A clathrate hydrate of carbon monoxide

Abstract

Since the mid-1950s it has been known that most simple clathrate hydrates belong to one of two cubic crystal forms known as type I and II^1–3. Only recently it was established that structure II hydrates are formed by small guests such as O₂, N₂, Kr and Ar⁴ as well as molecules larger than ∼0.58nm, with molecules of intermediate size forming type I hydrate. The existence of a hydrate of carbon monoxide, a molecule similar in size to O₂ and N₂, has been proposed since ∼ 1960^5,6. This hydrate is thought to play an important role in comets and the outer planets in the Solar System^7,8. We have now prepared carbon monoxide hydrate, and demonstrate its clathrate hydrate nature from dielectric and ¹³C NMR (nuclear magnetic resonance) measurements. Unexpectedly, X-ray powder diffraction shows that, unlike hydrates of O₂ and N₂, carbon monoxide has a type I cubic structure with a lattice parameter of 1.188 nm.