Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Kiel, Germany.
Phys Chem Chem Phys. 2012 Jan 14;14(2):1030-7. doi: 10.1039/c1cp22123j. Epub 2011 Aug 30.
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%).
首次直接测量了反应 NCN + O 的速率常数。根据 Fenimore 修正机理，该反应由 CH + N(2)→ NCN + H 引发，对于火焰中瞬态 NO(x)的形成起着关键作用。通过 NCN(3)和 N(2)O 的热解分别定量生成了 NCN 自由基和 O 原子。使用窄带宽激光吸收在波长 λ = 329.1302nm 处，在冲击波后监测 NCN 浓度-时间曲线。然而，在 709 mbar < p < 1861 mbar 的压力下，没有发现压力依赖性，而仅发现对应于 5.8 ± 6.0 kJ mol(-1)的活化能的几乎显著的温度依赖性。总的来说，在 1826 K < T < 2783 K 的温度下，速率常数可以表示为 k(NCN + O) = 9.6 × 10(13)× exp(-5.8 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (±40%)。作为准确测量高温速率常数的要求，首先从纯 NCN(3)/Ar 气体混合物的热解实验中推导出一致的 NCN 背景机理。据推测，双分子二次反应 NCN + NCN 生成 CN 自由基，从而引发有效地去除 NCN 的链式反应循环。从压力范围为 143 mbar 至 1884 mbar 且温度范围为 966 K 至 1900 K 的测量中确定了温度无关的 k(NCN + NCN) = (3.7 ± 1.5) × 10(12) cm(3) mol(-1) s(-1)。在较高的温度下，NCN 的单分子分解 NCN + M → C + N(2) + M 占主导地位。在 2012 K < T < 3248 K 的温度和 703 mbar < p < 2204 mbar 的总压力下的测量结果表明，单分子分解接近其低压极限。相应的速率常数可以表示为 k(NCN + M) = 8.9 × 10(14)× exp(-260 kJ mol(-1)/RT) cm(3) mol(-1) s(-1)(±20%)。
