Molecular conformation and methyl proton delocalization in triiodomesitylene: A combined density functional theory and single-crystal neutron diffraction study

First the conformations of various ortho and di-ortho substituted toluenes calculated by quantum chemistry methods are discussed as well as the hindering potentials deduced from the latter results and those established experimentally by microwaves and fluorescence techniques. It appears that methyl (Me) groups are much less hindered in di-ortho than in ortho substituted compounds. Then the study of the 1,3,5-triiodo-2,4,6-trimethylbenzene (triiodomesitylene or TIM) is reported. Density functional theory (DFT) calculations indicate that two conformations of the TIM molecule have the same formation energies. One has C3h symmetry, the other with the Cs symmetry is obtained from the C3h by a rotation of 60° for one Me. Experimentally, the TIM structure has been determined at 15 K using single-crystal neutron diffraction data. TIM crystallizes in the triclinic space group P−1. Molecules are stacked in an antiferroelectric manner along the oblique a axis. For two Me groups the experimental conformation is close to the Cs one, but the third Me group has a C–H bond nearly orthogonal to the ring plane. Such conformation is unstable in the gas state, but it is stabilized in the crystal by intermolecular interactions, nevertheless DFT predicts accurate bond lengths and angles when the Me conformations are constrained to the experimental ones. The three Me groups, having different environments, experience different hindering potentials, this explains why they are tunnelling at different energies as found by inelastic neutron scattering (INS). Using INS results, we deduced the potentials hindering the Me groups rotation in the crystal. The proton probability densities (PPD) calculated from these potentials are in concordance with the crystallographic results. Hence the quantum origin of the broad spreading of PPD observed at 15 K is established. Moreover, it is demonstrated that the crystal field is responsible for the larger part of the potentials hindering the Me groups in the solid state.