吡啶并环脲硝基衍生物结构和爆轰性能的量子化学研究

马丛明，侯可辉，刘祖亮，姚其正,3

(1.南京理工大学环境与生物工程学院，江苏南京210094; 2.南京理工大学化工学院，江苏南京210094；

3.中国药科大学药学院，江苏南京210009)



吡啶并环脲硝基衍生物结构和爆轰性能的量子化学研究

马丛明1，侯可辉2，刘祖亮2，姚其正2,3

(1.南京理工大学环境与生物工程学院，江苏南京210094; 2.南京理工大学化工学院，江苏南京210094；

3.中国药科大学药学院，江苏南京210009)

摘要：采用Guassian03程序，在DFT-B3LYP/6-31G**水平下得到吡啶并环脲硝基衍生物的分子几何构型、电子结构、理论密度和生成热，采用Kamlet-Jacobs方程计算了爆速和爆压值。结果表明，化合物1,3,5,7-四硝基-5,7-二氢二咪唑[4,5-b:4′,5′-e]吡啶-2,6(1H,3H)-二酮和8-氨基-1,3,5,7-四硝基-5,7-二氢二咪唑[4,5-b:4′,5′-e]吡啶-2,6(1H,3H)-二酮具有良好的爆轰性能，但化合物1,3,5,7-四硝基-2,6-二氧杂-1,2,3,5,6,7-六氢二咪唑[4,5-b:4′,5′-e]吡啶-4-氧化物、1,3,5,7,8-五硝基-2,6-二氧杂-1,2,3,5,6,7-六氢二咪唑[4,5-b:4′,5′-e]吡啶-4-氧化物(8)和8-氨基-1,3,5,7-四硝基-2,6-二氧杂-1,2,3,5,6,7-六氢二咪唑[4,5-b:4′,5′-e]吡啶-4-氧化物的结构不稳定。分子的对称性、空间位阻和氢键是影响分子稳定性的3个主要因素。

关键词：量子化学; 爆轰性能; 环脲硝胺; 吡啶环；硝基衍生物

Introduction

Energetic materials (explosives, propellants andpyrotechnics) are used extensively for civil as well as military applications. Today the variety and number of high energy materials for various applications become innumerable; synthesis, properties and other salient features are available in the literature[1-4]. There are strong requirements for explosives with good thermal stability, impact and shock insensitivity and better performance. However, explosives having good thermal stability and impact insensitivity usually exhibit poorer explosive performance. Therefore, the foremost objective at the stage of synthesizing new explosives consists of finding the molecule having both a good energy capability and optimal safety to those in current use.

Many studies reveal that pyridine ring based compounds have attracted renewed attention, and the potential use of nitro derivatives of pyridines and their bicyclic analogues have been reported for the synthesis of novel insensitive explosives[5-8]. The introduction of the alternate amino and nitro group in the pyridine ring system may increase the insensitivity of the parent molecule. Further increase in density and the thermal stability of the parent compound could be achieved by converting the tertiary amines into their corresponding N-oxide functionality.

At present time, cyclourea nitramines increasingly gain importance as perspective and highly energetic materials. And an alternative approach to increase the performance of cyclourea nitramines involves incorporating a carbonyl group in place of methylene group between two nitramines to generate a dinitrourea. Several mono- and dinitroureas have been synthesized as energetic materials and have attractive densities and predicted performance.

To date, information of the relationship between structure and property of nitro derivatives of pyrido-dicycloureas is very spare, and few systematic surveys are conducted to cover these compounds. In the present study, we report a systematic study on the density, heats of formation (HOF), thermal stability, and energetic properties of nitro derivatives of pyrido-dicycloureas by using density functional theory (DFT) method. Detonation velocities and pressures were predicted using the calculated HOF and densities. All energetic materials related were based on the concept of new nitro derivatives of pyrido-dicycloureas. So reacting with urea, 2,3,5,6-tetraaminepyridine yields a precursor with two five-membered rings, which provides more N-H sites for introducing nitro substituents, and thus generates a series of new energetic materials. These results provide theoretical support for molecular design of novel high energetic density compounds (HEDC).

1Computational methods

Calculations were carried out by using the Gaussian 09

program suite[9]. The geometry optimization of the structures and frequency analyses were carried out by using the B3LYP functional with the 6-31G** basis set[10]. All of the optimized structures were characterized to be true local energy minima on the potential-energy surface without imaginary frequencies. The gas phase heats of formation (HOF) of model molecules were calculated adopting isodesmic reactions[11].

