Karthikeyan S, Singh N Jiten, Kim Kwang S
Department of Chemistry, Center for Superfunctional Materials, Pohang University of Science and Technology, Namgu, Pohang, Korea.
J Phys Chem A. 2008 Jul 24;112(29):6527-32. doi: 10.1021/jp801678r. Epub 2008 Jun 26.
To understand the autoionization of pure water and the solvation of ammonia in water, we investigated the undissociated and dissociated (ion-pair) structures of (H2O) n and NH3(H2O)n-1 (n = 5, 8, 9, 21) using density functional theory (DFT) and second order Moller-Plesset perturbation theory (MP2). The stability, thermodynamic properties, and infrared spectra were also studied. The dissociated (ion-pair) form of the clusters tends to favor the solvent-separated ion-pair of H3O+/NH4+ and OH-. As for the NH3(H2O)20 cluster, the undissociated structure has the internal conformation, in contrast to the surface conformation for the (H2O)21 cluster, whereas the dissociated structure of NH3(H2O)20 has the surface conformation. As the cluster size of (H2O)n/NH3(H2O)n-1 increases, the difference in standard free energy between undissociated and dissociated (ion-pair) clusters is asymptotically well corroborated with the experimental free energy change at infinite dilution of H3O+/NH4+ and OH-. The predicted NH and OH stretching frequencies of the undissociated and dissociated (ion-pair) clusters are discussed.
为了理解纯水的自电离以及氨在水中的溶剂化作用，我们使用密度泛函理论（DFT）和二阶莫勒-普列斯特定理（MP2）研究了(H₂O)ₙ和NH₃(H₂O)ₙ₋₁（n = 5、8、9、21）的未离解和离解（离子对）结构。还研究了其稳定性、热力学性质和红外光谱。团簇的离解（离子对）形式倾向于形成溶剂分隔的H₃O⁺/NH₄⁺和OH⁻离子对。对于NH₃(H₂O)₂₀团簇，未离解结构具有内部构象，这与(H₂O)₂₁团簇的表面构象不同，而NH₃(H₂O)₂₀的离解结构具有表面构象。随着(H₂O)ₙ/NH₃(H₂O)ₙ₋₁团簇尺寸的增加，未离解和离解（离子对）团簇之间标准自由能的差异与H₃O⁺/NH₄⁺和OH⁻在无限稀释时的实验自由能变化渐近地得到了很好的证实。讨论了未离解和离解（离子对）团簇预测的NH和OH伸缩频率。
