Proton-enhanced NMR of dilute spins in solids

The NMR signals of isotopically or chemically dilute nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain in power sensitivity as compared with conventional observation of the rare spins approaches N_II(I+1)γI2/N_SS(S+1)γS2, or ∼ 10³ for S = ¹³C, I = ¹H in organic solids. The transfer of polarization is accomplished by any of a number of double resonance methods. High-frequency resolution of the S -spin signal is obtained by decoupling of the abundant spins. The experimental requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and experimental results are mentioned.