Detonation velocity and pressure are the most important parameters for evaluating detonation characteristics of energetic materials. For the explosives with CHNO elements, the Kamlet and Jacob empirical equations were used to determine these

parameters[12-13]:

p=1.558NM1/2Q1/2ρ2

(1)

D=1.01(NM1/2Q1/2)1/2(1+1.30ρ)

(2)

wherepis detonation pressure (GPa);Dis detonation velocity (km·s-1);Nis the number of moles of gaseous detonation products per gram of explosive;Mis the average molecular mass of the gaseous products;Qis the energy of explosion (J/g) of explosive andρis the crystal density (g/cm3);N,MandQare decided according to the largest exothermic principle, i.e., for the explosives with CHNO elements, all the N atom convert into N2, the O atom forms H2O with H atom first and the remainder forms CO2with C atom. The remainder of C atom will exist in solid state if O atom does not satisfy full oxidation of C atom. The remainder of O atom will exist in O2if O atom is superfluous.

Table 1 presents the methods for calculating theN,M, andQparameters of the CaHbOcNad explosives[14].

Table 1　Methods for calculating theN,MandQparameters of the CaHbOcNadexplosives

2Results and Discussions

2.1Optimized structures

Fig.1 is the molecular structures of the ten title compounds. At the outset, we have performed structure optimizations of molecules 1-10 at the B3LYP/6-31G** level, and selected optimized bond lengths of nitro derivatives of pyrido-dicycloureas are tabulated in table 2, and corresponding dihedral angles are listed in table 3.

Investigating the optimized geometries, variations (i.e. the differences between the maximum and minimum values) of the calculated results for the C-C, C-N, and N-N bond lengths and N-C-N, C-N-C, and C-N-N angles are much more different from all title compounds, indicating that these geometrical parameters are more sensitive to the environmental or molecular structures. Comparing to N11-N15 bond of molecule 1, which is 0.140 nm and can be seen as a original bond length with an introduction of one nitro group, N11-NO2, N13-NO2bond lengths of molecule 2 are 0.142, 0.145 nm, respectively, N9-NO2, N13-NO2bond lengths of molecule 3 are 0.141, 0.143 nm, and N11-NO2, N7-NO2are 0.141, 0.141 nm. It is of significance that two N-N bond lengths of molecules 2, 3 are different, while the same in molecule 4, and the values, showing that introduction of nitro group is the main energy origin of the series, and HOFs of molecules increase when the number of the nitro group increases, which may be attributed to repulsion of the nitro groups. For molecules 2, 3, and 4, it seems that the greater the steric hindrance is, the greater the HOF is, with two nitro groups attaching to pyrido-dicycloureas. For molecules 6 and 9, it can be seen that the HOF of substituted amino pyridine is higher than the corresponding pyridine, which indicates that -NH2group also improves HOF effectively. The result reveals that both nitro and amino groups are effective substituents for increasing the HOF of the nitro derivatives of pyrido-dicycloureas.

Fig.1　The molecular structures of the ten title compounds

CompoundBondBondlength/nmCompoundBondBondlength/nm1N11-N150.1406N9-N150.1431N7-C80.1406N7-N180.1461C4-N110.1416N11-N210.1431N11-C120.1456N13-N240.1461C8-O100.1216N11-C120.1442N11-C120.1447N11-N160.1432N13-C120.1427N9-N220.1432N13-N180.1457N9-C80.1442N11-N150.1427N13-N250.1522C4-N110.1417N7-N190.1523N9-C80.1458N9-C80.1443N13-C120.1448N9-N220.1493N11-C120.1398N7-N250.1523N13-N150.1438N11-N280.1493N9-N180.1418N13-N160.1514N11-N180.1419N11-N220.1434N11-C120.1459N11-C120.1454N9-C80.1459N13-N250.1464N9-N150.1419N9-N190.1434N9-C10.1419N7-N160.1465N11-N150.14210N9-N170.1445N11-C120.14410N11-N230.1445N7-C80.14410N13-N260.1465N13-N210.14510N16-C30.1355N7-N180.14410N7-N200.153

Table 3　Dihedral anglesa of cyclourea nitramine compounds

2.2Detonation performance

The detonation velocity (D) and detonation pressure (p) of molecules are computed by Kamlet-Jacobs empirical equations on the basis of their theoretical densities (ρ) and calculated gas phase heats of formation, which are the important parameters to evaluate performances of explosion of energetic materials. Table 4 shows the predicted detonation properties of nitro derivatives of pyrido-dicycloureas.

