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Quantum chemical studies on EGDN and its monovalent ions
Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14 www.elsevier.com/locate/theochem
Quantum chemical studies on EGDN and its monovalent ions Lemi Tu¨rker* Department of Chemistry, Middle East Technical University, Ankara 06531, Turkey Received 2 August 2004; accepted 6 September 2004 Available online 8 January 2005
Abstract An explosive material, ethylene glycol dinitrate (EGDN) and its mono ionic forms have been subjected to semiempirical AM1 (UHF) and ab initio 6-31G (UHF) type quantum chemical analyses. The AM1 treatment reveals that the neutral and anionic forms are exothermic whereas the cation is endothermic. All of the systems are found to be stable (by AM1 and 6-31G methods) but the calculations indicate that plus and minus charge developments in AM1 treatment and minus charge development in 6-31G treatment cause drastic geometrical changes in the molecular structures resulting in bond elongation, possibly bond cleavage in esteric N–O bond. q 2005 Elsevier B.V. All rights reserved. Keywords: EGDN; Explosives; Nitro compounds; Ions; Detonation
1. Introduction The possibility of an explosive is for obvious reasons a matter of prime importance both to the manufacturer and the user. There is no evidence to suggest that the initial step in the decomposition of an explosive is in any way different from that of a non-explosive of a similar chemical class. An explosive reaction differs from the others only by the large free energy of the molecule, the rapidity of its release and the accelerating effects of the decomposition products which produce an explosive reaction [1]. Ethylene glycol dinitrate (EGDN) (C2H4N2O6) is the inorganic ester of ethylene glycol with nitric acid. It is an oxygen sufficient (neither oxygen negative nor positive) secondary type explosives [1,2]. It is one of the explosives having quite high power index (182, picric acid as the standard). It explodes producing no oxygen according to the following reaction [2] C2 H4 N2 O6 / 2CO2 C 2H2 O C N2 with the heat of explosion value of 6658 kJ/kg, thus it has very high rank among the nitro explosives [2]. EGDN have recently attracted the attention of scientists not because of * Tel.: C90 3122 103 244; fax: C90 3122 101 280. E-mail address: [email protected]. 0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.09.033
its explosive character but also because of some others [3–6]. EGDN like pentaerythriol tetranitrate (PETN) is of considerable interest in ammunition industry since it can be used in the sheet form or mixed with RDX to make Semtex [7], a plastic explosive. Crellin et al., performed some quantum chemical calculations (AM1 level) on the species involved in the reaction of Si(CH3)C 3 with EGDN [4]. In the present treatment, some quantum chemical calculations were performed (semiempirical and ab initio level of the theory) on EGDN as well as its univalent cation and anion forms.
2. Method The initial geometry optimizations of all the structures leading to energy minima were achieved by using molecular mechanics (MMC) type calculations followed by semiempirical AM1 self-consistent fields molecular orbital (SCF MO) method [8,9] at the unrestricted Hartree–Fock (UHF) level [10]. The AM1 geometry optimizations were obtained by the application of the steepest-descent method followed by conjugate gradient methods, Fletcher–Reeves and Polak–Ribiere, consecutively (convergence limit of 4.18!10K4 kJ/mol (0.0001 kcal/mol) and RMS gradient of 4.18!107 kJ/mmol (0.001 Kcal/Amol)). The initial geometry optimizations were fallowed by, ab initio
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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treatment at the level of 6-31G [10,11] (UHF type, convergence limit 10K5 Kcal/mol and RMS gradient of 10K3 Kcal/Amol) by using Polak-Ribiere technique. The Raffenetti integral format (cut-off 1.10K10 Hartree) was used for two-electron integral control. For the molecular orbital initial guess, the core-Hamiltonian option [11,12] was applied with involvement of six d-orbitals. All these computations were performed by using the Hyperchem (release 5.1) package program.
