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J. Phys. Chem. B 2005, 109, 8978-8982
Investigation of Correlation between Impact Sensitivities and Nitro Group Charges in Nitro Compounds Chaoyang Zhang,* Yuanjie Shu, Yigang Huang, Xiaodong Zhao, and Haishan Dong Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-311, 621900 Mianyang, Sichuan, PR China ReceiVed: March 10, 2005
A new method of calculating the Mulliken net charges of the nitro group, QNO2, to assess impact sensitivities for nitro compounds is established. All calculations including optimizations and Mulliken population and frequency analyses are performed by density functional theory (DFT) and the general gradient approximation (GGA) method in Acceryls’ code Dmol3 with the Beck-LYP hybrid functional and the DNP basis set. As a result, the charges on nitro group can be regarded as a structural parameter to estimate the impact sensitivity on the bond strength, oxygen balance, and molecular electrostatic potential. The compound with more -QNO2 will be insensitive and gives a large value of impact sensitivity H50. This method considering the molecular structure is applicable for almost all nitro compounds when the C-NO2, N-NO2, or O-NO2 bond is the weakest in the molecule. According to the results in this paper, the compounds with -QNO2 >0.23e show H50 e 0.4 m.
1. Introduction Today, people pay more attention to the security of armament including explosives, so the relationship between the sensitivity and molecular structure of high-energy-density materials (HEDM) has become an important subject. The “trigger linkage” is a common but very important concept that considers that bond breaking is the key step in detonation initiation. Many researchers believe that C-NO2, N-NO2, and O-NO2 bonds are trigger spots in nitro explosives. Depluech and Cherville proposed that shock wave and thermal sensitivities in nitro compounds could be related to the electronic structure of molecules and properties of C-NO2, N-NO2, and O-NO2 bonds such as electrostatic potentials, lengths, strengths, and so forth.1-3 Xiao suggested that the stronger these bonds are, the more stable the molecules are.4 However, Bates thought that the sensitivity of tetrazole is related to the electron-attraction ability of substitution groups. The stronger the electron-attraction ability is, the more sensitive the compound is.5 Kamlet and Adolph concluded that the impact sensitivities of some nitro compounds increase when their oxygen balances are enhanced.6 Politzer concluded that sensitivity must be related to the electrostatic potential on the molecular surface, and more detailed overviews are presented in his paper.7 Nevertheless, these criteria and correlation methods are valid only for special cases or usually have certain limits. In addition, Zeman’s work is noticeable for its correlation between the characteristics of impact and electric spark sensitivities, detonation and thermal decomposition, and 13C and 15N NMR chemical shifts of polynitro compounds.8 Our recent work presents another way to assess the molecular stabilities of some nitro compounds by computing the electron-attraction ability of nitro group denoted by τ in eq 1, which is the sum of the Mulliken population of the nitro group. The larger the τ is, the less sensitive the compound is.9
τ)
1
∑ P + 2N,O,O ∑ Pi N,O,O
(1)
* Corresponding author. E-mail:
[email protected]. Tel: +86-816-2494904. Fax: +86-816-2281339.
