Molecular orbitals, patterns

FIGURE 5. Some molecular orbital patterns of the electronic ground state of the phenol molecule. Due to the Cj symmetry of phenol, its MOs are characterized by the a or a" irreducible representations of this group s denotes the corresponding orbital energy in eV... 

Just as (he ir orbitals of the linear three- and four-atom chains formed molecular orbital patterns identical to those of linear Il3 and II4 in Chapter 5, so there is one-to-one ctirrcspondence between the orbitals of cyclic jxdyencs and those derived for cyclic units. Figure 12.5 shows the orbitals of cyclic C3—C(5 sN stems for comparison. There are several patterns which emerge. 

A schematic molecular orbital pattern for a tetrahedral complex with a significant n bonding contribution is shown in Fig. 6.26. It is emphasized that the relative energies shown for the molecular orbitals in this figure are to be regarded as highly flexible. 

The alternating behavior of the annulenes between aromatic and antiaromatic had been predicted earlier by the theoretical chanist Hiickel, who formulated this (4n + 2) rule in 1931. Hiickel s rule expresses the regular molecular-orbital patterns calculated for planar, cyclic conjugated polyenes. The p orbitals mix to give an equal number of tt molecular orbitals, as shown in Figure 15-18. For example, the four p orbitals of cyclobutadiene result... 

Molecular Orbitals Possess Specific Topology, Symmetry, and Energy-Level Patterns... 

The Most Elementary Molecular Orbital Models Contain Symmetry, Nodal Pattern, and Approximate Energy Information... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

The Exclusion Principle is fundamentally important in the theory of electronic structure it leads to the picture of electrons occupying distinct molecular orbitals. Molecular orbitals have well-defined energies and their shapes determine the bonding pattern of molecules. Without the Exclusion Principle, all electrons could occupy the same orbital. 

Electrophilic Aromatic Substitution. The Tt-excessive character of the pyrrole ring makes the indole ring susceptible to electrophilic attack. The reactivity is greater at the 3-position than at the 2-position. This reactivity pattern is suggested both by electron density distributions calculated by molecular orbital methods and by the relative energies of the intermediates for electrophilic substitution, as represented by the protonated stmctures (7a) and (7b). Stmcture (7b) is more favorable than (7a) because it retains the ben2enoid character of the carbocycHc ring (12). 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

High-level molecular- orbital calculations of cyclobutadiene itself and experimentally measured bond distances of a stable, highly substituted derivative both reveal a pattern of alternating short and long bonds characteristic of a rectangular-, rather than square, geometry. 

According to a molecular orbital calculation of Veber and Lwowski, isoindole should be favored over its tautomer, isoindolenine, by about 8 kcal/mole. However, the calculated electronic distribution is markedly different in the two oases, particularly at position 1, and it is to be expected that the nature and pattern of substituents will play an important role in determining the position of tautomeric equilibrium between these two species. 

The photoelectron spectrum of nitrogen (N2) has several peaks, a pattern indicating that electrons can be found in several energy levels in the molecule. Each main group of lines corresponds to the energy of a molecular orbital. The additional "fine structure" on some of the groups of lines is due to the excitation of molecular vibration when an electron is expelled. 

This chapter covers the literatures of phosph(v)azenes. The general pattern of development in this area is similar to that observed in previous yearly reviews with additional interest being shown in polyphosphazenes and in a variety of molecular orbital calculations of both linear and cyclic phosphazenes. 

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