The shape and information content of high-field solid-state proton NMR spectra of methyl groups

The possible variation of the lineshape of the high-field 1 H spectrum of methyl groups is explored by simulation and experiment. The spectrum of an isolated methyl group depends, apart from the orientation of the applied field B 0 relative to the C 3-axis of the group, on its rotational tunnel frequency ν t and on its stochastic reorientation rate k. For quantitative analyses, the directional mobility of the C 3-axis must also be taken into account. A distinct but frequently occurring case arises when the methyl groups come as pairs of magnetically equivalent close neighbours. For the experiments, single crystals of four compounds I – IV were grown that were isotopically substituted such that they contained protons only in the methyl positions. The crystal symmetry of all compounds I – IV allowed us to record spectra with all methyl groups being orientationally and otherwise equivalent. I , acetonitrile in deuterated hydroquinone, represents the case of a well-isolated methyl group with a “high” tunnel frequency ν t . Its spectrum is (almost) independent of the temperature T. In II , monomethyl malonic acid, ν t is comparable in size with the strength of the intramolecular dipolar H – H interaction. All seven theoretically expected lines in the 1 H spectrum are clearly resolved in the spectra of II . ν t can be inferred with an uncertainty of only ±300 Hz . ν t ( T) is found to possess a (flat) maximum near 40 K . Compound III , l -alanine, allows the study of the case of a methyl group with an extremely low, although nonzero tunnel frequency ( ν t≈3 kHz) while IV , dimethylglyoxime, represents the case of a close pair of equivalent methyl groups. Its spectrum reflects intriguing structural implications.