Selective cross-polarization in solution state nuclear magnetic resonance of scalar coupled spin 12 and quadrupolar nuclei

Abstract

Semiselective heteronuclear cross-polarization for achieving coherence transfer between a spin I=12 scalar coupled to a spin S⩾12 in isotropic solution is considered. The expansion of the density operator as products of irreducible tensor operators provides a compact formalism for describing cross-polarization involving scalar coupled quadrupolar nuclei. An analytical description of cross-polarization is presented in the limit of strong radio-frequency (rf) fields, with respect to the scalar-coupling constant. Numerical simulations show that reducing the rf field amplitudes does not have a detrimental effect on the efficiency of the transfer provided they are comparable to or greater than the scalar-coupling constant. The use of weak rf fields largely circumvents the reduced efficacy due to Hartmann–Hahn mismatch. Applications of the method for selective observation of scalar-coupled quadrupolar nuclei are considered and experimental results are presented for a mixture of beryllium fluoride complexes.