Hydrogen switch mechanism

Polymerization of isobutylene, for example, may be explained in terms of this hydrogen switch mechanism by assuming that the hydrogen switch occurs between two molecules of the olefin ... 

The reaction mechanism presented here combines the evidence from X-ray structures (41,42) with elements of the affinity change mechanism (116) and of the catalytic switch mechanism (118). All electron transfer reactions occur between species when they are hydrogen bonded to each other therefore, electron transfer will be extremely rapid and most likely not rate limiting. 

Figure 8.11 Stiffness modification of the Cucumaria frondosa (a sea cucumber) in the presence of an external stimulus the sea cucumber is in a relaxed state (A) and when touched the stiffness ofthe dermis increases (B). In (C) is a schematic representation of the switching mechanism in the biomimetic mechanically adaptive nanocomposites. In a swell state the interactions between the NCC are switched of, while in a dry state the hydrogen bonds between NCC are switched on and a stiffer material is obtained [81]. Copyright 2008 with permission from The American Association for the Advancement of Science.

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

---