Guest determining step

Recently, Turro suggested that the rate-determining step of the host-guest association is the breakdown of the water structure inside CD and/or around the guest molecule. The direction of the inclusion by CD is also important in molecular dynamics of inclusion information about the... 

Hersey and Robinson also foundthat many guest species that show kinetic behavior apparently explicable in terms of a single-step binding, give a discrepancy between the values of the equilibrium constant determined kinetically and those determined from equilibrium studies. It was found that the equilibrium constant, deterrmned spectrophotometrically, was usually greater than the ratio of the forward and backward rate-constants, determined kinetically. They therefore suggested that this discrepancy could be adequately explained if the two-step mechanism just described was used to interpret the results. A similar proposal has also been made by Hall and coworkers, who observed a large discrepancy between AV° values for the inclusion of 1-butanol and 1-pentanol by alpha cyclodextrin, calculated from equilibrium-density measurements and kinetic, ultrasonic-absorption data. 

A study similar to that of Hersey and Robinson has been reported by Seiyama and coworkers.From a stopped-flow, kinetic study of the interaction of various azo dyes and some azo dye-metal complexes with alpha cyclodextrin, they observed two kinetic processes. The dependence of the observed rate-constants for these two processes on the alpha cyclodextrin concentration was found to be explainable in terms of a mechanism identical to that proposed earlier by Hersey and Robinson. In the case of the guests used by Seiyama and coworkers, however, values for the rate constants of the binding step could be determined from the concentration dependence of kobs for the faster process thus,... 

Host 2 was obtained in 10% yield by reacting isophthaloyl dichloride (3) with the cyclohexano-substituted diamine 4. The association constant (ATa) of host 2 and guest 1 was determined to be 103 M-1 in chloroform. When the synthesis of 2 was changed to a two-step procedure, the yield could be increased to 51% and at the same time two new products with the same molecular mass were isolated (Figure 3) [15]. 

Fluorescence correlation spectroscopy analyses the temporal fluctuations of the fluorescence intensity by means of an autocorrelation function from which translational and rotational diffusion coefficients, flow rates and rate constants of chemical processes of single molecules can be determined. For example, the dynamics of complex formation between /3-cyclodextrin as a host for guest molecules was investigated with singlemolecule sensitivity, which revealed that the formation of an encounter complex is followed by a unimolecular inclusion reaction as the rate-limiting step.263... 

In a broad family of cases the excited vibration loses its energy to the lattice in two steps. In the first step, which is frequently rate determining, it excites localised vibrations, in particular a rotational type of motion of the guest molecule or a translational motion of the same in the lattice cage 19K Secondly the localised excitation diffuses away to the farther reaches of the lattice. 

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