3. Results and discussion EGDN is a secondary type explosive. The secondary explosives are qualitatively different from the primary explosives with respect to their chemical structure as well as to their detonative behavior. The primary explosives immediately detonate when heat is added and a certain ignition temperature is reached without exhibiting any preceding nearly state deflagration [13]. They can be characterized as materials capable of a detonation but not of a deflagration. This behaviour may depend of course, not only on the chemical structure of the substance but also on
its physical state particularly density [13]. There exist mainly two mechanisms of detonation initiations: (i) initiation by means of heat addition (possible only in primary explosives); (ii) initiation through shock waves (possible for all explosives). Other mechanisms like radiation or chemical effects are fairly unimportant [13]. However, the propagation of detonation waves could be controlled by means of external magnetic and electrical fields [14]. The influence of applied fields on gaseous systems detonating at their threshold conditions has been observed years ago [14]. The influence of these fields on the explosive molecules might cause polarization or ionization, which may be involved by some means in the explosion mechanisms. Some explosives, like RDX and TNT, because of their ring shaped geometries are comparatively rigid molecules while nitrate esters (e.g. NG, EGDN, PETN) are not constrained in such a fashion and are ‘floppier’. These molecules have more conformational states and might be expected that this would leave the nitrate esters more susceptible to fragmentation. In the present study, firstly EGDN molecule and its mono ionic forms have been subjected to AM1 (UHF) type semiempirical quantum chemical analysis.
Fig. 1. The AM1 (UHF) geometry optimized structures and numbering scheme of EGDN species considered.
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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Table 1 Some bond lengths/distances (10K10 m, AM1 method) in EGDN species considered Neutral O(1)–N(2) O(1)–C(5) N(2)–O(3) N(2)–O(4) C(5)–C(6) C(5)–H(11) C(5)–H(12) C(6)–O(7) C(6)–H(13) C(6)–H(14) O(7)–N(8) N(8)–O(9) N(8)–O(10)
Cation 1.354 1.441 1.186 1.189 1.520 1.125 1.121 1.452 1.121 1.121 1.347 1.185 1.192
O(1)–N(2) O(1)–C(5) N(2)–O(3) N(2)–O(4) C(5)–C(6) C(5)–H(11) C(5)–H(12) C(6)–O(7) C(6)–H(13) C(6)–H(14) N(8)–O(9) N(8)–O(10) N(8)–O(7)
Anion 1.352 1.450 1.179 1.197 1.530 1.120 1.124 1.347 1.137 1.139 1.114 1.111 1.827
O(1)–C(5) C(5)–C(6) C(5)–H(11) C(5)–H(12) C(6)–O(7) C(6)–H(13) C(6)–H(14) O(7)–N(8) N(8)–O(9) N(8)–O(10) N(2)–O(3) N(2)–O(4) O(1)–N(2)
1.358 1.546 1.128 1.127 1.465 1.116 1.118 1.323 1.197 1.197 1.184 1.190 1.767
Fig. 2. The 6-31G (UHF) geometry optimized structures and numbering scheme of EGDN species considered.
Then, they have been the subject of 6-31G (UHF) type ab initio treatment. Fig. 1 shows the geometry optimized structures (AM1 (UHF)) and the numbering scheme of EGDN species. Table 1 includes some bond lengths/distances in these structures. The data reveal that as compared to the neutral structure of EGDN, O(7)–N(8) distance in the cationic form and O(1)–N(2) distance in the anionic form are quite long
indicative of bond rapture. Note that originally these bonds were ester oxygen-nitro (nitrogen) bonds. The ionic structures, geometry optimized at 6-31G (UHF) level of theory, are shown in Fig. 2. The ab initio method used (especially for the anionic case) predicts one of the esteric N–O bonds to be highly elongated (bond cleavage). Table 2 shows some of the bond lengths and distances calculated by the ab initio treatment.
