Molecular orbitals simple molecules

For large molecules, the equivalent-orbital analysis is the most convenient starting point for a molecular-orbital treatment. In a molecule such as a long-chain paraffin it is possible to write approximate equivalent orbitals corresponding to each bond and then to apply a transformation to obtain the delocalised molecular orbitals. Simple assumption about, the interaction of neighbouring bonds will then lead to estimates of the relative stability of the various energy levels.9... 

Transferable, partially localized MOs—group orbitals. Functional groups such as olefins, ketones, amides, aromatics, etc., contribute a set of transferable orbitals to a molecule. For example, a simple olefin always has -ir and tt orbitals. Even CH3 and CH2 groups contribute partially delocalized molecular orbitals to molecules, and importantly, these include tt(CH3) and tt(CH2) group orbitals that can interact strongly with conventional it systems. 

Another feature of advanced molecular orbital calculations that we can anticipate from this simple example is that calculating matr ix elements for real molecules can be a formidable task. 

Approximate Theoretical. The simplest molecular orbital problem is that of the hydrogen molecule ion (Pig KJ-3), is a preliminary example of all molecular orbital problems to come, w hich, although they may be very complicated, are elaborations on this simple example. 

For a molecule as simple as Fl2, valence bond and molecular- orbital theor-y produce very similar- pictures. The next two sections describe these two approaches. 

For a molecule as simple as Fl2, it is hard to see much difference between the valence bond and molecular orbital methods. The most important differences appear- in molecules with more than two atoms. In those cases, the valence bond method continues to view a molecule as a collection of bonds between connected atoms. The molecular- orbital method, however, leads to a picture in which the sane electron can be associated with many, or even all, of the atoms in a molecule. We ll have more to say about the similarities and differences in valence bond and molecular- orbital theory as we continue to develop their principles, beginning with the simplest alkanes methane, ethane, and propane. 

In Chapter 9, we considered a simple picture of metallic bonding, the electron-sea model The molecular orbital approach leads to a refinement of this model known as band theory. Here, a crystal of a metal is considered to be one huge molecule. Valence electrons of the metal are fed into delocalized molecular orbitals, formed in the usual way from atomic... 

In this section we start, as in valence-bond theory, with a simple molecule, H2, and in the following sections extend the same principles to more complex molecules and solids. In every case, molecular orbitals are built by adding together—the technical term is superimposing—atomic orbitals belonging to the valence shells of the atoms in the molecule. For example, a molecular orbital for Fi2 is... 

Let us first briefly review the construction of molecular orbitals in simple diatomic molecules, AB, using the linear combination of atomic orbitals (LCAO) scheme. The end product for the first long row of the periodic table is the well-known diagram in Fig. 6-1. We focus on two broad principles that are exploited in the construction of this diagram one has to do with symmetry and overlap, the other concerns energies. 

Even though molecular mechanics has given satisfactory results (i.e., results that agree with experimental measurements) for many molecules, it is still not totally reliable, since it does fail in certain cases. A further limitation is that it can be used only in cases for which transferable parameters can be obtained from simple molecules. Molecular orbital calculations do not have this limitation, but to some extent semiempirical MO methods do. 

Burdett JK (1976) The Shapes of Main-Group Molecules A Simple Semi-Quantitative Molecular Orbital Approach. 31 67-105... 

Sp and sp-carbon atoms such as a carbonyl group and an alkene are reactive cent-ersl in many kinds of reactions. The n orbitals of a simple molecule such as ethylene or formaldehyde are symmetric in magnimde and antisymmetric in sign with respect to reflection in the molecular plane. Various attempts have been made to rationalize stereoselectivities, and several general ideas have emerged, including a steric basis and an orbital basis. 

Parallel to this use of relatively simple approximations of the molecular orbital theory to the study of complex molecules Berthier has investigated the possible utilization of more refined molecular orbital procedures in the study of necessarily smaller molecules. We owe him the first application of the SCF method to the study of fulvene and azulene and also a pioneering extension, presented in 1953, of the SCF method to the study of molecules with incomplete electronic shells. 

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