Molecular orbital methods qualitative application

In this section, the conceptual framework of molecular orbital theory is developed. Applications are presented and problems are given and solved within qualitative and semi-empirical models of electronic structure. Ab Initio approaches to these same matters, whose solutions require the use of digital computers, are treated later in Section 6. Semi-empirical methods, most of which also require access to a computer, are treated in this section and in Appendix F. 

There are (at least) two major opportunities for research by those interested in this topic. On the computational side, there is definite room for improvement in simulation methods. Right now none of the simulation approaches has the user friendliness that has brought electronic-structure calculation into the realm of routine applicability by nonspecialists. Nor has the field seen the development of the qualitative or semiquantitative models that did so much to make the results of molecular orbital calculations useful to organic chemists. On the experimental side, it will be obvious to the reader that the techniques for detecting the effects of nonstatistical dynamics are still very rudimentary and indirect. There is clearly room for creative scientists to come up with techniques whose results can give us more direct insight into these issues. 

The latter three, (4)—(6), are utilized for qualitative elucidation of the observed ICD bands in signs, and the former three, (1)—(3), are successfully applicable for quantitative calculations of the observed ICD bands in both signs and magnitudes. The ICD of P-cyclodextrin complexes with benzene derivatives or azanaphthalenes162) has been analyzed by a molecular orbital calculation, using an approximation of PPP-type, which has been compared with the theoretical spectra calculated by using the CNDO/S-CI method on the basis of the MCD spectra 16S). 

Some general aspects related to the derivation, and interpretations of ELF analysis, as well as some representative applications have been briefly discussed. The ELF has emerged as a powerful tool to understand in a qualitative way the behaviour of the electrons in a nuclei system. It is possible to explain a great variety of bonding situations ranging from the most standard covalent bond to the metallic bond. The ELF is a well-defined function with a nice pragmatic characteristic. It does not depend neither on the method of calculation nor on the basis set used. Its application to understand new bond phenomenon is already well documented and it can be used safely. Its relationship with the Pauli exclusion principle has been carefully studied, and its consequence to understand the chemical concept of electron pair has also been discussed. A point to be further studied is its application to transition metal atoms with an open d-shell. The role of the nodes of the molecular orbitals and the meaning of ELF values below 0.5 should be clarified. 

A related method to interpret the diastereofecial selectivities of the reactions of double bonds has been proposed by Dannenberg and coworkers [8, 13, 14,]. Tins method also relies on the 7t frontier orbitals of non symmetrical molecules, and proposes breaking the symmetry of the n or it orbitals due to polarization induced by the substituents. Application of frontier molecular orbital theory, taking into account only the substrate MOs, gives a qualitative trend of stereoselection in a number of nucleophilic (reductions of carbonyl compounds) and electrophilic reactions. 

By far, the theoretical approaches that experimental inorganic chemists are most familiar with and in fact use to solve questions quickly and qualitatively are the simple Hiickel method and Hoffmann s extended Hiickel theory. These approaches are used in concert with the application of symmetry principles in the building of symmetry adapted linear combinations (SALCs) or group orbitals. The ab initio and other SCF procedures outlined above produce MOs that are treated by group theory as well, but that type of rigor is not usually necessary to achieve good qualitative pictures of the character and relative orderings of the molecular orbitals. 

Ab initio and semiempirical computational methods have proved extremely useful. But also needed is a simple conceptual scheme that enables one to predict the broad outlines of a calculation in advance, or else to rationalize a coii5)uted result in a fairly simple way. Chemistry requires conceptual schemes, simple enough to carry around in one s head, with which new information can be evaluated and related to other information. Such a theory has developed alongside the mathematical methods described in earlier chapters. We shall refer to it as qualitative molecular orbital theory (QMOT). In this chapter we describe selected aspects of this many-faceted subject and illustrate QMOT applications to questions of molecular shape and conformation, and reaction stereochemistry. 

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