Methyl Dynamics Flattens Barrier to Proton Transfer in Crystalline Tetraacetylethane

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• 作者：Gordon J. Kearley ; Jernej Stare ; Ramzi Kutteh ; Luke L. Daemen ; Monika A. Hartl ; Juergen Eckert
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：March 8, 2012
• 年：2012
• 卷：116
• 期：9
• 页码：2283-2291
• 全文大小：466K
• 年卷期：v.116,no.9(March 8, 2012)
• ISSN：1520-5215

文摘

We analyze the interplay between proton transfer in the hydrogen-bond bridge, O路路路H路路路O, and lattice dynamics in the model system tetraacetylethane (TAE) (CH₃CO)₂CH鈺怌H(COCH₃)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 脜. 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.