Rapid rotation explains unusual stability of C2 anion
In various settings, including chemical reactions in the lab, inside nuclear reactors, and in outer space, scientists have found C2– anions living for as long as three milliseconds before decaying to a more stable state — and they haven’t been able to explain why. Normally particles, atoms, and molecules make these transitions to lose energy and become more stable. And normally the C2– molecule has around 4 eV less energy than the C2– anion, so the latter decayed to the former within one-trillionth of a second. The puzzle was that scientists didn’t know of a mechanism that allowed C2– to not decay to C2 for more than 3 ms, a timespan more than a billion-times longer.
In two new papers published on October 31 (here and here), researchers from Austria, the Czech Republic, and Germany reported “strong evidence” for an idea scientists first had in the late 1990s: that the delay had something to do with rotation. Scientists have previously found rapidly rotating molecules in space — including when radiation breaks up water molecules in the interstellar medium and in the dynamic neighbourhood of a newborn star.
The study team found that when the C2– complex rotates fast enough to increase its rotation quantum number 𝑁 beyond 155, it acquires a “centrifugal potential” that rearranges the lower-energy states to which C2– can decay. In particular, the team’s theoretical calculations revealed that a different state other than the C2 state to which it normally decays has lower energy, and dropping to the C2 state becomes unfavourable. More specifically, if the C2– anion had 𝑁 values in the 165-183 range, the normal decay to C2 requires electrons to have at least six units of angular momentum. If 𝑁 is lower than 165, the rearrangement of energy states doesn’t forbid the rapid drop to C2.
In other words, the spinning molecule spits out a spinning electron to move to a more stable configuration — and even then not before living to the ripe old age of 3 ms. This so-called rotation-assisted stability of the C2– anion isn’t entirely new. Other scientists have previously found dihydrogen and dideuterium anions (H2– and D2–) to be more stable as well when 𝑁 = 20-40. Using and theory and experiments, the European team found C2– acquired the same stability gain at 𝑁 of 155 or more because it’s heavier and has a higher rotational constant (“a fundamental parameter describing the rotational energy levels of a molecule,” per Meta AI).
