ISOLATION OF SELECTED EXCHANGE PROCESSES IN NUCLEAR-MAGNETIC-RESONANCE

We have shown that it is possible to inhibit the transfer of magnetiza tion in a system with several exchanging sites in dynamic equilibrium, as in a mixture of cis- and trans-ZrCl(4)L(2) with excess free ligand L (L = (CH3O)(3)PO). The forward and backward reaction rates involvin g two selected sites can be studied while the effect of competing exch ange processes is ''quenched''. This can be achieved either by selecti ve inversion of the magnetization of the two chosen sites in the cours e of the reaction interval, or alternatively by inversion of the magne tization of all other sites in the exchange network. The rate of excha nge from the free to the cis site was determined to be k(cis<--free) = 0.018 s(-1). In the usual methods, this process would tend to be over shadowed by the almost two hundred times faster competing exchange pro cess from the cis to the trans site (k(trans<--cis) = 3.32 s(-1)).