Magnetic field effects on the far-infrared absorption in Mn 12 -acetate

We report the far-infrared spectra of the molecular nanomagnet ${\mathrm{Mn}}_{12}$-acetate $({\mathrm{Mn}}_{12})$ as a function of temperature (5--300 K) and magnetic field (0--17 T). The large number of observed vibrational modes is related to the low symmetry of the molecule, and they are grouped together in clusters. Analysis of the mode character based on molecular-dynamics simulations and model compound studies shows that all vibrations are complex; motion from a majority of atoms in the molecule contribute to most modes. Three features involving intramolecular vibrations of the ${\mathrm{Mn}}_{12}$ molecule centered at 284, 306, and $409 {\mathrm{cm}}^{\ensuremath{-}1}$ show changes with applied magnetic field. The structure near $284 {\mathrm{cm}}^{\ensuremath{-}1}$ displays the largest deviation with field and is mainly intensity related. A comparison between the temperature-dependent absorption-difference spectra, the gradual low-temperature cluster framework distortion as assessed by neutron-diffraction data, and field-dependent absorption-difference spectra suggests that this mode may involve Mn motion in the crown.