NH_4^+(H_2O)_n(n=1～9)的量子化学和ABEEM/MM的理论研究

Theoretical Studies on NH_4^+(H_2O)_n(n=1～9) in terms of Quantum Chemistry and ABEEM/MM

运用量子化学和ABEEM/MM浮动电荷分子力场,构建描述铵离子-水体系相互作用的精密势能函数,对NH+4-(H2O)n(n=1～9)簇合物的结构和稳定性等性质进行了研究.对团簇的结合能和电荷布居分析发现,当n≤4时,随着水分子数目的增加,NH+4与水分子间尽可能多地形成线型氢键,直至水分子在NH+4周围形成完整的第一水合层;当n≥5时,簇合物以NH+4为中心,通过氢键网络形成的环状和笼状结构为最稳定.NH+4与第一水合层水分子的相互作用强于水分子之间的相互作用.结果表明,ABEEM/MM方法的结果与量子化学方法得到的结果有很好的一致性.

By means of quantum chemistry and the ABEEM/MM fluctuating charge force field,an accurate interaction potential function for-H2O system was constructed,and then it was applied to study the structures and stabilities of hydrated ammonium clusters（H2O）n（n=1～9）.Analysis on the binding energies and charge distributions of the clusters shows that when n≤4,as the number of water molecules increases,tends to form linear hydrogen bonds as many as possible with water molecules until its first hydration shell is completely constructed.The clusters for 5～9 water molecules tend to form a ring or a cage around the central through the network of hydrogen bonds.The strength of H-bonds between and water molecules in its 1st hydration shell is stronger than that of the H-bonds between water molecules.The results of the ABEEM/MM model are well consistent with those from quantum chemistry calculations.