Aromatic compounds highest occupied molecular orbitals

Molecular orbital calculations have been performed on compounds 19 and 20 . The calculated PM3 equilibrium geometric structures show that these compounds are severely distorted from planarity in accordance with X-ray structural analysis (see Section 8.I2.3.I). On the other hand, PM3 calculations performed on both neutral and oxidized/reduced compounds show that oxidation and reduction induce a clear gain of aromaticity. Predictions using the nonempirical valence effective Hamiltonian (VEH) method have shown that the electronic charge density in the highest occupied molecular orbital (HOMO) is localized on the benzodithiin 19 or benzoxathiin 20 rings. 

In solvents of weak basicity (e.g. AN), aromatic hydrocarbons (AH) are oxidized, at least in principle, in two steps. In the first step, the compound gives an electron in its highest occupied molecular orbital (HOMO) to the electrode to form a radical cation (AH +), whereas, in the second step, it gives another electron to the electrode to form dication AH2+ ... 

The multiplicity of the ground state, p-stability ( aromaticity ), the order and energy of the electron-excited and ionic states of thieno[3,4-c]thiophene were studied (93ZN1621). In particular, this compound was demonstrated to have aromaticity analogous to that of thiophene, but it is much less stable due to a specific structure of its highest occupied molecular orbital. 

Another definition of aromaticity that does not depend explicitly on either experimental results or on comparison with reference compounds is derived from the concept of absolute hardness, 17, which is defined as one-half of the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a system (equation 4.70). According to Koopmans theorem, Ehomo is related to the ionization potential of the species, while Elumo is related to its electron affinity. A large gap between these two orbitals implies resistance to both oxidation and reduction, and low chemical reactivity is one of the defining characteristics of aromaticity. 

According to another strategy proposed by Lambert and Ferraris [294,295], monomers of reduced aromaticity, which decreases separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), are used so compounds 66 and 67 have been polymerized, resulting in polymers with values of 1.2 and 0.8 eV, respectively. However, no conductivity data have been reported for either material. 

Many of the novel aromatic compounds found present high symmetry. Symmetry is one of the usual features of aromatic compounds. Although not all aromatic species are symmetric, the most archetypal aromatic compounds are highly symmetric and possess degenerate highest-occupied molecular orbitals. These orbitals can be fuUy occupied resulting in a closed-shell structure or can be same-spin half-filled. This is the case of paradigmatic aromatic species, namely, benzene, BeHe closo borane cluster, AU , but also of triplet CsHs" or Ceo with a... 

TABLE 4.15 The Training (Calibration) and Test (Marked With Asterisk - to be chosen) Compounds Studied Along Their HOMA (Harmonic Oscillator Model of Aromatic) Index (Mosquera et al., 2007 Ciesielski et al., 2009) and of Associated Computed (Hypercube, 2002 (Semiempirical, AMI, Polak-Ribier optimization procedure)) Structural First, Second, and Third Order HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) Reactivity Indices (Tarko Putz, 2010)... 

Originally invoked for electroluminescent reactions, this idea has now been developed for the reaction of peroxides with fluorescent compounds of low ionisation potential [51]. Many of these reactions are discussed in Chap. XI, but Fig. 2 can be most succintly exemplified by the radical ion annihilation shown, where Ar is a fluorescent aromatic hydrocarbon such as diphenylanthracene, LUMO is the lowest, normally unoccupied molecular orbital and HOMO is the highest occupied molecular orbital. 

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