Selective cross-polarization in solution state NMR

In two influential papers, Ernst and coworkers argued that cross-polarizati on, originally proposed by Hartmann and Hahn, can be very useful in isotrop ic liquids to transfer coherence between scalar-coupled nuclei such as prot ons and carbon-13 (Maudsley, A. A., Muller, L., and Ernst, R. R., 1977; J. magn. Reson., 28, 463; Muller, L., and Ernst, R. R., 1979, Molec. Phys., 38 , 963). Unfortunately, the efficiency of cross-polarization in liquids tend s to be strongly attenuated if the radiofrequency fields are not perfectly homogeneous. In this paper, it is demonstrated by experiments and simulatio ns that imperfections in RF fields have little effect on the efficiency of magnetization transfer, provided that the RF amplitudes are comparable with the magnitudes of the heteronuclear scalar coupling constants. A compariso n between selective cross-polarization and selective INEPT shows clearly th at crosspolarization is more efficient. Selective cross-polarization does n ot require any careful calibration and is insensitive to experimental insta bilities.