Methyl dynamics flattens barrier to proton transfer in crystalline tetraacetylethane

We analyze the interplay between proton transfer in the hydrogen-bond bridge, O•••H•••O, and lattice dynamics in the model system tetraacetylethane (TAE) (CH ₃CO) ₂CH= CH(COCH ₃) ₂ using density functional theory. Lattice dynamics calculations and molecular dynamics simulations are validated against neutron scattering data. Hindrance to the cooperative reorientation of neighboring methyl groups at low temperatures gives a preferred O atom for the bridging proton. The amplitude of methyl torsions becomes larger with increasing temperature, so that the free-energy minimum for the proton becomes flat over 0.2 Å. For the isolated molecule, however, we show an almost temperature-independent symmetric double-well potential persists. This difference arises from the much higher barriers to methyl torsion in the crystal that make the region of torsional phase space that is most crucial for symmetrization poorly accessible. Consequently, the proton-transfer potential remains asymmetric though flat at the base, even at room temperature in the solid.