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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Formation and Growth of Molecular Clusters Containing Sulfuric Acid, Water, Ammonia, and Dimethylamine
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Formation and Growth of Molecular Clusters Containing Sulfuric Acid, Water, Ammonia, and Dimethylamine

机译：包含硫酸，水，氨和二甲胺的分子簇的形成和生长

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摘要

The structures and thermochemistry of molecular clusters containing sulfuric acid, water, ammonia, and/or dimethylamine ((CH_3)_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(HSO_4~-)_n(H_2O)_y], where B = NH_3 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.
机译：结合蒙特卡洛构型采样，半经验计算和密度泛函理论（DFT）计算，探索了包含硫酸，水，氨和/或二甲胺（（CH_3）_2NH或DMA）的分子簇的结构和热化学。聚类的一般形式为[[（BH〜+）_ n（HSO_4〜-）_ n（H_2O）_y]，其中B = NH_3或DMA，2≤n≤8，且0≤y≤10。在计算的结构上，该结构均匀地显示了质子从每个硫酸分子转移到基础分子，而水分子保持未电离。由于离子之间强烈的静电吸引，因此在能量上有利于团簇的形成。水对团簇形成的能量影响较小，这取决于团簇的大小和水分子的数量，将形成的自由能降低了约10％。对于无水和水合簇，簇生长（将一个碱基分子和一个硫酸分子添加到预先存在的簇中）和碱基取代（用DMA代替氨）也是在能量上有利的过程。但是，对于不同的碱，水的作用是不同的。水合硫酸氢铵簇具有比无水簇更有利于生长的自由能（即，以固定的y递增n），而对于二硫酸氢二铵铵簇则相反，其中生长的自由能对无水簇更有利。用DMA取代硫酸氢盐簇中的氨是有利的，但表现出复杂的水依赖性。水的存在会增强较小的硫酸氢盐簇中的碱取代，而较大的硫酸氢盐簇中的水合物则比无水簇的碱取代更不利。虽然DMA取代可以稳定包含一个或几个硫酸分子的小簇，但相对于氨簇，形成胺簇的自由能优势在较大尺寸时变得不那么明显，尤其是考虑到水的影响时。

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