Reaction mechanisms nonstatistical dynamics, effects

When a reaction involves multiple bonding changes, a question may arise whether the bonding changes occur by a stepwise or concerted pathway. An answer to such a question based on the classical reaction theory is that the reaction proceeds by a concerted pathway, by a stepwise pathway, or by a mixture of the two separate pathways. However, if one takes into account dynamic effects, the answer to the question of concerted versus stepwise may be much more complex. It is interesting to point out here that the case reported by Singleton for the ene reaction affords a case, where stepwise mechanism can dynamically operate on a concerted PES. This contrasts with the reactions described in section Nonstatistical Product Distribution , in which the... 

Schmittel and coworkers [420a] considered the kinetics of such reactions in the framework of a nonclassic mechanism. A series of enyne-allenes follow neither the classical concerted nor stepwise mechanism but rather comply with the laws of the nonstatistical dynamics. The computed potential energy surface and experimental kinetic isotope effects indicate that dynamic effects operate before as well as after the transition state. Similar effects seem significant for the enyne-heterocumulenes [421]. 

---