Direct measurements of the total rate constant of the reaction NCN + H and implications for the product branching ratio and the enthalpy of formation of NCN

Abstract

The overall rate constant of the reaction (2), NCN + H, which plays a key role in prompt-NO formation in flames, has been directly measured at temperatures 962 K < T < 2425 K behind shock waves. NCN radicals and H atoms were generated by the thermal decomposition of NCN3 and C2H5I, respectively. NCN concentration-time profiles were measured by sensitive narrow-line-width laser absorption at a wavelength of λ = 329.1302 nm. The obtained rate constants are best represented by the combination of two Arrhenius expressions, k2/(cm(3) mol(-1) s(-1)) = 3.49 × 10(14) exp(-33.3 kJ mol(-1)/RT) + 1.07 × 10(13) exp(+10.0 kJ mol(-1)/RT), with a small uncertainty of ±20% at T = 1600 K and ±30% at the upper and lower experimental temperature limits.The two Arrhenius terms basically can be attributed to the contributions of reaction channel (2a) yielding CH + N2 and channel (2b) yielding HCN + N as the products. A more refined analysis taking into account experimental and theoretical literature data provided a consistent rate constant set for k2a, its reverse reaction k1a (CH + N2 → NCN + H), k2b as well as a value for the controversial enthalpy of formation of NCN, ΔfH = 450 kJ mol(-1). The analysis verifies the expected strong temperature dependence of the branching fraction ϕ = k2b/k2 with reaction channel (2b) dominating at the experimental high-temperature limit. In contrast, reaction (2a) dominates at the low-temperature limit with a possible minor contribution of the HNCN forming recombination channel (2d) at T < 1150 K.

The rate constant of the reaction NCN + H2 and its role in NCN and NO modeling in low pressure CH4/O2/N2-flames

“…It has therefore been synthesized directly in high purity in the gas phase, according to BrCN(g) + NaN 3 (s) -NCN 3 (g) + NaBr(s), using a method described in detail in ref. 22. After an 8 h reaction time the remaining BrCN impurities were usually o3% according to FTIR analysis.…”

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

“…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].…”

“…In contrast to φ, the rate constant k 7 for the assumed reformation of NCN has been found to be nearly temperature independent. It is therefore unfeasible that NCN is formed through the simple reverse of reaction (6), which should be temperature dependent as well. Hence, it can be speculated that NCN reformation takes actually place through a reaction sequence according to 3 NCNOO → 1 NCNOO → 3 NCN + O 2 involving an intersystem crossing (ISC) process.…”

“…As already mentioned above, a rough estimation of the corresponding activation energy yields a lower limit of E a = 32 kJ mol -1 . Here, the CIISC process (5) was assumed to be temperature independent; allowing for a positive temperature dependence would yield an even higher E a value for k 6 . Apparently, the formation of the 3 NCNOO adduct exhibits a pronounced entrance barrier, hence similar to the 3 NCN + 3 O 2 → 1 NCNOO reaction.…”