TNT高温热解及含碳团簇形成的反应分子动力学模拟

ReaxFF-MD模拟三硝基甲苯(TNT)高温热解显示增加了伦敦耗散力项(E_lg)的ReaxFF/lg 势函数在含能材料平衡密度计算方面具有优越性. 产物识别分析得出TNT热解的主要产物为NO₂、NO、H₂O、N₂、CO₂、CO、OH以及HONO，且最终产物为H₂O、N₂和CO₂. 使用ReaxFF势函数模拟同样过程进行比较性分析显示，在主要产物和最终产物方面与ReaxFF/lg 作用结果具有一致性，但在化学反应动力学方面表现出一些差异. ortho-NO₂键断裂和C―NO₂→C―ONO重排布-断裂形成NO₂和NO是TNT热解的主要初级反应，且前者产生速率大于后者，NO₂和NO形成后很快参与次级反应并最终形成N₂. 高温热解中形成OH等小分子会促进H₂O的形成. 环上基团相互反应或直接脱落后，主环间C―C键才发生断裂，但温度升高会加快主环断裂，并进一步分解形成CO₂，这也是高温条件下CO₂分布产生波动的一个重要原因. 并且当晶胞中的TNT分子几乎完全分解时，系统的势能开始明显衰减. 与温度相比，密度对热解中最大含碳团簇形成的影响更明显. 并且，模拟结果显示，在TNT完全分解前已经出现含碳中间体的聚合现象. 此项工作表明使用ReaxFF/lg 反应力场研究TNT高温热解可以提供具体的动力学和化学方面的信息，并有助于理解含能材料的爆轰问题并可进行安全评估.

Reactive Molecular Dynamics Simulations of Carbon-Containing Clusters Formation during Pyrolysis of TNT

ReaxFF molecular dynamics simulations of trinitrotoluene (TNT) pyrolysis show that use of the ReaxFF/lg potential function, which adds the London dispersion term, gives superior results in equilibrium density calculation of energetic materials. According to our calculations using limited time steps, the main products are NO₂, NO, H₂O, N₂, CO₂, CO, OH, and HONO, and H₂O, N₂, and CO₂ are the final products. We also used ReaxFF potential functions to simulate the same process to conduct a comparative analysis. The main and final products are consistent with those obtained using ReaxFF/lg, but the kinetics are different. Both ortho-NO₂ homolytic cleavage and C―NO₂→C―ONO rearrangement homolysis are thermodynamically favorable pathways in the early thermal decomposition of TNT. However, C―NO₂→C―ONO rearrangement homolysis is less favorable kinetically than C―NO₂ homolysis, since C―NO₂ is the weakest bond in TNT. Soon after their formation, NO₂ and NO participate in secondary reactions and eventually form N₂. Pyrolysis to form OH and other small molecules promotes the formation of H₂O. Aromatic ring fission does not take place until most of the attached groups have interacted or are removed, and increasing the temperature accelerates main-ring fission and further decomposition to form CO₂; this is the major reason for CO₂ distribution fluctuations under high-temperature conditions. When the TNT molecules in the unit cell are almost completely decomposed, the potential energy of the system is significantly attenuated. The maximum amount of carbon-containing clusters formed in the thermal decomposition is more dependent on density than on temperature. Moreover, the simulation results show that coagulation of carbonaceous intermediates occurs before the TNT decomposes completely. These studies show that the simulation of TNT pyrolysis using the ReaxFF/lg reactive force field can provide detailed kinetic and chemical information, which are helpful in understanding the detonation of energetic materials and assessing their security.