Energy small HOMO-LUMO

The small HOMO-LUMO band gap and presence of other close-in-energy MOs results in fullerenes being easily polarized. They all give very intense Raman scattering lines and have relatively large x values useful for NLO applications (11). Indeed, C60 is one of the best materials known to date for optical limiting. 

Carbanions are tetrahedral, and inversion of configuration can be expected to be easy—since the LUMOs, the cr CH orbitals, are high in energy, the HOMO-LUMO gap is not small as it is in a phosphine. However, carbanions in the form of compounds containing C—Li bonds can often be configurationally stable by virtue of the <7 bond, which would have to be broken to allow inversion. 

While not completely accurate, the PFEO model provides an estimate of the first level excitation energy and explains the trend in HOMO-LUMO gap. As the molecule increases in size the HOMO-LUMO gap decreases in magnitude and it becomes easier to generate an excited state. Technologically this has some ramifications for example, hexacene s very small HOMO-LUMO gap makes its electrons easy to excite. The material is easily oxidized in air and unsuitable for air stable transistors. The HOMO and LUMO surfaces for pentacene are shown in Fig. 2.8. It is clear that the wavefunction indeed travels around the perimeter of the molecule and this distance is a significant determiner of the electronic properties of the material. 

The FMO definition also helps explain why Pearson s hard-hard and soft-soft interactions form stable complexes. Hard compounds have a large HOMO-LUMO gap, as shown in Figure 14.9 for F. Therefore, hard Lewis acid-base complexes tend to form strongly ionic compounds, such as LiF, where the interaction is dominated by electrostatic attractions. Soft compounds, on the other hand, have a small HOMO-LUMO gap, as shown in Figure 14.9 for I, so that these types of interactions form covalently bonded acid-base adducts, where the strength of the interaction is controlled primarily by the energies of the FMOs that participate in the bonding. 

The Suzuki reactions and the substituted PPP produced by tfiis route have also been used in order to obtain ladder-type polymers [85]. These polymers possess a two-dimensional structure and incorporation of the n chain into a rigid ladder allows full conjugation. 7t-Extended systems of the ladder type are expected to show small HOMO/LUMO energy differences and high hyperpolarizability. The optical, electrical, photoelectrical and non-linear optical properties of these materials appear to show great promise. An organic electroluminescent diode is an example of the expected future applications of such materials. 

Experimentally, palladium clusters with fewer than 10 atoms have been fonnd to have single layer structure (Kaden et al. 2009). To check the adsorption behavior of different isomers of Pdg cluster, we carried out calculations on the adsorption properties of a planar Pdg clnster and the optimized structure is shown in Figure 14.6(c) and (d). Note that the isolated planar isomer is energetically less stable by 2.1 eV compared to an octahedral Pdg clnster. Planar isomer has a small HOMO-LUMO gap of about 0.04 eV, and the total magnetic moment is 2 pg. This cluster is deposited on the channel of 3L surface slab. The adsorption energy of this cluster on 3L surface slab is 4.81 eV, which is... 

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