Homo level

In light of oxidative processes, the high degree of resonance stabilization that arises from the maximally occupied HOMO (10 electrons), makes it an extremely difficult task to remove an electron from the HOMO level [31], Thus, [60]fullerene can be considered mostly an electronegative entity which is much more easily reduced than oxidized. 

A < 640 nm (or 1.9 < E < 2.5 eV), weak absorption takes plaee, and is associated with electric dipole-forbidden transitions between the one-electron HOMO level w ith /i symmetry and the one-electron Uu LUMO level. 

Radical cations can be derived from aromatic hydrocarbons or alkenes by one-electron oxidation. Antimony trichloride and pentachloride are among the chemical oxidants that have been used. Photodissociation or y-radiation can generate radical cations from aromatic hydrocarbons. Most radical cations derived from hydrocarbons have limited stability, but EPR spectral parameters have permitted structural characterization. The radical cations can be generated electrochemically, and some oxidation potentials are included in Table 12.1. The potentials correlate with the HOMO levels of the hydrocarbons. The higher the HOMO, the more easily oxidized is the hydrocarbon. 

Calculated band structures of aU these compounds feature the fermi level above a density-of-state peak that is consistent with the 3d bands for nickel. The [BN]" anion in CaNi(BN) compromises an electronic situation with a filled 3(7 (HOMO) level that is B-N anti-bonding (Fig. 8.13). Any additional electron will... 

In the ground state of a covalent bond, the molecular orbital is occupied by at least one, usually two electrons with anti-parallel spins. This is said to be the HOMO level that is, the highest occupied molecular orbital. If the bond is slightly sheared, the kinetic energies of its electrons is not affected, but the... 

One more step provides an operational definition. The HOMO level lies, I = ionization energy, below the vacuum level, while the LUMO level lies, A = electron affinity, below it. Thus, the chemical hardness lies midway in the gap and usually is given in units of eV. 

The choice of the thiol anchoring groups ensures a strong chemical bonding to the leads, i.e., a stable junction configuration, with the current flow dominated by the molecular HOMO level [271]. 

The nature of the electronic states for fullerene molecules depends sensitively on the number of 7r-electrons in the fullerene. The number of 7r-electrons on the Cgo molecule is 60 (i.e., one w electron per carbon atom), which is exactly the correct number to fully occupy the highest occupied molecular orbital (HOMO) level with hu icosahedral symmetry. In relating the levels of an icosahedral molecule to those of a free electron on a thin spherical shell (full rotational symmetry), 50 electrons fully occupy the angular momentum states of the shell through l = 4, and the remaining 10 electrons are available... 

TABLE 19. HOMO level, energy values of several a-alkoxystannanes and silanes, with comparison to alkylmetals and to dimethyl... 

Before going into a detailed account of the chemistry of phanes, the author will touch on 3,4,7,8-tetrasilacycloocta-l,5-diyne briefly, since the compound illustrates the importance of a—it mixing. The ionization potential of the Si-Si bond is estimated by photoelectron spectroscopy to be 8.69 eV (9). Thus, the HOMO level of the Si-Si is comparable to most HOMOs of tt systems. Consequently, the Si-Si bond can conjugate efficiently with carbon-carbon double and triple bonds, benzene rings, and other tt systems. Most Si-Si bonds are stable enough to construct sophisticated structures by themselves and with organic molecules (10). 

In the cyclophane 1, although the overlap between the n-system (2p) and the bridging cr-bonds (2s2p) is most effective, these orbital energy levels match worst, the first ionization potentials being 9.25 eV for benzene and 12.1 eV for ethane. As a result, the HOMOs are the almost pure it MOs with the b2g and b3g combinations. Both the PE spectrum and theoretical calculation demonstrate the degeneracy of the two HOMO levels. The absorption bands are attributed to the 17-17 transitions associated with the HOMOs. 

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). 

It has been known for some time that the basicities of a heteroatom decrease upon a-silyl substitution [12], For example, alkyl silyl ethers (R3Si-0-R ) are less basic than dialkly ethers. Silylamines are weak bases compared to alkylam-ines. This electron-withdrawing effect of silyl groups has been explained in terms of the interaction between low lying vacant orbitals such as 3d orbitals of silicon or a orbitals with the nonbonding p orbitals (lone pairs) of the heteroatom (Fig. 4). This interaction decreases the HOMO level which in turn lowers the basicity of the heteroatom. Such effect may also cause the increase of the oxidation potentials, but little study has been reported on the electrochemical properties of this type of compounds. 

Although little information has been available for the effect of a-silyl substitution on the electrochemical properties of heteroatom compounds, extensive studies have been carried out on the effect of /f-silyl substitution [10,13]. For the -substituted heteroatom compounds (substitution at the a carbon), the introduction of a silyl group results in a significant decrease of the oxidation potentials, although the magnitude depends upon the nature of the heteroatom. This effect is explained in terms of the interaction between the C Si a orbital and the nonbonding p orbital of the heteroatom (Fig. 5). This interaction raises the HOMO level which in turn favours the electron transfer. 

Fig. 6. Plots of the HOMO level of SiH3CH2OH and that of CHjCH2OH vs. the torsion angle of Si-C-O-H or C-C-O-H [13]...

The introduction of an additional silyl-substituent in the /i-position to the heteroatom causes a further decrease in the oxidation potential of heteroatom-substituted organosilanes. The introduction of an additional silyl group results in the rise of the HOMO level. Increase in the population of the favorable conformers for the electron transfer also seems to be important. 

Although oxidation potentials of aldehydes and ketones are generally very high, silyl substitution at the carbonyl carbon results in a significant decrease in the oxidation potential [16]. The decrease in the oxidation potentials is attributed to the rise of the HOMO level by the interaction of the C Si cr-bond and the nonbonding p orbital (lone pair) of the carbonyl oxygen (Fig. 9). In the case of a-silyl-substituted ethers, the rotation around the C-O bond is free and,... 

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