RESOLUTION ENHANCEMENT IN MULTIPLE-QUANTUM MAS NMR-SPECTROSCOPY

Two techniques for resolution and sensitivity enhancement are introduc ed in multiple-quantum (MQ) MAS spectroscopy of rigid solids. The firs t makes use of ultrafast MAS with spinning frequencies of up to 35 kHz , while the second combines MAS at moderately fast spinning frequencie s of about 13 kHz with multiple-pulse (MP) dipolar decoupling. For the latter approach, a semiwindowless WHH-4 sequence is applied during th e MQ evolution period (MQ dimension) and/or detection period (single-q uantum dimension). In the MQ dimension, the MP sequence has to be supp lemented by two bracketing pulses in order to preserve the order and t he intensities of the evolving MQ coherences. Double-quantum H-1 NMR s pectra of L-alanine recorded using both decoupling techniques are show n and compared to each other. Triple-quantum H-1 NMR spectra under ult rafast MAS conditions are also presented, (C) 1998 Academic Press.