Gomez Diego T, Pratt Lawrence R, Rogers David M, Rempe Susan B
Department of Chemical & Biomolecular Engineering, Tulane University, New Orleans, LA 70118, USA.
National Center for Computational Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA.
Molecules. 2021 May 21;26(11):3087. doi: 10.3390/molecules26113087.
With a longer-term goal of addressing the comparative behavior of the aqueous halides F-, Cl-, Br-, and I- on the basis of quasi-chemical theory (QCT), here we study structures and free energies of hydration clusters for those anions. We confirm that energetically optimal (H2O)nX clusters, with X = Cl-, Br-, and I-, exhibit hydration structures. Computed free energies, based on optimized surface hydration structures utilizing a harmonic approximation, typically (but not always) disagree with experimental free energies. To remedy the harmonic approximation, we utilize single-point electronic structure calculations on cluster geometries sampled from an AIMD ( molecular dynamics) simulation stream. This procedure is broadly satisfactory and suggests unfavorable ligand crowding as the physical effect addressed. Nevertheless, this procedure can break down when n≳4, with the characteristic discrepancy resulting from a relaxed definition of clustering in the identification of (H2O)nX clusters, including ramified structures natural in With ramified structures, the central equation for the present rough-landscape approach can acquire some inconsistency. Extension of these physical cluster theories in the direction of QCT should remedy that issue, and should be the next step in this research direction.
基于准化学理论（QCT）研究卤离子F-、Cl-、Br-和I-在水溶液中的比较行为这一长期目标，我们在此研究这些阴离子水合簇的结构和自由能。我们证实，对于X = Cl-、Br-和I-的能量最优（H2O）nX簇呈现出水合结构。基于利用谐波近似优化的表面水合结构计算得到的自由能，通常（但并非总是）与实验自由能不一致。为了修正谐波近似，我们对从AIMD（分子动力学）模拟流中采样的簇几何结构进行单点电子结构计算。该过程总体上令人满意，并表明不利的配体拥挤是所涉及的物理效应。然而，当n≳4时该过程可能会失效，其特征差异源于在识别（H2O）nX簇（包括在具有分支结构时自然存在的分支结构）时对聚类的宽松定义。对于当前的粗略景观方法，核心方程可能会出现一些不一致。将这些物理簇理论朝着QCT方向扩展应能解决该问题，这应是该研究方向的下一步。
