Douady J, Calvo F, Spiegelman F
LCPQ, IRSAMC, Universite Paul Sabatier, 118 Route de Narbonne, F31062 Toulouse Cedex, France.
J Chem Phys. 2008 Oct 21;129(15):154305. doi: 10.1063/1.2987304.
The combined effects of size and temperature on the stable structures of water clusters doped with one ammonium molecule have been investigated theoretically using an empirical potential and density-functional theory (DFT) calculations. Global optimization with Monte Carlo methods has been performed using an explicit intermolecular potential based on the Kozack-Jordan polarizable model. Putative lowest-energy structures based on this empirical potential are reported. Our results indicate a high propensity for the NH(4)(+) impurity to be fully solvated by water molecules. Clathratelike patterns are formed for clusters containing more than 11 molecules. Local reoptimizations of candidate structures carried out at the DFT level with the B3LYP hybrid functional and the 6-311++G(d,p) basis set confirm the general trends obtained with the intermolecular potential. However, some reorderings between isomers often due to zero-point energy corrections are found in small clusters, leading to stable geometries in agreement with other first-principles studies. Temperature effects have been assessed using a simple harmonic superposition approximation for selected cluster sizes and using dedicated Monte Carlo simulations for (H(2)O)(20)NH(4)(+). The clusters are found to melt near 200 K, and possibly isomerize already below 50 K. The free energy barrier for core/surface isomerization of the impurity in the 21-molecule cluster is estimated to be only a few kcal/mol at 150 K. The vibrational spectroscopic signatures of the clusters obtained from the electronic structure calculations show the usual four O-H stretching bands. As the cluster size increases, the double acceptor-single donor band near 3700 cm(-1) increasingly dominates over the three other bands. While we do not find conclusive evidence for a O-H stretching spectroscopic signature of the ammonium impurity to be in the core or at the surface in the 20-molecule cluster, a possible signature via the N-H stretching bands is suggested near 2800-2900 cm(-1). In the larger (H(2)O)(49)NH(4)(+) cluster, the impurity is slightly more stable at the surface.
采用经验势和密度泛函理论（DFT）计算方法，从理论上研究了尺寸和温度对掺杂一个铵分子的水团簇稳定结构的综合影响。使用基于Kozack-Jordan极化模型的显式分子间势，通过蒙特卡罗方法进行了全局优化。报道了基于该经验势的假定最低能量结构。我们的结果表明，NH₄⁺杂质极易被水分子完全溶剂化。对于含有超过11个分子的团簇，形成了笼状结构。使用B3LYP杂化泛函和6-311++G(d,p)基组在DFT水平上对候选结构进行局部重新优化，证实了分子间势得到的一般趋势。然而，在小团簇中经常发现由于零点能校正导致的异构体之间的一些重新排列，从而得到与其他第一性原理研究一致的稳定几何结构。使用简单的简谐叠加近似对选定的团簇尺寸评估了温度效应，并对(H₂O)₂₀NH₄⁺进行了专门的蒙特卡罗模拟。发现团簇在200 K附近熔化，并且可能在50 K以下就发生异构化。在150 K时，21分子团簇中杂质的核/表面异构化自由能垒估计仅为几千卡/摩尔。从电子结构计算得到的团簇的振动光谱特征显示出通常的四个O-H伸缩带。随着团簇尺寸的增加，3700 cm⁻¹附近的双受体-单供体带相对于其他三个带越来越占主导地位。虽然我们没有找到确凿证据证明在20分子团簇中铵杂质的O-H伸缩光谱特征在核内或表面，但在2800 - 2900 cm⁻¹附近通过N-H伸缩带可能存在特征。在较大的(H₂O)₄₉NH₄⁺团簇中，杂质在表面稍微更稳定。
