二氧化碳水合物小晶穴和大晶穴的密度泛函研究

本文通过B3LYP方法,在6-31G(d,p)水平下对结构Ⅰ型二氧化碳水合物的十二面体小晶穴和十四面体大晶穴的结构进行优化,并计算了其振动频率.13C-NMR化学屏蔽常数及电子密度.结果表明,水合物晶穴的氢键键能强于水分子二聚体和冰晶格中的氢键键能.由此可以看出结构Ⅰ型二氧化碳水合物的晶穴是由氢键组成的稳定结构.通过CO2分子与大、小晶穴的范德华能比较,发现C02分子在大晶穴中更稳定.所计算出的C=O键的不对称伸缩振动频率和13C-NMR化学屏蔽常数与文献中的实验值基本一致.通过分析CO2与晶穴的电子密度,其结果与氢键键能计算结果一致,计算结果还表明,CO2在大晶穴中的稳定性较在小晶穴中的稳定性强.

Density functional studies on small and large cavities of carbon dioxide hydrate

Geometry optimizations of type I CO_2 hydrate structure by B3LYP/6-31G(d,p) have been performed on CO_2(H_2O)_(20) and CO_2(H_2O)_(24) clusters.Then the frequencies,~(13)C-NMR chemical shielding constants and electronic density are calculated.According to the results,the H-bond energies in each cavities are stronger than that in the water dimmer (H_2O)_2 and in hexagonal ice.Hence,both the cavities are stable units.The VDW interaction of CO_2 with the large cavity is slightly stronger than that with the small cavity by considering the VDW interaction energy between CO_2 and hydrogen-bonded water cavities.The calculated values of asymmetric C=O stretching frequencies and ~(13)C-NMR chemical shieldings of the CO_2 in the two clusters show good agreements with the experimental values in the references.Analysis of the electron density of CO_2 and two cavities shows agreement with trends of O-H...O hydrogen bonds and C=O bond lengths in two clusters.The calculated results also indicate that the stability for CO_2 in the large cavity should be stronger than that in the small cavity.