Table 4　Predicted detonation properties of cyclourea

It can be found that all nitro derivatives of pyrido-dicycloureas have good detonation properties. Considering of the results that reveal the existence probability of compounds 7, 8, 10, the calculated detonation velocities of compounds 1, 2, 3, 4, 5, 6, 9 are 6.49, 7.50, 7.49, 7.48, 8.23, 8.81, 8.78 km/s, respectively. The calculated detonation pressures of compounds 1, 2, 3, 4, 5, 6, 9 are 18.77, 25.57, 25.52, 25.55, 31.43, 36.70, 36.20 GPa, respectively. So molecule 6 is calculated to have the highestDandpvalues among cyclourea nitramine compounds, and a replacement of hydrogen atom of pyridine ring by amino groups bringsDandpvalues a little down in molecule 9. Meanwhile, with the number of the nitro group increasing from one to five,ρ,Q,D, andpvalues of the corresponding compounds increase. Fig. 2 shows the relationship betweenρ,V,D,pvalues and the number of nitro groups (n), indicating thatρ,V,ppresent a good linear relationship withn, butDdoes whennis equal to 2, 3 and 4 (n=1, representative of compound 1;n=2, representative of compound 2;n=3, representative of compound 5;n=4, representative of compound 6). This phenomenon gives a curtain account for the importance of the numbers of nitro groups in increasing detonation properties. The above predictions indicate that the molecules e.g. 6 and 9 are appearing to be the most promising candidates.

2.3Thermal stability

Energies (a.u.) of frontier molecular orbital and their gaps (ΔELUMO-HOMO) of the nitro derivatives of pyrido-dicycloureas at B3LYP/6-31G** level are listed in Table 5. It is seen that the ΔELUMO-HOMOvalues are different from different positions of substituted groups, and molecule 6 has the largest value of 0.14762 a.u., while molecule 9 has the smallest of 0.11486 a.u., despite of the three unstable structures 7, 8 and 10, which can easily draw a conclusion from the too small values of 0.04651, 0.06466, 0.04786 a.u., respectively, and in constitution with the data from optimized structures. The results also reveal a phenomenon that thermal stability of a molecule might be mostly affected by the combination of molecular symmetry and the obvious steric hindrance. With an incorporation of one nitro group into the cyclourea structure, molecule 1 has a value of 0.13121, which can be seen as a model molecule. When two nitro groups are attached, molecules 2, 3, and 4 have values of 0.12272, 0.13616, and 0.14570 a.u., respectively. When four nitro groups are introduced symmetrically, it comes to a maximum value in molecule 6.

Fig.2　Correlations between ρ, V, D, p and n for cycloureanitramine compounds

CompoundsE/(a.u.)HOMOLUMOΔELUMO-HOMO/(a.u.)1-0.23330-0.102090.131212-0.25829-0.135570.122723-0.26198-0.125820.136164-0.26402-0.118320.145705-0.28729-0.148430.138866-0.31064-0.163020.147627-0.28950-0.242990.046518-0.29903-0.234370.064669-0.27540-0.160540.1148610-0.25152-0.203660.04786

However, an introduction of the amino group makes ΔEUMO-HOMOvalue decrease, similar phenomenon occurs in molecules 6 and 9, in which N-N bond lengths are 0.143, 0.143, 0.146, 0.146 and 0.143, 0.143, 0.146, 0.146 nm, respectively, indicating that the molecular symmetry plays an important role in molecule stability.

It appears that some N-NO2bonds break in molecules 7, 8, 10, after a formation of pyridine N-oxide. Among three molecules, four N-NO2groups in molecule 8 all break, which attributes to the obvious steric hindrance effect. The result also shows that the high instability of cyclic dinitrourea in the pyridine N-oxide structure. The dihedral angles of the pyridine ring are almost zero and six atoms can be considered as nearly coplanar. With nitro groups introduced, N-NO2should rotate by some degree from the pyridine ring to avoid too large steric effect.

The hydrogen bonds between neighboring nonbonding atoms in cyclourea nitramine compounds are also investigated. As pointed out[15], in a moderate X-H……Y hydrogen bonding (H-bonding) system, normal Y……H separations are in the range of 0.15-0.22nm, while the separations betweenXandYare within 0.25-0.32nm. Hence, according to these criteria, there are moderate intramolecular H-bonding between neighboring O and H in molecules. For molecule 9 and 10, interatomic distance of O-H is more or less 0.20nm, which is substantially shorter than the sum of van der Waals radii and is known to be a typical distance for N-H……O hydrogen bond. There is no doubt that it is for this reason that molecule 10 can be more stable than 7 and 8 after introducing four nitro groups, one amino group and N-oxide, with only one nitro group breaking down. As a consequence, molecular symmetry, steric hindrance and hydrogen bonds are three main factors in contribution to molecular stability.

