Anharmonic Computations Meet Experiments (IR, Raman, Neutron Diffraction) for Explaining the Behavior of 1,3,5-Tribromo-2,4,6-trimethylbenzene
Résumé
In the present paper we first show the exptl. Raman, IR, and neutron INS spectra of tribromomesitylene (TBM) measured in the range 50-3200 cm-1 using cryst. powders at 6 or 4 K. Then, the bond lengths and angles detd. by neutron diffraction using a TBM single crystal at 14 K are compared to the computed ones at different levels of theory. Anharmonic computations were then performed on the relaxed structure using the VPT2 approach, and for the lowest normal modes, the HRAO model has led to a remarkable agreement for the assignment of the exptl. signatures. A particularity appears for frequencies below 150 cm-1, and in particular for those concerning the energy levels of "hindered rotation" of the three Me groups, they must be calcd. for one-dimensional sym. tops independent of the frame vibrations. This fact is consistent with the structure established by neutron diffraction: the protons of the Me groups undergoing huge "libration" motions are widely spread in space. The values of the transitions between the librational levels detd. by inelastic neutron scattering indicate that the hindering potentials are mainly due to intermol. interactions different for each Me group in the triclinic cell.