摘要
水溶液中Pb(Ⅱ)的水合结构缺乏实验数据,成键机理尚不明确.采用密度泛函理论、周期性边界计算水合物种Pb(HO)2+1-9的低能构象,探讨其结合能和稳定性,结合该离子在水溶液中的第一性原理分子动力学弛豫行为和扩展X射线吸收精细结构(EXAFS)谱特征,确定其最佳稳定构型和水合数,并通过Mulliken电荷布居、分波态密度及差分电荷密度分析其成键机理.低能优化结果表明,水合数6,7和8的构象均有可能存在,动力学弛豫过程Pb(Ⅱ)的第一和第二水合层不存在明显分界,其优势构型的水合数为6,属于偏半方位构型.22 6Pb(HO)+中Pb—O键的离子性较强,成键机理主要为Pb6p6d轨道与Pb6s-O2p反键态轨道进行耦合,存在反键态电子填充,表现为原子间电子密度的减少.
Experimental investigation of the hydration structure of Pb（Ⅱ） has been poorly documented, and the bonding mechanism is not clear. Geometries of Pb（H2O）1~92＋ were optimized using the Perdew-Burke-Ernzerhof generalized gradient approximation （PBE-GGA） of density functional theory with CASTEP code under the periodic boundary conditions. The interaction between nuclei and electrons was approximated with Vanderbilt ultrasoft pseudopotential. First-principles molecular dynamics （MD） study of Pb（H2O）52＋ in an aqueous medium was carried out in order to gain insight into solvation behavior of Pb（Ⅱ）, where the density of water in solution was 1.0×103 kg/m3. The MD simulation was performed in the NVT ensemble at 298 K, with a time step of 1.0 fs and simulation time of 8.0 ps. Then the Mulliken charge population, partial density of states （PDOS） and charge density difference of hydrated Pb（Ⅱ） complex were analyzed. Determination of bond length, binding energy and Mulliken charge population of equilibrium geometries of Pb（H2O）1~92＋ indicated that the primary hydration numbers of 6, 7 and 8 were all possible, and the geometry of Pb（H2O）62＋ was found to be the transition state between the holodirected and the hemidirected. According to the relaxation of Pb（H2O）52＋ in aqueous solution, there was always water exchange between the primary and secondary hydration shells of Pb（Ⅱ）. Based on the equilibrium structural parameters of Pb（H2O）1~92＋, relaxation of Pb（H2O）52＋ in aqueous solution and the experimental extended X-ray absorption fine structure （EXAFS） results, the most probable coordination number of the first hydration shell of aqueous Pb（Ⅱ） was 6 of a complex with approximately hemi-directed structure. Bonds of Pb—O in Pb（H2O）62＋ exhibit strong ionicity with few covalent character. The distortion of Pb（H2O）62＋ in structure does not originate from a 6s-6p hybridization on the heavy metal as the Pb 6s and 6p states are energetically too far apart to mix directly. It is the coupling of Pb6p6d with the antibonding Pb6s-O2p state that gives rise to the net asymmetry in the electron density on Pb, which results in the distorted structure of Pb（H2O）62＋.
出处
《化学学报》
SCIE
CAS
CSCD
北大核心
2013年第9期1307-1312,共6页
Acta Chimica Sinica
基金
国家自然科学基金(No.20677053)资助~~
关键词
Pb(Ⅱ)
水合结构
密度泛函理论
第一性原理分子动力学
态密度
Pb（Ⅱ）
hydration structure
density functional theory
first-principles molecular dynamics
density of states
