Acceptor LUMO

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 anion-jt interactions with electron-deficient (neutral) aromatic tt-acceptors, the halide lies preferentially over the periphery of the aromatic ring (as illustrated in Fig. 13a) and this is apparently related to the shape of the acceptor LUMO (presented for comparison in Fig. 13b). 

A = electron acceptor LUMO = lowest unoccupied 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 this equation the Cg represent the respective atomic coefficients for the orbitals of the nucleophile (HOMO) and those of the different atoms in the electron-acceptor (LUMO and LUMO + 1) ( homo E lumo) and (T homo umo + 1) represent the energy gaps between the HOMO and LUMO (LUMO + 1) levels of nucleophile and acceptor. 

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. 

Hydrogen bonding can also be considered as donor-acceptor interaction. In it the acceptor (LUMO) is the unoccupied antibonding orbital of hydrogen bonded to the electronegative element. 

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.

In the simplest example, a donor and acceptor pair is activated by electronic excitation of either the donor or the acceptor. In addition to photophysical deactivation or energy transfer, two processes can proceed subsequently—the electronically excited donor donates an electron from its SOMO into the acceptor LUMO or the... 

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. 

Hydrogen bonding (see Topic F2) can also be regarded as a donor-acceptor interaction in which the acceptor LUMO is the (unoccupied) antibonding orbital of hydrogen bonded to an electronegative element. 

As with protic-acid-catalyzed additions, the Lewis-acid-promoted additions are believed to occur by attack of an enol species on a Lewis acid complex (protonated) form of the acceptor. Complexation of the acceptor with the Lewis acid serves to lower the energy of the acceptor LUMO and polarize the 71-system. Neutral or anionic nucleophilic species with high HOMOs will then combine with the acceptor-Lewis acid complex to yield products. 

---