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

作者：Fassheber Nancy; Dammeier Johannes; Friedrichs Gernot^*

来源：Physical Chemistry Chemical Physics, 2014, 16(23): 11647-11657.

DOI：10.1039/c4cp01107d

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 lambda = 329.1302 nm. The obtained rate constants are best represented by the combination of two Arrhenius expressions, k(2)/(cm(3) mol(-1) s(-1)) = 3.49 x 10(14) exp(-33.3 kJ mol(-1)/RT) + 1.07 x 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 + N-2 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 k(2a), its reverse reaction k(1a) (CH + N-2 -> NCN + H), k(2b) as well as a value for the controversial enthalpy of formation of NCN, Delta H-f(298K)degrees = 450 kJ mol(-1). The analysis verifies the expected strong temperature dependence of the branching fraction phi = k(2b)/k(2) 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.

• 出版日期2014