Symmetry Demands on the Reaction Path

It is important to note that these rules are valid only on condition that the MERP is a continuous line without bifurcation points and all critical points lying on this line are nondegenerate [75, 76]. Next we give some examples to illustrate how the rules a), b), c) operate in chemical reactions. 

In the isomerization reaction of the T-shaped structures VIIa VIIb VIIc, all nuclear motions, both with the identical and the different ligands R, occur in a common plane. No pyramidal structures are present on the MERP. 

All nondegenerate reactions must obey the requirements a)-c). For the degenerate transformations (in which a simple exchange of positions of equivalent nuclei occurs), the symmetry considerations admit of supplementary, in comparison with the structures of the reactants and the products, symmetry elements in transition state structures provided that the symmetry operations corresponding to these elements lead to the mutual exchange between nuclear configurations of the reactants and the products [6]. 

This statement may be illustrated by the following example. The transition state XVIII of the isotopic exchange reaction (Sect. 1.3.3.2) has, over against the symmetry elements of nuclear configurations of the reactants and the products, only one more, viz., the fourth-order axis C4. But it is precisely a 90° rotation of the nuclear configuration XVIII about this axis which causes the 

Different is the case of the degenerate intramolecular isomerization of sulfuranes SR4 (each ligand is given its own index). The CNDO/2 calculations of the PES of this reaction [77] have shown that C2V structures of the type XXII are the stable forms of sulfuranes SH4 and SF4, which is in accord with the experimental data on SF4. The transition state XXIII in the topomerization XXII XXIIa characterized by mutual exchange between the pairs of the equatorial and the axial ligands (respectively, Nos. 3, 4 and 1, 2 in XXII) has C4V symmetry (Fig. 1.14). The calculated activation barrier of 15kcal/mol for this topomerization (Berry pseudorotation) reproduces quite well the experimental value. 

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