Having made many calculations, we find that the relationship between the impact sensitivities and electronic structures of some nitro compounds can be established by the charge analysis of the nitro group, and we try to extend this method to all nitrocontaining explosive compounds. Today, nitro compounds are still the main and most important explosives containing C, H, N, and O, so this study may be instructive to the security and the design of HEDM. 2. Theory and Computational Details In the chemical world, there is a common rule that a state with higher potential energy will transform to a state with lower potential energy automatically, and the system becomes more stable. For example, an oxidant with a lower standard electrode potential can easily react with a reductant with a higher electrode potential, and the total system becomes stable. The situation is similar for covalent compounds. Covalent compounds are composed of atoms or groups that show different abilities to donate or accept electrons. Hydrides such as HF, H2O, NH3, and CH4 are taken as candidates in the investigation, and their calculation results are listed in Table 1. In these hydrides, hydrogen atoms are electron donors. The bond dissociation energy BDE, Pauling electronegativity χ of central atoms (F, O, N and C atoms), and Mulliken atomic net charges QH on hydrogen atoms show consistent decreasing sequences for these hydrides (HF > H2O > NH3 > CH4), but their bond length shows the reverse order (HF < H2O < NH3 < CH4). In fact, people usually use these parameters to estimate the molecular stability. It has been found that the hydrogen atom in different hydrides has a different ability to donate electrons, so we can compare the stabilities of hydrides not only by the electronegativity of the central atom and the bond strength but also by charges on common atoms in hydrides (i.e., the hydrogen atoms). The hydride will become more stable when its hydrogen atom donates more electrons and has more positive charges. Here, the hydrogen atom’s ability to donate electrons is represented by its Mulliken atomic net charges. Namely, the more positive charges the hydrogen atom has, the lower the ability to donate an electron and therefore the more stable the hydride becomes.
10.1021/jp0512309 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/06/2005
Sensitivities, NO2 Group Charges in NO2 Compounds
J. Phys. Chem. B, Vol. 109, No. 18, 2005 8979
TABLE 1: Properties of Some Hydrides RHn hydrides
HF
H2O
NH3
CH4
BDE(298 K), kJ/mol H-R bond length, Å χ QH, e
557.16 0.925 3.98 0.358
480.60 0.965 3.44 0.305
434.08 1.018 3.04 0.239
433.47 1.091 2.55 0.118
As for nitro groups in nitro compounds, they are very strong electron acceptors. Namely, they have a strong ability to attract electrons, the same as the ability of hydrogen atoms in the above-mentioned hydrides. This ability can also be represented by the net charges of the nitro group. We can draw a conclusion similar to that described above: the more negative charges the nitro group possesses, the lower the electron-attraction ability and therefore the more stable the nitro compound becomes. In
nitro-containing covalent compounds, C-NO2, N-NO2, and O-NO2 bonds denoted by the R-NO2 bond are usually the weakest in the molecule, and their breaking is the initial steps in the decomposition or detonation. For a polynitro compound, the nitro group with the lowest negative charges will be discussed in the following text. Moreover, the impact sensitivity is used to show the stability of the nitro compound and is measured by the height H50, from where a given weight falling upon the compound gives a 50% probability of initiating an explosion. The shorter this drop height, the greater the sensitivity. To find the relationship between the nitro group charges and the impact sensitivity of the nitro compound, the Mulliken net charges of the nitro group and the length of the R-NO2 bond were investigated theoretically. The normal mode vibration analysis for each molecular structure gives six zero frequencies
TABLE 2: Properties of Nitrobenzenes Cited from Reference 11
a Location and Q NO2 are the location and charges of the nitro group whose negative charges are the lowest within the molecule. Data of H50 and Vmid,max (molecular electrostatic potential) are cited from ref 11, and LB,max is the maximum C-NO2 bond length in the molecule.
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Figure 1. Relationships between Vmid,max, -QNO2 and LB,max, H50 of nitrobenzenes.
and no imaginary frequencies for the remaining 3N - 6 vibrational degrees of freedom, where N is the number of atoms in the system. This indicates that the structure of each molecule corresponds to a local minimum on the potential energy surface, and these were calculated by Acceryls’ code Dmol3 using the general gradient approximation method (GGA) and the BeckLYP hybrid functional with the DNP (double-numeric-quality basis with polarization functions) basis set.10 The nitro group charge QNO2 is calculated by eq 2, where QN, QO1, and QO2 are the net charges on the N and O atoms in the nitro group, respectively.
QNO2 ) QN + QO1 + QO2
(2)
3. Results and Discussion 3.1. Nitrobenzenes. Structure-sensitivity relationships of nitrobenzenes have been studied by many scientists, and the work of Kamlet, Politzer, and Owen is famous and widely accepted. Here, the nitrobenzenes in ref 11 and 12 are taken as the objects of our investigation.
