The effects of off-resonance irradiation in 2D exchange NMR at zero-field are analysed. A theoretical treatment of the 2D exchange NMR pulse sequence is presented and applied to the quantitative study of exchange processes in molecular crystals. It takes into account the off-resonance irradiation, which critically influences the spin dynamics. The response of a system of spins I=3/2 in zero applied field to the three-pulse sequence is analysed. The mixing dynamics by exchange and the expected cross-peak intensities as a function of the frequency offset has been derived. It is shown that the off-resonance effects are of crucial importance for the quantitative description of the exchange spectra. The theory is successfully tested for the exchange spectra of hindered trichloromethyl groups of p-chloroanilinium trichloroacetate, where the conventional approach without taking into account the off-resonance phenomena has failed.
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