Study of static and dynamic structure of β‐quinol‐methanol clathrate by13C high‐resolution solid‐state NMR and protonT1measurements

Simulations of guest transport in clathrates of Dianin's compound and hydroquinone

Clathrates have been proposed for use in a variety of applications including gas storage, mixture separation and catalysis due to the potential for controlled guest diffusion through their porous lattices. Here molecular dynamics simulations are employed to study guest transport in clathrates of hydroquinone (HQ) and Dianin's compound (DC). Systems investigated were HQ with methanol and acetonitrile, and DC with methanol and ethanol. Simulations were set up with one guest in the pore, two guests in the pore and one vacancy in the pore and a filled pore, and free-energy barriers for movement between cavities of the pore were estimated for all cases. Comparison between these simulations indicates that guest transport most likely proceeds by molecules moving from full to empty cavities consecutively, one by one, rather than in a concerted manner. Thus, the presence of empty cavities is very important for guest transport, which becomes more energetically demanding in fully loaded systems. Flexibility of the host can assist guest transport. In the studied DC clathrates transport occurs via an intermediate conformation in which the hydroxyl group of the alcohol guest molecule participates in the hydrogen-bonded ring of the host. We also address the issue of the number of methanol guest molecules that DC accommodates, for which conflicting information exists. We found that this is likely to be temperature dependent and suggest that under some conditions the system is most likely non-stoichiometric.

Pursuing structure in microcrystalline solids with independent molecules in the unit cell using 1H-13C correlation data

The (1)H-(13)C solid-state NMR heteronuclear correlation (HETCOR) experiment is demonstrated to provide shift assignments in certain powders that have two or more structurally independent molecules in the unit cell (i.e. multiple molecules per asymmetric unit). Although this class of solids is often difficult to characterize using other methods, HETCOR provides both the conventional assignment of shifts to molecular positions and associates many resonances with specific molecules in the asymmetric unit. Such assignments facilitate conformational characterization of the individual molecules of the asymmetric unit and the first such characterization solely from solid-state NMR data is described. HETCOR offers advantages in sensitivity over prior methods that assign resonances in the asymmetric unit by (13)C-(13)C correlations and therefore allows shorter average analysis times in natural abundance materials. The (1)H-(13)C analysis is demonstrated first on materials with known shift assignments from INADEQUATE data (santonin and Ca(OAc)(2) phase I) to verify the technique and subsequently is extended to a pair of unknown solids: (+)-catechin and Ca(OAc)(2) phase II. Sufficient sensitivity and resolution is achieved in the spectra to provide assignments to one of the specific molecules of the asymmetric unit at over 54% of the sites.