To show the tendencies or relationships more clearly between Vmid,max, QNO2 and LB,max, H50, we convert the data in Table 2 into Figure 1, showing the tendencies on the top: the higher -QNO2, the lower Vmid,max, the lower LB,max, and the greater H50. To a certain extent, the bond strength can be approximately represented by the bond length. Particularly, -QNO2 and Vmid,max are discussed here. Along with decrease of the impact sensitivities of the nitrobenzenes, there is an increase in -QNO2 but a decrease in Vmid,max. There exist extreme points in the curves of -QNO2 and Vmid,max at the same locations for compounds M4, M5, M16, M17, and M20 shown in the lower part of Figure 1, which are hydroxy or amino nitrobenzenes. For hydroxy nitrobenzenes, one of the explanations of these extreme points must be due to the formation of unstable nitronic acid tautomers (i.e., a minor part of nitro-phenols (NP) can change into nitronic acids (NA) as shown in eq 3, which have higher total energies and higher sensitivities). Two isomers of PA and TNAP (Figure 2) were calculated, and they give evidence for the above conclusion from the data of the total energy: PA, -921.2358675 hartree (NP) and -921.1879466 hartree (NA); TNAP, -976.5969929 hartree (NP) and -976.5654405 hartree (NA). Therefore, it is no surprise that we find these exceptions, when -QNO2 is calculated according to the case of NP, whereas H50 is that for NA. Another explanation comes from the fact that the bond orders of C-OH bonds are usually less than those of C-NO2 bonds, that is, not the C-NO2 bond but the C-OH bond is the weakest in the molecule (Table 3).4 For amino nitrobenzenes, their low -QNO2 and high H50 must be due to their resonance structures, which can diminish the sensitivities.
It can be concluded from the above discussion that there is a relationship between the Mulliken charges of the nitro group and the impact sensitivity of nitrobenzenes. The nitrobenzene compound with low -QNO2 shows low H50, when the C-NO2 bond is the weakest and its breaking is the primary step in decomposition. It plays the same role in assessing the impact sensitivity as the molecular electrostatic potentials introduced by Murray and Politzer.11 The relationship between the C-NO2 bond dissociation energy (BDE) and impact sensitivity (H50) was discussed in Rice’s work.12 Here, the compounds in Figure 2 are taken as another group to study, even though some of them are mentioned above. Correlations between the impact sensitivity and chemical composition reported earlier by Kamlet and Adoplph are
Figure 2. Nitrobenzenes cited from ref 12. The nitro groups indicated in the circles have the lowest -QNO2 values.
Sensitivities, NO2 Group Charges in NO2 Compounds
J. Phys. Chem. B, Vol. 109, No. 18, 2005 8981
TABLE 3: Bond Orders of C-NO2 (BOC-N) and C-OH Bonds (BOC-O) in Hydroxy Nitrobenzenes at the CNDO/2 Level
TABLE 4: Properties of Nitrobenzenes Cited from Reference 12 compounds
HNB
PNA
TETNB
TETNA
PA
TNB
TNAP
TBN
TNT
TNA
DATB
TATB
H50, m BDE, kcal/mol QNO2, e RR-NO2, Å OB(CO),%[6]
0.12 50.1 -0.134 1.466 3.45
0.15 47.1 -0.135 1.473 1.89
0.27 50.3 -0.162 1.506 0.78
0.41 48.1 -0.163 1.511 0.37
0.87 60.1 -0.254 1.504 -0.44
1.00 64.0 -0.228 1.465 -1.41
1.38 62.8 -0.288 1.476 -0.82
1.40 57.0 -0.231 1.516 -1.68
1.60 58.9 -0.249 1.463 -3.08
1.77 66.5 -0.289 1.483 -1.75
3.20 69.2 -0.330 1.431 -2.06
4.90 69.4 -0.416 1.408 -2.33
probably the most widely applied in the area of energetic materials research.6 The oxygen balances of nitrobenzenes listed in Table 4 are calculated from eq 4
OB100 )
100(2nO - nH - 2nC - 2nCOO) M
TABLE 5: Relevant Calculation Results of Nitro Substitutes of Methane
(4)
