Molecular Zeeman interactions in NMR spectra of methyl groups

Abstract

Methyl groups in organic solids generally behave as uniaxial quantum rotors. The existing nuclear magnetic resonance (NMR) theory appears to be complete, capable of describing even the finest details of the temperature-dependent spectra of such objects. However, the once reported temperature effects in the carbon spectra of the [Formula: see text]-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, they 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. However, 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.