3-硝基-1,2,4-三唑-5-酮(NTO)热分解机理的对称破缺密度泛函理论研究

Broken-Symmetry Density Functional Theory Study on Pyrolysis Mechanisms of 3-Nitro-1,2,4-triazol-5-one(NTO)

共价键均裂在含能分子的热分解过程中普遍存在,因此极为重要.然而,以往的理论研究通常用键能(ΔH,BDE)估算均裂能垒,因忽略了熵效应,必然造成较大误差.采用对称破缺密度泛函方法(BS-UB3LYP/6-311+G**),对含能分子3-硝基-1,2,4-三唑-5-酮(NTO)的热分解机理进行了系统研究和梳理,计算了共价键均裂的过渡态及能垒.结果表明,C—NO_(2)键均裂和随后的自由基复合是最优途径,能垒为216.9 k J·mol^(-1)(523 K).随后产生的NO自由基通过多次“复合-均裂”过程促进三唑中间体开环,分解为HNCO,N_(2)O和CO等小分子,它们相互反应又生成NO_(2),N_(2)和CO_(2).这些分解产物与诸多实验观测结果一致.

Homolytic cleavage of covalent bonds is very common during the pyrolysis of energetic molecules.However,instead of locating the transition state and calculating the free energy barrierΔG,the bond dissociation energy (ΔH,BDE) is usually considered as the“energy barrier”of the homo-cleavage processes.This simplification brings large errors.In the present work,several pyrolysis pathways of 3-nitro-1,2,4-triazol-5-one (NTO) were studied using broken-symmetry density functional theory method (BS-UB3LYP/6-311+G**).Each transition state of homolytic cleavage was located.The results show that the best pyrolysis pathway under the experimental conditions proceeds via the homolytic cleavage of the C—NO_(2)bond and the subsequent radical recombination,the energy barrier of the rate-determining step is 216.9 kJ·mol^(-1)(523 K).Then,NO can promote the subsequent ring-opening,and finally yielding HNCO,N_(2)O and CO.Further reactions between these small molecules lead to NO_(2),N_(2)and CO_(2).These products are consistent well with the experimental observations.