HETERONUCLEAR SPIN DECOUPLING IN SOLID STATE NMR

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We have demonstrated TPPM decoupling at 500 and 750 MHz and observed dramatic narrowing as predicted by theory. Other studies have been described in previous abstracts, as quoted below: Due to increased chemical shift offsets from resonance among abundant spins (usually 'H), 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 that 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 (x-"C,"N-glycine. In inhomogeneous systems, since the self-decoupling effect due to homonuclear couplings within the 'H reservoir is minimal, CW decoupling is particularly ineffective. The reduction of the second-order recoupling described by Ernst et al. is significant with TPPM. In a_13C,l 'N-glycine, additional CW decoupling power improves resolution and sensitivity. However, even with improved probe technology, the contribution of strong dipolar couplings due to abundant 'H 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 150 kHz provides additional signal intensity, albeit without narrowing. A 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 125 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 ex pected at higher magnetic fields have been realized.