Table 2 Some bond lengths/distances (10K10 m, 6-31G method) in univalent EGDN species considered
C(1)–C(2) C(1)–O(7) C(1)–H(11) C(1)–H(12) C(2)–H(13) C(2)–H(14) N(8)–O(9) N(4)–O(6) O(3)–N(4) N(8)–O(10) O(7)–N(8) N(4)–O(5) C(2)–O(3)
Neutral
Cation
Anion
1.511 1.454 1.077 1.077 1.077 1.077 1.236 1.241 1.382 1.241 1.382 1.236 1.454
1.513 1.438 1.077 1.077 1.076 1.076 1.237 1.372 1.289 1.241 1.398 1.230 1.584
1.499 1.425 1.082 1.081 1.075 1.075 1.255 1.247 1.369 1.257 4.823 1.246 1.499
12
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
Fig. 3. The 3D-electrostatic field maps (AM1 (UHF)) of EGDN species considered.
Fig. 4. The 3D-electrostatic field maps (6-31G (UHF)) of EGDN species considered.
Fig. 5. The 3D-charge density maps (AM1 (UHF)) of EGDN species considered.
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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Table 3 Some calculated energies (kJ/mol) of EGDN species (AM1 method)
Total Binding Isolated atomic Electronic Core-core interaction Heat of formation
Fig. 6. The 3D-spin density maps (AM1 (UHF)) of ionic EGDN species considered.
The three dimensional (obtained by AM1 and 6-31G methods) electrostatic field maps of the species considered are shown in Figs. 3 and 4, respectively. Since, the optimized geometries by the AM1 and 6-31G methods are different, the electrostatic field maps are different for the same ionic specie. The distributions of the excess charges in the case of ionic species are shown in charge density maps (AM1) in Fig. 5. Figs. 6 and 7 show spin densities of the ionic species calculated at the level of AM1 (UHF) and 6-31G (UHF) methods, respectively. Some calculated energies (AM1) of the species are given in Table 3. According to these data, these systems are stable (the total and binding energies) and with the exception of
Neutral
Cation
Anion
K25,4736 K4966 K249,770 K1,003,415 748,679 K223.03
K253,809 K4039 K249,770 K1,013,434 759,625 703.56
K254,957 K5187 K249,770 K966,403 711,446 K444.04
cationic case they are exothermic in nature. The anionic system appears to be the most stable and the cationic system to be the least stable of all. The presence of nitro groups due to electron withdrawing nature of them made the molecule highly electron demanding. Thus, the cation of EGDN becomes the least stable (leading to bond cleavage) while the anion is the most stable structure. Note that the structure for the anionic form also has an elongated O–N bond. Thus, in that geometry the system is stable but it does not mean that the molecule is intact. The ab initio method used predicts the stability order of the systems as anionic formO neutral formOcationic form. Whereas, the stability order based on the total energy with MP2 energy is neutral formO anionic formOcationic form. This order is very logical in the light of principles of classical organic chemistry. The presence of NO2 groups makes the anionic case to be more stable than the cationic form because NO2 groups are electron demanding. However, insertion of an electron into the nitro group of the neutral form causes formation of a radicalic anion and thus, repulsion with the lone-pairs of esteric oxygen atom resulting in highly elongated O–N bond. Whereas, removal of an electron from the neutral system to form the cationic case, generates a radical cation in which esteric O–N bond is less affected. Some energies Table 4 Some calculated energies (kJ/mol) of EGDN species (6-31G method)
Total Electronic Kinetic Energy eK, ee and eN Energy The Virial (KV/T) MP2 Correlation Energy Total energy with MP2 energy
Neutral
Cation
Anion
K1,668,353 1,669,330
K1,667,350 1,668,224
K1,668,600 1,669,549
K3,081,040 1.9994 K3271
K3,063,963 1.9995 K2783
K1,376,340 1.9994 K2913
K1,671,559
K1,669,967
K1,671,513
Table 5 The calculated (AM1) HOMO and LUMO energies of the EGDN species HOMO
Fig. 7. The 3D-spin density maps (6-31G (UHF)) of ionic EGDN species considered.
