KNO_(3)溶液结构:密度泛函理论和分子动力学研究

Structure of KNO_(3)Solutions:Density Functional Theory and Molecular Dynamics

基于密度泛函理论,采用ωB97XD方法和6-311++G(d,p)基组,系统研究了[K(H_(2)O)_(n)]+,[NO_(3)(H_(2)O)_(n)]-和KNO_(3)(H_(2)O)_(n)水合团簇的结构和性质;并结合经典分子动力学、OPLS-AA全原子力场以及SPC/E水模型模拟了不同水盐比(WSR)KNO_(3)溶液,目的在于深入理解KNO_(3)溶液中K^(+)和NO_(3)^(-)的微观水合结构,以及K^(+)-NO_(3)^(-)离子间缔合作用特征。研究发现,对于KNO_(3)(H_(2)O)_(n)(n=6,8,10和12)水合团簇,K^(+)第一水合层稳定结构为四、五和六水合结构,其中五水合结构最稳定。K^(+)和NO_(3)^(-)主要以溶剂共享离子对形式存在。随着团簇尺寸减小,直接接触离子对(CIP)物种出现。同时,分子动力学模拟证实KNO_(3)溶液中形成了直接接触K^(+)-NO_(3)^(-)离子对,且随着溶液浓度增加,CIP物种形成需越过更大能垒。以上结果表明在KNO_(3)稀溶液中主要存在物种为水合K^(+)和NO_(3)^(-),随浓度增大,单齿直接接触离子对(MCIP)逐渐出现。在高浓溶液或过饱和溶液中,K^(+)-NO_(3)^(-)离子缔合主要以单齿CIP和少量双齿CIP结构存在。

The structures and properties of[K(H_(2)O)_(n)]+,[NO_(3)(H_(2)O)_(n)]-and KNO_(3)(H_(2)O)_(n)hydrated clusters are systematically investigated atωB97XD/6-311++G(d,p)basis level based on density functional theory(DFT)to understand the micro-hydrated structures of K^(+)and NO_(3)^(-),and the K^(+)-NO_(3)^(-)ion association characteristics in aqueous KNO_(3)solution.Meanwhile,the KNO_(3)solutions in different concentrations are also studied by molecular dynamics(MD)simulation with OPLS-AA all-atom force filed and SPC/E water model.The results showed that the four-,five-,and six-hydrated structures are stable for the first hydration layer of K^(+)in KNO_(3)(H_(2)O)_(n)(n=6,8,10 and 12)hydrated clusters,and the five-hydrated structure is the most stable.The solvent-shared ion pairs are the main form for K^(+)and NO_(3)^(-)in KNO_(3)(H_(2)O)_(n)hydrated clusters.The contact ion pairs(CIP)appear as cluster size decreasing.Meanwhile,MD simulation confirmed that direct contact K^(+)-NO_(3)^(-)ion pairs were formed in concentrated KNO_(3)solutions,and the formation of CIP species had to cross a larger energy barrier with the increase of solution concentration.It could be considered that the hydrated K^(+)and hydrated NO_(3)^(-)are the mainly species for dilute KNO_(3)solutions,the monodentate direct contact ion pair(MCIP)structure gradually appeared with the concentration increasing.In high concentration or supersaturated KNO_(3)solutions,the K^(+)-NO_(3)^(-)ion association is mainly as MCIP and a small amount of bidentate direct contact ion pair.