Electrochemical reduction LUMOs

Paradoxically, although they are electron-rich, S-N compounds are good electron acceptors because the lowest unoccupied molecular orbitals (LUMOs) are low-lying relative to those in the analogous carbon systems. For example, the ten r-electron [SsNs] anion undergoes a two-electron electrochemical reduction to form the trianion [SsNs] whereas benzene, the aromatic hydrocarbon analogue of [SsNs], forms the monoanion radical [CeHg] upon reduction. ... 

The formation of anion radicals by chemical or electrochemical reduction may provide information about electron delocalisation in an inorganic ring system through the determination of the EPR spectrum in conjunction with molecular orbital calculations. In practice, however, the LUMO of the homo- or heterocycle is often an antibonding orbital. Consequently, occupation of the LUMO... 

After electrochemical reduction electron is placed on the lowest unoccupied molecular orbital (LUMO) of the acceptor subunits of A-D molecule. In the electrochemical oxidation, an electron is correspondingly removed from the highest occupied molecular orbital (HOMO) of the donor moiety. In the diffusion-controlled reaction electrochemically generated ions A -D and A-D+ form an activated complex A-D + A -D for which the following reaction pathways are possible ... 

Fig. 3 HOMO / LUMO scheme for a modified molecular shift register in which bits are encoded and propagate as free electrons instead of the system based on hole transfer described in Figure 2. Data is encoded by electrochemical reduction of the donor moiety adjacent to the cathode, resulting in the electronic configuration shown in (a). Subsequent stepwise thermal electron transfer...

It is believed that the electrochemical reductive of aliphatic halides [58], benzyl halides and aryldialkylsulfonium salts [89] are concerted, i.e., electron acceptance is concomitant with bond cleavage, due in part to the a nature of the LUMO as well as the instability of the anion-radical species and stability of the products. If the anion-radical is not a discrete chemical entity back ET cannot take place. Therefore, the efficiency of PET bond cleavage reactions would be expected to be greater for the reasons mentioned above. However, due to the localized nature of the a molecular orbitals the probability for intermolecular and intramolecular ET, for example, to a a MO may be quite low. However, the overall efficiency of PET concerted bond cleavage reactions may approach unity provided that ET to the This topic clearly requires further consideration and research using fast kinetic laser spectrophotometric techniques to go beyond the qualitative discussion provided here. 

The LUMOs of conjugated pi systems are typically much lower in energy than that of ethene, so that the reduction of these systems to the corresponding anion radicals is more facile. The electrochemical reduction of 1,3-butadiene in liquid ammonia solution at —78 °C, in fact, yields an anion radical which is stable enough to permit observation by ESR spectroscopy (Scheme 66) [109]. 

The essential parameters which determine the electrochemical process are the electron affinity of the neutral compound, which correlates with the energy of the LUMO, the energies of interaction with the solvent and counterions, the electron-electron repulsion energies and stereochemical factors. A precondition for an electrochemical study is that the chemical reaction which may occur, e.g. with the solvent, is much slower than the electron transfer process, and that the electrochemical reaction is reversible 66). Correlation of half-wave potentials with the energies of Huckel LUMO s has been one of the early successes of the Huckel model 8>2°.67-88>. The power of the electrochemical method in the study of polycyclic anions has been demonstrated recently 69a). Studies on reactions occurring during electrochemical reductions report reductive alkylations of polycyclic systems and their mechanism 70,69b). 

An electron is a tiny and powerful nucleophile that is easily accepted into a CF3 -bearing tt-system because of the low-lying LUMO. Either electrochemical reduction [9 ] or magnesium metal reduction [10] is useful for the C—F bond activation via the formal supply of one... 

Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals." Benzene and naphthalene are examples. The EPR spectrum of the benzene radical anion was shown in Fig. 12.2a (p. 657). These reductions must be carried out in aprotic solvents, and ethers are usually used. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representitive compounds are given in Table 12.1. The potentials correlate with the energy of the LUMO as calculated by simple Huckel MO theory." A correlation that includes a more extensive series of compounds can be observed with the use of somewhat more sophisticated molecular orbital methods." ... 

The electrochemical reduction of benzophenone (xLvi) was studied in five different ionic liquids [43], In dry (<20 ppm H2O) and aprotic ILs, such as [C4mpyr][N(Tf)2] and [C4mpip][N(Tf)2], two reversible and well-resolved one-electron processes were observed (Fig. 15.4). These processes are related to the addition of an electron to the LUMO of the carbonyl moiety present in xLvi to form a radical anion (Eq. 15.45), in which the new charge is delocalized over the jt-system conferring stability to this redox process. The second reversible one-electron process, observed at 0.664 V more cathodic with respect to the parent reduction, is related to the formation of a dianion reduction product (Eq. 15.46). The magnitude of this second process was smaller than the first one. This was attributed to the presence of a following comproportionation reaction (Eq. 15.47) [43]. 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

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]. 

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