甲烷水合物(可燃冰)结构-I的分子间势能的量子化学研究

Quantum Chemical Study of Molecular Potential Energies in Methane Hydrate Structure-I Unit Cell

采用Hartree-Fork,4种DFT(BLYP,B3LYP,MPW1PW91,SVWN5)和MP2方法研究了甲烷水合物结构-Ⅰ的氢键和范德华能.甲烷分子取HF/6-31G(d,p)优化构型,水分子选用ST2模型.水分子间的氢键能Ebb(l)和甲烷-水分子间的范德华能Evdw(l)作为正十二面体边长l的函数,用HF/6-31G(d,p)和4种DFT方法做计算,保持水分子和甲烷分子的构型不变,在几个关键点上选用MP2方法做了计算.计算中选用6-31G(d,p)基组,分别用完全平衡校正和不完全校正法进行校正,这两种方法给出了基组重叠误差(BSSE)的上限和下限.DFT/B3LYP方法计算的氧-氧距离Ro-o=0.280 nm和碳-氧距离Rc-o=0.392 nm最接近于实验值0.282 nm和0.395 nm.所有计算方法(HF,DFT,MP2)都表明,甲烷水合物结构-Ⅰ是一个由超强氢键(30～36 kJ/mol)组成的稳定结构,其氢键能远大于水分子二聚体和冰Ⅰ4晶格中的氢键能((-22.6±2.9)kJ/mol和(-21.7±0.5)kJ/mol).这些数据为气体水合物的Lennand-Jones和Kihara势能函数提供了基本参数,可用于气体水合物的分子动力学模拟.

Hartree-Fock, DFT （BLYP, B3LYP, MPWIPW91, and SVWNS） and MP2 methods are used to study the intermolecular potential functions for the unit cell of methane hydrate structure-Ⅰ. The unit cell is treated as a regular dodecahedron including 20 water molecules and a methane molecule in the center. The geometry of methane is optimized by ab initio HF/6-31G（d, p） and the water molecules take the geometry of an ST2 model. The water-water hydrogen bond potential Ebb（l） and water-methane van der Waals potential Evdw （1） as functions of the side length l are calculated, keeping the molecular geometries of water and methane fixed. Then MP2 calculations are performed at several key points. In all calculations we use the 6-31G（d,p） basis set. Basis set superposition error （BSSE） is corrected and the upper and low- er limits of BSSE are determined for the water-water hydrogen bond energy. The oxygen-oxygen distance Ro o ： 0. 280 nm, and the carbon-oxygen distance Rc-o = 0. 392 nm obtained by BSSE-corrected B3LYP are closer to the experimental values 0. 282 nm and 0. 395 nm, comparing with other methods. The results indicate that the water-water pair hydrogen bond energy （30-36 kJ/mol） in gas hydrate structure-Ⅰ is stronger than the hydrogen bond energy （-22. 6±2.9） kJ/mol in the water dimer （H2O）2 and （21. 7± 0.5） kJ/mol in hexagonal ice, and the dodecahedron cell is a stable unit. These molecular interaction potentials provide a solid basis for the derivation of parameters in the Lennard-Jones 6-12 and Kihara poten tials, which are useful in molecular dynamics simulations of gas hydrates.