The ability of small molecules to form clathrate hydrates of structure II

Since gas hydrates were shown in the mid-1950s¹ to be water clathrates of two distinct crystallographic structures (Table 1), it has been generally accepted that structure I hydrates are formed by molecules with maximum van der Waals diameters of up to about 5.8 Å while structure II hydrates are formed by larger molecules, up to about 7.0 Å in size. No exception to this general rule has been observed among more than 100 individual species of molecules known to form clathrate hydrates^2,3. From X-ray and neutron diffraction studies we now find, however, that the two smallest molecules which form clathrate hydrates-argon and krypton-do so in the structure II modification. This finding supports a suggestion of Holder and Manganiello⁴.