Qualitative molecular orbital theory applications

A superb treatment of applied molecular orbital theory and its application to organic, inorganic and solid state chemistry. Perhaps the best source for appreciating the power of the independent-particle approximation and its remarkable ability to account for qualitative behaviour in chemical systems. 

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. 

SECTION 1.5. QUALITATIVE APPLICATION OF MOLECULAR ORBITAL THEORY... 

This review gives some indication of the applications of molecular-orbital theory to organometallic chemistry. The interaction between theory and experiment has been quite fertile in this field, and even qualitative treatments have been of predictive value. Quantitative treatments have been few in number, mainly because of the complexity of these large molecules. Unfortunately, even the available quantitative treatments differ considerably in... 

In this chapter, we discuss the various applications of group theory to chemical problems. These include the description of structure and bonding based on hybridization and molecular orbital theories, selection rules in infrared and Raman spectroscopy, and symmetry of molecular vibrations. As will be seen, even though most of the arguments used are qualitative in nature, meaningful results and conclusions can be obtained. 

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. 

The more recent treatment of the covalent bond, based on the application of the principles of wave mechanics, has developed in two distinct forms, usually termed the valence-bond and molecular-orbital theories, respectively. Although ultimately there is no inconsistency between these two theories, they do in fact approach the problem of chemical binding from different points of view, and we shall generally find that for our purposes the valence-bond treatment is the more suitable. This theory starts from concepts already familiar to the chemist and its conclusions can usually be expressed verbally in qualitative terms the molecular-orbital theory, on the other hand, is more mathematical in its approach and lends itself less readily to such an interpretation. We shall, therefore, first discuss the valency-bond theory, and refer only briefly to the molecular-orbital treatment later in the chapter. 

In contrast to the claim (10) that the ECW model "disguises the relationship between reactivity and periodic elemental properties", elementary application of frontier molecular orbital theory (H) can be used to understand the trends. Using qualitative trends in ionization energies, inductive effects, electronegativities and partial charge/size ratios, one can estimate trends in the HOMO-LUMO separation of the donor and acceptor. Increasing the separation decreases the covalent and increases the electrostatic nature of the interaction. Decreasing the separation has the opposite effect. Trends in the reported acid and base parameters as well as in the Ey E0 and C C0 products can be understood in this way. 

Fortunately, this issue has so far not appeared as a limitation in practical applications of COHP, and the reason is simple. The successes of one-electron theories in quantum chemistry (see Section 2.11.1) is due in no small part to the intimate connection between the sum of one-electron eigenvalues (COHP terms) and the total energy, and this connection is one of the pillars of qualitative molecular orbital theory. Experience also reveals that the most important chemical information contained in the COHP (and also COOP) analyses results from the shape of these bonding indicators. 

This discussion has been deliberately qualitative in order to avoid complicating the fundamental concepts of QTAIM with the mathematical equations necessary to describe it completely. Further details of the theory and its applications are detailed in the references. The point to be made here is that there is a way to reconnect the results of molecular orbital theory with the familiar concepts of structure and bonding that have served organic chemists well. The QTAIM approach is not yet available in computational packages readily accessible to organic chemists, however. 

Gimarc, B.M. (1974). Applications of qualitative molecular orbital theory. Accounts of Chemical Research, 1, 384-392. 

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