Abstract-The Raman, IR and INS spectra of 1,3-dibromo-2,4,6-trimethylbenzene (DBMH) were recorded in the 80-3200 cm -1 range. The molecular conformation and vibrational spectra of DBMH were computed at the MPW1PW91/LANL2DZ level. Except for the methyl 2 environment, the agreement between the DFT calculations and the neutron diffraction structure is almost perfect (deviations < 0.01 Å for bond lengths, < 0.2° for angles). The frequencies of the internal modes of vibration were calculated with the harmonic and anharmonic approximations; the later method yields results that are in remarkable agreement with the spectroscopic data, resulting in a confident assignment of the vibrational bands. Thus, no scaling is necessary. The coupling, in phase or antiphase, of the motions of symmetrical C-Br and C-Me bonds is highlighted. Our DFT calculations suggest that the torsion of methyl groups 4 and 6 is hindered in deep wells, whereas methyl group 2 is a quasi-free rotor. The failure of the calculations to determine the frequencies of the methyl torsional modes is explained as follows: DFT does not consider the methyl spins and assumes localization of the protons, whereas the methyl groups must be treated as quantum rotors.Keywords: Raman spectroscopy; IR spectroscopy; Inelastic neutron scattering; DFT calculations; 1,3-dibromo-2,4,6-trimethylbenzene 1-IntroductionA tremendous amount of work has been devoted to the spectroscopy of polysubstituted benzenes.Among those relative to penta-and hexa-substituted compounds, two extensive papers were published in 1985: one paper compared eleven polymethyl-benzenes (pMeB) [1] and the second described six chloromethyl-benzenes C 6 (CH 3 ,Cl) 6 (ClMeB) [2]. In these studies, the experimental spectra were assigned using normal coordinate calculations to determine a common multi-parameter force field. Because the molecular conformations were not accurately known the authors assumed that the aromatic ring in all molecules was a regular hexagon, that the bond angles were 120°, and the bond lengths were 1.396 Å for the carbon-carbon aromatic bond, 1.509 Å for the methyl carbonaromatic carbon bond, 1.083 Å for the aromatic carbon-hydrogen bond and 1.096 Å for the methylcarbon hydrogen bond. For the pMeB species, the Raman and infra-red measurements were performed with samples in the gaseous phase at approximately 550 K. A 46-parameter valence force field had been proposed, and the average error between the 736 observed and calculated wavenumbers was 6.4 cm -1 . This agreement suggests that the use of an excessive number of off-diagonal constants (interactions terms) allows us to compensate for the large errors done in the geometrical parameters. According to the authors in these previous studies: "there is a great stability of the internal methyl modes, in a limit of ± 5 cm -1 they are located at: 2930 cm -1 for the C-H stretching, 1445 cm -1 for the asymmetric C-H bending, 1380 cm -1 for the C-H symmetric bending, 1000 and 1060 cm -1 for the C-H rocking". For the "torsion mode" of the meth...