Neutral Cation Anion
LUMO
a-type
b-type
a-type
b-type
K20.4711 K25.1586 K7.6260
K20.4711 K24.6158 K7.7906
K0.7359 K12.3909 4.7014
K0.7359 K12.3910 4.7010
Energies in 10K19 J. All have A type symmetry.
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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Table 6 The HOMO and LUMO energies (6-31G) of monovalent EGDN ions a
Neutral Cation Anion
b
HOMO
LUMO
HOMO
LUMO
K20.5425 (AU) K21.3188 (BG) K26.0498 (A 00 ) K22.7286 (A 00 ) K8.4752 (A) K8.4747 (A)
4.1813 (BG) 1.6610 (BG) K7.3563 (A 00 ) K9.3794 (A 00 ) 9.3872 (A) 9.4889 (A)
K20.8217 (AU) K21.3188 (BG) K25.8471 (A 0 ) K24.7602 (A 0 ) K8.4771 (A) K8.4763 (A)
4.1021 (BG) 1.6610 (BG) K10.1909 (A 00 ) K12.7122 (A 00 ) 9.4482 (A) 9.3721 (A)
Energies in 10K19 J. The second entries in each case are for the values including MP2 correlation. Symmetries in parenthesis.
calculated at the level of 6-31G method are shown in Table 4. Note that the viral (KV/T) in every case is very close to 2. The HOMO and LUMO of the species considered are tabulated in Tables 5 and 6. The AM1 data shown in Table 5 indicate presence of degenerate a- and b-types of HOMO/ LUMO levels for the neutral EGDN. The ab initio data with and without MP2 correlation are shown in Table 6.
4. Conclusion The chemistry of explosives materials at the molecular level needs to be studied by means of quantum chemical methods to understand better the underlying phenomena behind the explosion reaction(s) for the considered explosive agent. However, one should keep in mind that an explosion is a dynamic process and the static methods can describe only a part of it.
References [1] J. Taylor, Solid propellent and exothermic compositions, George Newnes Ltd, London, 1959. [2] J. Akhavan, The Chemistry of Explosives, The Royal Society of Chemistry, Cambridge, 1998. [3] G.R. Asbury, J. Klasmeier, H.H. Hill Jr., Talanta 50 (6) (2000) 1291. [4] K.C. Crellin, N. Dalleska, J.L. Beauchamp, Int. J. Mass Spectrosc. Ion Processes 165/166 (1997) 641. [5] S. Kennedy, B. Caddy, J.M.F. Douse, J. Chromatogr. A 726 (1996) 211. [6] S.A. Forman, Toxicol. Lett. 43 (1–3) (1988) 51. [7] A. Fainberg, Science 255 (1992) 1531. [8] M.J.S. Dewar, E.G. Zoebish, E.F. Healey, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [9] K.B. Lipkowitz, D.B. Boyd, Rev. Comput. Chem., VCH, New York, 1990. [10] A.R. Leach, Molecular Modelling, Longman, Essex, 1997. [11] I.N. Levine, Quantum Chemistry, Prentice Hall, New Jersey, 2000. [12] Hyperchem Computational Chemistry, Hypercube, Canada, 1996. [13] H.D. Gruschka, F. Wecken, Gasdynamic Theory of Detonation, Gordon and Breach, New York, 1971. [14] M.A. Cook, The Science of High Explosives, Robert E. Krieger Pub, New York, 1971.
