Symmetry-Based Recoupling in Double-Rotation NMR Spectroscopy,

A. Brinkmann*, A. P. M. Kentgens, T. Anupõld, and A. Samoson,
J. Chem. Phys. 129, 174507, (2008).
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In this contribution we extend the theory of symmetry-​based pulse sequences of types CNnν and RNnν in magic-​angle-​spinning nuclear resonance spectroscopy [Levitt, Encyclopedia of NMR. Vol 9, 165 (2002)] to the case of rotating the sample simultaneously around two different angles with respect to the external magnetic field (double-​rotation). We consider the case of spin-1/2 nuclei in general and the case of half-​integer quadrupolar nuclei that are subjected to weak radiofrequency pulses operating selectively on the central-​transition polarizations. The transformation properties of the homonuclear dipolar interactions and J-couplings under central-​transition-​selective spin-​rotations are presented. We show that the pulse sequence R221R22-1 originally developed for homonuclear dipolar recoupling of half-​integer quadrupolar nuclei under magic-​angle-​spinning conditions [Edén, Chem. Phys. Lett. 431, 397 (2006)] may be used for the same purpose in the case of double-​rotation, if the radiofrequency pulses are synchronized with the outer rotation of the sample. We apply this sequence, sandwiched by central-​transition selective 90° pulses, to excite double-​quantum coherences in homonuclear spin-​systems consisting of 23Na and 27Al nuclei.