The orientations and dynamics of ethanol guest molecules in the clathrate of Dianin's compound

Abstract

In the clathrates of Dianin's compound the guests are trapped in cages formed by six host molecules. Two ethanol molecules can be enclosed by each cage. Deuteron nuclear magnetic resonance spectra of single crystals of Dianin's compound with ethanol-OD, ethanol-1,1-D₂ and ethanol-2,2,2-D₃ as a guest were recorded in the temperature range 10 K < T < 310 K. Only at T < 25 K the guests are found to be immobile enough to allow the determination of the bond directions of the different deuterons from the unique eigenvectors of the quadrupole coupling (qc) tensors. The measured qc tensors of the hydroxyl and of the methylene deuterons are used to determine the orientations of the ethanol molecules as a whole. These can occupy six distinct sites. The two ethanol molecules in a given cage are linked by a weak hydrogen bond. The dynamics of the ethanol guests is elucidated by line shape analyses and by deuteron two-dimensional exchange spectroscopy. It is found to be rather complex, consisting of three different types of large-angle reorientations of the entire ethanol molecules, of internal reorientations of the methyl groups about their local C₃ axes and of additional librational and torsional motions. The average equilibrium orientations of the ethanol molecules change with increasing temperature. The observed dynamics implies that the host lattice of the clathrate possesses a considerable degree of flexibility, in particular it must enable the two ethanol molecules within a given cage to exchange places, or to diffuse from one cage to the next.