Classical to Nonclassical Transition of Ether−HCN Clathrate Hydrates at Low Temperature

Guest molecules of typical clathrate hydrates are stabilized by weak nonspecific interactions with the cage walls of the host lattice. Despite their ability to form hydrogen bonds, this description also generally applies to encaged ether and other moderately strong proton acceptor molecules. However, on the basis of infrared spectroscopic and molecular dynamics results, an altered structure is indicated when guests such as HCN or SO₂, capable of binding to oxygen lattice sites, occupy the small cages. During cooling from 140 to 60 K, structure-II clathrate hydrates, with HCN in the small cages and either tetrahydrofuran or trimethylene oxide as the large-cage guests, convert to nonclassical structures in which most “guest” molecules establish hydrogen bonds to water. The nonclassical structure is stable at even higher temperatures for the dimethyl ether−HCN double clathrate hydrate.