Qualitative molecular orbital theory description

Qualitative molecular-orbital theory approaches (and related qualitative treatments) are discussed throughout the text (particularly in Chapters 4 and 6), and a more detailed discussion of the contributions of such approaches presented in Chapter 8. As with the experimental methods discussed in Chapter 2, the topics presented in the present chapter are associated with numerous abbreviations and acronyms (and alternative titles). Both to serve as a key to these abbreviations, and as a source of reference to the numerous theoretical approaches now available, they are listed along with brief descriptions and references to further information in Appendix C. 

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. 

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],... 

The same principles that we have used for the description of diatomic molecules will now be used in a description of the electronic structures of triatomic molecules. However, let it be clear that in using molecular-orbital theory with any hope of success, we first have to know the molecular geometry. Only in very rare cases is it possible from qualitative molecular-orbital considerations to predict the geometry of a given molecule. Usually this can only be discussed after a thorough calculation has been carried out. 

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. 

One-electron picture of molecular electronic structure provides electronic wavefunction, electronic levels, and ionization potentials. The one-electron model gives a concept of chemical bonding and stimulates experimental tests and predictions. In this picture, orbital energies are equal to ionization potentials and electron affinities. The most systematic approach to calculate these quantities is based on the Hartree-Fock molecular orbital theory that includes many of necessary criteria but very often fails in qualitative and quantitative descriptions of experimental observations. 

There are a number of different approaches to the description of molecular electronic states. In this section we describe molecular orbital theory, which has been by far the most significant and popular approach to both the qualitative and quantitative description of molecular electronic structure. In subsequent sections we will describe the theory of the correlation of molecular states to the Russell Saunders states of the separated atoms we will also discuss what is known as the united atom approach to the description of molecular electronic states, an approach which is confined to diatomic molecules. 

An alternative to the molecular orbital description of CO bonding to a transition metal is proposed here. The new description is based on ab initio calculations which include important electronic correlation effects neglected in molecular orbital theory. The resulting valence bond picture, which includes "bent-bonds" for CO2 rather than O and 7T bonds, has striking similarities to the description given in qualitative discussions by Pauling many years ago. 

A simple description of electrons in a solid is the model of a free electron gas in the lattice of the ions as developed for the description of metals and metal clusters. The interaction of electrons and ions is restricted to Coulomb forces. This model is called a jellium model. Despite its simplicity, the model explains qualitatively several phenomena observed in the bulk and on the surface of metals. For a further development of the description of electrons in solids, the free electron gas can be treated by the rules of quantum mechanics. This treatment leads to the band model. Despite the complexity of the band model, Hoffmann presented a simple description of bands in solids based on the molecular orbital theory of organic molecules that will also be discussed below. 

The preceding section focused mostly on qualitative description of the dependence of ORR activity on the structure of N4-metaUomacrocyclic complexes and some special modification procedures which can be used to improve activity. In this section, much of the discussion will focus on quantitative decription of the parameters which influence the ORR activity of N4-metallomacrocyclic complexes. Specifically, the dependence of activity on the properties of the central metal ion will be dicussed in relation to the driving force of the reaction. In addition to this, the molecular orbital theory and the concept of intermolecular are used to describe the interaction between oxygen the central metal ion in N4-macrocyclic complexes and how this interaction influences the ORR activity of the complex. 

A second concept which makes valence bond theory useful for the structural description of complex molecules is resonance theory. Resonance theory represents an extension of valence bond theory that applies to molecules for which more than one Lewis structure can be written. Its usefulness to organic chemistry lies in its being a convenient way of depicting electron delocalization, particularly in conjugated systems and in reactive intermediates. We will use resonance arguments in this qualitative way, rather than in their fullest form of development, which is an extensive mathematic treatment intended as an alternative to molecular orbital theory. The elements of resonance theory necessary for qualitative applications are simple and can be summarized as follows ... 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. 

Section treats the spatial, angular momentum, and spin symmetries of the many-electron wavefunctions that are formed as antisymmetrized products of atomic or molecular orbitals. Proper coupling of angular momenta (orbital and spin) is covered here, and atomic and molecular term symbols are treated. The need to include Configuration Interaction to achieve qualitatively correct descriptions of certain species electronic structures is treated here. The role of the resultant Configuration Correlation Diagrams in the Woodward-Hoffmann theory of chemical reactivity is also developed. 

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