Abstract
Calculations of NMR parameters (the absolute shielding constants and the spin-spin coupling constants) for 512, 51262 and 51264 cages enclathrating CH4, C2H6 and C3H8 molecules are presented. The DFT/B3LYP/HuzIII-su3 level of theory was employed. The 13C shielding constants of guest molecules are close to available experimental data. In two cases (the ethane in 512 and the propane in 51262 cages) the 13C shielding constants are reported for the first time. Inversion of the methyl/methylene 13C and 1H shielding constants order is found for propane in the 51262 cage. Topological criteria are used to interpret the changes of values of NMR parameters of water molecules and they establish a connection between single cages and bulk crystal.Electronic supplementary materialThe online version of this article (doi:10.1007/s00894-014-2511-2) contains supplementary material, which is available to authorized users.
Highlights
Clathrate hydrates are solid structures composed of a water lattice forming cages in which guest molecules reside
The crystalline structure of the clathrate hydrates (CHs) is made up of H-bonded water molecules forming a network with cages of different shapes and sizes
We have demonstrated the use of calculated NMR parameters for the analysis of molecular interactions of the methane and carbon dioxide with host-water molecules in static hydrates [42, 43]
Summary
Clathrate hydrates are solid structures composed of a water lattice forming cages in which guest molecules reside. Small guest molecules form in general sI structures with a unit cell composed of six large 14-sided cages 51262 and two cages 512 for a total of 46 water molecules [6]. Larger guest molecules form structure sII, with the unit cell composed of 136 water molecules ( small ones as Ar, Kr, O2 and N2 form sII). They are made up of 24 polyhedral cages: eight large 16-sided cages 51264 and 16 pentagonal small 12-sided cages 512.
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