Global picture of isomerization and dissociation of CN2O2: new metastable isomers

Abstract

Molecules containing carbon (C), nitrogen (N) and oxygen (O) atoms have received considerable attention due to their great relevance in astrophysical, atmospheric and high-energy-density material (HEDM) realms. Greatly differing from most studies that mainly center on the low-lying isomers, study of C, N and O systems appeals for the additional consideration of high-energy species. Thus, understanding the thermodynamic and kinetic stability of diversified isomers is of vital importance to assess their role in these processes. In this work, we investigated a pentatomic CN₂O₂ system, the isomeric study of which was initiated 20 years ago and up to now its three isomers OCNNO, CNNO₂ and NCNO₂ have been experimentally characterized. Based on our global search strategies for both isomers and transition states, we constructed hitherto the most comprehensive potential energy surface of CN₂O₂, covering 15 new isomers and 29 new transition states. The ring-containing isomers, i.e., 14, 22 and 29, were shown to possess considerable rate-determining Gibbs free energy barriers with respect to the radical–radical (P3 NCO + NO, P6 ³NCN + ³O₂ or P10 ³NNC + ³O₂) and lowest-energy product (P1 CO₂ + N₂) at the (U)CCSD(T)/CBS level. Thus, they are expected to be experimentally observable. After the experimentally known OCNNO, CNNO₂ and NCNO₂, the presently found three isomers 14, 22 and 29 warmly welcome future laboratory investigations. In addition, for CNNO₂ 09, we located a previously unreported transition state, which provides a new viewpoint on its kinetic stability.