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Theoretical Study on Nitroso-Substituted Derivatives of Azetidine as Potential H

时间:2024-09-03

LI Bu-Tong LI Lu-Lin YANG Chuan

Theoretical Study on Nitroso-Substituted Derivatives of Azetidine as Potential High Energy Density Compounds①

LI Bu-Tong②LI Lu-Lin YANG Chuan

(051008)

At the B3PW91/6-311+G(d,p)//MP2/6-311+G(d,p) level, molecular densities, detonation velocities, and detonation pressures of nitroso substituted derivatives of azetidine with their thermal stabilities were investigated to look for high energy density compounds (HEDCs). It was found that the azetidine derivatives had high heat of formation (HOF) and large bond dissociation energy (BDE). Intramolecular hydrogen bonds were located in three molecules (1, 4, and 5), and the molecular stability were improved markedly as well. For 5 and 6, the detonation performances (= 9.36km/s and 10.80km/s,= 44.42GPa and 60.70GPa, respectively) meet requirements as high energy density compounds. This work may provide basic information for further study of title compounds.

high energy density compounds, Kamlet-Jacobs equation, azetidine derivatives;

1 INTRODUCTION

The search for new high energy density com- pounds (HEDCs) is ongoing, which have been used widely for both military and civilian applications[1-8]. In the last decades, strained polynitro cyclic com- pounds have attracted a lot of attentions for their high energy density and low vulnerability. RDX (hexahydro-1,3,5-trinitro-1,3,5-trizine) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane) be-long to this class[9-13]. Such materials may find application in advanced propellants and explosive formulations. The derivatives of azetidines with strained four-member heterocyclic ring have made foray in the area of melt castable explosive since 1990s, particularly 1,3,3-trinitroazetidine (TNAZ)[14]. It is particularly attractive because of some excellent characters including low sensitiveity, easy management, and steam castability. Its melting point is 101 ℃ with density=1.84 g/cm3, and it is ther- mally stable (240.10 ℃) and solid at room tem- perature[15]. However, TNZA is with a somewhat lower output in energy than RDX and HMX[16]. Considering the importance of detonation perfor- mance (detonation velocity and detonation pressure), the way to improve them of azetidine derivatives is urgent for development.

In general, high-energy materials have large numbers of nitrogen atoms, so called “high nitro- gen” compounds. This character can derive energy from the dissociation reactions of energetic N−N and C−N bonds[17,18]. High-energy-density compounds with polynitro groups are an important class of energetic materials[19-23]. The presence of nitro groups tends to decrease the heats of formation but contributes markedly to energetic performance.Herein is why we are interested in azetidine derivatives, which contain high percentage of both oxygen and nitrogen atoms, and low amounts of carbon and hydrogen atoms. The high nitrogen content can lead to high crystal density, accompa- nied with increased denotation performance. The high oxygen content has an advantage in combustion reactions. For these reasons, another nitrogen-con- taining functional group interested in for this study is nitroso groups (=NO). The main difference among nitroso group and nitro group is the nitrogen content. In addition, the C=N bond is stronger than the C–N bond and can introduce more stability.

Therefore, we replaced the hydrogen atoms of azetidine molecule with nitroso groups to design new potential HEDCs. It is expected that our results provide some useful information for synthesis of polynitroso derivatives of azetidine.

2 COMPUTATIONAL METHODS

All calculations were performed at B3PW91/6- 311+G(d,p)//MP2/6-311+G(d,p) level by using Gaus- sian 03 program packages[24-26], along with the standard Gaussian basis set 6-311+G(d, p). The me- thod can deal with the system with large dynamic electron correlation effectively and provide more accurate electronic energy, which also emerged in title compounds, especially for the polynitroso- substituted azetidine[27]. Harmonic vibrational analy- ses were performed at the same level of theory to confirm the structural nature. Fig. 1 listed all object molecules.

Fig. 1. Structures of azetidine derivatives designed in this paper

The heats of formation (HOFs) are needed in the calculation of detonation energy. In present paper, the atomization reaction[28]was applied to calculate the HOFs of title compounds. For 1, 2, 3, 4, and 5, reaction (1) was used. For 6 compound, reaction (2) was used. HOFs are obtained via equation (3)~(5).