where nO, nH, and nN represent the numbers of atoms of the corresponding elements in the molecule. nCOO is the number of carboxyl groups and equals zero here because there are no carboxyl groups in these compounds, and M is the molecular weight. Similarly, Table 4 is changed into Figure 3 for intuition. Figure 3 verifies Kamlet’s suggestion that the lower the oxygen balance is, the lower the sensitivity becomes. However, for special cases such as TNAP and TNT there must be other decomposition mechanism. Figure 3 also shows a similar tendency for the cases of BDE and -QNO2. It seems that the dependence of the impact sensitivity upon the nitro group charge is more evident than that upon the bond dissociation energy because there are only two obvious extreme points at PA and TNAP on the curve. This can be explained in a way similar to that of hydroxy nitrobenzenes discussed above. Therefore, we can relate the impact sensitivity to the nitro group charges according to the investigation results of nitrobenzenes. The higher -QNO2, the larger H50. This means that -QNO2 can be regarded as another criterion for estimating the impact sensitivities of nitrobenzenes. 3.2. Other Explosive Molecules. Usually, the more substituted nitro groups the compound contains, the less stable it is.
Figure 3. Dependence of H50 upon -QNO2, OB, and LB,max for nitrobenzenes cited in ref 12.
For example, TNB, TETNB, and HNB have more nitro substituents and show higher sensitivities. Methane is regarded as another example in Table 5. It can be found that the increase in the number of substituted nitro groups will decrease -QNO2 and increase the R-NO2 bond length and OB. When the number of nitro groups increases, the acceptance of an electron by a nitro group becomes more and more difficult because fewer hydrogen atoms exist that donate electrons. Meanwhile, the nitro group’s electron-attraction ability gets higher and higher, and the compound becomes more and more unstable accordingly. Namely, as more nitro groups are competing for the available electronic charges, each can accept less negative charges.13 As a result, for nitro-substituted methane, QNO2 can also be used to assess the stabilities of compounds as RR-NO2 and OB. Two main nitric ester explosives PETN and NG, two main nitramine explosives RDX and HMX, and other explosives cited from refs 14 and 15 are depicted in Figure 4. It can be found that two nitric esters have positive nitro group charges due to the bonding between the nitro group and oxygen atom, which attracts electrons strongly. Apparently, it interferes with the electron attraction by the nitro group. Accordingly, the nitric esters are unstable and have smaller H50 values. Explosive molecules such as CL-20, RDX, HMX, 2#, and TNAZ show lower -QNO2, less than 0.12e, and higher impact sensitivities, H50 e 0.32 m, but other explosives such as FOX-7, LLM-105, NTO, and NQ show higher -QNO2, >0.26, and higher H50, >0.72 m. Therefore, for these explosive molecules, the fact that the molecule with high -QNO2 shows high H50 can be regarded as a rule, although there is not a monotonic relationship between -QNO2 and H50. For example, PETN has the smallest H50 but not the smallest -QNO2, and FOX-7 has the largest -QNO2 but not the largest H50. Also, -QNO2 can be used to predict the impact sensitivity for those compounds with -QNO2 values that are too small, such as ONC and TEX showing high impact sensitivities, but TNA and DAMN may be not too sensitive for their -QNO2 values that are not too small. Additionally, from the viewpoint of nitro group charges, we tentatively conclude that there is an increasing sensitivity order of C-NO2 > N-NO2
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Figure 4. Nitro group charges and impact sensitivities of other nitro compounds cited from refs 14 and 15.
Figure 5. Special cases for the rule.
