Orbital properties 77 electron systems

A complete mechanistic description of these reactions must explain not only their high degree of stereospecificity, but also why four-ir-electron systems undergo conrotatory reactions whereas six-Ji-electron systems undergo disrotatory reactions. Woodward and Hoifinann proposed that the stereochemistry of the reactions is controlled by the symmetry properties of the HOMO of the reacting system. The idea that the HOMO should control the course of the reaction is an example of frontier orbital theory, which holds that it is the electrons of highest energy, i.e., those in the HOMO, that are of prime importance. The symmetry characteristics of the occupied orbitals of 1,3-butadiene are shown in Fig. 11.1. 

Quantitative similarities of molecules can easily be recognized if it is possible to define quantities for molecular parts which are additive as well as transferable. Such quantities can be derived from transferable molecular orbitals because any one-electron property, such as dipole moment, quadrupole moment, kinetic energy, is a sum of the corresponding contributions from all molecular orbitals in a system, if such orbitals are chosen mutually orthogonal. Thus, for each transferable orthogonal molecular orbital there exists, e.g., a transferable orbital dipole moment. Since chemists appreciate additive decompositions of... 

L in Scheme 11.3) departs. Nucleophilic addition to the intermediate benzyne (step D) is readily explained by perturbative MO arguments. The extra and orbitals of benzyne are compared to those of ethylene in Figure 11.7. The aromatic n system is not involved in the special properties of benzyne. The third benzyne n bond is due to the overlap in fashion of the two sp2 hybrid orbitals which lie in the nodal plane of the intact 6 electron system. Two factors contribute to a very low LUMO for benzyne. First, the sp2 hybrid orbitals are lower in energy than the 2p orbitals from which the ethylene orbitals are constructed. Second, the intrinsic interaction between the two sp2 orbitals is less than the normal / cc since the orbitals have less p character and are tipped away from each other. The low LUMO of benzyne makes the molecule a strong Lewis acid, susceptible to attack by bases, and a reactive dienophile in Diels-Alder reactions, as we shall see later. 

Write a one-page essay on the properties and use of orbitals in the description of many-electron systems. Your answer should include at least ten distinct points. Use of equations is encouraged, but the equations should be verbally interpreted. 

The discovery of electrical conductivity and superconductivity in crystalline materials derived from conventional molecular species effectively introduces a new area of synthetic chemistry. The conductivity is associated with specific molecular arrays and synthesis of the materials requires the ability to conceive and implement the preparation of a particular crystalline state. Molecular conductors are derived from pairs of redox reagents, an area where heterocyclic systems are well established. Placement of heteroatoms of selected electronegativity at chosen positions in a delocalized electron system offers a subtle and effective means for altering the orbital energies which ultimately control the electron transfer properties of redox reagents. 

Because the /-orbitals are fairly well shielded from the valence effects it might appear that a relationship between the Dq and Fr parameters could be calculated. However, it seems that the 4/-orbital wave functions are not known sufficiently well for this purpose, and Dq and Fr are to be treated as independent variables. This makes the interpretation of /-electron system properties, even in high symmetry, a much less straightforward business than for -systems. 

The foregoing facts suggest that the unique spectral properties of polysilanes are due to the silicon-silicon bond acting as a chromophore, probably through the use of vacant d orbitals of the silicon atom. In addition, it seems likely that there exists an enhanced conjugation through overlap of d and it orbitals between substituents with 7r-electron systems, such as phenyl and vinyl, and polysilane chains (53, 81,153). 

Clear theoretical evidence is presented to show that the u-electron systems of benzenoid aromatic molecules are described well in terms of localized, non-orthogonal, singly-occupied orbitals. The characteristic properties of molecules such as benzene or naphthalene arise from a profoundly quantum mechanical phenomenon, namely the mode of coupling of the electron spins, rather than from any supposed delocalization of the orbitals. Other systems considered include azobenzenes, such as pyridine, five-membered rings, such as furan, and inorganic heterocycles, such as borazine ( inorganic benzene ). 

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