J-COMPENSATED HOMONUCLEAR CROSS POLARIZATION IN NMR-SPECTROSCOPY

We show how to modify homonuclear cross polarization or mixing sequences to compensate for a range of spin coupling constants. The basis of the method is an analogy between complete polarization transfer, represented as a 180-degrees pulse in a zero-quantum frame, and normal spin inversion pulses. The analogy lets us adapt existing compensation schemes for radiofrequency field inhomogeneity to the problem of J compensation in homonuclear cross polarization. We find that, in AX spin systems, complete polarization transfer can be made to occur for a broad range of J values, providing the chemical shift difference between the spins is known. A contour plot of cross polarization efficiency as a function of pulse flip angle and precession angle under the chemical shift difference shows the degree of compensation for different combinations of these parameters. In arbitrary AMX spin systems, the formal analogy breaks down, but some J compensation can still be achieved. These results can play an important role in maximizing cross peak intensity for 2D and 3D nuclear magnetic resonance applications.