Controlling spin diffusion by tailored rf-irradiation schemes

Spin-diffusion type polarization transfer among heteronuclei, e.g., 13C or 15N, plays an important role in many solid-state NMR experiments for structure determination of proteins. In such experiments, chemical-shift compensation is provided by the proton bath (‘proton-driven’) which can be improved by radio-frequency irradiation of the protons. Here, we address the problem that the polarization-transfer rates depend not only on the intermolecular distance but also on the chemical-shift difference of the two spins. We introduce rf-modulation schemes that allow eliminating the chemical-shift dependence to a large extent over a predefined range. At the same time, the rate constants are maximized.