Fig. 2. Highest achievable transfer efficiencies estimated by numerical optimizations.

An efficiency of 1 reflects a quantitative transfer of magnetization from nitrogen to carbon; zero means no transfer, and negative values reflect a change of sign upon magnetization transfer. (A) Transfer efficiency of ramp-CP (blue), adiabatic-CP (green), and optimal control (red) sequences. Pulse sequence parameters were optimized for each contact time independently. In the calculations, the assumed dipolar coupling and chemical shift anisotropy parameters correspond to a ¹⁵N-¹³Cα spin pair, measured in a 700-MHz NMR spectrometer. The sample is rotated with a MAS frequency of 16.5 kHz. Both CP methods suffer strongly from rf inhomogeneity. (B) Illustration of the spatial distribution of the magnetization transfer efficiency for ramp-CP and optimal control sequences optimized for a duration of 60 rotor periods (about 3.6 ms). In case of ramp-CP, magnetization transfer occurs in a restricted volume of the rotor with moderate efficiency, while optimal control yields efficient transfer in almost the full volume.