Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M

Abstract

The rate constant of the reaction NCN + O has been directly measured for the first time. According to the revised Fenimore mechanism, which is initiated by the NCN forming reaction CH + N(2)→ NCN + H, this reaction plays a key role for prompt NO(x) formation in flames. NCN radicals and O atoms have been quantitatively generated by the pyrolysis of NCN(3) and N(2)O, respectively. NCN concentration-time profiles have been monitored behind shock waves using narrow-bandwidth laser absorption at a wavelength of λ = 329.1302 nm. Whereas no pressure dependence was discernible at pressures between 709 mbar < p < 1861 mbar, a barely significant temperature dependence corresponding to an activation energy of 5.8 ± 6.0 kJ mol(-1) was found. Overall, at temperatures of 1826 K < T < 2783 K, the rate constant can be expressed as k(NCN + O) = 9.6 × 10(13)× exp(-5.8 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (±40%). As a requirement for accurate high temperature rate constant measurements, a consistent NCN background mechanism has been derived from pyrolysis experiments of pure NCN(3)/Ar gas mixtures, beforehand. Presumably, the bimolecular secondary reaction NCN + NCN yields CN radicals hence triggering a chain reaction cycle that efficiently removes NCN. A temperature independent value of k(NCN + NCN) = (3.7 ± 1.5) × 10(12) cm(3) mol(-1) s(-1) has been determined from measurements at pressures ranging from 143 mbar to 1884 mbar and temperatures ranging from 966 K to 1900 K. At higher temperatures, the unimolecular decomposition of NCN, NCN + M → C + N(2) + M, prevails. Measurements at temperatures of 2012 K < T < 3248 K and at total pressures of 703 mbar < p < 2204 mbar reveal a unimolecular decomposition close to its low pressure limit. The corresponding rate constants can be expressed as k(NCN + M) = 8.9 × 10(14)× exp(-260 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(±20%).

Thermal Decomposition of NCN: Shock-Tube Study, Quantum Chemical Calculations, and Master-Equation Modeling

“…In view of this combined uncertainty, we refrain from giving a three-parameter Arrhenius-Kooij-type equation despite the slight curvature of the Arrhenius plot discernible in Figure 4. The very good agreement between the rate coefficients determined in this work with those from ref [15] can be seen as an indication that our calibration procedure is adequate. It provides also strong evidence for reaction R1 being indeed the dominant decomposition channel of NCN.…”

“…Whereas in ref [15] time-resolved laser absorption was used to monitor the reactant NCN, we use atomic resonance absorption spectroscopy (ARAS)[…”

Collisional Relaxation of NCN: An Experimental Study with Laser-Induced Fluorescence

“…In combustion chemistry, the kinetics of the cyanonitrene radical, NCN, has recently attracted attention [4][5][6][7][8][9][10][11] because NCN is considered an important intermediate in the formation of so-called prompt NO in hydrocarbon flames under fuel-rich conditions [12][13][14][15][16][17][18][19][20]. Cyanonitrene is a linear diradical with a 3 − g electronic ground state [21,22] and was shown to be formed under combustion conditions in the spin-allowed reaction CH(…”

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

“…Its rate constant was inferred from Ref. [5]; however, instead of adopting the recommended value of 3.7 × 10 12 cm 3 mol…”

“…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].…”