Roaming mechanism

Wester term this process a roundabout mechanism, and they have some evidence for the occurrence of the double roundabout (two rotations of the methyl group about the iodine) as well  

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Reactions without wells can also exhibit multiple pathways due to deviation from the MEP. While many trajectories may follow the MEP over a saddle point, alternative pathways arise when forces on the PES steer away from the saddle point, typically into relatively flat regions of the PES, before finding an additional path to the same exit channel. The roaming mechanisms recently elucidated in the photodissociation of formaldehyde and acetaldehyde, and the reaction of CH3 + O, are examples of this phenomenon, and are discussed in Section V. 

However, as the CH3 + O reaction shows [67], the roaming atom mechanism is not limited to closed-shell adducts, nor is it limited to abstraction pathways that are barrierless (see Fig. 13). Finally, the photodissociation of CH3CHO implies that the roaming fragment may not be limited to H atoms [55]. It remains to be seen how universal roaming mechanisms are, but they have the potential to be competing pathways in a large number of reactions. 

Measurements by Feng and Hershberger" show that the reaction yield of CO + NO is 20%, and that of NCO + O is 80%. Very probably both sets of products arise from the initial formation of an energised NCOO complex with NO + CO being formed in the exit channel by a roaming mechanism." The barrier for the direct formation of NO + CO via a four-centre transition state is apparently too high for such a mechanism to be significant." ... 

The picture here is of uncoupled Gaussian functions roaming over the PES, driven by classical mechanics. The coefficients then add the quantum mechanics, building up the nuclear wavepacket from the Gaussian basis set. This makes the treatment of non-adiabatic effects simple, as the coefficients are driven by the Hamiltonian matrices, and these elements couple basis functions on different surfaces, allowing hansfer of population between the states. As a variational principle was used to derive these equations, the coefficients describe the time dependence of the wavepacket as accurately as possible using the given... 

When Fitz s eyes roamed, Garudin stared about him in covetous wonder-a scandalously close cousin to lust - at the TARDIS s interior. A truly magnificent timepiece, a mechanism that deserved pride of place in his collection. If his current host had sufficient knowledge of the vessel s operation, then it would have been simple to have Fitz kill the Doctor and deliver the ship to him. But no, Fitz was as useless in that respect as in every other. 

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