Nuclear magnetic resonance parameters for methane molecule trapped in clathrate hydrates

The calculations of the nuclear shielding and spin-spin coupling constants were carried out for two models of clathrate hydrates, 5(12) and 5(12)6(8), using the density functional theory three-parameter Becke-Lee-Yang-Parr method with the basis set aug-cc-pVDZ (optimization) and HuzIII-su3 (NMR parameters). Particular attention has been devoted to evaluate the influence of a geometrical arrangement, the effect of long-range interactions on the NMR shielding of methane molecule, and to predict whether (13)C and (1)H chemical shifts can distinguish between guests in two clathrate hydrates cages. The correlation of the changes in the (17)O shielding constants depend strongly on the hydrogen-bonding topology. The intermolecular hydrogen-bond transmitted (1h)J(OH) spin-spin coupling constants are substantial. The increase of their values is connected with the elongation of the intramolecular O-H bond and the shortening of the intermolecular O···H distance. These data suggests that hydrogen bonds between double donor-single acceptor (DDA)-type water molecules acting as a proton acceptor from single donor-double acceptor (DAA)-type water molecules are stronger than ones formed by DAA-type water molecules acting as an acceptor for a DDA water proton. These state-of-the-art calculations confirmed the earlier experimental findings of the cage-dependency of (13)C chemical shift of methane.