Two-stage feature of Hartmann-Hahn cross relaxation in magic-angle sample spinning

In magic-angle sample-spinning experiments the Hartmann-Hahn cross relaxation between protonated ¹³C and protons usually proceeds in two stages, except in fast internal rotating ¹³CH₃. The protonated ¹³C magnetization of powder samples changes very rapidly during the first tens of microseconds due to the fast energy exchange between each protonated ¹³C and its directly bonded ¹H spins; then it approaches at a much slower rate to a quasiequilibrium value via the energy exchange between these ¹³CHₙ subsystems and the remaining ¹H spins. This fact means that the whole ¹H spin system is not in a quasiequilibrium state and is not describable by a single spin temperature at least during the first stage of the cross relaxation. The two-stage feature has been obviously revealed by the depolarization experiment for ¹³C magnetization. The expression for protonated ¹³C magnetization as a function of depolarization time has been deduced, which reaches agreement with the experiments semiquantitatively. The depolarization experiment offers a reliable approach to distinguishing between ¹³CH and ¹³CH₂ signals in organic solids.