Oxidation potentials HOMO energies

Fig. 6. New developed electrolyte salts containing F element to obtain a high stability for electrochemical oxidation, and HOMO energy and experimentally determined oxidation potential (vs Li/Li+) (reproduced with permission from J. Power Sources, 68 (1997) 307 [48], Denki Kagaku, 65 (1997) 909 [50], Battery Technof, 6 (1994) 45 [49], 10 (1998) 106 [51]).

Table 1 Stmcture, oxidation potential, and energies for highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) of carbonates (EC and EMC), ethers (EPE), fluorinated carbonates (F-AEC and F-EMC), and fluorinated ethers (F-EPE)...

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

Boberski and Allred reported that oxidation potentials of permethylpoly-silanes determined by a.c. polarography decrease with increasing chain length, and that the oxidation potentials are correlated almost linearly with the energies of the HOMO as determined by MO calculations (Table 7) [63]. 

Energy Levels for Hole Injection. For the hole conductor TPD (6), measurements are available from different groups that allow a direct comparison of different experimental setups. The ionization potential that corresponds to the HOMO level under the assumptions mentioned above was measured by photoelectron spectroscopy to be 5.34 eV [230]. Anderson et al. [231] identified the onset of the photoelectron spectrum with the ionization potential and the first peak with the HOMO energy, and reported separate values of 5.38 and 5.73 eV, respectively. The cyclovoltammetric data reveal a first oxidation wave at 0.34 V vs. Fc/Fc+ in acetonitrile [232], and 0.48 V vs. Ag/0.01 Ag+ in dichloro-methane [102], respectively. The oxidation proceeds by two successive one-electron oxidations, the second one being located at 0.47 V vs. Fc/Fc+. 

For the sake of simplicity, the search for additive effects has oriented most of the research, and ligand additivity has been clearly recognized in some cases for many years. Hence, for the series of closely related 18-electron octahedral complexes [Mn(CO)6 x(CNR) ,]+ (x = 1-6) that undergo a single-electron reversible oxidation, the oxidation potential was shown to correlate linearly with the HOMO energy, a... 

This model was proposed by Bursten [82] in 1982. It assumes a linear correlation (with a negative slope) between the HOMO energy and the oxidation potential of octahedral d metal complexes of the type [ML Lg ] (L = stronger jr-acceptor than L ), this potential (viz. HOMO energy) being determined in an additive way by the effects B) of all the L and (6 — n)L ligands and by the effects (C) of the ligands, xL and (4 — x)L, that r-interact with the metal d,r orbital comprised in the HOMO of the complex (Eq. 29 in which depends on the metal atom, in particular its oxidation state, and trivially on the solvent and reference electrode). 

Pronounced differences between radical cation structures and their parents must be expected for strained ring compounds. The HOMO or LUMO of these systems may be localized mainly in one bond, which may be weakened or actually break upon ionization. The oxidation potentials of strained ring compounds are lower than those of unstrained substrates because strain energy is released, resulting in noticeable changes in structure. 

Radical stabilities may be measured experimentally by the determination of homo-lytic bond dissociation energies (BDEs) in the gas phase [140] or in solution by relating them empirically to the and the oxidation potentials, E ox(A ) of weak acids,... 

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). 

Table 12 shows oxidation potentials ( 1/2) of a series of permethylpolysilanes determined by a.c. polarography or cyclic voltammetry50,51. The 1/2 values decrease as the chain length increases. This reflects the HOMO energy levels of these permethylpolysilanes52. 

Disilenes undergo irreversible anodic oxidation at much less positive potentials, as shown in Table 1356. The oxidation potentials for these compounds are similar, indicating that the HOMO of each species lies at approximately the same energy level. 

It is easier to oxidize an alkene electrochemically than to reduce it, because it is easier to reach powerfully oxidizing anode potentials12 (at which one can remove an electron from the HOMO of the alkene) without interference from the solvent or electrolyte than it is with reductions, where one can normally not achieve sufficiently negative cathode potentials to be able to add an electron to the LUMO. Even so, anodic oxidation of alkenes is relatively rarely observed almost always the alkene is part of a conjugated system or it bears an electron-supplying substituent, which raises the HOMO energy. 

---