HOMO highest occupied substituent effects

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). 

Wong and coworkers37 studied a series of both symmetrical and unsymmetrical tetraalkyltin compounds with different alkyl substituents, focusing their attention on the effect of these on the ionization energy of the highest occupied MOs. It is useful to recall the type of molecular orbitals deriving from the triply degenerate HOMO in the symmetrical R4M (local symmetry Td) upon substitution of one or more R ... 

Correlation of the effect of substituents on the rates of reactions with early transition states often is best accomplished in terms of perturbational molecular orbital theory, and polar effects can play a major role for such reactions [100, 101]. Essentially this theory states that energy differences between the highest occupied molecular orbital (HOMO) of one reactant and the lowest unoccupied molecular orbital (LUMO) of the other reactant are decisive in determining the reaction rate the smaller the difference in energy, the faster the predicted rate of reaction [102,103]. Since the HOMO of a free radical is the SOMO, the energy difference between the SOMO and the alkene HOMO and/or LUMO is of considerable importance in determining the rates of radical additions to alkenes [84],... 

Within a molecular orbital approximation, the electron is ejected from the highest occupied molecular orbital (HOMO). Molecular orbital calculations at various levels of sophistication describe the highest occupied MOs of most yhdes as being strongly localized on tlie ylidic carbon. Exceptions to this are found for example in cyclopentadienide derivatives, where the orbital of corresponding symmetry is the HOMO-1 (IE2). In terms of reactivity, the low first ionization potentials of ylides reflect high oxidizabihty, high proton affinity, and basicity. UV photoelectron spectra in conjunction with detailed molecular orbital calculations for each individual ylide structure have made possible a rationalization of the different substituent and heteroatom effects. 

In Table 10 the first three IPs of some substituted anilines are summarized. Aniline and other aromatic amines are systems weU suited for studies of substituent effects on the electronic structure and the effects of stoic inhibition to resonance - . Since such effects mainly affect the electronic structure of the benzene ring it would lead too far covering these compounds hoe in detail. However, a few remarks seem to be adequate. Because of the electron delocalization thoe are several MOs with substantial UN contributions. However, in most cases the third highest occupied MO (HOMO-2), or the third IP, is mainly related to the electron lone-pair. Thoefore, the assignment of the IPs is generally the same as for the parent molecules IPi and IP2 are assigned to 71 and Jt2, respectively, and IP3 is assigned to /in. 

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