Molecular distortion shape change

The preceding discussion is relevant for vertical ionization potentials only, i.e. for ionization processes without a change in the shape of the molecule. The effect of molecular distortions due to ionization has to be estimated separately and added to AEt and Et— ). Recently, the validity of Koopmans theorem has been questioned within the SCF approximation itself. It should be noted that the orbitals of a Slater determinant are defined except for a unitary transformation, and the... 

No matter what type of host, the strongest attractions occur when combination with the guest causes the smallest amount of distortion of the host. That is, a fully preorganized host will bind better than a host whose molecular shape must change in order to accommodate the guest. 

The discussion of the distortion of the water molecule from a linear to a bent shape allows a tentative general conclusion to be reached. This is that if a distortion of a molecule from a particular symmetry allows two MOs to mix, so that the lower occupied orbital is stabilized at the expense of the higher vacant orbital, such a distortion will occur and will confer stability on the distorted molecule. A gain of stability will only occur if the two orbitals concerned in the stabilization process are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).. If both orbitals are doubly occupied, interaction between them does not lead to any change in stability. The generality of this conclusion is explored further in the next sections of this chapter and in Chapter 6. 

Despite the difficulties mentioned above, by now a critical mass of data has been compiled enabling us to formulate a concise picture of the nature of the IT effect in these fascinating materials. In the rest of this paper we would like to summarize such experimental data with respect to the following questions Are the fulleride ions found in various compounds distorted Is the distortion dominated by the molecular Jahn-Teller effect or by the potential field of the environment In which direction is the molecule distorted and what is the shape of the distortion Is the distortion static or dynamic The way we approach these questions is the study of symmetry change through vibrational and electronic transitions. The fundamental concepts of these methods will be summarized in the next section. 

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