Ab initio study of the oxidation of NCN by O₂

Abstract

The reaction of NCN with O 2 has been investigated by ab initio molecular orbital and transition state theory calculations. The mechanisms for formation of possible product channels involved in the singlet and triplet potential energy surfaces have been predicted at the highest level of the modified GAUSSIAN-2 (G2M) method, G2M (CC1). The stationary points of species on the low-energy paths were also examined with the third-order RayleighSchrödinger perturbation (CASPT3) and the multireference configuration interaction including Davidson's correction for higher excitations (MRCI + Q) theories at the CASPT3 (4, 4)/aug-cc-PVTZ//CASSCF(4,4)/cc-pVDZ(4,4) and MRCI+Q(4,4)/aug-cc-PVTZ //CASSCF (4, 4)/cc-pVDZ(4, 4) levels. The rate constants for the two low-energy channels, NCN + O 2 → cis,trans-NCNOO → NCO + NO (k 1 ) and NCN + O 2 → cis,trans-NCNOO → CNO + NO (k 2 ), have been calculated in the temperature range of 1000-3000 K based on the energies obtained at the G2M(CC1) level. The results show that the formation of NCO + NO is dominant, and its branching ratio k 1 /(k 1 + k 2 ) = 0.83 is almost temperature independent from 1000 to 3000 K. The total rate constant can be expressed by k t = 7.29 × 10 −15 T 0.51 exp(−12370/T) cm 3 molecule −1 s −1 in the 1000-3000 K range. Overall, the reaction is quite slow because of the high entrance and exit barriers. Based on the predicted heats of reaction and the known heats of formation of NCN and NO, the heats of formation of NCO and CNO at 0 K have been calculated to be 29.8 and 92.6 kcal/mol, respectively. The former agrees closely with the available experimental value, 30.4 ± 1.0 kcal/mol. C 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: [593][594][595][596][597][598] 2005

Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2

“…The high activation energy of 97 kJ mol -1 is in very good agreement with the transition state theory prediction of Zhu and Lin [14], but the absolute experimental rate constant values are about a factor of 5 above the theoretical estimate. Compared to other bimolecular NCN reactions, such as NCN + H/O/OH/H 2 , the reaction is still slow.…”

“…RT [14]. At T = 1500 K, this rate constant is five orders of magnitude lower than the estimated rate constant value of Glarborg et al [13].…”

“…However, as already mentioned above, k 7 and φ are important to reproduce the overall shape of the NCN profile; hence, we allow for an additional uncertainty of k 2 on the order of ±5%. Figure 5 compares k 2 (NCN + O 2 ) data of this work with the transition state theory prediction of Zhu and Lin (dashed curve) [14]. The outdated, several orders of magnitude higher rate constant estimate from Glarborg et al [13] is not shown.…”

“…These studies include the reactions NCN + NO, NO 2 [4], NCN + NCN, O, and M [5] as well as NCN + H [6] and NCN + H 2 [7]. The only other direct study reported in [14].…”

Diode laser absorption measurement and analysis of HCN in atmospheric-pressure, fuel-rich premixed methane/air flames

“…Reactions (R5)-(R8) were included in the modeling study of NO reburning in a flow reactor [37], where their rates were estimated. Recently, the rates of reactions (R5) and (R6) have been determined by ab initio calculations [38,39]. It should be noted that reaction (R6) between NCN and O 2 can be important in the fuelrich highly stabilized flames, where oxygen is removed relatively slowly, yielding a broad flame front in which the O 2 concentrations are one to two orders higher than those of O, H, and OH.…”