Donor HOMO

For electron movement to occur, the donor and acceptor molecules must approach so that the donor HOMO and acceptor LUMO can interact. For example, the LUMO of singlet methylene is a 2p atomic orbital on carbon that is perpendicular to the molecular plane. Donors must approach methylene in a way that allows interaction of the donor HOMO with the 2p orbital. 

In sharp contrast to the stable [H2S. .SH2] radical cation, the isoelectron-ic neutral radicals [H2S.. SH] and [H2S. .C1] are very weakly-bound van der Waals complexes [125]. Furthermore, the unsymmetrical [H2S.. C1H] radical cation is less strongly bound than the symmetrical [H2S.. SH2] ion. The strength of these three-electron bonds was explained in terms of the overlap between the donor HOMO and radical SOMO. In a systematic study of a series of three-electron bonded radical cations [126], Clark has shown that the three-electron bond energy of [X.. Y] decreases exponentially with AIP, the difference between the ionisation potentials (IP) of X and Y. As a consequence, many of the known three-electron bonds are homonuclear, or at least involve two atoms of similar IP. 

D = electron donor HOMO = highest occupied molecular orbital ... 

The main stabilization in reactions with activated reaction partners, viz. when one partner is electron-rich and the other electron-poor, arises through interaction between the donor HOMO and the acceptor LUMO which are much closer in energy than the acceptor HOMO and the donor LUMO. Figure 2 illustrates which interactions between the frontier orbitals cause the main stabilization in normal, neutral and inverse Diels-Alder reactions. For example, the main stabilization in the reaction between an electron-rich diene and an electron-poor dienophile stems from the interaction of the diene HOMO with the dienophile LUMO. 

In electrophilic substitution, the electrophile is the acceptor and the C—X bond the donor. Now frontside attack gives donor HOMO and acceptor LUMO of the same symmetry (14), and stabilization can occur (Figure 10.16). 

Interaction between orbitals will be strongest when the energy difference between the acceptor LUMO and the donor HOMO is least. 

Fig. 14 A schematic of the photoinduced charge separation and charge recombination processes in 18(w). A simple orbital diagram is provided which captures the essentials of the ET processes. HD, donor HOMO LD, donor LUMO HA, acceptor HOMO LA, acceptor LUMO. Note that all depicted processes are assumed to take place on the singlet multiplicity manifold.

Figure 1 Potential energy wells for the electron localized on the donor (D) and acceptor (A) sites. The parameter (A ) indicates the average energy gap for an instantaneous (Franck-Condon) transfer of the electron from the donor HOMO to the acceptor LUMO. The dotted lines show the electronic energies on the donor and acceptor at a nonequilibrium nuclear configuration with a nonequilibrium energy gap AE. The upper dashed horizontal line indicates the bottom of the conduction band of the electrons in the solvent.

Within the framework of this chapter, a section on stereodynamics must focus on the heart of harpoon reactions the electron transfer itself. Hence we review studies which directly inform on the extent to which the electron transfer is affected by the relative polarization of the donor (HOMO) and acceptor (LUMO) orbitals. 

The right side of Figure 12.13 shows what happens if a good pi donor is attached to a double bond. The pi donor HOMO interacts with both the double-bond HOMO and LUMO to produce a new ally lie system with a new HOMO and LUMO, which are raised... 

As we pointed out earlier in Chapter 3, Cr(CO)6 is a good example of a complex that obeys the 18-electron rule. The molecular orbitals of interest in this molecule are those that result primarily from interactions between the d orbitals of Cr and the a donor (HOMO) and n acceptor orbitals (LUMO) of the six CO ligands. The molecular orbitals corresponding to these interactions are shown in Figure 3-1.7 (See Chapter 2, Figure 2-5, for a molecular orbital picture of CO.)... 

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