Cross-polarization efficiency in INS systems using adiabatic RF sweeps

The theory describing nuclear magnetic resonance cross-polarization using adiabatic sweeps of the rf spin-lock fields through the Hartmann–Hahn matching condition is extended to small homonuclear coupled systems of the type INS . In particular, the connection is made between such experiments and the associated theoretical limits on polarization transfer—the ‘‘unitary bounds’’— demonstrating that these techniques can achieve the maximum transfer of polarization from the I spins to the S spins, subject to the constraint of angular momentum conservation imposed by spin-locking. Factors such as permutation symmetry of the spins, imperfect adiabaticity of individual crossings and fast sample spinning are shown to have no fundamental impact on the validity of these results. © 1997 American Institute of Physics. @S0021-9606~97!00445-5#