Abstract
The low-frequency modes of MIL-47(V) and MIL-53(Cr) frameworks have been measured by inelastic neutron scattering. The librational modes of benzene are very close in frequency in the two structures. First-principles calculations have been performed to assign the measured spectra. The calculated values of the rotational barriers of the benzene rings are in reasonable agreement with experimental data. The relatively large barriers (413- 588 meV) explain the absence of quasi-elastic broadening on the neutron time scale.