Formation of a long-lived cyclic isomer of ethylenedione

A century of unsuccessful attempts to identify the neutral ethylenedione molecule combined with the results of quantum-chemical computations resulted in the conclusion on the instability of this molecule. In this article, we demonstrate that although the lowest energy isomer of ethylenedione with linear geometry is indeed unstable, a higher energy three-membered cyclic isomer can be stabilized, and at low temperature has a life-time longer than one millisecond. In our study, the ethylenedione C₂O₂ molecule was synthesized in the low-temperature reaction CO₂ + C → C₂O₂ inside liquid helium nanodroplets. To study the reaction, a newly developed calorimetric technique was applied. Single pairs of reactants were incorporated into tiny helium droplets having a temperature of 0.37 K. The reaction energy was transferred to liquid helium stabilizing an intermediate gas-phase reaction product. The energy transfer also resulted in the evaporation of helium atoms. Therefore, the change of the helium droplets' size allowed precise calorimetry on a molecular scale.