ROTATING-FRAME EXCHANGE SPECTROSCOPY OF MACROCYCLIC SYSTEMS

Homonuclear Hartmann-Hahn phenomena in rotating-frame exchange spectro scopy complicate studies of exchange dynamics of systems possessing ex tensive scalar-coupling networks. With the homonuclear Hartmann-Hahn ( HH) expressions derived from spin simulations, one can optimize experi mental conditions to minimize HH processes and maximize the chemical-e xchange contribution to the cross-peak integral. This procedure, appli ed to a diamagnetic macrocyclic yttrium chelate, Y(Hp-DO3A), provides quantitative information on the fluxional dynamics of the pendant arms and of the ethylenic moieties. Exploiting the condition where the HH frequency is much larger than the exchange rate (omega(HH)/2 pi much g reater than K-i.j) and where the HH transition is of low relative inte nsity (I-i.j;HH much less than 1), exchange-weighted buildup curves ca n be extracted as a function of mixing time for the pendant arms and f or the macrocyclic gauche-gauche' interconversion. The presence of an asymmetric arm on the macrocycle also enables separation of exchange p rocesses of each pendant arm and of the four individual ethylenic moie ties. The results also suggest that rearrangement either in the ethyle nic moieties or in the acetate-arm systems does not occur in a concert ed manner at 20 degrees C. (C) 1995 Academic Press, Inc.