C3H7-2xN+1O→3C+(7-2)H+(+1)N+O (=1~3) (1)

C3N4O4→3C+4N+4O (2)

The bond dissociation energy with zero-point energy (ZPE) correction can be calculated by Eq. (8):

Where ΔZPE is the difference between the ZPEs of products and the reactants.

For each title compound, explosive reaction is designed in terms of the maximal exothermal principle, that is, all the N atoms were turned into N2, the O atoms react with H atoms to give H2O at first, and then form CO2with C atom. If the number of O atoms is more than needed to oxidize H and C atoms, redundant O atoms will be converted into O2. If the number of atoms is not enough to satisfy full oxidation of H and C atoms, and the remaining of H atoms will convert into H2O, and the C atoms will exist as solid-state C. For the CHNO explosives, the detonation velocity () and detonation pressure () were estimated by Kamlet-Jacobs equation (9) and (10)[33].

3 RESULTS AND DISCUSSION

3.1 Heats of formation

It is well-known that evaluation for detonation performance of energetic materials requires the knowledge of HOFs. Moreover, HOFs are of great importance for researchers involved in thermal chemistry. Higher are the molecular HOFs, more energy stores the molecule. However, to obtain HOFs via experimental methods is extremely hazar- dous and difficult, so, theoretical studies is parti- cularly important and quite necessary. In our work, an atomization reaction was applied to calculate the HOFs of title compounds. Table 1 presented the calculated total energies, the zero- point energies, the values of thermal correction, and HOFs of azetidine derivatives at the B3PW91/6-311+G(d,p)//MP2/ 6-311+G(d,p) level.

Table 1. Total Energy (E0, a.u.), Zero-point Energy (ZPE, a.u.), Heats of Formation (HOF, kJ/mol) and Thermal Correction (∆HT, a.u.) Calculated at B3PW91/6-311+G(d,p)//MP2/6-311+G(d,p) Level

Inspecting the values of HOFs, we found that all azetidine derivatives have high positive HOFs, which become larger as the numbers of nitroso group increased. Obviously, the contributions of nitroso group on HOFs of azetidine derivatives meet the group additivity rule. The higher the molecular HOFs, the more the molecule stores energy. Furthermore, for isomers, such as 1 and 2, it is found that the HOF value of 1 is higher than 2 about 40kJ/mol, which indicated that the thermochemical stability of 2 is better than 1. The reason is the steric interaction is weaker in 2 than that in 1, which increases the molecular stability compared to 1. For 3 and 4, the HOF value of 3 is higher than 4 about 2kJ/mol, which is caused by the steric hindrance of nitroso groups. The nitroso groups in 3 are closer each other than that in 4 insulting in the steric hindrance effect which raised HOF value of 3, meanwhile, the molecule stability is dropped down. Although high HOFs is negative for thermal stability, it can improve detonation energy. In addition, we also should notice that HOFs value in this paper are the superior limit in practice for their gas-phases state, as pointed out by Politzer and cooperators[34].

3.2 Bond dissociation energies

The sensitivity and stability of energetic com- pounds are directly relevant to the bond strength, which is commonly described by BDE[32,35,36]. In general, stronger are the weakest bond, more stable is the energetic material. Bond order reflect the electron accumulations in bonding region, and they can provide us with detailed information about the chemical bond. That is to say that the less bond order a bond has, the easier the bond breaks. In previous papers, the method had been applied successfully[37-39]. So the weakest bond order is selected as the trigger bond on the basis of the results of populations analysis[40]in this paper. The bond order and bond dissociation energies of trigger bond at B3PW91/6-311+G(d,p) level are listed in Table 2.

It can be found from Table 2 that the BDE values without zero point energy correction are larger than BDEZPE(including zero point energies correction). However, the sequence of dissociation energies are not affected by the zero-point energies. It should be pointed out that BDEs of the trigger bonds in all these azetidine derivatives are lower than that of TATB (276.93kJ/mol). However, their BDEZPEvalues are over 120kJ/mol, which means these compounds sufficient stability request of explosives. The BDEZPEof 2 is larger than 1, which indicate 2 is more stable than 1 kinetically consistent with the results from HOFs calculations in last section. For 3 and 4, the BDEZPEof 4 is larger than 3 about 10 kJ/mol. This is because that all atoms except hydrogen atoms of 4 are in the same plane, as well as strong conjugate action exists in 4 system. The correlation between the bond orders and the bond dissociation energies are positive from 1 to 5. However,6 has lower bond order but higher BDEZPEvalue compared to others. This shows that the kinetic stability of azetidine derivatives is not determined simply by the bond orders, and the BDEZPEmust be considered. Furthermore, on the consideration of the unobservable in experiment of bond order, the prediction based on bond dissocia- tion energies is more reliable.

