A model for a temperature-dependent frequency distribution of methyl group tunnelling splittings

Molecular Zeeman interactions in NMR spectra of methyl groups

Methyl groups in organic solids generally behave as uniaxial quantum rotors. The existing NMR theory appears to be complete, capable of describing even the finest details of the temperature-dependent spectra of such objects. However, the once re-ported temperature effects in the carbon spectra of the 13C-labeled methyl group in a single crystal of acetylsalicylic acid have still not been explained. As the temperature decreases, in the quartet corresponding to the rapid motional averaging regime, the inner lines first begin to broaden but then narrow again, so that at 6 K a pattern similar to that at room temperature was observed. In the present work, these puzzling effects are explained quantitatively by invoking the molecular Zeeman (MZ) interaction. Like the spin-rotation (SR) interaction long known to occur in methyl groups, it engages the magnetic moments generated by their torsional motions. How-ever, it has not been considered in NMR spectroscopy until now. This is a surprising situation because in the magnetic fields currently used in NMR spectroscopy, the MZ interaction is orders of magnitude stronger than the (magnetic field independent) SR effects.

A field-cycling NMR investigation of resonant spin-lattice relaxation features arising from tunnelling methyl groups

(1)H nuclear spin-lattice relaxation has been investigated in sodium acetate trihydrate and sorbic acid using field-cycling NMR in the solid state. The relaxation is dominated by the reorientation of the methyl groups. Resonant features arising from coherent tunnelling are observed in both the magnetic field dependence of the spin lattice relaxation rate, T(1)(-1)(B(z)) and in the inverse temperature dependence, T(1)(-1)(1/T). The two systems have different barrier heights and tunnelling frequencies, providing different perspectives on the tunnel resonance phenomena. The magnetic field dependence enables different spectral density components to be separately investigated and in the carboxylic acid, sorbic acid, concerted proton transfer in the hydrogen bonds is also identified at low field and low temperature. The methyl hindering barriers and the correlation times characterising the reorientational dynamics has been accurately determined in both materials.

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 1H 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 B0 relative to the C3-axis of the group, on its rotational tunnel frequency vt and on its stochastic reorientation rate k. For quantitative analyses, the directional mobility of the C3-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 vt. Its spectrum is (almost) independent of the temperature T. In II, monomethyl malonic acid, vt is comparable in size with the strength of the intramolecular dipolar H-H interaction. All seven theoretically expected lines in the 1H spectrum are clearly resolved in the spectra of II. vt can be inferred with an uncertainty of only +/- 300 Hz. vt(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 (vt approximately 3 kHz) while IV, dimethylglyoxime, represents the case of a close pair of equivalent methyl groups. Its spectrum reflects intriguing structural implications.

Molecular dynamics in solid pyridoxine as studied by 1H NMR

Spin-lattice relaxation times T1 and T1d as well as NMR second moment were employed to study molecular dynamics of pyridoxine (vitamin B6) in the temperature range 10-350 K. The T1 minimum observed at low temperatures at 200 MHz is attributed to a motion of methyl group. The motion is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. At low temperatures, where T1 is temperature independent, occupation of the ground state only is assumed. A motion of proton of the hydroxyl groups or CH2OH groups probably provides additional mechanism of relaxation, in the high-temperature region.