Fast-exchange limit problem

NMR spectroscopy is a useful new tool to investigate kinetic problems. Attention has been focused on cyclopentadienyl and related complexes, and in a number of cases thermodynamic parameters have been derived in spite of experimental difficulties. First, in a typical experiment, on increasing the temperature, two or more sharp signals broaden, vanish into the noise, then a broad singlet appears and sharpens. Thus from line shape analysis it is possible to determine rates of exchange near the slow and fast exchange limits, but not in the tem-... 

The second role of the chemical exchange phenomena can be seen in Eq. (2) the exchange lifetime competes with the in-complex nuclear spin-lattice relaxation time and can become a limiting factor in the attainable PRE. This aspect of the problem is highly relevant in practical consideration in the case of Gd(III) complexes as a potential contrast agent, because the water exchange in these systems is not too fast. This issue is considered to be outside of the scope of this article and we refer to recent literature on the subject 5,160) and to other contributions in this volume. 

The procedure is complicated by a relatively fast proton exchange between the starting material and the lithiated intermediate [105]. This problem can be partially overcome via addition of LiCl which accelerates the lithiation or by quenching with an electrophile in situ (typically limited to Me3SiCl) [49,105]. 

Under polymer synthesis there is considerable number of processes proceeding under phase separation conditions, in particular in such systems like "liquid-liquid" (high-viscosity polymer solution - low-molecular fluid), "gas-liguid" (polymer-analogous processes) and "solid-liquid" (catalytic polymerization). Necessity of substance transfer from one phase to another by mass-exchange [140], i.e. removal of diffusive limitations is the feature of heterogeneous processes. In this case one have to deal with serious problems while realizing fast chemical reactions [141]. 

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