Correspondence Between Anasymmetrized Reactant and Transition Structure

2 Correspondence Between Anasymmetrized Reactant and Transition Structure 

At this writing, even the proper model to use within the one-dimensional framework is in dispute as a result,the tunneling rates calculated by different authors vary widely [61, 63, 64, 65]. 

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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