Calculations of NMR properties for sI and sII clathrate hydrates of methane, ethane and propane

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.