Oganesson difluoride

The single rarest molecule I can imagine is almost certainly one containing a rare superheavy element.

A strong contender is oganesson difluoride (OgF₂).

In human history there have been fewer than 20 total atoms of oganesson ever made

Og has a half-life of about 0.7 milliseconds before the nucleus decays. So even if the molecule OgF2 is chemically stable, it isn’t, because one of the atoms blows up. That’s not something chemists usually have to worry about.

To make OgF₂, you would first create oganesson atoms in a heavy-ion accelerator by fusing californium-249 with calcium-48, then have the recoiling atoms stop directly in a reaction zone containing a dense source of fluorine atoms or radicals.

OgF₂ should be at least metastable against decomposition in the fleeting interval before nuclear decay. The +2 oxidation state is favoured due to relativistic stabilization of Og’s 7p₁/₂ electrons, which behave almost like an inert core.

Even so, success would require super fast in-situ chemistry. The process would need to be diffusion-limited, with radical fluorine available to drive rapid bond formation within microseconds. The most plausible route would be zero-transport implantation into a fluorine-radical solid matrix, where cage capture and vibrational quenching occur on the picosecond scale.

If the spatial universe is infinite and there is a nonzero density of civilizations that ever run heavy-ion fusion experiments followed by fast chemistry, let r be the average rate density of “attempts that briefly make an OgF₂ molecule” per cubic light-year per year, and τ its mean lifetime.

The expected number present at any instant is r × τ × Volume. With infinite volume, any r > 0 implies an infinite expected count.

In plain terms: if even a tiny fraction of space hosts labs like ours, then somewhere, right now, OgF₂ exists. But by density (rτ), the rarest here-ish is also the rarest anywhere-ish.