HOMO-LUMO properties

The MINDO/3 procedure was applied to calculate HOMO-LUMO properties of simple and conjugated enamines240, partial optimization of the molecular geometry of... 

Exercise 1.3. Consider a coordination compound formed from BH3 and C2H4. From the HOMO-LUMO properties of each species predict the geometric structure of the Lewis acid-base adduct. Now predict the structure of a compound formed by replacing one CO ligand of Fe(CO)5 with C2H4. Note the parallelism between the main group and transition metal examples. The second compound is a stable and isolatable compound, whereas the first is a transient intermediate in the hydro-boration of ethylene to ethyl borane and has only been characterized as a transient intermediate in a fast-flow system by modulated mass spectrometry. 

As shown in Fig. 7, a large increase in optical absorption occurs at higher photon energies above the HOMO-LUMO gap where electric dipole transitions become allowed. Transmission spectra taken in this range (see Fig. 7) confirm the similarity of the optical spectra for solid Ceo and Ceo in solution (decalin) [78], as well as a similarity to electron energy loss spectra shown as the inset to this figure. The optical properties of solid Ceo and C70 have been studied over a wide frequency range [78, 79, 80] and yield the complex refractive index n(cj) = n(cj) + and the optical dielectric function... 

HOMO-LUMO. This may be supported by low Zeff of technetium atoms in tetragonal-prismatic clusters [103,127] and formation of compounds with fer-ricinium cations with some unusual physico-chemical properties, as is the case of trigonal-prismatic clusters [108,128]. 

AMI semi-empirical and B3LYP/6-31G(d)/AMl density functional theory (DFT) computational studies were performed with the purpose of determining which variously substituted 1,3,4-oxadiazoles would participate in Diels-Alder reactions as dienes and under what conditions. Also, bond orders for 1,3,4-oxadiazole and its 2,5-diacetyl, 2,5-dimethyl, 2,5-di(trifluoromethyl), and 2,5-di(methoxycarbonyl) derivatives were calculated <1998JMT153>. The AMI method was also used to evaluate the electronic properties of 2,5-bis[5-(4,5,6,7-tetrahydrobenzo[A thien-2-yl)thien-2-yl]-l,3,4-oxadiazole 8. The experimentally determined redox potentials were compared with the calculated highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energies. The performance of the available parameters from AMI was verified with other semi-empirical calculations (PM3, MNDO) as well as by ab initio methods <1998CEJ2211>. 

Due to higher variety of possible structures, copolymers allow a better control of the HOMO LUMO levels necessary to optimize the EL properties of the PPV, compared to homopolymers. Often the optical and electronic properties in copolymers can be finely tuned by simply changing the feed ratio of comonomers (although the structure-property relationship in these systems is even more complex than in homo-PPV polymers). Using different comonomer units, various PPV-based materials with tuned optical and electronic properties have been prepared. 

Beside the excellent optical properties and suitable HOMO-LUMO energy levels, the PFs possess great charge-transport properties. Time-of-flight (TOF) measurements of PFO showed nondispersive hole transport with a room temperature mobility of holes of fi+ = 4 x 10-4 cm2/(V s) at a field of li 5 x 105 V/cm that is about one order of magnitude higher than that in PPV [259]. The polymer revealed only a weak-field dependence of the mobility, from /r+ = 3 x 1(U4 cm2/(V s) at E= 4 x 104 V/cm to /r+ = 4.2 x 1(V4 cm2/(V s) at E= 8 x 105 V/cm. 

Introducing heteroatoms such as O, N, S in the backbone of conjugated polymers is routinely used to modify their electronic properties, and particularly, the HOMO-LUMO energies. Other heteroatoms only recently have been studied in this aspect [703],... 

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