Selective cross-polarization in solution state NMR

In two influential papers, Ernst and coworkers argued that cross-polarization, originally proposed by Hartmann and Hahn, can be very useful in isotropic liquids to transfer coherence between scalar-coupled nuclei such as protons and carbon-13 (Maudsley, A. A., Müller, L., and Ernst, R. R., 1977; J. magn. Reson., 28, 463; Müller, L., and Ernst, R. R., 1979, Molec. Phys., 38, 963). Unfortunately, the efficiency of cross-polarization in liquids tends to be strongly attenuated if the radiofrequency fields are not perfectly homogeneous. In this paper, it is demonstrated by experiments and simulations 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 comparison between selective cross-polarization and selective INEPT shows clearly that crosspolarization is more efficient. Selective cross-polarization does not require any careful calibration and is insensitive to experimental instabilities.