2.4Density and oxygen balance

In the present study, single-point molecular volume calculations at B3LYP/6-31G** level were performed based on geometry optimized structures. The densities and oxygen balance were calculated and listed in Table 6. As for the urea moiety has an inherent high molecular density, all mono- and nitrourea compounds containing the pyridine ring have attractive molecular densities[1], suggesting that they will make excellent candidates as highly energetic materials.

Table 6　Predicted densities and detonation properties

Table 6 predicted densities and oxygen balance of cyclourea nitramine compounds.The cyclourea nitramine compounds with different numbers or positions of nitro groups have different ρ values ranging from 1.81-2.08g/cm3, respectively. The calculated results indicate that compound 8 has the largest density bearing five nitro groups and N-O functionality, while 1 has the smallest density. However, compound 8 is not stable due to the most obvious steric hindrance effect, neither do 7 and 10. As a result, compound 6 has the largest density of 2.01g/cm3among all nitro derivatives of pyrido-dicycloureas, which will be a novel potential candidate for HEDC when it is successfully synthesized. It is clear that the density increases as an introduction of nitro groups, and there is not much difference with the same amount of nitro groups in different positions in the nitro derivatives of pyrido-dicycloureas. Besides, density is an essential factor in determining detonation properties of energetic materials, and detonation velocity increases with the increasing of density.

It is found from Table 6 that all title compounds have a negative oxygen balance, and when the amount of nitro group increases, the oxygen balance is close to zero, ignoring of three unstable compounds 7, 8, 10, proving that the nitro group is a good substituent for improving oxygen balance in designing potential HEDC.

2.5Heats of formation

Heat of formation reflects to the nature of substituents, and high positive HOF is usually required for an effective energetic material. The zero point energies (ZPE), thermal correction to enthalpy (HT) and electronic energies calculated at B3LYP/6-31G** level for nitro derivatives of pyrido-dicycloureas are listed in Table 7. The result reveals that all HOFs of cyclourea nitramine compounds vary from negative to positive values mainly due to steric hindrance effect. The stability here refers to the chemical or photochemical processes with electron transfer or electron leap.

In nitro compounds,N-NO2bond is the weakest in the molecule and the rupture of this bond is the initial step in the decomposition or detonation. The property ofN-NO2bond, i.e., charge is used to show the relationship with the impact sensitivity of compounds, and may reflect the ability of -NO2attracting electrons[16-17]. In the present study, the charge on nitro group (-QNO2) is considered for its correlation to impact sensitivity.

QNO2=QN+QO1+QO2

(3)

The charge on nitro group (-QNO2) is calculated by the sum of atomic charges on nitrogen (QN) and oxygen (QO1andQO2) atoms in nitro group. Computed -QNO2values of molecules are presented in Table 8.

Table 7　Calculated electronic energies (E0), zero-point

Table 8　Computed nitro group charge (-QNO2) of molecules 1-6 and 9

The higher the -QNO2, the larger the impact insensitivity, and hence, -QNO2can be regarded as the criterion for estimating impact sensitivities. Based on -QNO2values in Table 7 and the frame structure of the pyrido-dicyclourea in Fig.3, -QNO2values at β position are higher than α position, indicating that -NO2at β position is more stable than α position, probably because of electron deficiency of the pyridine ring. However, with an introduction of the amino group, the -QNO2value of compound 9 falls down, probably due to the steric hindrance, which is consistent with the result based on ΔELUMO-HOMO. The above investigations provide important theoretic information for molecular design of novel high energetic density nitramine explosives containing pyridine ring.

Fig.3　The frame structure of the pyrido-dicyclourea

3Conclusions

(1)The full geometrical optimizations of nitro derivatives of pyrido-dicycloureas were performed using density functional theory at B3LYP/6-31G** level, without any symmetry restriction. The systematic structure property studies were performed on compounds 1-10 to achieve energetic performance for the first time.

(2)Calculaed results indicate that molecular symmetry, steric hindrance and hydrogen bonds were three main factors in contribution to molecular stability, and molecule 6 (ρ= 2.01g/cm3,D=8.81km/s) and 9 (ρ=1.98g/cm3,D=8.78km/s) performs well as energetic materials, and can nearly satisfy the quantitative criteria for the energy as HEDCs.

(3)These results provide theoretical support for molecular design of novel high energetic density compounds based on the successful experimental synthesis.

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