二氟氨基二硝甲基芳香杂环含能材料的理论研究

doi:10.6023/A24010017

摘要/Abstract

近几十年以来, 传统含能材料的发展遇到了瓶颈. 如何继续提高能量水平, 打破瓶颈, 是这一领域亟待攻克的难题. 氟是比氧更强的氧化剂, 预期分子内引入氟可以进一步提高能量水平, 因此设计了13种二氟氨基二硝甲基取代的芳香杂环含能材料分子. 为了保证这些分子合成的可能性, 所有结构设计都是从已有的中间体出发, 并且原则上均可经由成熟的合成方法转化为目标分子. 对它们的分子结构、初始热分解机理以及能量特性进行的理论研究表明, 多数分子具有足够的动力学稳定性. 本工作通过深入分析, 揭示了分子结构与动力学稳定性之间的关系. 使用硝酸酯增塑聚醚(NEPE)固体推进剂配方对这些分子的能量特性进行了理论评价, 最终优选出4种分子, 其中最好的一个不仅动力学稳定性较好, 而且配方比冲高达280.1 s, 比传统的环四亚甲基四硝胺(HMX)配方提高了8.4 s.

关键词: 二氟氨基化合物, 含能材料, 固体推进剂, 比冲, 理论研究

In recent decades, the development of traditional energetic materials has encountered a bottleneck. How to continue to improve the energy level and break the bottleneck has become an urgent problem in the field of energetic materials. Fluorine is a stronger oxidizing agent than oxygen, and theoretically the introduction of fluorine can further increase energy density. Thirteen kinds of energetic molecules containing (difluoramino)dinitromethyl substituted heteroaromatic rings were designed. To ensure the possibility of synthesis, all the structural designs are based on existing intermediates and could be transformed into target molecules through mature synthesis methodologies. The molecular structure, initial thermal decomposition mechanism and energy characteristics were studied theoretically with density functional theory (DFT) methods (B3LYP/6-311+G(d,p) and M06-2X/6-311+G(d,p)) using Gaussian16 program. By calculating the mechanism of the initial decomposition reaction, the trigger bond was determined to be one of the C—NO₂ bonds in the (difluoramino)dinitromethyl group. Dynamic stability is evaluated by the energy barrier of the trigger bond breaking. The results show that most of these molecules have sufficient dynamic stability, the initial decomposition reaction barriers are around 30 kcal/mol. The relationship between the molecule structure and the dynamic stability is revealed. The carbon radical center in the transition state is connected with the strong electron-withdrawing groups (-NO₂ and -NF₂) and the heterocyclic ring with a certain electron-donating ability. This is a typical “push-pull” electronic structure, which makes the free radical particularly stable. Therefore, the homo-cleavage energy barrier of the trigger bond is determined by the stabilizing effect of heterocyclic rings on the methyl free radicals. The energy properties of these molecules were theoretically evaluated with nitrate ester plasticized polyether (NEPE) solid propellant formulations using EXPLO5 program. The results show that the specific impulse of one dynamic stable molecule is up to 280.1 s, which is about 8.4 s higher than that of the traditional HMX (Octogen) formulations.

Key words: difluoramino compound, energetic material, solid propellant, specific impulse, theoretical research

引用此文

崔勇康, 成守飞, 凌琳, 李玉学, 吕龙. 二氟氨基二硝甲基芳香杂环含能材料的理论研究[J]. 化学学报, 2024, 82(4): 377-386.

Yongkang Cui, Shoufei Cheng, Lin Ling, Yuxue Li, Long Lu. Theoretical Study on Energetic Materials Containing (Difluoramino)dinitromethyl Substituted Heteroaromatic Rings[J]. Acta Chimica Sinica, 2024, 82(4): 377-386.

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图/表 13

图1. (a)化合物A的合成路线; (b)化合物B的合成路线

Figure 1. (a) The synthetic route of A; (b) the synthetic route of B

图2. 目标分子的构建和筛选

Figure 2. The construction and screening of target molecules

Compd.	1-NF₂	2-NF₂	3-NF₂	4-NF₂	5-NF₂	6-NF₂	7-NF₂
ρ/g/cm³)	2.04	2.00	2.04	2.07	2.04	2.04	1.98
C-NF₂ (Å)	1.495	1.490	1.495	1.487	1.488	1.494	1.494
C-NO₂^a (Å)	1.589	1.568	1.593	1.566	1.577	1.565	1.566
Compd.	8-NF₂	9-NF₂	10-NF₂	11-NF₂	12-NF₂	13-NF₂
ρ/g/cm³)	1.93	1.96	1.98	2.10	2.05	2.08
C-NF₂ (Å)	1.484	1.493	1.492	1.489	1.492	1.491
C-NO₂^a (Å)	1.599	1.569	1.556	1.589	1.569	1.558

表1. 新化合物的密度(ρ)和-C(NO2)2NF2中C─NF2和C─NO2的键长

Table 1. The density (ρ) and the C─NF2, C─NO2 bond length in -C(NO2)2NF2 group for 1-NF2 to 13-NF2

图3. 典型的二氟氨基二硝甲基化合物优化的分子结构示例. 键长单位: ?. 计算在B3LYP /6-311+G(d,p)水平上进行

Figure 3. Selected optimized molecular structures of the (difluoramino)dinitromethyl compounds. Selected bond lengths are in angstroms (?), calculated at the B3LYP/6-311+G(d,p) level

图4. 1-NF2的初始分解机理. (1) 化合物1-NF2的共价键种类. 键长单位: ?. 计算在M06-2X/6-311+G(d,p)水平上进行

Figure 4. The initial decomposition mechanism of 1-NF2. (1) The covalent bond types of the 1-NF2. Selected bond lengths are in angstroms (?), calculated at the M06-2X/6-311+G(d,p) level

