Lowest unoccupied molecular orbital analysis

Methylcyclohexanone, pK 20, is typical of a weak acid that undergo H/D exchange. Identify the acidic protons of 2-methylcyclohexanone, i.e., those most susceptible to attack by base, as positions for which the value of the lowest-unoccupied molecular orbital (LUMO) is large. Use a LUMO map (the value of the LUMO mapped onto the electron density surface). Does this analysis correctly anticipate which of the anions obtained by deprotonation of 2-methylcyclohexanone is actually most stable Are any of the other ions of comparable stability, or are they aU much less stable ... 

The Woodward-Hoffmann rules for pericyclic reactions require an analysis of all reactant and product molecular orbitals, but Kenichi Fukui at Kyoto Imperial University in Japan introduced a simplified version. According to Fukui, we need to consider only two molecular orbitals, called the frontier orbitals. These frontier orbitals are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In ground-state 1,3,5-hexa-triene, for example, 1//3 is the HOMO and excited-stale 1,3,5-hexatriene, however, 5 is the LUMO. 

The COMPACT (computer-optimized molecular parametric analysis of chemical toxicity) procedure, developed by Lewis and co-workers [92], uses a form of discriminant analysis based on two descriptors, namely, molecular planarity and electronic activation energy (the difference between the energies of the highest occupied and lowest unoccupied molecular orbitals), which predict the potential of a compound to act as a substrate for one of the cytochromes P450. Lewis et al. [93] found 64% correct predictions for 100 compounds tested by the NTP for mutagenicity. 

Each reaction species must have molecular orbitals available and with the correct symmetry to allow bonding. These will be called frontier orbitals composed of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In addition to their involvement in bonding between species, these orbitals are of considerable interest in that they are largely responsible for many of the chemical and spectroscopic characteristics of molecules and species and are thus important in analytical procedures and spectroscopic methods of analysis [5-7],... 

In recent years, molecular descriptors such as the energy of the highest occupied molecular orbital (EHomo) ar d the energy of the lowest unoccupied molecular orbital ( IUMO) have gained in popularity for QSAR analysis, as these descriptors are readily calculated from PC-based software such as SPARTAN. Before we discuss EHomo ar d ELumo further, a brief discussion of quantum chemistry is necessary. 

Regioselectivity is quite predictable, and consistent in a simple way with typical electrophilic activation of an alkene (Markovnikov s rule). Just as in bromination of an nnsymmetrical alkene, initial coordination of an electrophile (M+, Br+) activates the alkene toward nncleophilic addition of a nncleophile, the addition is preferred at the end of the alkene that best stabilizes a cation. Electronic effects dominate over steric effects. An molecnlar orbital (MO) analysis has been pnt forward ( the slip mechanism ) to rationalize the activating effect of the metal and, in a secondary way, the regioselectivity. It focnses on the reactants and prodncts, and notes that the metal moves dnring the reaction from the approximate midpoint of the alkene to one end. As that slip occurs toward one end of the alkene, the lowest unoccupied molecular orbital (LUMO) for the complex changes and a large coefficient develops at the other end. 

We observed (Fig. 3) that in the absence of surface quinones, the relaxation of QD absorption bleach band (A,ex=528 nm) corresponding to 1 lSelSh> state reflects the trapping excited charge carriers at the surface. The picosecond kinetic analysis shows that in the presence of tCl-l,2-BQ the short time component of the transient bleach formation at A.reg=530 nm is additionally shortened from 93 ps down to 27 ps. It reflects the appearance of the additional non-radiative relaxation channel for electrons from QD conduction band to the lowest unoccupied molecular orbitals of quinone (LUMO). These results are in an agreement with calculations presented in [4]. We believe that long component (r> 3 ns) may reflect the electron shuttling from LUMO of the quinone to the QD valence band. 

LSwdin population analysis -> charge descriptors (O atomic charge) lowest unoccupied molecular orbital -> quantum-chemical descriptors lowest unoccupied molecular orbital energy -> quantum-chemical descriptors LUMO electron density on the ath atom -> charge descriptors (O net orbital charge)... 

Cycloadditions are controlled by orbital symmetry (Woodward-Hoffman rules) and can take place only if the symmetry of all reactant molecular orbitals is the same as the symmetry of the product molecular orbitals. Thus, an analysis of all reactant and product orbitals is required. A useful simplification is to consider only the frontier molecular orbitals. These orbitals are the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO). The orbital symmetry must be such that bonding overlap of the terminal lobes can occur with suprafacial geometry that is, both new bonds are formed using the same face of the diene. 

With the progress in the chemical calculation of the molecule, analyses with HOMO/LUMO (highest occupied molecular orbital/lowest unoccupied molecular orbital) energy, absolute hardness/absolute electron negativity, and research into these new and old descriptors have been reported recently. Furthermore, new methods with neural network computers, as well as multiregression analysis, cluster analysis, and major component analysis have been applied to investigate the relationship between the property and function of the molecule and of each descriptor. 

Lovasz-Pelikan index spectral indices (0 eigenvalues of the adjacency matrix) LOVIs = LOcal Vertex Invariants local invariants Lowdin population analysis quantum-chemical descriptors Lowest-Observed-Effect Level biological activity indices (0 toxicological indices) lowest unoccupied molecular orbital quantum-chemical descriptors lowest unoccupied molecular orbital energy quantum-chemical descriptors LUDI energy function scoring functions Lu index —> hyper-Wiener-type indices... 

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