Methyl dynamics flattens barrier to proton transfer in crystalline tetraacetylethane
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Date
2012-03-08
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Publisher
American Chemical Society
Abstract
We analyze the interplay between proton transfer in the hydrogen-bond bridge, O center dot center dot center dot H center dot center dot center dot O, and lattice dynamics in the model system tetraacetylethane (TAB) (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 angstrom. 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. © 2012, American Chemical Society.
Description
Keywords
Protons, Neutron diffraction, Crystallization, Approximations, Density, Crystal lattices
Citation
Kearley, G. J., Stare, J., Kutteh, R., Daeman, L. L., Hartl, M. A., & Eckert, J. (2012). Methyl dynamics flattens barrier to proton transfer in crystalline tetraacetylethane. Journal of Physical Chemistry A, 116(9), 2283-2291. doi:10.1021/jp210212q