Simulated Annealing and Density Functional Theory Calculations of Structural and Energetic Properties of the Ammonium Chloride Clusters (NH4Cl)n, (NH4+)(NH4Cl)n, and (Cl–)(NH4Cl)n, n = 1–13

Simulated annealing Monte Carlo conformer searches using the “mag-walking” algorithm are employed to locate the global minima of molecular clusters of ammonium chloride of the types (NH4Cl)n, (NH4+)(NH4Cl)n, and (Cl–)(NH4Cl)n with n = 1–13. The M06-2X density functional theory method is used to refine and predict the structures, energies, and thermodynamic properties of the neutral, cation, and anion clusters. For selected small clusters, the resulting structures are compared to those obtained from a variety of models and basis sets, including RI-MP2 and B3LYP calculations. M06-2X calculations predict enhanced stability of the (NH4+)(NH4Cl)n clusters when n = 3, 6, 8, and 13. This prediction corresponds favorably to anomalies previously observed in thermospray mass spectroscopy experiments. The (NH4Cl)n clusters show alternations in stability between even and odd values of n. Clusters of the type (Cl–)(NH4Cl)n display a magic number distribution different from that of the cation clusters, with enhanced stability predicted for n = 2, 6, and 11. None of the observed cluster structures resemble the room-temperature CsCl structure of NH4Cl(s), which is consistent with previous work. Numerous clusters have structures reminiscent of the higher-temperature, rock-salt phase of the solid ammonium halides.

本研究采用"磁游走（mag-walking）"算法开展模拟退火蒙特卡洛（simulated annealing Monte Carlo）构象搜索，以定位三类氯化铵分子团簇的全局极小值：(NH4Cl)n、(NH4+)(NH4Cl)n 与 (Cl–)(NH4Cl)n，其中n的取值范围为1至13。本研究采用M06-2X密度泛函理论（density functional theory）方法，对中性、阳离子及阴离子团簇的结构、能量与热力学性质进行优化与预测。针对部分选定的小型团簇，将其优化得到的结构与多种模型及基组（包括RI-MP2与B3LYP计算结果）进行对比分析。M06-2X计算结果预测，当n=3、6、8及13时，(NH4+)(NH4Cl)n类团簇的稳定性显著提升，该预测结果与此前热喷雾质谱（thermospray mass spectroscopy）实验中观测到的异常现象高度吻合。(NH4Cl)n类团簇的稳定性随n的奇偶取值呈现交替变化规律。(Cl–)(NH4Cl)n类团簇则展现出与阳离子团簇截然不同的幻数分布特征，其稳定性在n=2、6及11时显著增强。所有观测到的团簇结构均未呈现氯化铵固态的室温氯化铯（CsCl）结构，这与既往研究结论一致；诸多团簇的结构与卤化铵固态的高温岩盐相结构具有较高相似性。