  We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate
(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 Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes
with applied magnetic field. The structure near 284 cm$^{-1}$ 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.
