烷烃客体分子(CnHm，n≤6，m≤14)在5^126^2和5^126^4水笼中的稳定性与拉曼光谱的密度泛函理论计算

Stability and Raman Spectroscopy of Alkane Guest Molecules（C_nH_m, n≤6, m≤14）in 5^(12)6~2 and 5^(12)6~4 Water Cavities by Density Functional Theory Calculations

可燃冰矿藏中气体成分非常复杂,通过谱学分析对水合物样品成分进行指认具有重要意义.基于B97-D/6-311++G(2d,2p)的密度泛函理论(DFT)计算,我们系统地探索了构成水合物的两种标准水笼(51262和51264)包络十八种不同烷烃客体分子的稳定性.从计算结果可以看出,除了3-甲基戊烷和2,3-二甲基丁烷两个烷烃客体分子,其它16个烷烃客体分子都可以被容纳在51262笼中;但是与51262笼不同,十八种烷烃客体分子都可以被容纳在尺寸较大的51264笼中.同时,我们也模拟了五种直链烷烃和四种环状烷烃在51262和51264笼中相应的谱学特征,从拉曼谱图上可以看出,随着碳原子数量的增多,直链烷烃客体分子C―H键伸缩振动区的多数拉曼谱带向高波数移动,而环状烷烃客体分子C―H键伸缩振动区的拉曼谱带则向低波数移动.这些结果为实验上通过拉曼谱测量指认水合物矿藏的成分提供理论参考.

The gas composition in natural gas hydrate deposits is complex, and therefore the use of spectroscopic analysis to elucidate the chemical composition is of great significance. Using density functional theory （DFT） calculations at the B97-D/6-311 ＋＋G（2d, 2p） level, we systematically explored the stability of 18 alkane guest molecules in two standard water cavities （5^126^2 and 5^126^4）. The results indicated that most alkane guest molecules can be stored in the 5^126^2 cage, with the exception of 3-methylpentane and 2,3-dimethylbutane, while all 18 alkanes can be encapsulated in the 5^126^4 cage. The Raman spectroscopic characteristics of five straightchain and four cyclic alkane guest molecules in the 5^126^2 and 5^126^4 cages were also simulated. The majority of the Raman bands of the straightchain alkanes in the C—H stretching region were found to move to higher wavenumbers as the number of carbon atoms increased, while most bands of the cyclic molecules in this region transitioned to lower wavenumbers. These theoretical results should prove helpful with regard to identifying hydrate deposits from experimental Raman spectroscopic data.