Frontier orbital interactions between

Fig. 12.5. Frontier orbital interactions between different combinations of substituted radicals and aikenes.

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

Figure 5. Frontier orbital interaction between a transition metal peroxo group and an olefin.

The frontier orbital interaction between the olefin HOMO 7t(C-C) and the orbitals with c (0-0) character in the LUMO group of the metal peroxo moiety controls the activation of 0-0 bond. Electron donating alkyl substituents at the olefin double bond raise the energy of the HOMO, with the epoxidation barrier dropping concomitantly. On the other hand, a base coordinated at the metal center pushes the a (0-0) LUMO to higher energies and thus entails a higher barrier for epoxidation. 

Cycloadditions with reactive ketenes therefore can he observed only when they are prepared in situ and in the presence of the alkene to which they shall he added. Dichloroketene generated in situ is the best reagent for intermolecular [2+2]-cycloadditions. Dichloroketene is poorer in electrons than the parent ketene and therefore more reactive toward the relatively electron-rich standard alkenes. The reason is that the dominating frontier orbital interaction between these reactants involves the LUMO of the ketene, not its HOMO (see Section 15.2.4). 

Theoretical analysis of this [4% + 27r]-cycloaddition reaction by consideration of frontier-orbital interactions between the electron-rich olefin (highest occupied molecular orbital, HOMO) and the electron-poor 5-nitropyrimidine (LUMO) has shown that the FMO perturbation theory correctly predicts an exclusive regiospecific addition of the enamine to N-l and C-4 of the pyrimidine ring (86JOC4070). 

Consider first the frontier orbital interactions between two ethylene molecules that approach one another in parallel planes ( face to face ). Their HOMOs and LUMOs are indicated in Figure 7-8 on the left and right, respectively. Also shown is the behavior of these orbitals with respect to the symmetry plane bisecting the two... 

Fig. 2. Frontier orbital interactions between 16e-L5M and a diaminophosphenium cation.

Frontier orbital interactions between a photochemically excited molecule and a ground-state molecule... 

S2N2 dimerizes rapidly to S4N4 in the presence of nucleophiles and this process can be understood in terms of the frontier orbital interactions between the singly occupied molecular orbital (SOMO) of an S2N2 radical anion and the LUMO of an S2N2 molecule. The polymerization of crystalline S2N2 is a nondiffusive process that produces a mixture of monoclinic (90%) and orthorhombic (10%) (SN). ... 

Consider first the frontier orbital interactions between two ethylene molecules that approach one another in parallel planes ( face to face ). Their HOMOs and LUMOs are indicated in Figure 7-8 on the left and right, respectively. Also shown is the behavior of these orbitals with respect to the symmetry plane bisecting the two breaking ir bonds. Since the HOMOs are symmetric and the LUMOs are antisymmetric with respect to this operation, there is a symmetry mismatch between the HOMO of one molecule and the LUMO of the other. The symmetry-allowed combination is between the two filled HOMOs. Since the interaction of two filled molecular orbitals of the same energy is destabilizing, the reaction will not occur thermally. 

FIGURE 10.2. Frontier orbital interactions between the two AH3 fragments in staggered and eclipsed A2II6 (a) staggered and (b) eclipsed. 

Fig. 10 (a) Schematic illustration of the frontier orbital interactions between Pt4CH2 and O2. (b) Contours of related SOMOs of Pt4CH2 and O2... 

Hudson and Klopman proposed an equation to describe the effect of orbital perturbation of two molecules on chemical reactivity. Their hypothesis is that the initial perturbation determines the course of a reaction or an interaction. They applied quantum-mechanical method to treat the encounter of two interacting systems as reactivity. Their equation for interaction energy can be simplified by including only two terms [10.2.2] the Coulombic interaction and the frontier orbital interaction between HOMO and LUMO. 

The bfetime of Sj is too short and so oxetane formation is much faster than C-C bond rotation. The formation of oxetane can be explained from FMO approach. The frontier orbital interactions between half occupied n orbital of carbonyl oxygen atom (LUMO) with the k orbital of electron rich alkene (HOMO) take place to form a C,C-diradical. 

TABLE 82.2 Estimated Frontier Orbital Interactions between 15 and 19 by the PM3-CI Calculation (yVeV)... 

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