“…The existence of clathrate hydrates (CHs) under high and ultrahigh vacuum (UHV) and cryogenic conditions is now well established. − While CH of CO 2 at 120 K in a vacuum of 10 –6 to 10 –7 Torr has been known since 1991, the formation of CHs of CH 4 , , C 2 H 6 , CO 2 , , acetone, formaldehyde, and tetrahydrofuran (THF) in UHV has been observed in the past few years in the range of 10–130 K. CHs are crystalline host–guest inclusion compounds and have drawn wide interest due to their scientific and technological implications. − CHs often coexist along with water ice and are mainly classified into structures I (sI, consisting of two small 5 12 and six large 5 12 6 2 cages per unit cell) and II (sII, consisting of 16 small 5 12 and eight large 5 12 6 4 cages per unit cell). While CO 2 is known to be encased in small 5 12 and large 5 12 6 2 cages, THF due to its larger van der Waals radius occupies the large 5 12 6 4 cage exclusively. ,,, In UHV, spectroscopic signatures of molecules in different hydrate cages (5 12 , 5 12 6 2 , and 5 12 6 4 ) were reported, ,− although these structures have not been confirmed by X-ray diffraction experiments.…”