A theoretical study of the NCN (3Σ−) biradical thermochemical properties: Implications for combustion chemistry

Theoretical calculations are performed to obtain thermochemical properties ( Δ f H 298 K ∘ , S 298 K ∘ , CP = f(T)) for the electronic ground state of the NCN (3Σ−) biradical. In order to validate our methodology, the thermochemical properties ( Δ f H 298 K ∘ , S 298 K ∘ , CP = f(T)) are determined also for seventeen species involved in the prompt-NO mechanism. The standard enthalpies of formation at 298 K are estimated using atomization reaction based on the CBS-QB3, CBS-APNO, G3B3, G3, and G4 calculation levels. In the case of the NCN biradical, an isogyric set of four reactions is also used to assess its standard enthalpy of formation at 298 K. Standard molar entropies at 298 K and heat capacities at constant pressure are estimated at the B3LYP/cc-pVQZ level of theory. Based on our results, we recommend the use of our calculated thermochemical properties for the modeling of the prompt-NO mechanism: Δ f H 298 K ∘ (NCN) = (448.7 ± 3.4) kJ mol−1, S 298 K ∘ = 225.8 J mol−1 K−1, CP (in J mol−1 K−1) = 41.9, 45.8, 49.0, 51.6, 55.2, 57.3, 59.9, 60.9, 61.7, 62.0, 62.1 at 300, 400, 500, 600, 800, 1000, 1500, 2000, 3000, 4000, 5000 K, respectively.