Piccini GiovanniMaria, Sauer Joachim
Institut für Chemie, Humboldt Universität zu Berlin , Unter den Linden 6, 10099 Berlin, Germany.
J Chem Theory Comput. 2014 Jun 10;10(6):2479-87. doi: 10.1021/ct500291x. Epub 2014 May 22.
The effect of anharmonic corrections to the vibrational energies of extended systems is explored. Particular attention is paid to the thermodynamics of adsorption of small molecules on catalytically relevant systems typically affected by anharmonicity. The implemented scheme obtains one-dimensional anharmonic model potentials by distorting the equilibrium structure along the normal modes using both rectilinear (Cartesian) or curvilinear (internal) representations. Only in the latter case, the modes are decoupled also at higher order of the potential and the thermodynamic functions change in the expected directions. The method is applied to calculate ab initio enthalpies, entropies, and Gibbs free energies for the adsorption of methane in acidic chabazite (H-CHA) and on MgO(001) surface. The values obtained for the adsorption of methane in H-CHA (273.15 K, 0.1 MPa, θ = 0.5) are ΔH = -19.3, -TΔS = 11.9, and ΔG = -7.5 kJ/mol. For methane on the MgO(001) (47 K, 1.3 × 10(-14) MPa, θ = 1) ΔH = -14.4, -TΔS = 16.6, and ΔG = 2.1 kJ/mol are obtained. The calculated desorption temperature is 44 K, and the desorption prefactor is 4.26 × 10(12) s(-1). All calculated results agree within chemical accuracy limits with experimental data.
研究了非谐校正对扩展体系振动能量的影响。特别关注小分子在通常受非谐性影响的催化相关体系上吸附的热力学。所实施的方案通过使用直线（笛卡尔）或曲线（内坐标）表示沿简正模式扭曲平衡结构来获得一维非谐模型势。仅在后一种情况下，模式在势的高阶项中也解耦，并且热力学函数沿预期方向变化。该方法用于从头计算酸性菱沸石（H-CHA）和MgO(001)表面上甲烷吸附的焓、熵和吉布斯自由能。在H-CHA中甲烷吸附（273.15 K，0.1 MPa，θ = 0.5）得到的值为ΔH = -19.3，-TΔS = 11.9，以及ΔG = -7.5 kJ/mol。对于MgO(001)上的甲烷（47 K，1.3×10⁻¹⁴ MPa，θ = 1），得到ΔH = -14.4，-TΔS = 16.6，以及ΔG = 2.1 kJ/mol。计算得到的脱附温度为44 K，脱附前因子为4.26×10¹² s⁻¹。所有计算结果在化学精度范围内与实验数据一致。
