Improved magnetization transfer in solid-state NMR with fast magic angle spinning

The efficiency of magnetization transfer between different spins S such as chemically inequivalent carbon-13 nuclei in solid samples that are spinning at high frequencies about the magic angle can be enhanced by a phase-alternated recoupling irradiation scheme (PARIS). Dipolar recoupling is assisted by a radio-frequency (rf) field applied to the abundant I (proton) spins. In contrast to rotary resonance-based recoupling schemes, the new method does not depend critically on the rf amplitude, which need not be matched with the spinning frequency. Modest rf amplitudes suffice to bring about efficient magnetization transfer even at high spinning speeds, thus avoiding excessive sample heating. The new method compensates efficiently for rf field inhomogeneity, so that the full sample volume is used more effectively.