Transition-state structure, degenerate reactions

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]. 

However, a reaction coordinate can never be parallel to a degenerate normal mode in a transition state as this would imply that there would be more than one direction of negative curvature in the transition state. In such a situation one can always find a lower potential energy path that goes around the hill (61). The structure... 

Scheme 41. (a) Transition State for the Degenerate Semibullvalene Rearrangement, (b) Avoided Crossing of the Corresponding Kekule Structures Along the Reaction Coordinate Mode (t>2), and the Generation of the Twin States by Avoided Crossing, (c) Frequencies of the b2 Mode in the Two States (Ref 225). Delocalized Bonds in c Are Removed for Clarity... 

Reactions 48 (M, M and M" are Si, Ge and Sn) were investigated for both degenerate and nondegenerate situations. Back-side (132) and front-side (133) approaches of MH3 were considered. Both approaches involve pentacoordinate transition states (Scheme 5). The optimized structures of 132 and 133, M = M = M" = Si are shown in Figure 39a. 

A similar problem arises in H atom loss from CH4. The TS structure consists of the methane molecule with an elongated C—H bond, which gives the TS C3, symmetry. Thus, the symmetry factor for the neutral reaction is 12/3 = 4, which is as expected. On the other hand, the methane ion is electronically degenerate, which must be taken into account if the methane ion states do not all correlate with the lowest energy H loss transition state. 

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