HETERONUCLEAR SPIN DECOUPLING IN SOLID STATE NMR

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Due to increased chemical shift offsets from resonance among abundant spins (usually 1H), decoupling becomes more demanding at high field. In addition, the high rates of MAS, required to average the anisotropic interactions, render the proton spin reservoir inhomogeneous. Under these conditions, the effects of insufficient decoupling become dramatic. The two-point phase-modulated (TPPM) decoupling sequence which we have developed demonstrates significant improvement over continuous-wave (CW) decoupling in inhomogeneous spin systems such as calcium 13C-formate, as well as more homogeneous cases such as ~-'3C. 5N-glycine. In inhomogeneous systems, the self-decoupling effect due to homonuclear couplings within the 1H reservoir is minimal, and so CW decoupling is particularly ineffective. The reduction of the second-order recoupling described by Ernst et al. is significant with TPPM. In u- 13C,1 5N-glycine, additional CW decoupling power improves resolution and sensitivity. However, even with improved probe technology, the contribution of strong dipolar couplings due to abundant 1H nuclei cannot be effectively removed with CW decoupling alone. Increasing from 75 kHz to 125 kHz CW decoupling provides an improvement in linewidth from 66 to 25 Hz, and an additional increase to tSO kHz provides additional signal intensity albeit without narrowing. The more dramatic improvement is observed when switching to the TPPM method; here a factor of six in narrowing and a factor of ten in sensitivity (peak height) is observed when comparing the 75 kHz CW decoupling and 12S kHz TPPM decoupling results. The combination of improved probe technology and decoupling methodology has potential to provide significant increases in the sensitivity and resolution of direct and indirect chemical shift dimensions. and the improvements in these categories expected at higher magnetic fields have been realized.