Ionization potential and HOMO

When parameters of the Pariser-Parr-Pople configuration interaction molecular orbital (PPP-CI MO) method were modified so as to reproduce the Aol)s values for l,3-di(5-aryl-l,3,4-oxadiazol-2-yl)benzenes 16 and 17, the calculated HOMO and LUMO energy levels corresponded with the experimental ionization potential and electron affinity values. The relationships between the electrical properties and molecular structures for the dyes were investigated. The absorption maximum wavelengths for amorphous films were found to be nearly equal to those for solution samples <1997PCA2350>. 

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+. 

The simplest method for calculating the ionization potentials and the electron affinities is the 7T-HMO method. Such an evaluation is particularly suitable for the determination of the relative electron donor-acceptor properties of the molecules. The appropriate indices are the energies of the highest occupied molecular orbitals (HOMO) for the electron donor capacity and the energies of the lowest empty molecular orbitals (LEMO) for the electron acceptor abilities. These simple theoretical predictions gave an excellent interpretation1,256 of the... 

The energy of the HOMO (EHomo) is directly related to the ionization potential and characterizes the susceptibility of the molecule to attack by electrophiles. On the other hand, EHOMO is directly related to the electron affinity and characterizes the susceptibility of the molecule toward attack by nucleophiles. Both the E, IOMO and LUMO energies are important in radical reactions. The concept of hard and soft nucleophiles and electrophiles has... 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

Cyclic voltammetry studies showed the ionization potential and electron affinity of each component of the molecule in solution. The HOMO and LUMO energy levels were estimated from the equations Ehomo = E x + 4.4 eV and Elumo = T 14+4.4 eV, where E(r]x and E%, were oxidation and reduction potentials with respect to the standard hydrogen electrode (SHE) and the value of 4.4 is the ionization potential for hydrogen in eV [94,95], The HOMO and LUMO energy levels of the methine dye (compound 6) (Scheme 13) were determined to be -5.82 and -3.48 eV, respectively, with respect to the vacuum level from... 

The ionization potential and electron affinity of Pd clusters converge to a common value at small size, as shown in Table VII. The HOMO (IP) and LUMO (EA) are composed of antisymmetric d molecular orbitals in both cases and,... 

The extension of an aromatic 7r-system by, for example, benzo-annelation or coplanar phenyl substitution, stabilizes the system overall, raises the energy of the HOMO, and decreases that of the LUMO. The consequent decrease in ionization potential and increase in electron affinity imply the formation of cation-radicals for lower energy cost, and of anion-radicals with greater energy gain, the more extensive the 7t-system of the diamagnetic aromatic precursor. 

The extended Hiickel calculations also indicate that the HOMO in N2F2 is not a w-type orbital however, lying immediately above and below the HOMO are two r-type orbitals. This property, if confirmed by further calculations, should be significant in evaluating the ionization potential and electron affinity of this molecule. 

Semiempirical MNDO calculations have been carried out on model pyrylium and thiopyrylium systems (88MI1). The calculated HOMO-LUMO gap in the gas phase correlates well with experimental absorption maxima obtained in solution. Ionization potentials and electron affinities predicted by Koopmans theorem with MNDO orbital energies do not track the observed trends in the experimental redox values. In contrast these are paralleled by the trends predicted by A// values calculated by MNDO and AMI for the open-shell and closed-shell species. 

The ionization potential and electron affinity of naphthalene were determined experimentally as IP = 8.2 eV and EA 0.0 eV. According to Koopmans theorem it is possible to equate minus the orbital energies of the occupied or unoccupied MOs with molecular ionization potentials and electron affinities, respectively (IP, = - s, and EA = - ). Thus, in the simple one-electron model, the excitation energy of the HOMO->LUMO transition in naphthalene may be written according to Equation (1.22) as... 

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