Temperature dependence of the proton hyperfine interaction in HC60 and the proton-muon hyperfine anomaly.

Abstract

The decrease in the proton hyperfine interaction of ${\mathrm{HC}}_{60}$ with increasing temperature is analyzed on the basis of quantum-chemical calculations of the vibrational force field of the radical. The changes of the hyperfine interaction with respect to normal-mode displacements are evaluated and used to simulate the temperature dependence. It is shown that the modulation by low-frequency A'' (${\mathit{C}}_{\mathit{s}}$ symmetry) vibrational modes of the radical that correspond to the lowest ${\mathit{h}}_{\mathit{g}}$ and ${\mathit{h}}_{\mathit{u}}$ vibrations of ${\mathrm{C}}_{60}$ dominates the observed temperature dependence. The experimentally observed negative hyperfine anomaly, vis-\`a-vis the muon coupling, is also attributed to vibrational motions, specifically to the three modes that describe hydrogen motions.