title: Methyl dynamics flattens barrier to proton transfer in crystalline tetraacetylethane.
abstract: 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(3)CO)(2)CH═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.
authors.affiliation: The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales, Australia. gordon.kearley@ansto.gov.au
authors.forename: Gordon J
authors.initials: GJ
authors.lastname: Kearley
authors.forename: Jernej
authors.initials: J
authors.lastname: Stare
authors.forename: Ramzi
authors.initials: R
authors.lastname: Kutteh
authors.forename: Luke L
authors.initials: LL
authors.lastname: Daemen
authors.forename: Monika A
authors.initials: MA
authors.lastname: Hartl
authors.forename: Juergen
authors.initials: J
authors.lastname: Eckert