclic aromatic hydrocarbons (PAH) in dipolar aprotic solvents [46,47], It can be shown that AE1/2 is related to the equilibrium constant for disproportionation of the aromatic radical anion into neutral species and dianion, that is,... 

The half-wave potentials (corrected for changes in liquid-junction potential) for the one-electron reduction of aromatic hydrocarbons generally become more positive (the reduction is easier) as the dielectric constant of the solvent increases.44 This is in accord with the direction of the variation in solvation energy of the radical anions that is predicted by the simple Bom theory... 

Indeed the polarographic half-wave potential is a direct measure of the ease with which the electron transfer is effected, provided that it is the potential of the reversible, one-electron process that is measured. Available data for the half-wave potentials for the oxidation of series of aromatic hydrocarbons is extensive and series of the early 1/2 values were shown to... 

FIGURE 4.9. Plot of half-wave oxidation potentials vs. ionization potentials for aromatic hydrocarbons. The electrochemical potentials are from Ref. 5 and the ionization potentials from Ref 6. 

Polarographic half-wave potentials for oxidation and reduction of aromatic hydrocarbons are given in Table P8-14. 

The rates of reduction of the aromatic compounds (Table I), appear to increase as AEg g and I decrease, in contrast with the behavior of k2. This trend is to be expected since k5 reflects the electron affinity which decreases when I and AEg g increase. The values of k5 also can be related to the polarographic half-wave potentials, Ei, of the aromatic hydrocarbons and the general trend is, of course, a decrease in k5 when Ei is more negative. 

TABLE I. Half-wave potential and quantum mechanical data for aromatic hydrocarbons ... 

Fig. 6. Correlation of half-wave potentials of aromatic hydrocarbons with the coefficient of the l.e.m.o. (taken from references 21 and 22) ...

Constit. of the sponge/tunicate composite, Batzella sp./Lissoclinum sp. High-boiling polar aprotic solv. Used for extraction of aromatics from hydrocarbon mixts. Used as solvent for measuring polarographic half-wave potentials for alkali metals and Ba. Bitter... 

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]. 

Two important methods for verifying the relative values of the electron affinities obtained from the ECD method were introduced in an article cautiously entitled, Potential Method for the Determination of Electron Affinities of Molecules Application to Some Aromatic Hydrocarbons, with comparisons to half-wave reduction potentials and SCF calculations [18, 21]. The relative ECD values agreed with the half-wave reduction potential order from two independent sets of measurements. From this correlation the relative values had an error of 10 to 15%, or for a value of 0.6 eV an absolute error of 0.1 eV, because the electron affinity is logarithmically related to the K value. The agreement was within the experimental and calculation error. It was suggested that electronic absorption spectra, ionization potentials (through the constant electronegativity concept), and... 

The theoretical calculation of the electron affinities of aromatic hydrocarbons was advanced by the development of the MINDO/3, MNDO, AMI, and PM3 semi-empirical techniques. These procedures gave the adiabatic electron affinities of molecules obtained from the ECD and from half-wave reduction potentials that agreed with the experimental values to within the experimental error. A different semi-empirical procedure yielded consistently lower values than the experimental values partially because they were adjusted to the lower values [67-69]. 

The electron affinities of many of the molecules determined in the ECD or NIMS have been verified by half-wave reduction potentials and charge transfer complex data. These methods were developed in the 1960s but have been significantly improved. The relationship between the electronegativity and the electron affinities and ionization potentials for aromatic hydrocarbons can be used to support the Ea. The use of the ECD model and these techniques to estimate the electron affinities of aromatic hydrocarbons are illustrated for selected compounds. We will also describe the use of charge transfer complex data to obtain the electron affinities of acceptors. 

This method was first applied to relative electron affinities of substituted nitro-benzenes. All but one of these has been measured by HPMS TCT studies. However, the Ea of s-butyl nitrobenzene has only been determined by collisional ionization and is still listed in the NIST tables as 2.17(20) eV. This value is referenced to a high value for nitrobenzene and should be about 1 eV lower [60]. The electron affinities of aromatic hydrocarbons have been reported using the collisional ionization method. The value for biphenylene is larger than that obtained from half-wave reduction potentials. The values for pyrene, anthracene, and c-CgHg are consistent with other reported values, but the values for benzanthracene, coronene, and benzo[ghi]perylene are significantly lower than the largest precise value and are attributed to excited states. 

Fewer than 300 Ea for organic molecules have been determined in the gas phase. The majority of the Ea have been determined by the ECD and/or TCT methods. The direct capture magnetron, AMB, photon, and collisional ionization methods have produced fewer than 40 values. Only the Ea of p-benzoquinone, nitrobenzene, nitromethane, azulene, tetracene, and perylene have been determined by three or more methods. Excited-state Ea have been obtained by each of these methods. Half-wave reduction potentials have determined the electron affinities of 50 aromatic hydrocarbons. The electron affinities of another 50 organic compounds have been determined from half-wave reduction potentials and the energies of charge transfer complexes. It is a manageable task to evaluate these 300 to 400 Ea. 

The electron affinities of organic halides and environmental pollutants are evaluated in Chapter 11 and those of biological molecules in Chapter 12. Many of the Ea measured in the gas phase are tabulated in the NIST tables [1], These are listed in the appendices according to molecules containing CHX, CHNX, CHOX, and CHONX with references. The ECD values for some of the aromatic hydrocarbons in NIST have not been updated. L. G. Christophorou s compilation includes Ea from half-wave reduction potentials and charge transfer complexes [2]. Some of these Ea will be revised and evaluated in this chapter based on gas phase measurements. 

This book is based on the reactions of thermal electrons with molecules. The ECD, negative-ion chemical ionization (NICI) mass spectrometry, and polaro-graphic reduction in aprotic solvents methods are used to determine the kinetic and thermodynamic parameters of these reactions. The chromatograph gives a small pure sample of the molecule. The temperature dependence of the response of the ECD and NIMS is measured to determine fundamental properties. The ECD measurements are verified and extended by correlations with half-wave reduction potentials in aprotic solvents, absorption spectra of aromatic hydrocarbons and donor acceptor complexes, electronegativities, and simple molecular orbital theory. 

Compounds with reducible functional groups predominate. Polyaromatic hydrocarbons, aromatic hydroxy compounds and amines, as well as amides and various nitrogen heterocyclic compounds can be determined anodically. These processes are in many cases pH dependent. Determinations based on redox processes are therefore carried out in buffered solutions. The composition and concentration of the buffer systems generally has no influence on the position of the half-wave or peak potentials. If its concentration is sufficiently high, the buffer simultaneously performs the function of the supporting electrolyte. 

Ruoff RS, Kadish KM, Boulas P, Chen ECM (1995) Relationship between the electron affinities and half-wave reduction potentials of fullerenes, aromatic hydrocarbons, and metal complexes. J Phys Chem 99 8843-8850... 

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