SPIN LOCKING & COHERENCE TRANSFER IN HALF INTEGER QUADRUPOLAR NUCLEI

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The study of quadrupolar nuclei under magic-angle spinning and radio-frequency irradiation has resulted in new experimental schemes including the demonstration of rotation induced adiabatic coherence transfer (RIACT), useful for high-resolution MQMAS experiments for half-integer quadrupolar nuclei. The complexity of spin-locking and coherence transfer characteristics in half-integer quadrupolar nuclei under RF irradiation and MAS has led us to pursue the efficiency and optimization of these effects considering high spinning rates, offset dependence and amplitude-modulated rf pulses. We have gained new insights into the spin-locking of quadrupolar nuclei that contradict many aspects of the conventionally accepted theory, which is based on an adiabatic description of the eigenstates of a quadrupolar nucleus under magic-angle spinning. We find that the interconversion of the highest symmetric coherence and the central-transition coherence via RIACT has desirable performance characteristics at very high MAS rates. We also have determined pulse shapes that provide improved signal-to-noise and complete quantitative excitation and reconversion of multiple quantum coherences for nuclei with quadrupole couplings in the range (0.2 - 5 MHz) in S=3/2 spin systems. We analyze these results in the context of an effective Hamiltonian recently developed in this group.