Heteronuclear recoupling in solid-state magic-angle-spinning NMR via overtone irradiation

A heteronuclear dipolar recoupling scheme applicable to I-S spin pairs unde rgoing magic-angle-spinning (MAS) is introduced, based on the overtone irra diation of one of the coupled nuclei. It is shown that when I is a quadrupo le, for instance N-14, irradiating this spin at a multiple of its Larmor fr equency prevents the formation of MAS dipolar echoes. The ensuing S-spin si gnal dephasing is significant and dependent on a number of parameters, incl uding the I-S dipolar coupling, the magnitude of I's quadrupolar coupling, and the relative orientations between these two coupling tensors. When appl ied to a spin-1 nucleus, this overtone recoupling method differs from hithe rto proposed recoupling strategies in that it involves only the \+/- 1) I-z eigenstates. Its dephasing efficiency becomes independent of first-order q uadrupolar effects yet shows a high sensitivity to second-order offsets. A constant-time/variable-offset recoupling sequence thus provides a simple ro ute to acquire, in an indirect fashion, N-14 overtone spectra from rotating powders. The principles underlying this kind of S-N-14 experiments and dif ferent applications involving S = C-13, Co-59 sites are presented.