A NOVEL-APPROACH FOR THE DETERMINATION OF FAST EXCHANGE-RATES

A novel technique is described to monitor fast exchange rates using NMR. This approach relies on the effects of partial saturation of the longitudinal magnetization with a saturation RF field at various offset frequencies on the observed resonance lines. A complete quantum mechanical approach for the two-site chemical exchange of nuclei is presented, which yields access to rate constants from slow to very fast exchange rates using the described technique. The model system evaluated using this approach was the hydroxyl proton exchange in water-ethanol solutions. Exchange rate constants from the intermediate range (about 170 liters/s) at pH 7.5 to very fast rate constants up to 106 liters/s at pH 5 were measured. © 1991.