Fluxional isomerism

In some cases, the two stereoisomers can interconvert. In cis- and trans-disubstituted cyclopropanones, for example, there is reversible interconversion that favors the more stable trans isomer. This fluxional isomerization occurs via ring opening to an unseen oxyallyl valence bond isomer. ... 

Bradford, A.M., Kristof, E., Rashidi, M., Yang, D.S., Payne, N.C. and Puddephatt, R.J. (1994) Isocyanide and diisocyanide complexes of a triplatinum cluster fluxionality, isomerism, structure, and bonding. Inorg. Chem., 33, 2355. 

An IGLO NMR study and MO calculations have been used to show that the ground-state structures of compounds like (50) and (51) in solution as well as the solid state are as shown and not of the symmetrical bridging type (53). Nevertheless, the stmcture represented by (53) is likely to be involved in degenerate, fluxional isomerization processes. Koster has also prepared a series of compounds illustrated by (54) where X=0, S, Se. There has also recently been some snccess at preparation of new triazaboles. ... 

Up to this point we have managed to sidestep the issue of spin [1, 2] entirely, because the reactions treated so far have all involved closed shell singlet reactants and products. Even in those few cases where the transition state is essentially open shell, as in the fluxional isomerization of cyclobutadiene (Section 8.4), its singlet state lies sufficiently far below the corresponding triplet [3, p. 363] that electron spin can be ignored. This is no longer the case in photochemical reactions, several of which will be dealt with in the following chapter, or in the less common - but by no means rare - thermal reactions in which the spin state of the product differs from that of the reactant. 

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