The Effect of Nonbonded Interactions on Molecular Structure

We are now prepared to examine how attractive or repulsive nonbonded interactions determine molecular geometry. We shall discuss representative examples where pi and/or sigma nonbonded interactions obtain. In each case, we provide computational data in support of general theoretical arguments as well as pertinent experimental results. It should be mentioned that only crucial indices of nonbonded interactions are provided and the survey of the experimental work is by necessity incomplete, i.e., it would take volumes to consider all available data. Nonetheless, at the end of this chapter, the reader should be able to apply the key ideas to problems of direct interest to him. 

It should be emphasized that OEMO theory does not treat explicitly interelectronic repulsions, which are reproduced by the two electron part of a complete hamiltonian operator, as well as internuclear repulsions. These effects are partially accounted for by virtue of the empirical evaluation of matrix elements in the OEMO method and will be grouped undo-the heading steric effects . It is obvious that steric effects will tend to favor uncongested structures. It is then apparent that the OEMO theory will lead to incorrect predictions when steric effects become a dominant influence. 

These indices depend on MO symmetry properties which ate well reproduced by any type of calculation. On the other hand, orbital energies and total energies do depend crucially on the type of calculation employed. For example, the three occupied pi MO s of cis 1,2-difluoroethylene are calculated as 

We shall first consider how nonbonded interactions influence bond angles in molecules. Our approach will be illustrated by reference to the model systems difluoro-methane and 1,1-difluoroethylene. In these problems, we shall consider not only stabilizing orbital interactions but also overlap repulsion in order to demonstrate some interesting trends which obtain in these an e problems. 

We first consider pi nonbonded interactions in difluoromethane and employ the dissection shown below. The appropriate interaction diagram for the pi system only is shown in Fig. 13. From this diagram it is obvious that only the symmetric 

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