Structure of trihalogenomesitylenes and tunneling of the methyl groups protons

The crystal structure of protonated 1,3,5-tribromo-2,4,6-trimethyl benzene is studied at 295 and 14 K by single-crystal neutron diffraction. In this temperature range, the structure is always triclinic: P(1) over bar and Z = 2. All atoms are in the plane of the aromatic ring except two staggered protons of each methyl group. Consequently, deviation from C-3h symmetry is negligible for the whole molecule. In the aromatic ring, a significant shortening of the C-C bond facing the eclipsed C-H bond is observed, while the C-ar-C-ar-C-me angle is increased to 123.3 degrees. The apparent conformation of the proton density in the methyl groups varies drastically as a function of temperature. At 14 K, the proton density shows three maxima for each methyl group, located on a circle of radius 1.006 Angstrom. Solving the Schrodinger equation, we have found the potentials compatible with the excitations measured in the incoherent inelastic neutron scattering spectrum. The eigenvectors corresponding to the two lower states allowed us to calculate a proton density very close to that measured by neutron diffraction at 14 K. These results establish a strong correlation between coherent and incoherent neutron scattering, for methyl groups treated as uniaxial rotors. However, at 295 K, four maxima of proton density were found, which were attributed to a complex coupling between the methyl rotor and various motions of the molecule. Therefore, the anisotropic spreading of proton density at low temperature is basically of quantum nature while, at higher temperatures, the apparent deformation of the methyl group is the result of complex thermal motions. (C) 2000 Elsevier Science B.V. All rights reserved.