Diabatic surfaces for two-bond cycloaddition reactions are examined in terms of a diabatic surface analysis which includes the computation of the resonance interaction between the reactant-like and product-like diabatic surfaces. A qualitative analysis and rigorous numerical computations are presented for a concerted synchronous mechanism (a two-bond process), a concerted asynchronous mechanism (a concerted one-bond process), and the first step of a two-step mechanism (a nonconcerted one-bond process) for both “allowed” and “forbidden” processes. The results illustrate that the resonance interaction is the dominant factor which controls the mechanistic preference between two-bond and one-bond processes. For a Woodward-Hoffmann forbidden process, the magnitude of the resonance interaction is found to be much smaller for the (forbidden) synchronous process than for the one-bond process; this leads to the expected preference for the one-bond process. For a Woodward-Hoffmann allowed process in the comparison of a concerted two-bond mechanism and the first step of a two-step mechanism, it is found that magnitude of the resonance interaction at the transition structure geometry can lead to a preference for the concerted process.
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Bernardi et al. (1987) studied this question.