Institute for Theoretical Chemistry, University of Vienna, Währinger Strasse 17, 1090 Vienna, Austria.
J Phys Chem B. 2011 Mar 3;115(8):1807-16. doi: 10.1021/jp107989k. Epub 2011 Feb 4.
The solvation properties of methyl and pentyl chloride were studied in a microhydrated environment with up to 10 explicit water molecules and at the liquid water-vapor interface. Geometry optimizations were performed in the former case using the density functional based tight binding (DFTB), DFTB-D, and Møller-Plesset perturbation theory (MP2) levels of theory. The microhydrated alkyl chloride complexes were characterized in terms of hydrogen bonding and energetic stability. The DFTB and DFTB-D results were verified by comparison with those obtained by MP2. Good agreement between the MP2 and DFTB-D results is found. Complexes where the alkyl chloride molecule is attached to an edge of the water cluster are found to be most stable. Pronounced stability is also observed for cubic arrangements of the alkyl chloride-water complexes. Molecular dynamics simulations based on the DFTB and DFTB-D methods were used to study the adsorption process of the alkyl chloride molecules to a water surface. The dynamics simulations show that the methyl chloride molecule is located at the water surface preferentially with the methyl group oriented toward the water surface, while for pentyl chloride, owing to the longer nonpolar hydrocarbon chain, a parallel alignment at the water surface is found with the hydrocarbon chain pointing slightly to the gas phase. Despite some quantitative differences, the present work provides confirmation of the somewhat surprising preferential orientation of the methyl chloride molecule at the water-vapor interface predicted in a recent study using molecular dynamics simulations based on an empirical force field (Harper et al., J. Phys. Chem. A 2009, 113, 2015-2024). The observed difference in preferred alignment at the aqueous surface between the methyl chloride and the longer-chain alkyl chloride is likely to have consequences for the chemistry of alkyl halides adsorbed on the surface of aqueous and ice particles in the atmosphere.
在含有多达 10 个显式水分子的微水环境中和在液态水-蒸汽界面处研究了甲基氯和戊基氯的溶剂化性质。在前者的情况下，使用密度泛函基紧束缚（DFTB）、DFTB-D 和 Møller-Plesset 微扰理论（MP2）理论水平进行了几何优化。根据氢键和能量稳定性对微水合烷基氯配合物进行了表征。通过与 MP2 获得的结果进行比较，验证了 DFTB 和 DFTB-D 的结果。发现 MP2 和 DFTB-D 的结果之间存在良好的一致性。发现将烷基氯分子附着在水簇边缘的配合物最稳定。还观察到烷基氯-水配合物的立方排列具有明显的稳定性。基于 DFTB 和 DFTB-D 方法的分子动力学模拟被用于研究烷基氯分子向水面的吸附过程。动力学模拟表明，甲基氯分子优先位于水面上，甲基基团朝向水面，而对于戊基氯，由于较长的非极性碳氢链，发现其与水面平行排列，碳氢链略微指向气相。尽管存在一些定量差异，但本工作证实了最近使用基于经验力场的分子动力学模拟（Harper 等人，J. Phys. Chem. A 2009，113，2015-2024）预测的在水-蒸汽界面处甲基氯分子的优先取向有些令人惊讶。在水表面上观察到的甲基氯和较长链烷基氯之间的优先取向的差异可能对吸附在大气中水溶液和冰粒子表面上的卤代烷烃的化学性质产生影响。
