Highest occupied molecular orbital redox properties

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>. 

Commonly used descriptor variables for QSARs involving redox reactions include substituent constants (o), ionization potential, electron affinity, energy of the highest occupied molecular orbital (EHOMO)or lowest unoccupied molecular orbital (ELUMO), one-electron reduction or oxidation potential (E1), and half-wave potential (E1/2)- One descriptor variable (D), fit to a log-linear model, is usually sufficient to describe a redox property of P. Such a QSAR will have the form... 

More recently, it has become clear that many polynuclear compounds undergo electron-transfer reactions, and several reviews devoted wholly or partly to this subject have appeared 1-4). Studies of the redox properties of polymetallic species can provide information on the nature of the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO), on the possible cooperativity between metal sites, on the existence of mixed-valence compounds, and on the ways in which one-electron (or, more rarely, two-electron) changes affect structure and reactivity. 

Close inspection of the solution phase redox peaks for the four species under analysis obtained in a thin layer cell in deaerated non-aqueous media (not shown here) yielded values for the onset of the reduction of the first Co site very similar to one another. Hence, it can only be surmised that the differences in specificity are related to the presence of the peripheral substituent and seemingly unrelated to the redox properties of the metal sites. Evidence that the differences in the specificity between the meso-substituted and non-meso-substituted materials are due to subtle electronic effects was provided by quantum mechanical calculations. These showed that upon addition of the weso-substituents, the electronic charge density associated with the HOMO (highest occupied molecular orbital) localized on the dioxygen moiety is markedly reduced, thereby weakening its Lewis acid character and hence its ability to coordinate a proton (see Figure 3.67). 

We extend our considerations of the micro-environment imposed by the protein to the thesis that axial ligands, hydrogen bonds and neighboring residues of photo synthetic chromophores help to define a scaffolding that in turn controls the conformations of the molecules. Theoretical calculations indicate that conformational variations would shift the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of (B)Chls and thereby modulate their redox and light-absorption properties [17,32]. We consider here the evidence for and the consequences of such conformational differences in (B)Chls, and test our conclusions with a series of synthetic porphyrins that are significantly puckered. 

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