Electron Affinities of Charge Transfer Complex Acceptors

5 ELECTRON AFFINITIES OF CHARGE TRANSFER COMPLEX ACCEPTORS 

TABLE 10.15 Calibration Data (in eV) for Charge Transfer Complex Acceptors 

TABLE 10.16 Electron Affinities (in eV) from Charge Transfer Complex Data 

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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. 

Figure 4.15 Electron affinities of charge transfer complex acceptors calculated from C2 = 2.9 versus the current best adiabatic electron affinities. This is a precision and accuracy plot. The zero intercept slope indicates that the same quantities are measured. The compounds are maleic anhydride, tetrachlorophthalic anhydride, benzoquinone, trinitro-flourenone, s-trinitrobenzene, chloranil, tetracyanoquinodimethane, and tetracyanoethylene in order of their electron affinities.

Chen ECM, Wentworth WE (1975) A comparison of experimental determinations of electron affinities of 7i-charge-transfer-complex acceptors. J Chem Phys 63 3183-3191... 

Generally, it is the interaction of a donor (D) and an acceptor (A) involving the transfer of one electron. The probability of one-electron transfer is determined by thermodynamics namely, by the positive difference between the acceptor electron affinity and donor IP. The electron transfer is accompanied by a change in the solvate surroundings—charged particles are formed, and the solvent molecules (the solvent is usually polar) create a sphere around the particles thereby promoting their formation. Elevated temperatures destroy the solvate shell and hinder the conversion. Besides, electron transfer is often preceded by the formation of charge-transfer complexes by the sequence D A D A (D +, A -) (D+, A ) D+ A . ... 

Both t-1 and c-1 form ground state charge-transfer complexes with strong electron acceptors (72-79). Equilibrium constants and absorption data for their complexes with several electron-poor alkenes are given in Table A. The absorption maxima of a family of charge-transfer complexes can be related to the donor ionization potential (IPp) and acceptor electron affinity (EA ) using eq. 16 (79). 

Electron transfer reactions and spectroscopic charge-transfer transitions have been extensively studied, and it has been shown that both processes can be described with a similar theoretical formalism. The activation energy of the thermal process and the transition energy of the optical process are each determined by two factors one due to the difference in electron affinity of the donor and acceptor sites, and the other arising from the fact that the electronically excited state is a nonequilibrium state with respect to atomic motion (P ranck Condon principle). Theories of electron transfer have been concerned with predicting the magnitude of the Franck-Condon barrier but, in the field of thermal electron transfer kinetics, direct comparisons between theory and experimental data have been possible only to a limited extent. One difficulty is that in kinetic studies it is generally difficult to separate the electron transfer process from the complex formation... 

According to the Mulliken theory of charge transfer complexes, the vertical electron affinity (VEa) of an acceptor and the vertical ionization potential (VIP) of a donor are related to the energy of maximum absorption of the complex (Ect) by the following equation ... 

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. 

The theory of charge transfer complexes relates the maximum in the absorption spectrum, the charge transfer energies Ect, and energies for complex formation AGct to the vertical ionization potential of the donor and the vertical electron affinities of the acceptor. The relationship uses constants related to the geometry of the complexes. Mulliken described the theory of charge transfer as follows ... 

Appendix II presents the structures of organic compounds. Structure 1 provides the number, names, and adiabatic electron affinities of the Bergman Dewar set. Structure 2 gives the adiabatic electron affinities, gas phase acidities, and names of the DNA and RNA bases. Structure 3 shows the charge transfer complex acceptors. Structure 4 gives the numbering system of naphthalene and biphenyl and compares the structures of acenaphthylene and biphenylene. 

Dozens of charge-transfer complexes of aliphatic peraminoethylenes with organic acceptor molecules of widely differing electron affinity... 

The electronic spectrum of the complex consists of a combination of the spectra of the parent compounds plus one or more higher wavelength transitions, responsible for the colour. Charge transfer is promoted by a low ionization energy of the donor and high electron affinity of the acceptor. A potential barrier to charge transfer of Va = Id — Ea is predicted. The width of the barrier is related to the intermolecular distance. Since the same colour develops in the crystal and in solution a single donor-acceptor pair should be adequate to model the interaction. A simple potential box with the shape... 

The most prospective donors are those with ionization potentials of ID < 6.6 eV. Acceptors with electron affinities of EA > 2.6 eV are suitable. When / EA < 4 eV, donor-acceptor interaction leads to strong molecular complexes with a charge-transfer degree >0.5. Donor-acceptor charge transfer often results in the formation of ion radical salts having metallic conductivity. In terms of charge-transfer degree, ion radical salts have values >0.7. 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

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