Antibonding a*-molecular orbital

The ketone receives an electron in the antibonding -rr molecular orbital to form it anion radical that is transformed into a (—C—Cl) particle by an intramolecular electron transfer to the antibonding a molecular orbital of the —C—Cl bond. The (—C—Cl)-bond then fragments to form a chloride ion and the corresponding radical (Scheme 8-2). 

Just as bonding and antibonding a molecular orbitals result from the combination of two s atomic orbitals in H2 (Section 1.6), so bonding and antibonding tt molecular orbitals result from the combination of two p atomic orbitals in ethylene. As shown in Figure 1.17, the n bonding MO has no node between nuclei and results from combination of p orbital lobes with... 

Figure 1.1. Representation of the geometry and energy of the bonding and antibonding ( a) molecular orbitals formed by covalent bonding between the Is atomic orbitals of two H atoms, A and B. The probability function, indicates electron density along the internuclear A-B axis.

Figure 1.9. A schematic representation of the formation of bonding (o) and antibonding (a ) molecular orbitals of hydrogen (Hj) by the combination of two equivalent Is hydrogen atomic orbitals. The signs (+) and (-) do not refer to charges but rather to the sign of the wave function /, whose square (t f ) gives the probabiUty of finding the electron(s) in the volume shown.

FIGURE 2.7 The formation of a carbon-hydrogen bond through the overlap of an s/> hybrid orbital with a hydrogen It atomic orbital. Note the formation of both the bonding (o) molecular orbital and the antibonding (a ) molecular orbital. 

Fig. 2.5 Schematic representations of (a) the bonding (cr ) and (b) the antibonding (a ) molecular orbitals in the H2 molecule. The H nuclei are represented by black dots. The red orbital lobes could equally well be marked with a -I- sign, and the blue lobes with a — sign (or vice versa) to indicate the sign of the wavefunction. (c) More realistic representations of the molecular orbitals of H2, generated computationally using Spartan 04, Wavefunction Inc. 2003.

A molecular orbital description of benzene has three tt orbitals that are bonding and three that are antibonding Each of the bonding orbitals is fully occupied (two electrons each) and the antibonding orbitals are vacant... 

As is tr-ue for all orbitals, a tt orbital may contain a maximum of two electrons. Ethylene has two tt electrons, and these occupy the bonding tt molecular- orbital, which is the HOMO. The antibonding tt molecular orbital is vacant, and is the LUMO. 

In ethylene, both the HOMO and LUMO are formed primarily from p orbitals from the two carbons. The carbons lie in the YZ-plane, and so the p,j orbitals lie above and below the C-C bond. In the HOMO, the orbitals have like signs, and so they combine to form a bonding n molecular orbital. In contrast, in the LUMO, they have opposite signs, indicating that they combine to form an antibonding Tt molecular orbital. 

Antibonding Molecular Orbital. A Molecular Orbital that is andbonding between particular atomic centers. The opposite is a Bonding Molecular Orbital. 

Nonbonded Molecular Orbital. A molecular orbital that does not show any significant bonding or antibonding characteristics. Nonbonded molecular orbitals often correspond to Lone Pairs. 

Figure 1.18 A molecular orbital description of the C=C tt bond in ethylene. The lower-energy, tt bonding MO results from a combination of p orbital lobes with the same algebraic sign and is filled. The higher-energy, -tt antibonding MO results from a combination of p orbital lobes with the opposite algebraic signs and is unfilled.

Antibonding MO (Section 1.11) A molecular orbital that is higher in energy than the atomic orbitals from which it is formed. 

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