Qualitative MO theory

The axial C—H bonds are weaker flian the equatorial C—H bonds as can be demonstrated by a strongly shifted C—H stretching frequency in the IR spectrum. Axial C-2 and C-6 methyl groins lower the ionization potential of the lone-pair electrons on nitrogen substantially more than do equatorial C-2 or C-6 methyl groups. Ehscuss the relationship between these observations and provide a rationalization in terms of qualitative MO 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. 

For problems of structure the CM model is not the first choice. The development of qualitative MO models over recent years has made an enormous impact on these questions so that, as a whole, this subject is relatively well understood. With regard to questions of reactivity, however, the situation is quite different. By its very design, qualitative MO theory is most readily applied to static problems where the interacting groups are stationary with respect to one another. There is no simple way in which motion along a reaction co-ordinate can be readily accommodated within such a framework. 

Gimarc has specified a set of rules for the application of qualitative MO theory [73], particularly for the determination of molecular structures and conformations, following the pioneering work of Walsh [74]. These are repeated here ... 

For properties which can be explained by qualitative MO theory, only the valence electrons need be considered. 

Ligand Field Theory as Extended CFT2-6 9 6.2.2.1 The identification of A in qualitative MO theory... 

We summarize here a procedure to predict the feasibility and the stereochemistry of thermally concerted reactions involving cyclic transition states. The 1,2 rearrangement of carbocations will be used to illustrate the approach. This is a very important reaction of carbocations which we have discussed in other chapters. We use it here as an example to illustrate how qualitative MO theory can give insight into how and why reactions occur ... 

MO-THEORETICAL ANALYSIS OF CHEMICAL BONDING BEYOND A QUALITATIVE MO THEORY... 

The most important purpose of this tutorial is, however, to show that it is possible to extend the traditional qualitative MO theory to arrive at a more detailed analysis of molecular interactions. Explicit calculation of physically well-defined energies enables one to obtain a more complete view of the physics... 

For a situation where we have four electrons on the two centers (10), VB theory predicts a doubling of the Pauli repulsion, and the following expression is obtained by complete analogy to qualitative MO theory ... 

Whenever the VB and MO wave functions of an electronic state are equivalent, the VB energy can be estimated using qualitative MO theory. 

Pauli repulsion The repulsion of two electrons with identical spins on two centers, (e.g., A B). This repulsion also appears in VB structures bearing three and four electrons, that is, A B, A B, A B. The Pauli repulsion is precisely the same as the overlap repulsion known from qualitative MO theory. (See Table 3.1.)... 

Structures of A anions and XA molecular anions. Tossell (1983) was able to rationalize a number of structural properties of polyatomic anions using qualitative MO theory in minerals, for example,... 

Qualitative MO theory (extended Hiickel molecular orbital ((EHMO) method) was employed for a rationalization of the electron density distributed over the three centers S-I-I in diiodine adducts of 4,5-ethylenedithio-l,3-dithiole-2-thione <1999IC4626>. 

Snatzke, G. (1979), Circular Dichroism and Absolute Conformation—Application of Qualitative MO Theory to Chiroptical Phenomena", Angew. Chem. Int. Ed. Engl. 18, 363. 

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