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Mass Spectrometry

Scientists find elusive SF6+

Helium nanodroplets allow detection of the fragile cation

by Sam Lemonick
April 29, 2021 | A version of this story appeared in Volume 99, Issue 16

A drawing of the structure of the SF6+ cationic complex.
Credit: Paul Scheier
SF6+. The gray sphere represents a nanodroplet of helium atoms. (Not to scale.)

Scientists have spent decades searching for the sulfur hexafluoride cation. Calculations say it should be stable enough to exist, but attempts by several methods since the 1980s to spot it have failed. Researchers report they’ve finally made SF6+ inside charged, nanosized droplets of helium and measured its presence using mass spectrometry (J. Phys. Chem. Lett. 2021, DOI: 10.1021/acs.jpclett.1c01024).

Neutral SF6 is stable and used as an insulator and spark suppressor in electrical infrastructure. It’s also one of the most potent greenhouse gases known. But when it loses an electron and becomes ionized, SF6+ is more like a complex of SF5+ and a fluorine atom, bound by van der Waals forces, says study coauthor Paul Scheier of the University of Innsbruck. This complex is unstable at the moment of ionization, however, because SF5+ and F repel each other and push apart like a small explosion. Encapsulating SF6 inside a helium nanodroplet helps contain the species and allow the cationic complex to stabilize.

Another challenge was observing the cation. The group used slow-moving helium atoms to gently strip away the nanodroplet until bare SF6+ or SF6+ and a small number of He atoms remained; researchers then recorded the mass spectrometry signature.

Deepak Mathur of the Tata Institute of Fundamental Research, who has also attempted to find the cation, says he’s convinced that this team has done it, but he wants more explanation of how the He nanodroplet prevents SF6+ dissociation.

Scheier says searching for SF6+ was about curiosity rather than a use for the cation. But the researchers say the technique could be used to find other short-lived cations, including some that may exist in interstellar space.



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