Qualitative orbital theory

Frontier Molecular Orbital theory is closely related to various schemes of qualitative orbital theory where interactions between fragment MOs are considered. Ligand field theory, as commonly used in systems involving coordination to metal atoms, can be considered as a special case where only the d-orbitals on the metal and selected orbitals of the ligands are considered. 

Although a separation of electronic and nuclear motion provides an important simplification and appealing qualitative model for chemistry, the electronic Sclirodinger equation is still fomiidable. Efforts to solve it approximately and apply these solutions to the study of spectroscopy, stmcture and chemical reactions fonn the subject of what is usually called electronic structure theory or quantum chemistry. The starting point for most calculations and the foundation of molecular orbital theory is the independent-particle approximation. 

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. 

Frontier Orbital theory is qualitative, so there is no need for great accuracy in the calculation as long as it produces the approximate form of the orbitals. 

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

It is important to realize that whenever qualitative or frontier molecular orbital theory is invoked, the description is within the orbital (Hartree-Fock or Density Functional) model for the electronic wave function. In other words, rationalizing a trend in computational results by qualitative MO theory is only valid if the effect is present at the HF or DFT level. If the majority of the variation is due to electron correlation, an explanation in terms of interacting orbitals is not appropriate. 

Hartree-Fock (HF), molecular orbital theory satisfies most of the criteria, but qualitative failures and quantitative discrepancies with experiment often render it useless. Methods that systematically account for electron correlation, employed in pursuit of more accurate predictions, often lack a consistent, interpretive apparatus. Among these methods, electron propagator theory [1] is distinguished by its retention of many conceptual advantages that facilitate interpretation of molecular structure and spectra [2, 3, 4, 5, 6, 7, 8, 9]. 

The period 1930-1980s may be the golden age for the growth of qualitative theories and conceptual models. As is well known, the frontier molecular orbital theory [1-3], Woodward-Hoffmann rules [4, 5], and the resonance theory [6] have equipped chemists well for rationalizing and predicting pericyclic reaction mechanisms or molecular properties with fundamental concepts such as orbital symmetry and hybridization. Remarkable advances in aeative synthesis and fine characterization during recent years appeal for new conceptual models. 

If molecules or atoms form a chemical bond with the surface upon adsorption, we call this chemisorption. To describe the chemisorption bond we need to briefly review a simplified form of molecular orbital theory. This is also necessary to appreciate, at least qualitatively, how a catalyst works. As described in Qiapter 1, the essence of catalytic action is often that it assists in breaking strong intramolecular bonds at low temperatures. We aim to explain how this happens in a simplified, qualitative electronic picture. 

The first pair of examples we would like to discuss occurs in a field which lends itself naturally to be conquered by theory. Indeed, the past three decades have seen the exploration of mechanistic details of pericyclic reactions as one of the major success stories of computational chemistry. Rooted in qualitative molecular orbital theory, the key concept of... 

L. Salem, The Molecular Orbital Theory of Conjugated Systems, W. A. Benjamin, New York, 1966 B. M. Gimarc, Molecular Structure and Bonding The Qualitative Molecular Orbital Approach, Academic Press, New York, 1979 T. A. Albright, J. K. Burdet and M. H. Whangbo, Orbital Interactions in Chemistry, Wiley, New York, 1985. 

In this section we give a simple and qualitative description of chemisorption in terms of molecular orbital theory. It should provide a feeling for why some atoms such as potassium or chlorine acquire positive or negative charge upon adsorption, while other atoms remain more or less neutral. We explain qualitatively why a molecule adsorbs associatively or dissociatively, and we discuss the role of the work function in dissociation. The text is meant to provide some elementary background for the chapters on photoemission, thermal desorption and vibrational spectroscopy. We avoid theoretical formulae and refer for thorough treatments of chemisorption to the literature [2,6-8],... 

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