Magic angle spinning technique pulse sequence

The D-HMQC (dipolar heteronuclear multiple-quantum coherence) technique is a recently developed NMR pulse sequence particularly suitable for the investigation of spatial proximity between quadrupolar and spin-1/2 nuclei. Compared to the crosspolarisation magic-angle spinning technique applied to a quadrupolar nucleus, D-HMQC does not require time-consuming optimisations and exhibits on the quadrupolar spin a better robustness to irradiation offset and to Cq values and... 

Figure 8.3 Illustration of HR-MAS techniques applied to a resin-bound trisaccharide (a) static XH spectrum of the solvent swollen sample (b) XH spectrum with magic-angle spinning at 3.5 kHz (c) H spectrum with MAS and spin echo pulse sequence.

Spectra of solids arc complicated, because of the multiplicity of parameters involved, especially by anisotropic interactions as dipolar coupling. Many techniques both experimental or computational have been developed for interpretation. A technique particularly useful for catalysis permits to selectively suppress anisotropic contributions by mechanical rotation of the sample (magic angle spinning, MASNMR) and by rotation of the nuclear magnetic dipoles with sequences of radiofrcqucncy pulses. 

Si-NMR spectra were recorded on a Bruker VJM-250 (liquid) or a Bruker CXP-300 Fourier transform magic-angle-spinning (FT MAS) solid state spectrometer. Resonances are relative to tetramethyl-silane (TMS). Dynamics of the silicate solutions were studied by selective excitation techniques by using DANTE-type (131 pulse sequences. 

C). MAS spectrum (magic angle spinning, at vr = 3 kHz), no decoupling. (D). MAS spectrum, with decoupling. (E). MAS spectrum, cross polarised and decoupled. (F). MAS spectrum, cross polarised and decoupled, with spinning side bands elimination by special pulse technique (TOSS-sequence). 

The development of new ID and 2D pulse sequences enables the spectroscopist to obtain structure and dynamic information about systems that were previously very hard to study. As an example is reported in Fig. 3.2.13 the 2D spectrum of erythromycin A measured with the FIREMAT (Five p Replicated Magic Angle Turning ) technique [30]. The slow spinning speed of 390 Hz produces a spinning sideband pattern for each peak in one dimension, whereas a multipulse sequence in combination with a special processing method produces isotropic lines in the second dimension. 

D solid-state NMR techniques, such as the 2D-TOSS pulse sequence [39], can allow the separation of isotropic and anisotropic chemical shift information over two dimensions, making it possible to distinguish individual carbon atoms under moderate magic angle spinning (MAS) conditions. 

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