Isomerism of X4Y2 and X2H2Y2 Molecules

In this section we shall consider conformational isomerism in typical hexaatomic molecules, X4Y2 and X2H2Y2. A diagram which depicts the MO s spannend by the 2px lone pairs of the heteroatoms X and the MO s of the coupling unit Y—Y in planar Y2X4 is shown in Fig. 20. We distinguish two cases  

We note here, that the eclipsed conformation of N2O4 can be viewed as being a sigma aromatic system geometrically similar to the pi aromatic system napthalene as illustrated below  

4 interaction was eiiminated by setting the appropriate off diagonal matrix element, Hjj, along with the corresponding overlap matrix element, Sy, to zero. 

Ab initio calculations furthermore, predict, in agreement with experiment, the staggered conformation of B2F4 to be more stable than the planar conformation c) Results of CNDO/2 calculations are shown below and confirm oiu analysis disscussed above. 

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