The potential of the thermal decomposition of cyanogen azide (NCN3) as a high-temperature cyanonitrene (NCN) source has been investigated in shock tube experiments. Electronic ground-state NCN(3Σ) radicals have been detected by narrow-bandwidth laser absorption at overlapping transitions belonging to the Q1 branch of the vibronic 3Σ+−3Π subband of the vibrationally hot 3Πu(010)−3Σg−(010) system at = 30383.11 cm(-1) (329.1302 nm). High-temperature absorption cross sections σ have been directly measured at total pressures of 0.2−2.5 bar, log[σ/(cm2 mol(-1))] = 8.9−8.3 × 10(-4) × T/K (±25%, 750 < T < 2250 K). At these high temperatures, NCN(3Σ) formation is limited by a slow electronic relaxation of the initially formed excited NCN(1Δ) radical rather than thermal decomposition of NCN3. Measured temperature-dependent collision-induced intersystem crossing (CIISC) rate constants are best represented by kCIISC/(cm3 mol(-1) s(-1)) = (1.3 ± 0.5) × 1011 exp[−(21 ± 4) kJ/mol/RT] (740 < T < 1260 K). Nevertheless, stable NCN concentration plateaus have been observed, showing that NCN3 is an ideal precursor for NCN kinetic experiments behind shock waves.

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%).

The thermal decomposition of cyanogen azide (NCN 3 ) and the subsequent collision-induced intersystem crossing (CIISC) process of cyanonitrene (NCN) have been investigated by monitoring excited electronic state 1 NCN and ground state 3 NCN radicals. NCN was generated by the pyrolysis of NCN 3 behind shock waves and by the photolysis of NCN 3 at room temperature. Falloff rate constants of the thermal unimolecular decomposition of NCN 3 in argon have been extracted from 1 NCN concentrationtime profiles in the temperature range 617 K < T < 927 K and at two different total densities: k(ρ ≈ 3 × 10 −6 mol/cm 3 )/s −1 = 4.9 × 10 9 × exp (−71 ± 14 kJ mol −1 /RT ) (±30%); k(ρ ≈ 6 × 10 −6 mol/cm 3 )/s −1 = 7.5 × 10 9 × exp (−71 ± 14 kJ mol −1 /RT ) (±30%). In addition, high-temperature 1 NCN absorption cross sections have been determined in the temperature range 618 K < T < 1231 K and can be expressed by σ/(cm 2 /mol) = 1.0 × 10 8 − 6.3 × 10 4 K −1 × T (±50%). Rate constants for the CIISC process have been measured by monitoring 3 NCN in the temperature range 701 K < T < 1256 K resulting in k CIISC (ρ ≈ 1.8 × 10 −6 mol/cm 3 )/s −1 = 2.6 × 10 6 × exp (−36 ± 10 kJ mol −1 /RT ) (±20%), k CIISC (ρ ≈ 3.5 × 10 −6 mol/cm 3 )/s −1 = 2.0 × 10 6 × exp (−31 ± 10 kJ mol −1 /RT ) (±20%), k CIISC (ρ ≈ 7.0 × 10 −6 mol/cm 3 )/s −1 = 1.4 × 10 6 × exp (−25 ± 10 kJ mol −1 /RT ) (±20%). These values are in good agreement with CIISC rate constants extracted from corresponding 1 NCN measurements. The observed nonlinear pressure dependences reveal a pressure saturation effect of the CIISC process. C 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: [30][31][32][33][34][35][36][37][38][39][40] 2013

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.