Table 2. Bond Dissociation Energies (kJ/mol) and Bond Orders of N−NO2 Calculated at the B3PW91/6-311+G(d,p)//MP2/6-311+G(d,p) Level

3.3 Detonation performance

Detonation velocity and detonation pressure are two important performance parameters for high energetic compounds. Several empirical methods have been applied to estimate these parameters[41], although the errors of density lead toandsomewhat deviating from experiments[42]. The Kamlet-Jacobs equation has been proved to be reliable[43]and used in this paper. The molecular density, heats of detonation, detonation velocity and detonation pressure were listed in Table 3 with that of RDX and HMX for comparison.

Table 3. Molecular Density (g/cm3), Explosive Heats (kJ/g), Detonation Pressure (GPa) and Detonation Velocity (km/s) of Azetidine Derivatives Together with RDX and HMX

As is evident in Table 3, theof azetidine deriva- tives rises as introduction of substituent groups. The maximal and leastvalue are 1.93 and 1.47 g/cm3, respectively. Inspecting thevalues of 1 and 2, it is found that thevalue of 1 is close to that of 2. As the introduction of nitroso groups, the molecular density are rising from 1.79 to 2.39, which is better than that of HMX. As we known, a molecular density better than 2.0g/cm3is desirable until now in the field of explosive, therefore, the molecule 6 is glamorous from the viewpoint of density. Forand, we find that the detonation performances of 5 and 6 are better than RDX, especially 6 compound, theandhave over HMX, one of the most widely used energetic compound in the field of high-performance explosive. Although 3 and 4 also have good detona- tion performance, they can not meet the requirement as HEDCs (= 9.0 km/s and= 40 GPa). In addition, 1 and 2 have the worstandin all compounds, and further research is not reasonable. 1,3,3-Trinitroazetidine (TNAZ) is a well charac- terized high energy density compound, and the explosive heat, the molecular density, the detonation velocity and the detonation pressure are 1160J/g, 1.84g/cm3, 9.60km/s, and 36.4GPa, respectively[44]. Compared to TNAZ, the tri-substituted derivative 5 and tetra-substituted derivative 6 have better deto- nation characters and others are inferior. Evidently, NO group is more effective to improve the mole- cular density than nitro group, because dini- troso-substituted derivatives have larger density than trinitro-substituted derivatives. On the consideration the importance of molecular density for explosive characters, nitroso group is a better energetic group than nitro group. In summary, 5 and 6 are recom- mended as candidates of HEDCs in this study.

4 CONCLUSION

Based on our calculations, it is shown that all azetidine derivatives possess large positive HOFs, which increase with the introduction of nitroso groups. The predicted detonation velocities and detonation pressures indicate that nitroso group is an effective substituent group to enhancing detonation performance. Particularly 5 (= 9.36km/s and= 44.42GPa) and 6 (= 10.80km/s and= 60.70GPa) may be the promising candidates of HEDCs.

The kinetic stability and pyrolysis mechanism were evaluated by using bond dissociation energies. For azetidine derivatives, the homolysis of C−N is the initial step in explosion reactions. Moreover, the BDEs of all molecules are over 120kJ/mol, which meet the criterion of HEDCs. The steric hindrance has also influence on the molecular stability.

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17 June 2019;

11 September 2019

① This work was supported by the Natural Science Foundation of Guizhou Province (Nos. QKHPTRC[2018]5778-09 and QKHJC[2020]1Y038) and the Natural Science Foundation of Guizhou Education University (Nos. 14BS017 and 2019ZD001).

. Li Bu-Tong (1977-). E-mail: libutong@hotmail.com

10.14102/j.cnki.0254–5861.2011–2501

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