Calculations of the molecular interactions in 1,3-dibromo-2,4,6-trimethyl-benzene: which methyl groups are quasi-free rotors in the crystal?

Dibromomesitylene (DBMH) is one of the few molecules in which a methyl group is a quasi-free rotor in the crystal state. Density functional theory calculations - using the Born-Oppenheimer approximation (BOa) - indicate that in isolated DBMH, Me4 and Me6 are highly hindered in a 3-fold potential > 55 meV while Me2 symmetrically located between two Br atoms has a small 6-fold rotation hindering potential: ∼ 8 meV. Inelastic neutron scattering studies have shown that this is also true in the crystal, the Me2 tunneling gap being 390 μeV at 4.