> O-NO2 for nitro compounds in which those bonds are the weakest. Obviously, this has wide applications in estimating the impact sensitivity by the charges on the nitro group for almost all nitrocontaining molecules in which the C-NO2, N-NO2, or O-NO2 bond is the weakest. It is important to emphasize that the rule does not work when those bonds are not involved in the primary decomposition step. These exceptions include the hydroxy nitrobenzenes described above and the special cases shown in Figure 5. According to the rule of nitro group charges, there is an increasing order for H50 for compounds in Figure 5: b < d < a < c. It is clear that the order is not consistent with that from experiment. References 16 and 17 suggest that the loss of nitrogen (N1-N2) might conceivably be a possibility in the decomposition of 1-picryl isomers. However, for compounds c and d, the rule is valid. Moreover, compounds a and c and b and d should show equal sensitivity according to the rule of OB, but this is not true. It reveals its limits that only the chemical composition had been considered but not the structure, which is probably the main factor affecting the sensitivity. For compound e, it is very sensitive because the N1-O2 bond is the weakest and its rupture is the initial decomposition step, even though it has QNO2 ) -0.239e. 4. Conclusions The nitro group charges can be regarded as a structural parameter used to assess the impact sensitivity. The compound will have a high H50 when it has a high -QNO2, which is the same trend as that found for the bond length, oxygen balance, and molecular electrostatic potential. The method considering the molecular structure has wide applications in almost all nitro compounds when the C-NO2, N-NO2, or O-NO2 bond is the weakest in the entire molecule. According to the data in this paper (Tables 2 and 4, Figures 4 and 5), the compound may
have H50 e 0.4 m when its nitro group has lower negative charges than about 0.23e. Acknowledgment. We are very grateful for the financial support from CAEP (nos. 2002Z0501 and 42101030404). Note Added after ASAP Publication. This article was published on the Web on 4/6/2005. The author names are now presented as given name followed by surname. The corrected version appeared in print and on the Web on 5/5/2005. References and Notes (1) Delpuech, A.; Cherville, J. Proc. Symp. Chem. Probl. Connected Stab. Explos. 1976. (2) Delpuech, A.; Cherville, J. Propellants Explos. 1978, 3, 169. (3) Delpuech, A.; Cherville, J. Propellants Explos. 1979, 4, 121. (4) Heming, X. Molecular Orbital Theory of Nitro-compound; Publishing House of Defense Industry: Peking, 1994; (in Chinese). (5) Bates, L. R. Proc. 13th Symp. Explos. Protech. 1986. (6) Kamlet, M. J.; Adolph, H. G. Propellants, Explos., Pyrotech. 1979, 4, 30. (7) Computational Chemistry: ReViews of Current Trends; Leszczynski, J., Ed; World Scientific: River Edge, NJ, 1999; pp 271-286. (8) Zeman, S. In Energetic Materials; Politzer, P., Murray, J. S., Eds.; Elsevier: Amsterdam, 2003; Vol. 2, pp 25-52. (9) Zhang, C.; Shu, Y.; Huang, Y.; Zhao, X.; Wang, X.; Dong, H. Sci. Technol. Energ. Mater. 2004, 65, 191. (10) Material Studio3.0; Acceryls Inc, 2003. (11) Murray, J.; Politzer, P. Chemisty and Physics of Energetic Materials; Kluwer Academic Publishers: The Netherlands, 1990; pp 157-173. (12) Rice, B. M.; Sahu, S.; Owens, F. J. J. Mol. Struct: THEOCHEM 2002, 583, 69. (13) Murray, J.; Lane, P.; Politzer, P. Mol. Phys. 1998, 93, 187. (14) Dobratz, B. M.; Crawford, P. C. LLNL ExplosiVes Handbook: Properties of Chemical ExplosiVes and ExplosiVe Simulants; LLNL, 1985; pp 187-188, (UCRL-52997-Chg.2) (translated into Chinese). (15) Phillp, F. P.; Gregory, S. L.; Alexander, R. M.; Robert, D. S. Thermochim. Acta 2002, 384, 187. (16) Neuman, P. N. J. Heterocycl. Chem. 1989, 8, 51. (17) Storn, C. B.; Ryan, R. R.; Ritchie, J. P. J. Phys. Chem. 1989, 93, 1000.
