Chemical exchange effect on the intensities of the 2D NOE cross peaks

It has been observed experimentally that in NMR 2D NOESY experiments, when a nucleus is involved in both the Overhauser effect and chemical exchange, the intensities of the NOE cross peaks are influenced by the rate of the chemical exchange. Calculations show that when the exchange rate is 6–7 times faster than the cross relaxation rate, the NOE cross peaks become too weak to be observed, although the internuclear distances are fixed.