Ab initiocoupled-cluster and multi-reference configuration interaction studies of the low-lying electronic states of 1,2,3,4-cyclobutanetetraone

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

The unimolecular dissociation of hydrogen squarate and the squarate radical anion has been studied by electrospray ionization mass spectrometry (including 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 the 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 HC_nO_n^- and C_nO_n^•-, respectively, being larger for the former than for the latter. Cyclooligomerization leading to a neutral product is endothermic, while the analogous one-electron reductive coupling is exothermic. The analysis shows that the addition of an electron is essential for cyclooligomerization to give the cyclic four-CO squarate structure.

Global Isomeric Survey of Elusive Cyclopropanetrione: Unknown but Viable Isomers

Despite the great interest in energy storage application, stable neutral C_nO_n (n > 1) structures either in thermodynamics or kinetics have yet been largely limited due to the rather high tendency to release the very stable CO molecule. The neutral cyclopropanetrione (C₃O₃) cluster has long remained elusive since no isomer with sufficient kinetic stability has been found either experimentally or theoretically. In this work, we constructed the first global potential energy surface of singlet C₃O₃ at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level, from which the kinetic stability of a wide range of C₃O₃ isomers can be determined by investigating their isomerization and fragmentation pathways. Amongst, a three-membered ring structure 01 is the global C₃O₃ isomer with a barrier of 10.6 kcal/mol at the sophisticated W1BD level. In particular, two carbene-type isomers 02 and 04 possess appreciable destruction barriers of 20.3 and 24.7 kcal/mol at W1BD, respectively. Thus, 02 and 04 can be useful building blocks for constructing larger high-energy density carbon-oxygen clusters. Moreover, with the carbene center, both might effectively functionalize various nano-materials while retaining the electrochemical active carbonyl and epoxyl moieties that are very desirable in alkali metal-ion batteries.