Quantum chemical studies on EGDN and its monovalent ions Lemi Tu¨rker* Department of Chemistry, Middle East Technical University, Ankara 06531, Turkey Received 2 August 2004; accepted 6 September 2004 Available online 8 January 2005
Abstract An explosive material, ethylene glycol dinitrate (EGDN) and its mono ionic forms have been subjected to semiempirical AM1 (UHF) and ab initio 6-31G (UHF) type quantum chemical analyses. The AM1 treatment reveals that the neutral and anionic forms are exothermic whereas the cation is endothermic. All of the systems are found to be stable (by AM1 and 6-31G methods) but the calculations indicate that plus and minus charge developments in AM1 treatment and minus charge development in 6-31G treatment cause drastic geometrical changes in the molecular structures resulting in bond elongation, possibly bond cleavage in esteric N–O bond. q 2005 Elsevier B.V. All rights reserved. Keywords: EGDN; Explosives; Nitro compounds; Ions; Detonation
1. Introduction The possibility of an explosive is for obvious reasons a matter of prime importance both to the manufacturer and the user. There is no evidence to suggest that the initial step in the decomposition of an explosive is in any way different from that of a non-explosive of a similar chemical class. An explosive reaction differs from the others only by the large free energy of the molecule, the rapidity of its release and the accelerating effects of the decomposition products which produce an explosive reaction [1]. Ethylene glycol dinitrate (EGDN) (C2H4N2O6) is the inorganic ester of ethylene glycol with nitric acid. It is an oxygen sufficient (neither oxygen negative nor positive) secondary type explosives [1,2]. It is one of the explosives having quite high power index (182, picric acid as the standard). It explodes producing no oxygen according to the following reaction [2] C2 H4 N2 O6 / 2CO2 C 2H2 O C N2 with the heat of explosion value of 6658 kJ/kg, thus it has very high rank among the nitro explosives [2]. EGDN have recently attracted the attention of scientists not because of * Tel.: C90 3122 103 244; fax: C90 3122 101 280. E-mail address: [email protected]. 0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.09.033
its explosive character but also because of some others [3–6]. EGDN like pentaerythriol tetranitrate (PETN) is of considerable interest in ammunition industry since it can be used in the sheet form or mixed with RDX to make Semtex [7], a plastic explosive. Crellin et al., performed some quantum chemical calculations (AM1 level) on the species involved in the reaction of Si(CH3)C 3 with EGDN [4]. In the present treatment, some quantum chemical calculations were performed (semiempirical and ab initio level of the theory) on EGDN as well as its univalent cation and anion forms.
2. Method The initial geometry optimizations of all the structures leading to energy minima were achieved by using molecular mechanics (MMC) type calculations followed by semiempirical AM1 self-consistent fields molecular orbital (SCF MO) method [8,9] at the unrestricted Hartree–Fock (UHF) level [10]. The AM1 geometry optimizations were obtained by the application of the steepest-descent method followed by conjugate gradient methods, Fletcher–Reeves and Polak–Ribiere, consecutively (convergence limit of 4.18!10K4 kJ/mol (0.0001 kcal/mol) and RMS gradient of 4.18!107 kJ/mmol (0.001 Kcal/Amol)). The initial geometry optimizations were fallowed by, ab initio
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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treatment at the level of 6-31G [10,11] (UHF type, convergence limit 10K5 Kcal/mol and RMS gradient of 10K3 Kcal/Amol) by using Polak-Ribiere technique. The Raffenetti integral format (cut-off 1.10K10 Hartree) was used for two-electron integral control. For the molecular orbital initial guess, the core-Hamiltonian option [11,12] was applied with involvement of six d-orbitals. All these computations were performed by using the Hyperchem (release 5.1) package program.
