Experimental and theoretical insights into the structure and molecular dynamics of...

The methoxy analogue of a trans-stilbene compound - 2,3,3',4'-tetramethoxy-trans-stilbene - was selected to characterize its crystallographic structure, intermolecular interactions and molecular dynamics. The sample was studied using single-crystal X-ray diffraction (XRD), infrared spectroscopy (FT-IR), liquid and solid-state ¹H and ¹³C nuclear magnetic resonance (NMR) and quasielastic neutron scattering (QENS). The compound crystallized in the orthorhombic Pbca space group. The experimental methods were supported by theoretical calculations, density functional theory (plane-wave DFT) and molecular dynamics simulations (MD) methods. Combining several experimental and simulation techniques allowed the detailed analysis of molecular reorientations and provided a consistent picture of the molecular dynamics. The internal molecular mobility of the studied compound can be associated with the reorientational dynamics of four methyl groups. Interestingly, a large diversity of the energy barriers was observed - one methyl group reoriented across low activation barriers (∼3 kJ mol^-1), while three methyl groups exhibited a high activation energy (10-14 kJ mol^-1) and they are characterised by very different correlation times differing by almost two orders of magnitude at room temperature. The intramolecular interactions mainly influence the activation barriers.