Orbitals anti-bonding

The LUMO is similar to the HOMO, but the lobes do not align. Thi> appears more like an anti-bonding orbital. 

The same conclusion may again be reached by considering only the HOMO orbital. Figure 15.24. For the conrotatory path the orbital interaction leads directly to a bonding orbital, while the orbital phases for the disrotatory motion lead to an anti-bonding orbital. 

There is (a) cr-donation from a filled oxygen orbital to an empty platinum orbital and (b) 7r back-bonding from a filled metal d orbital into an empty oxygen 7r -anti-bonding orbital. 

Jorgensen CK (1975) Partly Filled Shells Constituting Anti-bonding Orbitals with Higher Ionization Energy than Their Bonding Counterparts. 22 49-81 Jorgensen CK (1975) Photo-Electron Spectra of Non-Metallic Solids and Consequences for Quantum Chemistry. 24 1-58... 

Much theoretical work went into trying to understand these results more completely. Newns constructed a simple ID model that incorporated the idea of electrons hopping in and out of the tt anti-bonding orbital of NO as the means by which hot electron-hole pairs could transfer energy to the NO molecule.26 Using reasonable assumption, he was able to quantitatively reproduce the experimentally-observed surface temperature and incidence energy dependence. 

The group ofWalborsky probably has described one of the first true anionic/radi-cal domino process in their synthesis of the spirocyclopropyl ether 2-733 starting from the tertiary allylic bromide 2-730 (Scheme 2.161) [369]. The first step is a Michael addition with methoxide which led to the malonate anion 2-731. It follows a displacement of the tertiary bromide and a subsequent ring closure which is thought to involve a SET from the anionic center to the carbon-bromine anti bonding orbital to produce the diradical 2-732 and a bromide anion. An obvious alternative Sn2 halide displacement was excluded due to steric reasons and the ease with which the reaction proceeded. 

Figure 16.1 The chemical hardness of an atom, molecule, or ion is defined to be half. The value of the energy gap between the bonding orbitals (HOMO—highest orbitals occupied by electrons), and the anti-bonding orbitals (LUMO—lowest orbitals unoccupied by electrons). The zero level is the vacumn level, so I is the ionization energy, and A is the electron affinity, (a) For hard molecules the gap is large (b) it is small for soft molecules. The solid circles represent valence electrons. Adapted from Atkins (1991).

The bonding in the XeF2 molecule can be explained quite simply in terms of a 3-center, 4 electron bond that spans all three atoms in the molecule. The bonding in this molecular orbital description involves the filled 5pz orbital of Xe and the half-filled 2pz orbitals of the two F-atoms. The linear combination of these three atomic orbitals affords one bonding, one non-bonding and one anti-bonding orbital, as depicted below ... 

Lewis acid sites have empty orbitals able to accept electron density from the occupied orbitals of a Lewis base, in parallel with back-donation from the catalyst to the empty anti-bonding orbitals of the base [33]. This interaction leads to the formation of an activated acid-base adduct. In the case of alkanes activation may proceed by hydride abstraction [38]. Y and Beta are good examples of zeolites with Lewis acidity, often quite significant for catalysis [39, 40]. 

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