Improved Proton Decoupling in NMR Spectroscopy of Crystalline Solids Using the Spinal -64 Sequence
The performance of three different spin decoupling schemes, CW, Tppm , and Spinal -64 is compared, by recording proton decoupled 13C NMR spectra of a crystalline glycine sample, with 20% isotopic labelling. At a magnetic field of B 0 = 14.1 T, the two pha
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Introduction Spin decoupling is one of the most important techniques in Nuclear Magnetic Resonance (NMR) spectroscopy, because it allows the acquisition of highly resolved and simplified spectra. In NMR, spectra of rare spins such as 13e are usually observed with simultaneous decoupling of the abundant spins, which are most often protons, IH. This is done to remove spin-spin interactions (J couplings in liquid-state, heteronuclear dipolar interactions in solid-state), which otherwise might have deleterious effects on the resolution of the 13e spectra. The most straightforward approach to decoupling is continuous-wave (eW) irradiation at the resonance frequency of the target nuclei. In liquid-state NMR, ew decoupling with a sufficiently strong radio frequency (RF) field can give satisfactory results. However, much better decoupling performances are obtained by purpose-designed pulse sequences such as MLEV-4 [1], WALTZ-16 [2] or PAR-75 [3], which are less sensitive to offsets of the decoupler frequency. This robustness with respect to the frequency offset is one major criterion to judge the performance
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Corresponding author. E-mail: [email protected]
N. Müller et al. (eds.), Current Developments in Solid State NMR Spectroscopy © Springer-Verlag Wien 2003
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T. Brauniger et al.
of a decoupling technique, together with the desire to obtain good results for low RF power. As opposed to liquid-state NMR, where decoupling aims to remove the relatively small scalar J couplings, the dominating effect in solid-state NMR is the dipolar interaction, which is generally comparable or even larger than the chemical shift range. For solids, much stronger RF fields are needed for CW decoupling, and the 'broad-band' pulse sequences devised for liquid-state NMR [1-3] are found mostly inefficient. In addition, the time dependence imposed on the proton resonance frequencies by the routinely used Magic Angle Spinning (MAS) technique [4] makes matters more complex. For rigid (crystalline) samples at modest MAS speeds and moderate magnetic fields, spin diffusion is still efficient enough for the proton lineshape to be largely unresolved. If the transmitter frequency is set to the middle of this broad 1H line, CW usually gives better performance than other decoupling schemes. However, with the introduction of faster MAS speeds, and higher magnetic fields (and also for mobile samples), the proton lineshape starts to break up, rendering the spin diffusion process less efficient. This makes it essentially impossible to set the decoupler frequency precisely 'on resonance' [5], and decoupling performance tends to deteriorate. The thus arising need for a decoupling technique more efficient than CW was addressed by the development of the Two Pulse Phase Modulated (TpPM) sequence [6]. This scheme relies on a windowless train of phase modulated pulses on the IH channel (see Fig. 1), and because of its significantly improved performance over CW decoupling, TpPM has found fairly widespread applications [7] . It has also been shown t
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