Uncoupled bonding changes

In chemical reaction kinetics, there may be thermodynamic contributions arising from any change(s) in the degrees of freedom of the atoms involved in bond-breaking and bond-making steps. Coupled vibratory motions can also be a factor whenever the frequencies of two uncoupled vibrators are sufficiently close to each other, introduction of a physical connection between the vibrators will result in a frequency shift for the coupled system. 

The change from a stepwise preassociation mechanism through a triple ion intermediate to an uncoupled concerted reaction occurs as the triple ion becomes too unstable to exist in an energy well for the time of a bond vibration ( 10 s). The borderline between these two reaction mechanisms is poorly marked, and there are no clear experimental protocols for its detection. These two reaction mechanisms cannot be distinguished by experiments designed to characterize their transition states, which lie at essentially the same position in the inner upper right hand corner of Figure 2.3. Only low yields of the nucleophilic substitution product are obtained from both stepwise preassociation and uncoupled concerted reactions, because for formation of the preassociation complex in water is small... 

Attempts have been made to correlate these characteristics with the rate of photodimerization of a series of pyrimidine bases.467 1 Among these characteristics, two appeared to be particularly important for the photodimerization problem (for review see ref. 466), namely the density of the uncoupled electrons (or spin density) pf at C-5 and C-6 and the change of the C(5)-C(6) bond order P56 upon excitation. The proposal that the high concentration of spin density on the C(5)-C(6) bond as well as its low bond order in the excited states were factors responsible for the case of photodimerization was stated about ten years ago.464,471 The first calculations, within the framework of the 77-HMO approximation, showed the following findings (i) The concentration of uncoupled... 

In the a-diadsorbed form of an alkene (Table 4.2, structure 4), the state of hybridisation of the carbon atoms has changed from sp to sp the jr-bond between the carbons has been uncoupled, and two covalent bonds are then formed between them and two metal atoms ... 

Figure 8.4 Free-energy changes in bonding and solvation for a proton-transfer reaction in water (schematic). See text. Only the proton-transfer step (pt in Figure 8.3) is shown.The motions of the solvent molecules and of the proton are assumed to be uncoupled.

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