Vaghjiani Ghanshyam L, Sun Hongyan, Chambreau Steven D
In-Space Propulsion Branch, Air Force Research Laboratory, AFRL/RQRS, Edwards Air Force Base, California 93524, United States.
J Phys Chem A. 2020 Dec 17;124(50):10434-10446. doi: 10.1021/acs.jpca.0c07985. Epub 2020 Dec 2.
The NH + NO reaction plays a key role during the early stages of hypergolic ignition between NH and NO. Here for the first time, the reaction kinetics of NH in excess NO was studied in 2.0 Torr of N and in the narrow temperature range 298-348 K in a pulsed photolysis flow-tube reactor coupled to a mass spectrometer. The temporal profile of the product, HONO, was determined by direct detection of the / +47 amu ion signal. For each chosen [NO], the observed [HONO] trace was fitted to a biexponential kinetics expression, which yielded a value for the pseudo-first-order rate coefficient, ', for the reaction of NH with NO. The slope of the plot of ' versus [NO] yielded a value for the observed bimolecular rate coefficient, , which could be fitted to an Arrhenius expression of (2.36 ± 0.47) × 10 exp((520 ± 350)/) cm molecule s. The errors are 1σ and include estimated uncertainties in the NO concentration. The potential energy surface of NH + NO was investigated by advanced ab initio quantum chemistry theories. It was found that the reaction occurs via a complex reaction mechanism, and all of the reaction channels have transition state energies below that of the entrance asymptote. The radical-radical addition forms the NHNO adducts, while roaming-mediated isomerization reactions yield the NHONO isomers, which undergo rapid dissociation reactions to several sets of distinct products. The RRKM multiwell master equation simulations revealed that the major product channel involves the formation of -HONO and -NH below 500 K and the formation of NO + NHNHO above 500 K, which is nearly pressure independent. The pressure-dependent rate coefficients of the product channels were computed over a wide pressure-temperature range, which encompassed the experimental data.
NH + NO反应在NH与NO自燃点火的早期阶段起着关键作用。在此，首次在与质谱仪相连的脉冲光解流动管反应器中，于2.0托的N₂环境及298 - 348 K的狭窄温度范围内，研究了过量NO中NH的反应动力学。通过直接检测/+47原子质量单位的离子信号来确定产物HONO的时间分布。对于每个选定的[NO]，将观测到的[HONO]曲线拟合为双指数动力学表达式，从而得到NH与NO反应的准一级速率系数k'的值。k'对[NO]作图的斜率得出观测到的双分子速率系数k的值，该值可拟合为(2.36 ± 0.47) × 10 exp((520 ± 350)/T) cm³ molecule⁻¹ s⁻¹的阿伦尼乌斯表达式。误差为1σ，包括NO浓度的估计不确定性。采用先进的从头算量子化学理论研究了NH + NO的势能面。发现该反应通过复杂的反应机理进行，且所有反应通道的过渡态能量均低于入口渐近线的能量。自由基 - 自由基加成形成NHNO加合物，而漫游介导的异构化反应产生NHONO异构体，这些异构体经历快速解离反应生成几组不同的产物。RRKM多阱主方程模拟表明，主要产物通道在500 K以下涉及生成-HONO和-NH，在500 K以上涉及生成NO + NHNHO，且几乎与压力无关。在涵盖实验数据的宽压力 - 温度范围内计算了产物通道的压力依赖速率系数。
