AXIAL ASYMMETRY IN THE NUCLEAR-MAGNETIC-RESONANCE SPECTRA OF DEUTERATED METHYL-GROUPS - AN ALTERNATIVE EXPLANATION

The deuterium nuclear magnetic resonance (NMR) powder spectra of methy l groups at ambient temperatures are invariably averaged by rotation o r hopping about the C-3 axis. The canonical result for a perfectly thr eefold symmetric methyl is an axially symmetric Pake doublet whose spl itting, in the case of perfect tetrahedral angles around the group, is averaged by a factor of exactly three. However, deuterium NMR spectra of methyl groups are often somewhat axially asymmetric, indicating a significant deviation either of the methyl group, or of its environmen t, from threefold symmetry. Previous explanations for this phenomenon have hinged on either an additional motional mechanism along a differe nt axis, or on a perturbation of the field gradient around the methyl group by electronegative groups. We propose a simpler explanation; it may arise in a distortion of the methyl geometry itself, so that the g roup no longer has perfect rotational symmetry. We analyze some experi mental examples of distorted methyls, and compare the extent of distor tion required by the NMR data with those observed in published neutron structures.