图5. MTNMT的C─NO2均裂反应路径. 键长单位: ?. 计算在M06-2X/6-311+G(d,p)水平上进行

Figure 5. The initial decomposition mechanism of MTNMT. Selected bond lengths are in angstroms (?), calculated at the M06-2X/6-311+G(d,p) level

Compd.	1-NF₂	2-NF₂	3-NF₂	4-NF₂	5-NF₂	6-NF₂	7-NF₂
Energy Barrier ΔG	30.5	32.8	30.5	30.0	29.0	32.6	33.2
BDE ΔH	39.0	41.2	38.7	37.3	36.3	40.3	40.6
Compd.	8-NF₂	9-NF₂	10-NF₂	11-NF₂	12-NF₂	13-NF₂
Energy Barrier ΔG	22.1	32.3	31.4	26.1	31.7	31.8
BDE ΔH	27.3	41.3	40.9	33.1	41.3	41.1

表2. 新化合物中引发键Bond-a均裂的能垒和键能

Table 2. The homo-cleavage energy barrier and BDE (Bond Dissociation Energy, ΔH) of Bond-a for 1-NF2 to 13-NF2 (in kcal/mol)

图6. 二氟氨基自由基?NF2和二氧化氮?NO2的自旋密度分布

Figure 6. The spin density distribution of ?NF2 and nitrogen dioxide ?NO2

图7. 过渡态中的自旋密度分布示意图

Figure 7. Diagram of the spin density distribution in the transition state

图8. C─NO2键均裂能垒与甲基C原子上自旋密度的关系

Figure 8. The C─NO2 bond cleavage barrier vs. the spin density on the methyl carbon atom

图9. (A) 8-NF2和9-NF2的最高占据分子轨道(HOMO); (B)过渡态TS1-8-NF2和TS1-9-NF2的自旋密度分布

Figure 9. (A) The highest occupied molecular orbital (HOMO) of 8-NF2 and 9-NF2; (B) the spin density distribution of transition state TS1-8-NF2 and TS1-9-NF2

Compd.	Formula	ΔH_f/ (kJ/mol)	K-J Function		EXPLO5
Compd.	Formula	ΔH_f/ (kJ/mol)	D/(m/s)	P/GPa	D/(m/s)	P/GPa
1-NF₂	C₃F₂N₆O₇	180.1	9117	38.5	8752	35.4
2-NF₂	C₆F₄N₁₀O₁₁	213.7	9294	40.2	8663	34.6
3-NF₂	C₆F₄N₁₂O₁₀	585.7	9515	42.1	9066	38.5
4-NF₂	C₄F₄N₈O₉	–12.9	8927	37.1	8437	33.1
5-NF₂	C₆F₄N₁₀O₁₀	56.2	9190	39.1	8736	35.6
6-NF₂	C₆F₄N₁₀O₁₀	103.0	9227	39.4	8762	35.6
7-NF₂	C₇H₂F₄N₁₀O₁₀	16.2	9232	38.8	8855	39.2
8-NF₂	C₃H₂F₂N₆O₅	–98.5	9160	37.6	8757	37.5
9-NF₂	C₃HF₂N₇O₆	106.2	9203	38.4	8885	36.6
10-NF₂	C₆H₂F₄N₁₂O₈	187.8	9205	38.6	8910	39.0
11-NF₂	C₃F₄N₆O₅	41.3	9217	40.0	8452	34.3
12-NF₂	C₃F₃N₇O₆	177.7	9127	38.7	8640	34.8
13-NF₂	C₆F₆N₁₂O₈	299.2	9356	41.0	8847	37.1

表3. 13种目标化合物的生成焓(ΔHf)、爆速(D)和爆压(P)

Table 3. The enthalpy of formation (ΔHf), detonation velocity (D) and detonation pressure (P) of the 13 target compounds

Compd.	NF₂-Ox/%	AP/%	Al/%	ρ_p/(g/cm³)	I_sp	F/%
1-NF₂	60	0	15	1.910	276.499	14.08
2-NF₂	60	0	15	1.889	274.814	16.38
3-NF₂	62.5	0	12.5	1.899	280.064	15.97
4-NF₂	60	0	15	1.926	274.861	20.00
5-NF₂	60	0	15	1.910	271.657	16.97
6-NF₂	60	0	15	1.910	272.612	16.97
7-NF₂	64	0	11	1.859	269.561	16.45
8-NF₂	44	14	17	1.864	267.604	15.83
9-NF₂	60	0	15	1.867	273.919	14.13
10-NF₂	44	16	15	1.875	269.259	17.04
11-NF₂	70	0	5	1.902	280.308	27.52
12-NF₂	64	0	11	1.898	277.928	19.86
13-NF₂	70	0	5	1.891	276.083	23.65
HMX	42	15	18	1.860	271.638	0.00
3-NO₂	60	0	15	1.817	275.686	0.00

表4. 13种目标化合物的NEPE推进剂配方组成(质量分数)、配方密度(ρp)、理论比冲(Isp)和F含量. NEPE推进剂粘合剂体系由聚乙二醇(PEG, 7%)和硝酸甘油-丁三醇三硝酸酯(NG/BTTN=9%:9%)增塑剂组成

Table 4. The NEPE propellant formulations, formulation density (ρp), theoretical specific impulse (Isp) and the F content of the 13 target compounds. The NEPE propellant binder system is composed of polyethylene glycol (PEG, 7%), and plasticizer of nitroglycerine-butanetrioltrinitrate (NG/BTTN=9%:9%)

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