Sensitivity enhancement in ²⁹Si solid-state NMR signals by CPMG detection

The sensitivity in ²⁹Si NMR spectroscopy is greatly enhanced with a factor of 5 to 10 by utilizing a Carr-Purcell-Meiboom-Gill (CPMG) echo train. It is possible to detect multiple echoes even with a single excitation pulse if the T2* relaxation time is sufficiently shorter than the T₂’ relaxation time; T₂* is the life time of an FID calculated from an apparent linewidth, while T₂’ is the coherence life time under the CPMG sequence. While the ²⁹Si T₂* of most silicate compounds is typically short, resulting from structural distributions, the T₂’ is the order of seconds, governed by ¹H-²⁹Si heteronuclear dipolar interactions as well as ²⁹Si-²⁹Si homonuclear interactions. The former interactions can be removed by ¹H decoupling, whereas the latter reduced by fast MAS. A series of CPMG echoes thus obtained are summed up, leading to the remarkable sensitivity enhancement.

JNM-ECA600 with 3.2 mm HXMAS probe at 20 kHz MAS. 
Sample: zeolite sigma-2 (SGT)

Sample courtesy of

Prof. Toshiyuki Yokoi of Tokyo Institute of Technology

Reference

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J. W. Wiench, V.S.-Y. Lin, M. Pruski, J. Magn. Reson. 193 (2008) 233-242.