Heteronuclear polarization transfer represents one of the key elements for structure elucidation in spin 1/2 applications. We have investigated extensions of this approach to heteronuclear spin systems involving quadrupolar nuclei. In the quadrupolar case, a more detailed analysis of the influence of rf and dipolar interactions under MAS is needed. We have shown that an effective Hamiltonian can be derived to describe the central transition spin locking behaviour of a quadrupolar nucleus. In the same formalism, the RIACT concept to excite and reconvert multiple quantum coherence can be understood as a MAS induced rotation between central and triple quantum transition subspaces of a spin S=3/2 spin system. In the context of high-resolution MQMAS applications, it is possible to directly transfer spin 1/2 polarization into multiple-quantum polarization. The effective Hamiltonian derived for an isolated quadrupolar spin under MAS also provides analytical insight into the dynamics of an I,S spin system subjected to rf fields and MAS. We have shown that, by employing medium size (i.e. in the order of 50 kHz) rf fields on the quadrupolar nucleus, it is possible to directly cross polarize into triple-quantum coherence with good efficiency. Thus, TQCP transfer can act as a dipolar filter for the high-resolution MQMAS experiment. This scheme can also be adapted for a 2D HETCOR experiment (displaying high resolution in 2 dimensions). Comparative studies between a conventional single quantum (SQ) CP experiment and the TQCP method indicate that the latter approach leads to a 2-5 times higher overall transfer efficiency for large or medium-size dipolar couplings. In addition, adiabatic TQCP techniques employing shaped spin-locking pulses can substantially improve the transfer efficiency. This TQCP technique has many advantages over the single-quantum approach and will be a valuable method for acquiring dipolar filtered spectra of quadrupolar nuclei in biological macromolecules.
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