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

doi:10.6023/cjoc202206027

摘要/Abstract

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—NO₂键均裂和随后的自由基复合是最优途径, 能垒为216.9 kJ•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, N₂O和CO等小分子, 它们相互反应又生成NO₂, N₂和CO₂. 这些分解产物与诸多实验观测结果一致.

关键词: 3-硝基-1,2,4-三唑-5-酮(NTO), 含能材料, 热分解机理, BS-UDFT (Broken Symmetry Density Functional Theory), 开壳层单重态双自由基, 共价键均裂, 理论研究

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 barrier ΔG, 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—NO₂ 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, N₂O and CO. Further reactions between these small molecules lead to NO₂, N₂ and CO₂. These products are consistent well with the experimental observations.

Key words: 3-nitro-1,2,4-triazol-5-one (NTO), energetic materials, pyrolysis mechanism, BS-UDFT (Broken Symmetry Density Functional Theory), open-shell singlet diradical, homolytic cleavage, theoretical study

引用此文

凌琳, 王健, 李婧, 李玉学, 吕龙. 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[J]. 有机化学, 2023, 43(1): 285-294.

Lin Ling, Jian Wang, Jing Li, Yuxue Li, Long Lu. Broken-Symmetry Density Functional Theory Study on Pyrolysis Mechanisms of 3-Nitro-1,2,4-triazol-5-one (NTO)[J]. Chinese Journal of Organic Chemistry, 2023, 43(1): 285-294.

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图/表 8

表1. 已发表的NTO热分解实验和理论研究结果

Table 1. Experimental and theoretical studies on NTO pyrolysis

图1. NTO的可能分解路径

Figure 1. Possible initial pathways of NTO pyrolysis

图2. C—NO2均裂引发分解及后续反应过程的第一部分(Path-a)

Figure 2. C—NO2 homo-cleavage initiated decomposition pathways (Path-a) (Part I) Energies are in kJ?mol–1, and selected bond lengths are in nanometers (nm)

图3. 关键过渡态TS-Homo-CN-a和TS-Recom-CO-a的IRC计算(A)、(B)和自旋密度图(C)、(D)

Figure 3. IRC calculation (A), (B) and the spin density distribution (C), (D) of the key transition states TS-Homo-CN-a and TS-Recom- CO-a

图4. 第二部分: NO的产生及其促进的开环过程(Path-a)

Figure 4. Part II: formation of NO and NO promoted ring-opening (Path-a) Energies are in kJ?mol–1, and selected bond lengths are in nanometers (nm)

图5. 小分子气体产物之间的进一步反应和转化

Figure 5. Further reactions/transformations between small intermediates/products Energies are in kJ?mol–1, and selected bond lengths are in nanometers (nm)

图6. 质子转移及其后续反应路径(Path-b)

Figure 6. The proton transfer initiated decomposition pathways (Path-b) Energies are in kJ?mol–1, and selected bond lengths are in nanometers (nm)

图7. NTO直接开环的反应路径(Path-c)

Figure 7. Ring-opening initiated decomposition pathways (Path-c) Energies are in kJ?mol–1, and selected bond lengths are in nanometers (nm)

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