Monocyclic and bicyclic CO4: how stable are they?

Abstract

Seeking promising molecular species with huge energy release and significant kinetic stability continues to be a hot topic and a great challenge in the field of high-energy density materials (HEDMs). CO₄ is the first high-order carboxide that has the potential as an energetic molecule. However, the intrinsic kinetic stability of its two most studied energy-rich isomers, i.e., ¹1 (monocyclic) and ¹2 (bicyclic), has remained quite unclear in spite of numerous studies. This has greatly hindered the quantitative stability assessment of ¹1 and ¹2 under various conditions as well as the justification of their prospect as energetic candidates. In this work, for the first time we report the rate-determining transition states associated with the CO₂-elimination from ¹1 and ¹2. The thermodynamics of ¹1 and ¹2 was described using G3B3, CBS-QB3, G4, W1BD, CCSD(T)/CBS and CASPT2/CBS, while the kinetic stability was analyzed based on broken-symmetry UCCSD(T)/CBS and CASPT2/CBS single-point energy calculations on UB3LYP geometries. The rate-determining barriers for the dissociation of ¹1 and ¹2 into CO₂ + ¹O₂ at 298 K were found to amount to 28.7 and 14.7 kcal mol⁻¹ at the CASPT2(18e,12o)/CBS level of theory, and 23.5 and 21.1 kcal mol⁻¹ at the UCCSD(T)/CBS level of theory, respectively. ¹1 is a kinetically stable energetic molecule, which releases 45.2 kcal mol⁻¹ upon dissociation into CO₂ + ¹O₂ at the CASPT2(18e,12o)/CBS level and 38.9 kcal mol⁻¹ at the UCCSD(T)/CBS level, and could serve as a rigid energetic building block for larger oxocarbons. The bicyclic ¹2 releases much higher energy, 79.3 kcal mol⁻¹ at the CASPT2(18e,12o)/CBS level and 73.4 kcal mol⁻¹ at the CASPT2-corrected UCCSD(T)/CBS level whereas the barrier for dissociation is lower than that of monocyclic ¹1.