Symmetry breaking in thermally activated NH 4 rotation studied by NMR level crossing spectroscopy

Abstract

Hindered ammonium ions and level crossing NMR are used to study thermally activated rotation. Hindered rotation is a quantum trajectory occurring with a symmetry breaking deformation of the ion which creates rotational coordinates with an associated vector potential. The coordinates label the protons and modify the proton spin symmetry rules which apply to the wavefunctions of an ion in a static hindering potential. The degeneracy of the Kramers doublets is lifted and twelve orientations of the tetrahedron are identified instead of the six distinguished by the proton spin in the static case. Level crossing NMR spectra of ammonium persulphate between 4 and 20 K demonstrate the splitting through the large difference in the thermal broadening of transitions involving doublet and singlet states. New discrete structure which appears in the tunnelling spectrum with increasing temperature may be due to frequency modulation due to precession of the rotation axis about the crystal field symmetry axis. The symmetry breaking makes the quantum and classical descriptions of hindered rotation compatible.