Slow-exchange conditions

For substituted cyclohexanes, the slow-exchange condition is met at temperatures below about —50 C. Table 3.5 presents data for the half-life for conformational equilibration of cyclohexyl chloride as a function of temperature. From these data, it can be seen that conformationally pure solutions of equatorial cyclohexyl chloride could be maintained at low temperature. This has been accomplished experimentally. Crystallization of cyclohexyl chloride at low temperature affords crystals containing only the... 

Mislow and Bickart (258) have recently discussed the properties, and specified the limitations and essential features, of models that can be used for the prediction of chirality of a molecular system. In the simplified and idealized representation of molecular stracture, nonessential features are deliberately left out the model summarizes some selected aspects of the system and completely disregards or even falsifies, others. The model must be adequate to the time scale in which the phenomenon is observed. In particular, in mobile conformational systems it should refer to a time-averaged structure. In other words, the model can have a higher symmetry than that observed under static conditions (e.g., by X-ray diffraction in the crystalline state or by NMR under slow exchange conditions) (259). 

Slow Exchange Conditions k < 10 2 s 1. The species can be observed as if no exchange were taking place. 

Chemical Shift Measurements. Under slow-exchange conditions, the ionization ratio cannot be measured. In fact, one of the major advantages of the superacidic media is the ease with which weak bases can be fully protonated and directly observed by NMR. Because it is known that the protonation rates are practically diffusion-controlled ( 409 liter mol-1 s-1), under these conditions (< 10 2 s 1) the indicator is totally in the acidic form described by the NMR spectmm and no variable is available to measure the ionization ratio. 

With this information, we can determine the value of K under slow-exchange conditions by first rewriting Eq. (10.9) as... 

Solution-state NMR studies of mercury complexes are most meaningful when taken under slow-exchange conditions. Fast-exchange spectra involving equilibria between several species in solution are common due to the relatively high lability... 

For substituted cyclohexanes, the slow-exchange condition is met at temperatures below about -50° C. Data for the half-life for conformational equilibration of... 

The F relaxation times depend on both copper(II) and F concentration. 1/Tip is linearly dependent on the enzyme concentration between pH 7.8 and pH 10. l/Tg is independent of frequency between 7 and 56 MHz and strongly influenced by the temperature, a behavior typical of slow exchange condition. IT2p is governed by the rate of chemical exchange of fluoride ions between the bulk and the binding site, whereas 1/T 1 is only partially dependent on t. ... 

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