3. Results and discussion EGDN is a secondary type explosive. The secondary explosives are qualitatively different from the primary explosives with respect to their chemical structure as well as to their detonative behavior. The primary explosives immediately detonate when heat is added and a certain ignition temperature is reached without exhibiting any preceding nearly state deflagration [13]. They can be characterized as materials capable of a detonation but not of a deflagration. This behaviour may depend of course, not only on the chemical structure of the substance but also on
its physical state particularly density [13]. There exist mainly two mechanisms of detonation initiations: (i) initiation by means of heat addition (possible only in primary explosives); (ii) initiation through shock waves (possible for all explosives). Other mechanisms like radiation or chemical effects are fairly unimportant [13]. However, the propagation of detonation waves could be controlled by means of external magnetic and electrical fields [14]. The influence of applied fields on gaseous systems detonating at their threshold conditions has been observed years ago [14]. The influence of these fields on the explosive molecules might cause polarization or ionization, which may be involved by some means in the explosion mechanisms. Some explosives, like RDX and TNT, because of their ring shaped geometries are comparatively rigid molecules while nitrate esters (e.g. NG, EGDN, PETN) are not constrained in such a fashion and are ‘floppier’. These molecules have more conformational states and might be expected that this would leave the nitrate esters more susceptible to fragmentation. In the present study, firstly EGDN molecule and its mono ionic forms have been subjected to AM1 (UHF) type semiempirical quantum chemical analysis.
Fig. 1. The AM1 (UHF) geometry optimized structures and numbering scheme of EGDN species considered.
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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Table 1 Some bond lengths/distances (10K10 m, AM1 method) in EGDN species considered Neutral O(1)–N(2) O(1)–C(5) N(2)–O(3) N(2)–O(4) C(5)–C(6) C(5)–H(11) C(5)–H(12) C(6)–O(7) C(6)–H(13) C(6)–H(14) O(7)–N(8) N(8)–O(9) N(8)–O(10)
Cation 1.354 1.441 1.186 1.189 1.520 1.125 1.121 1.452 1.121 1.121 1.347 1.185 1.192
O(1)–N(2) O(1)–C(5) N(2)–O(3) N(2)–O(4) C(5)–C(6) C(5)–H(11) C(5)–H(12) C(6)–O(7) C(6)–H(13) C(6)–H(14) N(8)–O(9) N(8)–O(10) N(8)–O(7)
Anion 1.352 1.450 1.179 1.197 1.530 1.120 1.124 1.347 1.137 1.139 1.114 1.111 1.827
O(1)–C(5) C(5)–C(6) C(5)–H(11) C(5)–H(12) C(6)–O(7) C(6)–H(13) C(6)–H(14) O(7)–N(8) N(8)–O(9) N(8)–O(10) N(2)–O(3) N(2)–O(4) O(1)–N(2)
1.358 1.546 1.128 1.127 1.465 1.116 1.118 1.323 1.197 1.197 1.184 1.190 1.767
Fig. 2. The 6-31G (UHF) geometry optimized structures and numbering scheme of EGDN species considered.
Then, they have been the subject of 6-31G (UHF) type ab initio treatment. Fig. 1 shows the geometry optimized structures (AM1 (UHF)) and the numbering scheme of EGDN species. Table 1 includes some bond lengths/distances in these structures. The data reveal that as compared to the neutral structure of EGDN, O(7)–N(8) distance in the cationic form and O(1)–N(2) distance in the anionic form are quite long
indicative of bond rapture. Note that originally these bonds were ester oxygen-nitro (nitrogen) bonds. The ionic structures, geometry optimized at 6-31G (UHF) level of theory, are shown in Fig. 2. The ab initio method used (especially for the anionic case) predicts one of the esteric N–O bonds to be highly elongated (bond cleavage). Table 2 shows some of the bond lengths and distances calculated by the ab initio treatment.
Table 2 Some bond lengths/distances (10K10 m, 6-31G method) in univalent EGDN species considered
C(1)–C(2) C(1)–O(7) C(1)–H(11) C(1)–H(12) C(2)–H(13) C(2)–H(14) N(8)–O(9) N(4)–O(6) O(3)–N(4) N(8)–O(10) O(7)–N(8) N(4)–O(5) C(2)–O(3)
Neutral
Cation
Anion
1.511 1.454 1.077 1.077 1.077 1.077 1.236 1.241 1.382 1.241 1.382 1.236 1.454
1.513 1.438 1.077 1.077 1.076 1.076 1.237 1.372 1.289 1.241 1.398 1.230 1.584
1.499 1.425 1.082 1.081 1.075 1.075 1.255 1.247 1.369 1.257 4.823 1.246 1.499
12
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
Fig. 3. The 3D-electrostatic field maps (AM1 (UHF)) of EGDN species considered.
