Orbital interactions four electron

Fig. XVIII-16. A four-electron two-orbital interaction that a) has no net bonding in the free molecule but can be bonding to a metal surface if (b) the Fermi level is below the antibonding level. In the lower part of the figure, a zero-electron two-orbital situation (c) has no bonding but there can be bonding to a metal surface as in (d) if the Fermi level is above the bonding level. (From Ref. 160.)...

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

In the interaction of a pair of atomic orbitals, two electrons form a bond and four electrons form no bond (Sect. 1.1). The snbstitnted carbocations are stabilized by the electron delocalization (hyperconjngation and resonance) through the interaction of the doubly occupied orbitals on the snbstitnents with the vacant p-orbital on the cation center. The exchange repulsion (Sect. 1.5) is cansed by four electrons. Now... 

The particpation of one through three electrons in the orbital interaction gives rise to stabilization. The destabilization occurs when four electrons participate. 

The alkene with the electron donating group has the HOMO (n) raised by the interaction with the occupied orbital of the substiment. The low-lying SOMO (n ) interacts with the HOMO of the alkene more effectively. The frontier orbital interaction is the interaction (Scheme 30b), which is impossible at the four-membered... 

Steric repulsions come from two orbital-four electron interactions between two occupied orbitals. Facially selective reactions do occur in sterically unbiased systems, and these facial selectivities can be interpreted in terms of unsymmetrical K faces. Particular emphasis has been placed on the dissymmetrization of the orbital extension, i.e., orbital distortions [1, 2]. The orbital distortions are described in (Chapter Orbital Mixing Rules by Inagaki in this volume). Here, we review the effects of unsymmetrization of the orbitals due to phase environment in the vicinity of the reaction centers [3]. 

All the atoms of butadiene lie in a plane defined by the s p hybrid orbitals. Each carbon atom has one remaining p orbital that points perpendicular to the plane, in perfect position for side-by-side overlap. Figure 10-42 shows that all four p orbitals interact to form four delocalized molecular orbitals two are bonding MOs and two are antibonding. The four remaining valence electrons fill the orbitals, leaving the two p orbitals empty. 

Through an interaction of the two singly occupied ji orbitals, a transannular six-center four-electron 71 -ji bond with a formal bond order of 0.67 results... 

Figure 3.58 The mnemonic energy-level-splitting diagram for the four-electron destabilizing interaction of two occupied non-orthogonal orbitals.

For the simple case of one unpaired electron (5 = 1/ 2), the associated magnetic moment is not a simple number but is directionally oriented—that is, is anisotropic. Taking spin-orbit interactions into account for EPR spectra leads to four limiting cases ... 

In other papers408, the discussion of conjugative destabilization has been based on an incorrect assumption, L e that the destabilizing interaction of filled orbitals increases as their energy separation decreases, while, as we have seen in Eq. (5 ), four electron overlap repulsions depend not on the energy separation of the two interacting MO s but rather on the sum of them. Thus, situation A is less destabilizing than situation B (constant Sy). 

Numerous reports published in recent years have focused on carbon-centered radicals derived from compounds with selected substitution patterns such as alkanes [40,43,47], halogenated alkanes [43,48,49,51-57], alkenes [19], benzene derivatives [43,47], ethers [51,58], aldehydes [48], amines [10,59], amino acids [23,60-67] etc. Particularly significant advances have been made in the theoretical treatment of radicals occurring in polymer chemistry and biological chemistry. The stabilization of radicals in all of these compounds is due to the interaction of the molecular orbital carrying the unpaired electron with energetically and spatially adjacent molecular orbitals, and four typical scenarios appear to cover all known cases [20]. 

---