| Molecular Dynamics Simulation of NMR Lineshape Anisotropy of Guest Molecules within Inclusion Complexes |
Saman Alavi, Peter Dornan, and Tom K. Woo
Department of Chemistry, University of Ottawa, Ottawa, Ontario Canada |
| Nonspherical cages in inclusion compounds can result in non-uniform motion of guest species in these cages and anisotropic lineshapes in NMR spectra of the guest. We present a methodology to calculate lineshape anisotropy of guest species in cages based on molecular dynamics simulations of the inclusion compound. The methodology is valid for guest atoms with spin 1/2 nuclei and does not depend on the temperature and type of inclusion compound or guest species studied. As an example, the nonspherical shape of the structure I (sI) clathrate hydrate large cages leads to preferential alignment of linear CO2 molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO2 guests in terms of a polar angle θ and azimuth angle φ and small amplitude vibrational motions in the large cage are characterized by molecular dynamics simulations at different temperatures in the stability range of the CO2 sI clathrate. The experimental 13C NMR lineshapes of CO2 guests in the large cages show a reversal of the skew between the low temperature (77 K) and the high temperature (238 K) limits of the stability of the clathrate. In this work, we determine the angular distributions of the guests in the cages by the classical MD simulations of the sI clathrate and calculate the 13C NMR lineshapes over a range of temperatures. Good agreement between experimental lineshapes and calculated lineshapes is obtained. No assumptions regarding the nature of the guest motions in the cages are required. |
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