DePalma Joseph W, Doren Douglas J, Johnston Murray V
Department of Chemistry and Biochemistry, University of Delaware , Newark, Delaware 19716, United States.
J Phys Chem A. 2014 Jul 24;118(29):5464-73. doi: 10.1021/jp503348b. Epub 2014 Jul 9.
The structures and thermochemistry of molecular clusters containing sulfuric acid, water, ammonia, and/or dimethylamine ((CH3)2NH or DMA) are explored using a combination of Monte Carlo configuration sampling, semiempirical calculations, and density functional theory (DFT) calculations. Clusters are of the general form [(BH(+))n(HSO4(-))n(H2O)y], where B = NH3 or DMA, 2 ≤ n ≤ 8, and 0 ≤ y ≤ 10. Cluster formulas are written based on the computed structures, which uniformly show proton transfer from each sulfuric acid molecule to a base molecule while the water molecules remain un-ionized. Cluster formation is energetically favorable, owing to strong electrostatic attraction among the ions. Water has a minor effect on the energetics of cluster formation, lowering the free energy of formation by ∼ 10% depending on the cluster size and number of water molecules. Cluster growth (addition of one base molecule and one sulfuric acid molecule to a pre-existing cluster) and base substitution (substituting DMA for ammonia) are also energetically favorable processes for both anhydrous and hydrated clusters. However, the effect of water is different for different bases. Hydrated ammonium bisulfate clusters have a more favorable free energy for growth (i.e., incrementing n with fixed y) than anhydrous clusters, while the reverse is observed for dimethylammonium bisulfate clusters, where the free energy for growth is more favorable for anhydrous clusters. The substitution of DMA for ammonia in bisulfate clusters is favorable but exhibits a complex water dependence. Base substitution in smaller bisulfate clusters is enhanced by the presence of water, while base substitution in larger bisulfate clusters is less favorable for hydrated clusters than that for anhydrous clusters. While DMA substitution can stabilize small clusters containing one or a few sulfuric acid molecules, the free energy advantage of forming amine clusters relative to ammonia clusters becomes less pronounced at larger sizes, especially when the effect of water is considered.
采用蒙特卡罗构型采样、半经验计算和密度泛函理论（DFT）计算相结合的方法，探索了包含硫酸、水、氨和/或二甲胺（(CH3)2NH或DMA）的分子簇的结构和热化学性质。簇的一般形式为[(BH(+))n(HSO4(-))n(H2O)y]，其中B = NH3或DMA，2 ≤ n ≤ 8，且0 ≤ y ≤ 10。根据计算得到的结构写出簇的化学式，这些结构一致显示每个硫酸分子向一个碱分子发生质子转移，而水分子保持未电离状态。由于离子之间强烈的静电吸引力，簇的形成在能量上是有利的。水对簇形成的能量学有较小影响，根据簇的大小和水分子数量，使形成自由能降低约10%。对于无水簇和水合簇，簇的生长（向预先存在的簇中添加一个碱分子和一个硫酸分子）和碱取代（用DMA取代氨）在能量上也是有利的过程。然而，水对不同碱的影响不同。水合硫酸氢铵簇比无水簇具有更有利的生长自由能（即固定y时增加n），而对于硫酸氢二甲铵簇则观察到相反的情况，其中无水簇生长的自由能更有利。在硫酸氢盐簇中用DMA取代氨是有利的，但表现出复杂的水依赖性。较小的硫酸氢盐簇中的碱取代因水的存在而增强，而较大的硫酸氢盐簇中，水合簇的碱取代比无水簇更不利。虽然DMA取代可以稳定含有一个或几个硫酸分子的小簇，但在较大尺寸时，相对于氨簇形成胺簇的自由能优势变得不那么明显，特别是考虑水的影响时。
