Heteronuclear recoupling in solid-state magic-angle-spinning NMR via overtone irradiation.
A heteronuclear dipolar recoupling scheme applicable to I-S spin pairs undergoing magic-angle-spinning (MAS) is introduced, based on the overtone irradiation of one of the coupled nuclei. It is shown that when I is a quadrupole, for instance (14)N, irradiating this spin at a multiple of its Larmor frequency prevents the formation of MAS dipolar echoes. The ensuing S-spin signal dephasing is significant and dependent on a number of parameters, including the I-S dipolar coupling, the magnitude of I's quadrupolar coupling, and the relative orientations between these two coupling tensors. When applied to a spin-1 nucleus, this overtone recoupling method differs from hitherto proposed recoupling strategies in that it involves only the +/-1> I(z) eigenstates. Its dephasing efficiency becomes independent of first-order quadrupolar effects yet shows a high sensitivity to second-order offsets. A constant-time/variable-offset recoupling sequence thus provides a simple route to acquire, in an indirect fashion, (14)N overtone spectra from rotating powders. The principles underlying this kind of S-(14)N experiments and different applications involving S = (13)C, (59)Co sites are presented.
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Department of Chemical Physics, Weizmann Institute of Sciences, 76100 Rehovot, Israel.
Frydman
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D000409:Alanine / Q000737:chemistry; D002247:Carbon Isotopes; D004151:Dipeptides / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D009584:Nitrogen; D014633:Valine / Q000031:analogs & derivatives* / Q000737:chemistry
D002247:Carbon Isotopes; D004151:Dipeptides; D014633:Valine; D009584:Nitrogen; D000409:Alanine; C055589:N-acetylvaline