Methane caged inside C₆₀

The cage-like molecule C₆₀ has incarcerated a hefty new prisoner. Methane is the first organic molecule, and the largest of any kind, to be encapsulated in this way (Angew. Chem., Int. Ed. 2019, DOI: 10.1002/anie.201900983). Researchers have previously caged small molecules like H₂ and H₂O by opening a hole in C₆₀, putting the prisoner inside, and then chemically stitching the entrance closed. But larger occupants need a bigger opening, of a size researchers have previously been unable to close. “It’s not that hard to make holes in C₆₀ and put molecules inside, but actually closing them up is incredibly difficult,” says Richard J. Whitby, the University of Southampton chemist who led the research. His team created a fullerene cage with a relatively large opening—a 17-membered-ring that included a sulfur atom—and forced methane inside at high pressure. Then the researchers oxidized the sulfur and used a photochemical reaction to eject sulfur monoxide, which contracted the ring. Further reactions closed the hole completely, trapping a single methane molecule that could rotate freely in its cage. Isolating molecules inside C₆₀ enables researchers to investigate their quantum properties. X-ray crystallography revealed that the methane’s hydrogen atoms were smeared into a spherical shell, corresponding to their delocalized nuclear wave function. The team now hopes to snare other molecules, including O₂ and NH₃.