Molecular deformations of halogeno-mesitylenes in the crystal: structure, methyl group rotational tunneling, and numerical modeling

• Published in: Chemical physics 2002-12, Vol.285 (2), p.299-308
• Main Authors: Plazanet, M., Johnson, M.R., Cousson, A., Meinnel, J., Trommsdorff, H.P.
• Format: Article
• Language: English
• Subjects: Crystal structure; Halogeno-mesitylenes; Rotational tunneling; Steric hindrance
• ISSN: 0301-0104
• DOI: 10.1016/S0301-0104(02)00817-0

In crystals of halogeno-mesitylenes, steric hindrance between methyl groups and halogen atoms results in a small out-of-plane deformation of the heavy atoms. Even though these deformations are of very small amplitude, their effect on the rotational potential of the methyl groups is very large because the large threefold contributions to the potential due to the halogens next to a methyl group do not cancel as they do in a planar structure. An investigation of these effects by a combination of computational and experimental methods is presented here for compounds for which precise single crystal structure determinations at low temperatures are available, namely tribromomesitylene and triiodomesitylene, as well as dibromomesitylene for which such a determination was made recently. Information about the rotational potential of the methyl groups is derived from the observed proton densities as well from inelastic and quasi-elastic neutron scattering studies of the tunneling dynamics. DFT calculations reproduce well the molecular structures in the crystal, while a planar structure is predicted for the isolated molecules. The rotational potential of the methyl groups was characterized by DFT and force field calculations at different levels of approximation. The correct calculation of the amplitude of the rotational potential requires the inclusion of the relaxation of all atomic positions during the methyl group rotation, indicating that a one-dimensional model of the rotation is inadequate.