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Determination of NMR Lineshape Anisotropy of Guest Molecules within Inclusion Complexes from Molecular Dynamics Simulations
Author(s) -
Alavi Saman,
Dornan Peter,
Woo Tom K.
Publication year - 2008
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200700805
Subject(s) - clathrate hydrate , anisotropy , molecular dynamics , chemistry , chemical physics , molecule , crystallography , atmospheric temperature range , hydrate , computational chemistry , molecular physics , thermodynamics , physics , organic chemistry , quantum mechanics
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. Herein, we develop 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 CO 2 molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO 2 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 CO 2 sI clathrate. The experimental 13 C NMR lineshapes of CO 2 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. We determine the angular distributions of the guests in the cages by classical MD simulations of the sI clathrate and calculate the 13 C 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.