Lowest unoccupied 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. 

Cyclic voltammetry (CV) can provide information about the thermodynamics of the redox process, kinetics of heterogeneous electron transfer reactions and coupled chemical reactions [32]. The reversible electron transfer steps inform us about the compound s ability to accept electrons however, experimental conditions, such as solvent and temperature also influence the voltammogram. The structure of the lowest unoccupied molecular orbital (LUMO) levels of the compound can be determined from the number of CV waves and reduction potentials ( 1/2)- Moreover, the CV can serve as a spectroscopy as demonstrated by Heinze [32], since the characteristic shapes of the waves and their unequivocal positions on the potential scale are effectively a fingerprint of the individual electrochemical properties of the redox system. 

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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