Quantum Chemical Investigations On Structure And Stability Of Energy-rich Molecules

Carbon（C）, nitrogen（N） and oxygen（O） are important elements on the earth and in space. They can form diversified organic or inorganic species that are important intermediates during atmospheric, catalytic and interplanetary processes. Due to the huge demand for energy in today’s society, scientists are trying to find new type energetic species to replace the traditional fossil fuels. In the field of energetic materials, energetic compounds containing these light elements（i.e., C, N, O） are superior in explosiveness compared to heavy-metal-based explosives and could release environment friendly gases. Theoretical study on isomerization and decomposition of molecules consisting of C, N, O atoms contributes not only to the discovery of energetic molecules that can be experimentally detected or synthesized, it helps to understand reacting processes of these molecules in the atmosphere and interplanetary space as well. In this thesis, we studied the structure, properties and stability of several classes of [Cx, Ny, Oz] systems by using the quantum chemistry method. We constructed the most comprehensive potential energy surface based on our global search strategies for both isomers and transition states, which will determined the most interesting and worthwhile isomers. The study will also provide important theoretical information of these isomers for further observation, synthesis, and characterization in experiment and help us understanding the related molecules reaction behavior in the process of interstellar and atmosphere. The main results are summarized as follows1) Constructing the potential energy surface of the C₃O₃ of chemical bonding carbon monoxide trimer. Of the（CO）n systems, C₃O₃ appears to be quite elusive, for which available experimental and theoretical studies either have shown the hilltop or fleeting structures, or just have proposed isomers with high uncertainty in stability. We showed the first computational evidence that several previously unreported C₃O₃ isomers. A comprehensive C₃O₃ potential-energy surface covering 65 novel minimum V isomers and 97 novel transition states was constructed. All isomers lie high above the fragmentation limit（³CO） by at least 71.4 kcal/mol. Amongst, 13 isomers were shown to possess considerable rate-determining barriers ranging from 10.3 to 24.3 kcal/mol, implying their high possibility to be obtained in laboratory. For the first time, we identified the global isomer of C₃O₃, 101 with a barrier of 10.3 kcal/mol. Two isomers 102 and 104 are quite stable with the respective barriers of 17.8 and 24.3 kcal/mol towards fragmentation/isomerization. Finally, we showed that a previously unknown singlet isomer 101 b could have been generated in the very recent negative ion photoelectron spectrum. The present comprehensive potential-energy surface should also be valuable for understanding the complex CO^-catalytic processes that involve C₃O₃.2) Study on kinetics stability of Bicyclic CN₂O₂. Two isomers predicted to be energetic molecules in the [Cx, Ny, Oz] system, i.e., Diazirinone（[C,N₂,O] and Nitryl cyanide（[C,N₂,O2], which are detected in the recent years（Angew. Chem., Int. Ed., 2011, 50, 1720 and Angew. Chem., Int. Ed., 2014, 53, 6893）. Bicyclic CN₂O₂ also have been predicted to possess both large exothermicity（190 kcal/mol） and a large decomposition barrier to N2 + CO2（29 kcal/mol） at the CCSD（T）/TZ2P//MP2/6-31G（d） level, which appears to well deserve a synthetic trial for energetic molecule. By re-evaluating the stability of bicyclic CN₂O₂, we located a new rate-determining transition state for decomposition（TS2）. With TS2, a significantly reduced decomposition barrier was arrived at, i.e., 11.6–13.0 kcal/mol at the G3B3, CBS-QB3, G4, W1 BD and CCSD（T）_CBS//B3LYP/aug-cc-p VTZ composite energy calculated levels using a restricted wave function. Strikingly, TS2 possesses a significant open-shell feature and the barrier was further reduced to be 6.9 kcal/mol at the UCCSD（T）_CBS//UB3LYP/aug-cc-p VTZ level. Thus, the bicyclic isomer is unlikely to be a energetic molecule, though its spectroscopic detection could still be feasible.3） Study on kinetics stability of nitrile oxide ONCNO. Fulminates containing the CNO^- ion have been widely utilized as high-energy density materials（HEDMs） for more than 120 years. Yet no purely covalently bound CNO molecule, i.e., nitrile oxide, VI is known to behave as a energetic molecule. We performed a thorough investigation of the potential energy surface of nitrile oxide ONCNO and related isomers, applying various sophisticated methods including G4, CBS-QB3, W1 BD, CCSD（T）/CBS, and CASPT2/CBS. The Gibbs free energy calculations showed that the decomposition of ONCNO to the considerably endothermic products CNO + NO is favored compared to that into the highly exothermic products CO₂ + N₂. Thus, ONCNO fails to be the long expected nitrile oxide energetic molecule. However, with the rate-determining barrier of 23.3 kcal/mol at the W1 BD level, ONCNO should be experimentally accessible4） Construction of the potential energy surface and discovery of new isomers of a pentatomic molecule CN₂O₂ system. The isomeric study of this system 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 posse 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. After the experimentally known OCNNO, CNNO₂ and NCNO₂, the presently found three isomers 14, 22 and 29 welcome laboratory investigations. In addition, for CNNO₂ 09, we located a previously unreported transition state, which provides a new viewpoint on its kinetic stability.5) Kinetic stability study of high-order carboxide CO₄. 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., monocyclic CO₄ and bicyclic CO₄, has remained quite unclear in spite of numerous studies. This has greatly hindered the quantitative stability assessment of them under various conditions as well as the justification of their prospect as energetic candidates. For the first time we report the rate-determining transition states associated with the CO₂-elimination from these two isomers. Thermodynamic stability was described using G3B3, CBS-QB3, G4, W1 BD, CCSD（T）/CBS and CASPT2/CBS, while kinetic stability was analyzed based on brokensymmetry UCCSD（T）/CBS and CASPT2/CBS single-point energy calculations on UB3 LYP geometries. The rate-determining barriers for the dissociation of these two isomers into CO₂ + ¹O₂ at 298 K were found to amount to 28.7 and 14.7 kcal/mol at the CASPT2（18e,12o）/CBS level of theory, and 23.5 and 21.1 kcal/mol at the UCCSD（T）/CBS level of theory, respectively. Monocyclic CO₄ is a kinetically stable energetic molecule, which releases 45.2 kcal/mol upon dissociation into CO₂+¹O₂ at the CASPT2（18e,12o）/CBS level and 38.9 kcal/mol at the UCCSD（T）/CBS level, and could serve as a rigid energetic building block for larger oxocarbons. Bicyclic CO₄ releases much higher energy, 79.3 kcal/mol at the CASPT2（18e,12o）/CBS level and 73.4 kcal/mol at the CASPT2-corrected UCCSD（T）/CBS level whereas the barrier for dissociation is lower than that of monocyclic CO₄.