Lowest unoccupied molecular orbital interaction with highest occupied

The Bradsher reaction is formally a [4 + 2] Diels-Alder reaction. However, as a consequence of the aza cationic nature of the diene, this reaction proceeds by the inverse electron demand manifold. The classical Diels-Alder reaction employs the partnering of an electron-rich diene and an electron-deficient dienophile to provide the proper interaction of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as prescribed by frontier molecular orbital theory (FMO) to generate the observed adducts. Thus FMO theory interprets this reaction proceeding via the HOMO of the diene with the LUMO of the dienophile. In the case of the inverse electron demand reaction, the electronics of the reaction are inverted. Therefore, in the Bradsher reaction, the electron-deficient aza cation diene s LUMO interacts with the HOMO of an electron rich dienophile. This mechanistic pathway provided a rationalization for the lack of reactivity of the electron-deficient tetracyanoethylene (TCNE), while electron-rich styrenes afforded the predicted product from reaction of 1 to generate 2. ... 

This means that the tunnehng current is proportional to the local density of states (LDOS) of the sample at the center of the sphere (tip) and therefore, a constant current image reflects the LDOS of the sample. This demonstrates that an STM image does not display the mere topography of a sample surface. Instead, the electronic properties of the surface play an important role—which holds especially true for molecules adsorbed onto a surface. The electronic states of the molecules (HOMO, highest occupied molecular orbital and LUMO, lowest unoccupied molecular orbital) mix with those of the substrate surface, but modified by molecule-substrate interactions. Therefore, depending on the substrate site and material, an image of the molecules is obtained. 

Boron Tetraalkoxydiboranes, activated by a Lewis base, generate a nucleophilic sjp-carbene-type boryl moiety that can attack non-activated C=C bonds. Computational studies identified the interaction as the overlap of the strongly polarized B—B a bond (highest occupied molecular orbital, HOMO) with the tt orbital (lowest unoccupied molecular orbital, LUMO) of the C=C bond. According to this scenario, the normally electrophilic boron becomes nucleophilic and forces the olefin to act as an electrophile. ... 

In more detail, the interaction energy between donor and acceptor is determined by the ionisation potential of the donor and the electron affinity of the acceptor. The interaction energy increases with lowering of the former and raising of the latter. In the Mulliken picture (Scheme 2) it refers to a raising of the HOMO (highest occupied molecular orbital) and lowering of the LUMO (lowest unoccupied molecular orbital). Alternatively to this picture donor-acceptor formation can be viewed in a Born-Haber cycle, within two different steps (Scheme 3). 

An unusual observation was noted when ethanolic solutions of 2-alkyl-4(5)-aminoimidazoles (25 R = alkyl) were allowed to react with diethyl ethoxymethylenemalonate (62 R = H) [92JCS(P1)2789]. In addition to anticipated products (70), which were obtained in low yield ( 10%), the diimidazole derivatives (33 R = alkyl) were formed in ca.30% yield. The mechanism of formation of the diimidazole products (33) has been interpreted in terms of a reaction between the aminoimidazole (25) and its nitroimidazole precursor (27) during the reduction process. In particular, a soft-soft interaction between the highest occupied molecular orbital (HOMO) of the aminoimidazole (25) and the lowest unoccupied molecular orbital (LUMO) of the nitroimidazole (27) is favorable and probably leads to an intermediate, which on tautomerism, elimination of water, and further reduction, gives the observed products (33). The reactions of amino-imidazoles with hard and soft electrophiles is further discussed in Section VI,C. 

An analogous picture is drawn for analysis of XA in Fig. 4B. It should be remembered that the highest occupied molecular orbital of the xanthylidene-substituent is the lowest unoccupied orbital for the BA system. Interaction of this occupied aryl orbital with those of the prototype carbene gives the orbitals of XA shown in the center of Fig. 4B. The energy... 

Figure 1 shows the electron attachment energies (AE) and ionization potentials (IP) of silyl substituted 7t-systems and related compounds [4], AE can be correlated with the energy level of the LUMO (lowest unoccupied molecular orbital) and IP can be correlated with the energy level of the HOMO (highest occupied molecular orbital). For a-substituted 7t-systems, the introduction of a silyl group produces a decrease in the tc -(LUMO) level. This effect is attributed to the interaction between a low-lying silicon-based unoccupied orbital such as the empty d orbital of silicon and the it orbital (d -p interaction) as shown in Fig. 2. Recent investigations on these systems, however, indicate that d orbitals on silicon are not necessarily required for interpreting this effect a-effects of SiR3 can also be explained by the interaction between Si-R a orbitals and the 7r-system.

The mechanism of the Diels-Alder reaction involves a-overlap of the n-orbitals of two unsaturated systems. One molecule must donate electrons, from its highest occupied molecular orbital (HOMO), to the lowest unoccupied molecular orbital (LUMO) of the other. Also, the two interacting orbitals must have identical symmetry i.e. the phases of the terminal p-orbitals of each molecular orbital must match. There are two possible ways for this to happen the HOMO of the diene combining with the LUMO of the dienophile, and the LUMO of the diene with the HOMO of the dienophile (Figure 7.1). 

In order to simplify mathematical treatment, less important contributions from interactions between orbitals with large energy differences are neglected. The procedure is limited to the interaction of the frontier orbitals, viz. the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs), as illustrated in Figure 2. 

AMI calculations were performed using SPARTAN software, and these FMO predictions are consistent with the fact that the observed cycloaddition regiochemistry is generally /t -FMO . The HOMO/LUMO energies are listed in Table 9, and Figure 10 depicts the coefficients for the favored HOMO(miinchnone) - - LUMO(nitroindole) interaction (HOMO = highest, occupied molecular orbital LUMO = lowest unoccupied molecular orbital). 

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