Lowest unoccupied molecular orbital structure

The most extensive calculations of the electronic structure of fullerenes so far have been done for Ceo- Representative results for the energy levels of the free Ceo molecule are shown in Fig. 5(a) [60]. Because of the molecular nature of solid C o, the electronic structure for the solid phase is expected to be closely related to that of the free molecule [61]. An LDA calculation for the crystalline phase is shown in Fig. 5(b) for the energy bands derived from the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for Cgo, and the band gap between the LUMO and HOMO-derived energy bands is shown on the figure. The LDA calculations are one-electron treatments which tend to underestimate the actual bandgap. Nevertheless, such calculations are widely used in the fullerene literature to provide physical insights about many of the physical properties. 

Examine the lowest-unoccupied molecular orbital (LUMO) in benzenium ion. On which carbon(s) is it most concentrated Draw a Lewis structure (or series of Lewis structures) for benzenium ion, and locate the positive charge. Is it located on the same carbons where the LUMO is concentrated Explain. 

LUMO, see Lowest unoccupied molecular orbital Lyase, 1041-1042 Lycopene, structure of, 483 Lysergic acid diethylamide, structure of, 963... 

Conjugated polymers are generally poor conductors unless they have been doped (oxidized or reduced) to generate mobile charge carriers. This can be explained by the schematic band diagrams shown in Fig. I.23 Polymerization causes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the monomer to split into n and n bands. In solid-state terminology these are the valence and conduction bands, respectively. In the neutral forms shown in Structures 1-4, the valence band is filled, the conduction band is empty, and the band gap (Eg) is typically 2-3 eV.24 There is therefore little intrinsic conductivity. 

FIGURE 3.4 Molecular level alignment diagrams constructed using the HOMO and vacuum levels measured using UPS. The lowest unoccupied molecular orbital LUMO positions are inferred assuming a HOMO/LUMO gap equal to the onset of optical absorption. The chemical structure of CuPc is shown. (From Hill, I.G. and Kahn, A., J. Appl. Phys., 86, 2116, 1991. With permission.)... 

HOMO) and the subjacent HOMO - 1 depicted in Chart 3. As such, the centrosymmetric structures above accord with the optimum overlap of the naphthalene HOMO and HOMO - 1 of au and bIu symmetry, respectively, with the degenerate pair of tropylium e i lowest unoccupied molecular orbitals (LUMOs) of the same gross symmetry. On the basis of similar considerations of orbital symmetry, the EDA complexes of tropylium with neither benzene nor anthracene donors would be centrosymmetric. 

The concept of molecular orbitals (MOs) helps to explain the electron structure of ion-radicals. When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. HOMO is a bonding orbital. If one electron is introduced externally, it takes the lowest unoccupied molecular orbital (LUMO), and the molecule becomes an anion-radical. LUMO is an antibonding orbital. Depending on the HOMO or LUMO involved in the redox reaction, organic donors appear as n, a, or n species, whereas organic acceptors can be tt or a species. Sometimes, a combination of these functions takes place. 

The features in Figure 15.2 are believed to be due to electron transitions from the highest occupied molecular orbital states to lowest unoccupied molecular orbital states (HOMO-LUMO). Specifically, for single molecules and dilute solutions, the absorption in the blue part of the spectrum is proposed to be caused by the aromatic structure of TNT [4], probably involving it to it transitions.1 In the solid state, where the molecules are stacked up on top of each other, interactions between the molecules occur causing the energy levels to split into higher... 

The optical and electronic functions of polysilanes owe to their delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) that are occupied by holes and conduction electrons, respectively. The polymer does not show high conductivity or optical nonlinearity if the electrons or holes are localized on a small part of the polymer chain. To elucidate the structure of HOMO and LUMO is therefore important for the molecular design of polysilanes as functional materials. 

The fact that the electrochemical oxidation of ds-[M2(cp)2(/r-SR)2(CO)4] and of [M2(/r-SR)2(CO)8] is reversible despite the amount of structural reorganization involved suggests that these changes require low activation energy. Extended Htlckel Molecular Orbital (EHMO) calculations on the model complex ds-13 (R = H) indicated that the Lowest Unoccupied Molecular Orbital (LUMO) was the level [47]. Weakening of the... 

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