Stability and Raman Spectroscopy of Alkane Guest Molecules (CnHm, n <= 6, m <= 14) in 5(12)6(2) and 5(12)6(4) Water Cavities by Density Functional Theory Calculations

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(12)6(2) and 5(12)6(4)). the results indicated that most alkane guest molecules can be stored in the 5(12)6(2) cage, with the exception of 3-methylpentane and 2,3-dimethylbutane, while all 18 alkanes can be encapsulated in the 5(12)6(4) cage. the raman spectroscopic characteristics of five straight-chain and four cyclic alkane guest molecules in the 5(12)6(2) and 5(12)6(4) cages were also simulated. the majority of the raman bands of the straight-chain 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 wavenunnbers. these theoretical results should prove helpful with regard to identifying hydrate deposits from experimental raman spectroscopic data.