Dispersion curves of 12 external and 10 internal phonon modes have been measured in the a*, b* and c* directions. The internal dispersion curves correspond to the 5 low-frequency vibrations of the free molecule and they exhibit a significant proportion of lattice mode character involving both translational and librational motions. Recent lattice dynamical calculations, in which several internal modes are included, have yielded neutron scattering cross-sections which have made possible to assign the observed molecular vibrations
