Formula omitted] of quadrupolar [formula omitted] nuclei

Abstract

The spin dynamics of Hartmann-Hahn cross-polarization from I = 1 2 to quadrupolar S = 3 2 nuclei is investigated. A density-matrix model applicable to cases where the quadrupole frequency ν Q is much larger than the rf amplitude ν 1 S of the S spins, predicts the time development of the spin state of an isolated I, S spin pair in static situations and in three distinct cases of magic-angle-spinning speed ν R . These cases are characterized as slow, intermediate, and fast, depending on the magnitude of the parameter α = ν 1S 2 ν Qν R relative to the intermediate value of 0.4. The model predictions are supported by numerical simulations. The polarization transfer from I to S is efficient in the limits of slow and fast sample spinning. When α ⪡ 1, the Hartmann-Hahn condition is shifted over once or twice ν R . When the spinning rate is intermediate, poor spin-locking of the quadrupolar spins prevents the accumulation of a cross-polarization signal and, in addition, depletes the spin-locked I magnetization. Experimental CP MAS data obtained in NaOH show that the concepts developed for isolated spin pairs are also applicable to cross-polarization in a strongly coupled multi-spin system.