为探究固相CL-20热分解反应机理,本文采用反应分子动力学ReaxFF MD模拟研究了含有128个CL-20分子的超胞模型在800-3000 K温度下的热分解过程.借助作者所在课题组研发的反应分析及可视化工具VARxMD得到了热分解过程中多种反应中间物和较为全面的反应路径.氮氧化物是CL-20初始分解的主要中间产物,其中NO2是数量最多的初始分解产物,观察到的中间物NO3的生成量仅次于NO2.统计CL-20初始分解的所有反应后发现,在所有考察温度下CL-20初始分解路径主要是N—NO2断裂反应和C—N键断裂引起开环的单分子反应路径.N—NO2断裂反应数量在高温下显著增多,而C—N键断裂引起的开环反应数量随温度升高变化不大.在低温热分解模拟中还观察到CL-20初始分解阶段生成的NO2会发生双分子反应—从CL-20分子中夺氧生成NO3.对CL-20热分解过程中环结构演化进行分析后发现,CL-20分解的早期反应中间物主要为具有3元或2元稠环结构的吡嗪衍生物,随后它们会分解形成单环吡嗪.吡嗪六元环结构在热分解过程中非常稳定,这一模拟结果支持Py-GC/MS实验中提出吡嗪存在的结论.CL-20中的咪唑五元环结构相对不稳定,在热分解过程中会发生开环分解而较早消失.由ReaxFF MD模拟得到的3000 K高温热分解产物N2,H2O,CO2和H2的数量与爆轰实验的测量结果定量吻合.本文获得的对CL-20热分解机理的认识表明ReaxFF MD结合VARxMD有可能为深入了解热刺激下含能材料复杂化学过程提供一种有前景的方法.%The thermal decomposition of condensed CL-20 was investigated using reactive force field molecular dynamics (ReaxFF MD) simulations of a super cell containing 128 CL-20 molecules at 800-3000 K.The VARxMD code previously developed by our group is used for detailed reaction analysis.Various intermediates and comprehensive reaction pathways in the thermal decomposition of CL-20 were obtained.Nitrogen oxides are the major initial decomposition products,generated in a sequence of NO2,NO3,NO,and N2O.NO2 is the most abundant primary product and is gradually consumed in subsequent secondary reactions to form other nitrogen oxides.NO3 is the second most abundant intermediate in the early stages of CL-20 thermolysis.However,it is unstable and quickly decomposes at high temperatures,while other nitrogen oxides remain.At all temperatures,the unimolecular pathways of N—NO2 bond cleavage and ring-opening C—N bond scission dominate the initial decomposition of condensed CL-20.The cleavage of the N— NO2 bond is greatly enhanced at high temperatures,but scission of the C— N bond is not as favorable.A bimolecular pathway of oxygen-abstraction by NO2 to generate NO3 is observed in the initial decomposition steps of CL-20,which should be considered as one of the major pathways for CL-20 decomposition at low temperatures.After the initiation of CL-20 decomposition,fragments with different ring structures are formed from a series of bond-breaking reactions.Analysis of the ring structure evolution indicates that the pyrazine derivatives of fused tricycles and bicycles are early intermediates in the decomposition process,which further decompose to single ring pyrazine.Pyrazine is the most stable ring structure obtained in the simulations of CL-20 thermolysis,supporting the proposed existence of pyrazine in Py-GC/MS experiments.The single imidazole ring is unstable and decomposes quickly in the early stages of CL-20 thermolysis.Many C4 and C2 intermediates are observed after the initial fragmentation,but eventually convert into stable products.The distribution of the final products (N2,H2O,CO2,and H2) obtained in ReaxFF MD simulation of CL-20 thermolysis at 3000 K quantitatively agrees with the results of the CL-20 detonation experiment.The comprehensive understanding of CL-20 thermolysis obtained through this study suggests that ReaxFF MD simulation,combined with the reaction analysis capability of VARxMD,would be a promising method for obtaining deeper insight into the complex chemistry of energetic materials exposed to thermal stimuli.
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