Encounter complex formation

Recall that the diffusion coefficient of a molecule will decrease with increasing viscosity of the solvent. Thus, the rate of encounter complex formation will decrease in a viscous medium. Since viscosity is itself temperature dependent, such encounters in solution will have their own activation energy. 

The free energy cost of encounter complex formation, if there are neither favorable nor unfavorable interactions between the species coming together, can be calculated based on the probability of being there as opposed to anywhere in solution. This is essentially the approach suggested by Hine17 ... 

Fig. 9. (A) Free-energy dependence of the second- the formation product (/

By leaving all other terms constant one cannot expect accurate predictions of the self-exchange rates, and the magnitude of variations in the rates due to various terms (electronic coupling, encounter complex formation etc.) has been estimated 541. Therefore, the calculated rates presented in Table 11.2 are surprisingly... 

This definition of ko creates a problem of units because k is in units of M-1 s-1 and k-i in s-1. A possible solution to the problem was suggested by Hine (17) but this solution suffers from the disadvantage of having to assume the same equilibrium constant for encounter complex formation between nucleophile and electrophile in all reactions. In terms of relative ko values, little difference exists between our and Hines definitions. 

Diffusion control occurs if the reactions inside the encounter complex are faster than the rate of encounter complex formation or decomposition (i.e. Ac b- or... 

The nitrosylation of [Fe (CN)5(N02)] led to a particularly interesting result a notoriously fast conversion to nitroprusside was observed in the stopped-flow time scale. As E° for the [Fe" (CN)5(N02)] couple is also 0.4 V (83), we can still anticipate similar rates for the encounter-complex formation and the electron transfer reaction steps (analogs of 4-5). However, fe Kio2- cannot be high enough to account for the fast conversion to final products (its rate constant should be comparable to fe py, s ). Instead, the final step might involve a fast proton-assisted N02 /N0 interconversion (cf Section 2.2.1), which would yield the product without rupture of the initial Fe —N02 bond ... 

Combining equation 6 with 10, and 8 with 11, the equilibrium constants characterizing encounter-complex formation are obtained. These are, for neutral species ... 

Calculated rate and equilibrium constant values for encounter-complex formation... 

---