School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Dr., Atlanta, Georgia 30332-0100, USA.
J Chem Phys. 2011 Jul 21;135(3):034703. doi: 10.1063/1.3610420.
Experimental studies have reported that glycine is adsorbed on the Cu(110) and Cu(100) surfaces in its deprotonated form at room temperature, but in its zwitterionic form on Pd(111) and Pt(111). In contrast, recent density functional theory (DFT) calculations indicated that the deprotonated molecules are thermodynamically favored on Cu(110), Cu(100), and Pd(111). To explore the source of this disagreement, we have tested three possible hypotheses. Using DFT calculations, we first show that the kinetic barrier for the deprotonation reaction of glycine on Pd(111) is larger than on Cu(110) or Cu(100). We then report that the presence of excess hydrogen would have little influence on the experimentally observed results, especially for Pd(111). Lastly, we perform Monte Carlo simulations to demonstrate that the aggregates of zwitterionic species on Pt(111) are energetically preferred to those of neutral species. Our results strongly suggest that the formation of aggregates with relatively large numbers of adsorbed molecules is favored under experimentally relevant conditions and that the adsorbate-adsorbate interactions in these aggregates stabilize the zwitterionic species.
实验研究表明，甘氨酸在室温下以去质子化的形式吸附在 Cu(110) 和 Cu(100) 表面上，但在 Pd(111) 和 Pt(111) 上以两性离子的形式存在。相比之下，最近的密度泛函理论 (DFT) 计算表明，在 Cu(110)、Cu(100) 和 Pd(111) 上，去质子化的分子在热力学上是有利的。为了探究这种分歧的根源，我们检验了三种可能的假设。我们首先使用 DFT 计算表明，甘氨酸在 Pd(111)上的去质子化反应的动力学势垒大于在 Cu(110)或 Cu(100)上的势垒。然后，我们报告说，过量的氢对实验观察到的结果几乎没有影响，特别是对于 Pd(111)。最后，我们进行蒙特卡罗模拟，以证明在 Pt(111)上两性离子物种的聚集体在能量上优先于中性物种的聚集体。我们的结果强烈表明，在实验相关条件下，形成具有相对大量吸附分子的聚集体是有利的，并且这些聚集体中的吸附物-吸附物相互作用稳定了两性离子物种。
