Torsional Isomerism of Cations and Anions

Recent ab initio calculations have shown, in agreement with the conclusions reached by OEMO theory, that the most stable conformation of methyl vinyl ether is the Cs conformer The calculated relative energies of the four conformations of methyl vinyl ether at the STO-3G and 4-3IG levels are shown above along with the appropriate indices of nonbonded attraction. As can be seen, the greater stability of the Cs conformation can be attributed to an attractive 1,5 pi nonbonded interaction. 

Finally, experimental measurement as well asab initio computation ow that the methyl rotational barrier is also higher in the cis than the trans conformation. These results are shown in Table 17. 

The theoretical approach we have developed for determining the preferred conformation of uncharged molecules is also applicable to those cases where the molecule bears a net positive or neptive charge. In this section we wUl examine the importance of nonbonded interactions in torsional isomerism of selected cations and anions. 

We first consider the electronic factors which influence torsional isomerism in cations. Our first model system will be the 2-propyl cation and the two conformations which we will compare are diown below  

The dissection of this cation, illustrated for the staggered conformation, is shown above and the interaction diagrams for the staggered and eclipsed conformations are shown in Fig. 31. It is concluded that the eclipsed conformation will be favored over the staggered conformation since the two electron stabilizing 0i interaction is greater for the former case. Furthermore, change transfer from to i/zj results 

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