Electron affinity aromatic hydrocarbons, determination

Huckel theory was used to confirm the electron affinities of aromatic hydrocarbons determined with the ECD. Figure 6.18 shows the Ea for several aromatic hydrocarbons calculated from a linear correlation of the Huckel coefficients of the lowest unoccupied molecular orbital (LUMO) versus the EvV from gas phase... 

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 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]. 

In equation 3.1 the spin terms of the negative species have been canceled out. The quantity 12.43 is obtained from fundamental constants and the translation partition function of the electron. Qim is the ratio of the remaining partition function of the anion to that of the neutral. If the partition function ratio for the anion and neutral are assumed to be the same, this term is zero. With one value of the equilibrium constant the electron affinity of the molecule can be estimated. The statistical mechanical expression for Keq refers to the absolute zero of temperature so that no temperature correction to Ea is necessary. Unfortunately, there were no values for the equilibrium constants or electron affinities. Thus, the value of Keq for one molecule, anthracene, was determined and the electron affinity of other aromatic hydrocarbons referenced to that value. If the partition function ratios are equal,... 

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 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. 

When the ECD electron affinities were measured, the AAG for the aromatic hydrocarbons (2.0 eV) and the aromatic aldehydes and ketones (2.3 eV) were observed to be approximately constant within the class of molecules, but different from each other. The fiillerenes ( AAG = 1.76 eV) have a lower charge density and lower AAG than the majority of other compounds. With the determination of more gas phase electron affinities, the AAG values range from 1.7 eV for larger fullerenes to 2.7 eV for small anions [3, 10]. 

Table 4.3 lists AAG values, the ECD Ea, and the Ea from 1/2, of several aromatic hydrocarbons obtained in this manner. The electron affinity for pentacene was determined by TCT, while that of coronene is the value obtained from reduction potentials [6]. The Ea are verified using CURES-EC. The calculated values are given in Table 4.3. Also listed are the weighted average of the values that cluster about the current evaluated values from a 1983 compilation [27]. The consistency of the Ea values in this table support the gas phase experiment and the assignments of lower values to excited states.

The energy for the formation of a charge transfer complex is related to the VIP and VEa of the donor. To examine this relationship for the aromatic hydrocarbons with the measured electron affinities, the energies of complexes of methylbenzenes as the donors and aromatic hydrocarbons as the acceptors were determined. The charge transfer bands for these complexes were not observed [29]. Therefore, only the relation between the energy for complex formation and the VIP and VEa could be examined. The equation is... 

The electron affinities of the aromatic hydrocarbons have been calculated using Huckel theory and MINDO/3 procedures. The electron affinities of benzene, naphthalene, anthracene, and tetracene have been calculated by density functional and ab initio procedures [8]. The relationship between the experimental and calculated values is examined. The electron affinities of other organic compounds have been calculated using MNDO, density functional, and ab initio procedures [9]. A more thorough discussion of these experimental and theoretical methods can be found in Electron and Molecule Interactions and Their Applications, Volume 2, Chapter 6. The experimental and theoretical electron affinities of atoms, molecules, and radicals up to 1984 are listed but not evaluated [10]. The NIST site briefly discusses the various methods for determining electron affinities and gives an... 

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 numerous F, Cl, —CH3, —CF3, —OCHs, —C=N substituted aromatic hydrocarbons and aromatic aldehydes and ketones have been determined (4). For the most part the TEA occurred by Mechanism I. However, the chlorine derivatives can dissociate at higher temperatures according to Mechanism III. The EA can be evaluated from the temperature dependence in the intermediate temperature region. 

Various thermodynamic and kinetic problems concerning the chemistry and physics of radical anions and dianions were investigated quantitatively by electron photoejection. The approaches described allow the determination of the relative electron affinities of various aromatic hydrocarbons and the thermodynamics and kinetics of disproportionation of their radical anions into dianions. These approaches also allow the observation of unstable radical anions and their isomerization or dimerization. 

In 1956, Hoijtink et al. (I) reported relative electron affinities of some aromatic hydrocarbons. They determined these values by using a potentio-... 

Studies of gaseous electron capture taking place in a plasma were reported by Wentworth et al. (18). The equilibrium established in a plasma between e, A, and A " allowed the determination of the absolute electron affinities of the aromatic hydrocarbons in the gas phase. 

The polarographic reduction of aromatic hydrocarbons is a useful method for determining the electron affinities of conjugated hydrocarbons in solution. Investigations carried out in weakly proton-active solvents have shown that the reduction is governed by the following primary reactions (18-27) M+e fM and M +e45 M ... 

More recently, we have chosen the ionization potential of molecules as a representative parameter to be correlated to acid-base properties in several reactions. This parameter has already been used by Ai [54] to compare reactions, as well as by Richardson [55] who has correlated the degree of ionization of aromatics determined from ESR spectroscopy data with the electron affinity of cations in cationic zeolites. The ionization potential / of a C ( = number of carbons) hydrocarbon depends on n and on the type (linear or branched) of the isomer considered. The 1 potential decreases when n increases, because the greater the n, the higher is the HOMO orbital, the weaker is the ionization energy, and then the more reactive is the molecule. This is verified with a series of C2-C4 paraffins and olefins, respectively (Figure 10.5[a]). When oxygen is present in a molecule, / increases with the amount of O in the same series (saturated C-C or unsaturated C=C compounds). For example, / = 9.95 and 10.52 eV for propanal and propanoic acid, while / = 10.1 and 10.6 eV for acrolein and acrylic acid, respectively. 

In an attempt to determine the factors which control the rate of protonation, we excunined the correlation of these rates with some basic properties of the molecules, such as electron affinity, ionization potential, and singlet energy, which are known to be interrelated for alternant aromatic hydrocarbons. 

Serendipity played again some role in that development of our research activities. Shortly after publishing the results of our studies of methyl affinities of aromatic hydrocarbons I met Prof. S. I, Weissman who told me how his series of electron affinities of aromatic hydrocarbons parallels the methyl affinity series. He determined the relative electron affinities by studying electron transfers involving two aromatic hydrocarbons and their radical anions, e.g.,... 

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