Skeleton bonding molecular orbitals

Fig. 2.30. Linear atomic orbital combinations giving rise to skeleton bonding molecular orbitals in the anion (a) Orbital sketches (b) qualitative Molecular Orbital energy diagram...

The delocalization of molecular orbitals lies at the heart of modern chemistry. The concept that the tt orbitals of benzene or naphthalene cover the entire carbon skeleton promoted the successful understanding of conjugated molecules. The work of R. Hoffmann and others has proven that in saturated molecules [Pg.1]

Another example which illustrates beautifully the mixing of a group orbitals to form delocalized molecular orbitals is benzene. First of all the six crcc bond orbitals interact to give six linear combinations which are delocalized over the entire carbon skeleton. The amplitudes of the various bond orbitals in each [Pg.23]

Note, however, that even in such a simple case as this, molecular orbitals do not correspond one-to-one with bonds. For example, the highest-energy a orbital in acetylene is clearly made up of both CC and CH bonding components. The reason, as pointed out in Chapter 2, is that molecular orbitals are written as linear combinations of nuclear-centered basis functions, and will generally be completely delocalized over the entire nuclear skeleton. 

Hiickel molecular orbitals in porphin were investigated by Longuet-Higgins et al. (68), and the extended Hiickel molecular orbital model was applied to metalloporphyrins in attempts by Pullman et al. (93), Ohno et al. (86), and Zerner et al. (120) to explain various experimental observations. Let us briefly consider a description of cyanoferriporphin. According to the Hiickel theory all but the -orbitals of each carbon and nitrogen atom of porphin are used up to form the relatively inert skeleton of single bonds. To describe the -bonding twenty-four molecular orbitals of porphin can then be formed as linear combinations of... 

The broken lines in the diagrams show the trace of the plane of the n orbitals. A reaction will occur readily if X lies on this plane and makes an obtuse angle with the C=Y bond. Molecular models show that the carbon backbone is long and flexible enough to satisfy both of these criteria for the exo reactions. The 6-endo reaction poses problems. If X lies in the n plane, the carbon skeleton has to adopt a boat conformation, leading to a perpendicular attack. However, if X moves slightly out of the n plane, an acceptable compromise can be achieved the attack trajectory becomes non-perpendicular, with a fair nucleophile-n overlap. However, neither condition can be satisfied for a 5-endo reaction. Note that a direct application of Baldwin s empirical rules would have masked these subtleties. 

A theory which shows greater applicability to bonding in cluster compounds is the Polyhedral Skeletal Electron Pair Theory (PSEPT) which allows the probable structure to be deduced from the total number of skeletal bond pairs (400). Molecular orbital calculations show that a closed polyhedron with n vertex atoms is held together by a total of (n + 1) skeletal bond pairs. A nido polyhedron, with one vertex vacant, is held together by (n + 2) skeletal bond pairs, and an arachno polyhedron, with two vacant vertices, by (n + 3) skeletal bond pairs. Further, more open structures are obtainable by adding additional pairs of electrons. This discussion of these polyhedral shapes is normally confined to metal atoms, but it is possible to consider an alkyne, RC=CR, either as an external ligand or as a source of two skeletal CR units. So that, for example, the cluster skeleton in the complex Co4(CO)10(RCCR), shown in Fig. 16, may be considered as a nido trigonal bipyramid (a butterfly cluster) with a coordinated alkyne or as a closo octahedron with two carbon atoms in the core. 

When one (or more) hydrogen atom(s) of the polyethylene monomeric unit is (are) replaced by a heteroatom, an aliphatic or an aromatic group, modifications are induced in the valence band spectrum new peaks, band shift and/or splitting, redistribution of the electronic population among the molecular orbitals will denote the new bonds created in the molecule. Similar effects will be observed for the insertion of heteroatom(s) between two carbon elements of the polymer skeleton. 

The interaction (bonding) of oxygen with the carboneous skeleton is seen e.g. in the C-C bonding band as new structures that are related to the overlap between the C2s and 02p molecular orbitals to form the new C-0 bonds (they are best seen in the PTMO spectrum as the two new structures appearing in the middle of the C-C band). The discussion and assignment of the M.O. of the presented spectra are given elsewhere (, ). We want... 

The current understanding of the structure of benzene is based on molecular orbital (MO) theory. The six carbon atoms of benzene are sp hybridized. The three sp- hybrid orbitals of each carbon atom, which are arranged at angles of 120°, overlap with those of two other carbon atoms and with the s orbital of a hydrogen atom to form the planar a-bonded skeleton of the benzene ring. The p orbital associated with each carbon contains one electron and is perpendicular to the plane of the ring. 

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