Proton-enhanced NMR of dilute spins in solids

Abstract

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 $N_I{\text{I(I+1)γ}}_I^2{\mathrm{/N}}_S{\text{S(S+1)γ}}_S^2$ , or ${\text{∼ 10}}^3$ 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.