Electron Affinities from Reduction Potentials

Once the predictions were in place, the next step was to measure the Ea. They were obtained by measuring the half-wave reduction potentials of the bases. These relative values were scaled to measured gas phase values for acridine and anthracene [11]. To validate the experimental procedure, the Ea for molecules with different solution energy differences and experimental gas phase electron affinities were determined. These values all agreed with values found in the literature within the experimental uncertainty of 0.03 V. Recall that the general classes of charge densities, q and —AAG, are [EL, 0,1.7] [F, 0.05, 1.8] [A, 0.2, 2.0] [B, 0.4, 2.2] [C, 0.6, 2.4] [D, 0.8, 2.6] [EH, 1, 2.7] and Ea = Emi - 2.20 - w(0.20) E1/2, where —2.20 — w(0.20) is — AAG (see Chapter 7). Thus, the — AAG for anthracene and acridine are different from those of the nitrocompounds by about 0.4 eV, the difference between groups A(n = —1) and C(n = 1). The localized charge densities in anions of A, G, C, U, and T are about the same as for the acridine anion so the 

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Electron Affinities from Reduction Potentials and CURES-EC... 

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

When two conjugate redox pairs are together in solution, electron transfer from the electron donor of one pair to the electron acceptor of the other may proceed spontaneously. The tendency for such a reaction depends on the relative affinity of the electron acceptor of each redox pair for electrons. The standard reduction potential, E°, a measure (in volts) of this affinity, can be determined in an experiment such as that described in Figure 13-14. Electrochemists have chosen as a standard of reference the half-reaction... 

A major objective of this book is to evaluate the reported values of molecular electron affinities and their errors and to assign them to specific states. Prior to 1970 the magnetron and ECD methods were used to measure the majority of gas phase molecular electron affinities. An extensive compilation of unevaluated experimental, empirical, and theoretical electron affinities of atoms, molecules, and radicals was published before 1990 [9]. The electron affinities measured in the gas phase are now available on the Internet but have not been evaluated [26]. The molecular Ea in this list is defined and evaluated in Appendix IV. Values that are significantly lower than the selected values will be assigned to excited states. Semi-empirical calculations and the CURES-EC technique support these assignments. Unpublished electron affinities and updated electron affinities from charge transfer complex data and half-wave reduction potentials are given in Appendix IV. 

Wang and Charles Han calculated the electron affinities of aldehydes and ketones by using the parameterized Huckel theory. Eight parameters were used to calculate the electron affinities of 16 compounds with a deviation of only 0.05 eV. However, some of the data were not published until the 1970s [35]. By measuring relative electron capture coefficients and scaling to the acetophenone data, more precise electron affinities could be obtained. This was further support for the validity of the ECD model. M. J. S. Dewar reproduced the experimental electron affinities of aromatic hydrocarbons using the MINDO/3 method and calculated Ea from reduction potentials [36]. 

The general least-squares procedures can now be implemented in spreadsheets programmed with macros. Adjustments once impossible are now trivial. The classification of molecules to obtain electron affinities from half-wave reduction potentials is an example of a linear least-squares adjustment. The determination of the adiabatic electron affinity for acetophenone is an example of a nonlinear two-parameter least-squares procedure. The nonlinear least-squares adjustment of ECD to the expanded kinetic model is one of the major advances of the 1990s. 

In 1966 the relative electron affinities of charge transfer complex acceptors were calculated from spectral data and half-wave reduction potentials. Unfortunately, at the time, no accurate electron affinities of typical n charge transfer complex acceptors existed so one could obtain absolute electron affinities from either half-wave reduction potentials or charge transfer complexes. Thus, the magnetron Ea of 1.40 eV for the electron affinity of benzoquinone was selected. This is now known to be about 0.5 eV too low, making all the values low. This emphasizes the difference between the determination of relative electron affinities that depend on the absolute electron affinity of a reference compound and absolute ones from experimental measurements and fundamental constants. 

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 data obtained from the ECD and reduction potentials can be used to interpret PES data. Three examples for molecules are the lower values for the electron affinities of nitromethane, anthracene, and coronene. Based on the observation of excited states in anthracene and tetracene in the ECD data, it is reasonable to assume that the lower value for coronene derives from the population of an excited state. In Figure 6.8 the PES of coronene is shown with two sets of peaks. If the ground-state Ea for coronene is taken from the initial onset, it is much lower than the value obtained from reduction potential or electronegativity data. In addition, the second onset must be explained [38—42]. 

TABLE 10.7 Electron Affinities (in eV) of Aromatic Hydrocarbons from Reduction Potentials [56]... 

The electron affinities of the chlorobenzene isomers have been determined by scaling half-wave reduction potentials [22], With higher gas phase values higher values are obtained from reduction potentials. These are compared to the ECD and CURES-EC values in Table 11.9. The CURES-EC-calculated values for the above compounds support experimental quantities and suggest that the Ea of all halogenated benzenes can be calculated. The CURES-EC values are listed in Table 11.10. The Ea... 

TABLE 11.11 Electron Affinities of Chloronaphthalenes from Reduction Potentials and CURES-EC [22 and this work]... 

The electron affinities of halogenated aromatic and aliphatic compounds and nitro compounds have been evaluated. Additional electron affinities for halogenated benzene, freons, heterocyclic compounds, dibenzofuran, and the chloro- and fluoroben-zenes are reported from ECD data. The first positive Ea for the fluorochloroethanes were obtained from published ECD data. The Ea of halogenated aromatic radicals have been estimated from NIMS data. The AEa of all the halobenzenes have been calculated using CURES-EC. The Ea of chlorinated biphenyls and chlorinated napthalenes obtained from reduction potentials have been revised based on variable solution energy differences. 

ELECTRON AFFINITIES OF BIOLOGICAL MOLECULES FROM REDUCTION POTENTIALS... 

Figure 6 Electron affinities and ionization potentials of the DNA base OH and H adduct radicals calculated by scaling the Koopmans EAs and Koopmans IPs to experiment. These scales represent estimated vertical EAs and IPs. Double headings on the IP scale correspond to species with equal ionization potentials. Radical with electron affinities above 1.4eV are predicted to undergo reduction by thiols. Reproduced with piermission from ref. [145].

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

As the cation becomes progressively more reluctant to be reduced than [53 ], covalent bond formation is observed instead of electron transfer. Further stabilization of the cation causes formation of an ionic bond, i.e. salt formation. Thus, the course of the reaction is controlled by the electron affinity of the carbocation. However, the change from single-electron transfer to salt formation is not straightforward. As has been discussed in previous sections, steric effects are another important factor in controlling the formation of hydrocarbon salts. The significant difference in the reduction potential at which a covalent bond is switched to an ionic one -around -0.8 V for tropylium ion series and —1.6 V in the case of l-aryl-2,3-dicyclopropylcyclopropenylium ion series - may be attributed to steric factors. 

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