MOLECULAR-DYNAMICS AND MAGIC-ANGLE-SPINNING NMR

The observation of ''disappearing lines'' or the inability to observe expected resonances in magic angle spinning (MAS) NMR spectra suggest an interference phenomenon between molecular dynamics and the characte ristic frequencies of pulse trains and radiofrequency (RF) fields empl oyed in the NMR experiments. In this paper, we present a systematic st udy of 3-fold hopping of the -NH3+ group in L-alanine, and the motion' s interference with two widely-used techniques for homo- and heteronuc lear decoupling. It is demonstrated experimentally and theoretically t hat rates of 3-fold hopping in the vicinity of the cycle times of homo nuclear pulse trains or the reciprocal of the heteronuclear decoupling field strengths lead to dramatic broadening of NMR lines in MAS spect ra.