Reduction potentials LUMO energies

The electrochemical features of the next higher fullerene, namely, [70]fullerene, resemble the prediction of a doubly degenerate LUMO and a LUMO + 1 which are separated by a small energy gap. Specifically, six reversible one-electron reduction steps are noticed with, however, a larger splitting between the fourth and fifth reduction waves. It is important to note that the first reduction potential is less negative than that of [60]fullerene [31]. 

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). 

Hexacyano[3]radialene (50) is a very powerful electron acceptor according to both experiment23,24 35 and MNDO calculations of LUMO energy and adiabatic electron affinity25. The easy reduction to the stable species 50" and 502- by KBr and Nal, respectively, has already been mentioned. Similarly, the hexaester 51 is reduced to 512-by Lil24. Most [3]radialenes with two or three quinoid substituents are reduced in two subsequent, well-separated, reversible one-electron steps. As an exception, an apparent two-electron reduction occurs for 4620. The reduction potentials of some [3]radialenes of this type, as determined by cyclic voltammetry, are collected in Table 1. Due to the occurrence of the first reduction step at relatively high potential, all these radialenes... 

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... 

V negatively to the first reduction, provided that the supporting electrolytes used were tetraalkylammonium salts. Therefore, these reduction potentials were also correlated with the LUMO energies of the HMO model [3]. It was suggested that the energy difference of 0.55 eV corresponds to the repulsion energy between both electrons in the LUMOs of the dianions [1], despite the differences in their structures. 

Chemical Reviews, 2004, Vol. 104, No. 10 Table 10. LUMO Energy Level and Reduction Potentials of Solvents and Additives... 

The most powerful reducing agent is the solvated electron with a reduction potential of-3.05 V vs. see [60], The LUMO of ethene is too high in energy to permit electron attachment to this molecule but introduction of an electron withdrawing substituent such as carbonyl or nitrile lowers the energy of the LUMO sufficiently that the rate of electron attachment becomes close to diffusion control [61]. Benzene reacts with solvated electrons more slowly than the diffusion controlled limit... 

The potential difference between the first reduction and the first oxidation is a good measure of the HOMO-LUMO energy difference (the HOMO-LUMO gap) in solution. For Geo and G70, these gaps are 2.32 and 2.22 V, respectively, at room temperature in TGE [36]. 

Finally, cycloadducts (22a-c) exhibit an addend-based first reduction wave, which is very close to the first reduction potential of pristine Ceo- Remarkably, as shown in Table 14, the choice of substituent on the p-benzoquinone moiety affects the redox behavior of these derivatives. By controlling the relative energy of the LUMO of the organic addend with respect to the LUMO of Ceo, the first reduction process may be forced to occur at either the C60 cage or the organic addend, allowing for tunability of these electron-acceptor... 

Redox potential data frequently correlate with parameters obtained by other spectroscopic measurements. The correlation of E° potentials with gas-phase ionization potentials has already been briefly discussed. Electronic transitions observed by UV-visible spectroscopy involve the promotion of an electron from one orbital to another and this can be viewed as an intramolecular redox reaction. If the promotion involves the displacement of an electron from the HOMO to the LUMO, then the redox potentials for the reduction of the compound, °REd, and for its oxidation, °ox, are of importance. For a closely related series of compounds, trends in oxidation and reduction potentials can be related to shifts in the absorption frequency, v. If the structural perturbation causes the HOMO and the LUMO to rise or fall in energy in tandem, then (E°RED — E°ox) will remain constant in such cases the HOMO—LUMO frequency (energy) will be essentially independent of the structural perturbation. Where there is a differential influence of the perturbation on the HOMO and the LUMO, then ( °red E°ox) will vary as will the energy of the electronic transition. In such cases a linear correlation of °red or E°0x may result. In the limit the energy of the HOMO, or more usually the LUMO, will be unaffected by structural perturbation where the acceptor orbital is pinned, direct linear correlation of E°Gx with v should be apparent. With E°ox and v in a common energy unit, the plot E°0x versus v should have a slope close to one.33-36... 

For main group metallophthalocyanines, the ring centred redox is the only process to occur. The separation between the first oxidation and reduction potentials corresponds to the energy difference of the HOMO and LUMO, hence to the Q(0,0) absorption band at 670 nm, and is about 1.56 V. Deviation from the mean value becomes large when the size of the metal significantly exceeds the cavity of the Pc ring. The first reduction and oxidation potentials themselves (E° vs. NHE) depend on the polarizing power of the metal ion (Zejy) and are approximated by equation (32). 

The electronics and structure of the acceptors, especially a, 3-unsaturated ketones, is also a determinant in 1,2- vs. 1,4-addition processes. In general, substitution of aryl or large groups at the carbonyl unit increases the preference for 1,4-addition,16ab while a, 3-unsaturated aldehydes afford exclusive 1,2-addition and [3,(3-disubstitution suppresses 1,4-addition,l6c presumably due to steric hinderance. House and Seyden-Penne have established good correlations between chemoselectivity and either the half-wave electrolytic reduction potentials,17 or the energy levels of the LUMO of various a, 3-unsaturated ketones.17b... 

Cyclic voltammetry studies showed the ionization potential and electron affinity of each component of the molecule in solution. The HOMO and LUMO energy levels were estimated from the equations Ehomo = E x + 4.4 eV and Elumo = T 14+4.4 eV, where E(r]x and E%, were oxidation and reduction potentials with respect to the standard hydrogen electrode (SHE) and the value of 4.4 is the ionization potential for hydrogen in eV [94,95], The HOMO and LUMO energy levels of the methine dye (compound 6) (Scheme 13) were determined to be -5.82 and -3.48 eV, respectively, with respect to the vacuum level from... 

Figure 3. Regression line of the solution reduction potential versus the hmo-lumo energy (eLUM0, / negative) for a series of benzenoid hydrocarbons (Streitwieser Schwager 1962 Streitwieser 1962). The half-wave potentials for azulene (1), acepleiadylene (2), pyracyclene (3), and C60 are also shown (see text).

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