The Unimolecular Dissociation of Hydrogen Squarate (HC4O4−) and the Squarate Radical Anion (C4O4•−) in the Gas Phase and Relationship to CO Cyclooligomerization.

The unimolecular dissociation of hydrogen squarate and the squarate radical anion has been studied by electrospray ionization mass spectrometry (including energy resolved collisionally induced dissociation) and quantum chemical calculations, providing consistent reaction models. In both cases, consecutive decarbonylations are observed as the dominating fragmentations. The reverse of these reactions corresponds to successive cyclooligomerization of CO, which constitutes the most atom efficient route to the cyclic oxocarbons. The reaction models indicate moderate barriers for CO addition to HCnOn− and CnOn•−, respectively, larger for the former than the latter. Cyclooligomerization leading to a neutral product is endothermic, while the analogous one-electron reductive coupling is exothermic. Analysis shows that addition of an electron is essential for cyclooligomerization to give the cyclic four-CO squarate structure.