Frontier Orbitals HOMO and LUMO

Fukui [7-1] recognized the importance of the symmetry properties of HOMOs and LUMOs perhaps for the first time in connection with the Diels- 

Alder reaction. According to his Nobel lecture [7-12], however, it was only after the appearance of the papers by Woodward and Hoffmann in 1965 that he became fully aware that not only the density distribution but also the nodal property —that is, symmetry— of the particular orbitals have significance in. . . chemical reactions.  

The concept of frontier orbitals simplifies the MO description of chemical reactions enormously, since only these MOs of the reactant molecules need to be considered. Several examples of this approach will be given in Section 7.3. 

---

Fig. 8.10 Visualization of the relevant MOs, 16 and 17, for the active space of a CASSCF(2,2) calculation on 1,4-butanediyl the algorithm will recognize the active space as consisting of the two frontier orbitals (HOMO and LUMO the molecule has 32 electrons) we ensure by visual inspection that these are the two MOs that are localized on the end carbons. If a desired orbital is not a frontier orbital to start with, it can be switched with one (see text). NBO localization was used here. Calculated with the HF/STO-3G basis and localized by the NBO method...

Donor/acceptor interactions in charge-transfer (CT) complexes are determined primarily by the symmetry and the energetics of the frontier orbitals (HOMO and LUMO) [20]. A... 

When — E 0 interaction between frontier orbitals (HOMO and LUMO) is important and a frontier-controlled reaction involving electron transfer occurs. A totai may then be written as... 

Frontier Orbitals HOMO and LUMO 2.2.1 Transition States... 

Understand the concept of frontier orbitals, HOMO and LUMO, and he able to determine the HOMOs and LUMOs of organic molecules... 

It turns out that we can usually neglect all other orbital interactions between molecules and focus our attention only on these frontier orbitals (HOMOs and LUMOs) when considering interactions between molecules, and in particular pericyclic reactions. This insight gained the relatively unknown Japanese chemist Kenichi Fukui a share in the Nobel Prize for Chemistry in 1981. 

To summarize how acid-base reactions do work on the basis of molecular orbitals perturbation theory, we have reported on Figure 10.2.1, the relative energies (as perturbed by the field of the other reactant) of the frontier orbitals HOMO and LUMO of a hypothetical species A and of the frontier orbitals of several hypothetical reaction partners B, C, D, E and F. This figure is intended to represent possible variations of donor-acceptor properties in the broadest possible context i.e. not only those species encountered in aqueous solution but also those stabilized by non-aqueous environments. 

Note The concerted mechanism in the Diels-Alder reaction implies the 43t-2o cycloaddition of two reacting partners, diene and dienophile [3-7]. To affect the productive interaction, the formation of two o bonds, they should possess the proper electronic properties of their frontier orbitals HOMO and LUMO. The energy barrier for this reaction is lowered by the closer energies of diene s HOMO and dienophile s LUMO. This is achieved by electron-donating groups (EDO) in diene and electron-withdrawing groups (EWG) in dienophile (Scheme 2.6). 

For the quinoid ground state, however, the value of the bandgap will increase. This phenomenon is interpreted in Figures 107 and 108 by treating the butadiene groups as successive perturbations to tbe frontier orbitals (HOMO and LUMO) of polythiophene. 

---