Density Functional Theory Research on Decomposition Mechanisms of Nitroglycerin

doi:10.6023/A25030081

Abstract

M06-2X/6-311＋G** method has been used to systematically study the decomposition mechanism of nitroglycerin (NG), including unimolecular thermal decomposition, non-catalytic hydrolysis, HNO₃ catalyzed hydrolysis, and decomposition involving metal salt/base impurities (combinations of Zn²^＋/Mg²^＋ cations with OH^－/ $NO 3 －$ /Cl^－ anions). Consistent with previous reports, the homolytic cleavage of the O—NO₂ to generate •NO₂ free radicals was identified as the optimal initial pathway in unimolecular decomposition. In the presence of acidic impurities, several reaction modes were explored. The optimal pathway involves proton-activated heterolytic cleavage of the O—NO₂ bond in the nitrate ester group, releasing the nitronium ion (NO²^＋), which subsequently reacts with water to form nitric acid. The energy barrier for acid-catalyzed hydrolysis was calculated as 28.1 kcal/mol, 5.8 kcal/mol lower than the O—NO₂ homolysis barrier in unimolecular decomposition, making it a more plausible mechanism for the slow room-temperature decomposition. Notably, the continuous release of HNO₃ during hydrolysis creates an autocatalytic acceleration effect. For reactions of Zn(OH)₂ and moderately strong base Mg(OH)₂ with NG, the energy barriers were 25.3 and 21.3 kcal/mol, respectively. In this reaction mode, there is a cooperation effect of Lewis acid activation and the direct attack by anionic ligands on the nitrate ester, enabling relatively rapid decomposition. Although stoichiometric in nature, the generated nitric acid intermediates exhibit higher collision probabilities with NG molecules than with alkaline species for neutralization, potentially triggering acid-catalyzed decomposition. Once initiated, this acid-catalyzed process becomes self-accelerating. Zn²^＋ and Mg²^＋ nitrates/chlorides showed limited catalytic activity. We conclude that the acid-catalyzed mechanism most likely represents the true pathway for NG’s gradual decomposition at ambient temperatures. Previous theoretical studies have predominantly focused on the unimolecular decomposition pathways of nitroglycerin and neglected the influence of impurities, failing to adequately account for the observed slow decomposition at ambient temperatures. Our research provides valuable references on the study of nitroglycerin.

Key words: nitroglycerin, nitroglycerin (NG), decomposition mechanism, density functional theory (DFT), theoretical research

Cite this article

Shoufei Cheng, Jing Li, Lin Ling, Yuxue Li, Long Lu. Density Functional Theory Research on Decomposition Mechanisms of Nitroglycerin[J]. Acta Chimica Sinica, 2025, 83(8): 833-843.

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Fig. & Tab. 11

Figure 1. Possible unimolecular decomposition mechanisms of nitroglycerin

Figure 2. Unimolecular decomposition mechanism of nitroglycerin (O—NO2 bond homolysis and NO2 catalized mechanism. The green and red numbers are spin densities. The bond lengths are in angstroms)

Figure 3. Mode-3: Unimolecular decomposition mechanism of nitroglycerin (HONO elimination. The bond lengths are in angstroms)

Pathway	Gas phase		In NG		In H₂O
Pathway	α	β	α	β	α	β
Path-Mode-1	32.8	33.9	35.9	35.5	35.0	33.9
Path-Mode-2	61.7	64.0	63.6	64.4	58.9	60.3
Path-Mode-2-2	65.3	66.1	62.9	64.4	61.0	62.0
Path-Mode-2-3	67.1	65.8	67.2	66.2	64.9	64.4
Path-Mode-3	44.9	45.1	47.0	47.8	45.7	46.1

Table 1. Energy barriers of nitroglycerine unimolecular decomposition [ΔG/(kcal•mol－1)]

Figure 4. Hydrolysis reaction modes of NG

Figure 5. Catalysis free/acid-catalyzed nitroglycerin hydrolysis mechanisms (The bond lengths are in angstroms)

Figure 6. Orbital interaction of nitryl cation and surrounding oxygen atoms in transition state (The blue and pink italic numbers are NBO atomic charges. The bond lengths are in angstroms)

Pathway	SMD in NG		SMD in H₂O
Pathway	α	β	α	β
Path-H-Mode-1	51.9	54.2	56.5	52.5
Path-H-Mode-2	49.3	50.9	42.1	43.1
Path-H-Mode-3	54.5	51.5	47.1	44.4
Path-H-Mode-4	39.0	32.5	29.4	28.1

Table 2. Energy barriers of hydrolysis reaction pathways of nitroglycerin [ΔG/(kcal•mol－1)]

Figure 7. Mechanism of metal complex promoted nitroglycerine hydrolysis

Figure 8. Mechanism of metal complex promoted nitroglycerine hydrolysis (The bond lengths are in angstroms)

	M=Zn^2＋						M=Mg^2＋
	OH^－		$NO 3 −$		Cl^－		OH^－		$NO 3 −$		Cl^－
	α	β	α	β	α	β	α	β	α	β	α	β
M-Mode-1	47.2	44.1	45.0	44.5			45.9	42.1	43.5	38.6
M-Mode-2	28.7	26.3	54.1	48.3			29.2	30.1	52.6	46.1
M-Mode-3	26.2	25.3					21.3	22.0
M-Mode-4	27.0	27.5			36.4	39.9	27.3	26.7			43.3	47.7

	M=Zn^2＋						M=Mg^2＋
	OH^－		$NO 3 −$		Cl^－		OH^－		$NO 3 −$		Cl^－
	α	β	α	β	α	β	α	β	α	β	α	β
M-Mode-1	47.2	44.1	45.0	44.5			45.9	42.1	43.5	38.6
M-Mode-2	28.7	26.3	54.1	48.3			29.2	30.1	52.6	46.1
M-Mode-3	26.2	25.3					21.3	22.0
M-Mode-4	27.0	27.5			36.4	39.9	27.3	26.7			43.3	47.7

Table 3. Comparison of hydrolysis pathways of nitroglycerin

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