Fig. 4. The 3D-electrostatic field maps (6-31G (UHF)) of EGDN species considered.
Fig. 5. The 3D-charge density maps (AM1 (UHF)) of EGDN species considered.
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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Table 3 Some calculated energies (kJ/mol) of EGDN species (AM1 method)
Total Binding Isolated atomic Electronic Core-core interaction Heat of formation
Fig. 6. The 3D-spin density maps (AM1 (UHF)) of ionic EGDN species considered.
The three dimensional (obtained by AM1 and 6-31G methods) electrostatic field maps of the species considered are shown in Figs. 3 and 4, respectively. Since, the optimized geometries by the AM1 and 6-31G methods are different, the electrostatic field maps are different for the same ionic specie. The distributions of the excess charges in the case of ionic species are shown in charge density maps (AM1) in Fig. 5. Figs. 6 and 7 show spin densities of the ionic species calculated at the level of AM1 (UHF) and 6-31G (UHF) methods, respectively. Some calculated energies (AM1) of the species are given in Table 3. According to these data, these systems are stable (the total and binding energies) and with the exception of
Neutral
Cation
Anion
K25,4736 K4966 K249,770 K1,003,415 748,679 K223.03
K253,809 K4039 K249,770 K1,013,434 759,625 703.56
K254,957 K5187 K249,770 K966,403 711,446 K444.04
cationic case they are exothermic in nature. The anionic system appears to be the most stable and the cationic system to be the least stable of all. The presence of nitro groups due to electron withdrawing nature of them made the molecule highly electron demanding. Thus, the cation of EGDN becomes the least stable (leading to bond cleavage) while the anion is the most stable structure. Note that the structure for the anionic form also has an elongated O–N bond. Thus, in that geometry the system is stable but it does not mean that the molecule is intact. The ab initio method used predicts the stability order of the systems as anionic formO neutral formOcationic form. Whereas, the stability order based on the total energy with MP2 energy is neutral formO anionic formOcationic form. This order is very logical in the light of principles of classical organic chemistry. The presence of NO2 groups makes the anionic case to be more stable than the cationic form because NO2 groups are electron demanding. However, insertion of an electron into the nitro group of the neutral form causes formation of a radicalic anion and thus, repulsion with the lone-pairs of esteric oxygen atom resulting in highly elongated O–N bond. Whereas, removal of an electron from the neutral system to form the cationic case, generates a radical cation in which esteric O–N bond is less affected. Some energies Table 4 Some calculated energies (kJ/mol) of EGDN species (6-31G method)
Total Electronic Kinetic Energy eK, ee and eN Energy The Virial (KV/T) MP2 Correlation Energy Total energy with MP2 energy
Neutral
Cation
Anion
K1,668,353 1,669,330
K1,667,350 1,668,224
K1,668,600 1,669,549
K3,081,040 1.9994 K3271
K3,063,963 1.9995 K2783
K1,376,340 1.9994 K2913
K1,671,559
K1,669,967
K1,671,513
Table 5 The calculated (AM1) HOMO and LUMO energies of the EGDN species HOMO
Fig. 7. The 3D-spin density maps (6-31G (UHF)) of ionic EGDN species considered.
Neutral Cation Anion
LUMO
a-type
b-type
a-type
b-type
K20.4711 K25.1586 K7.6260
K20.4711 K24.6158 K7.7906
K0.7359 K12.3909 4.7014
K0.7359 K12.3910 4.7010
Energies in 10K19 J. All have A type symmetry.
L. Tu¨rker / Journal of Molecular Structure: THEOCHEM 717 (2005) 9–14
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Table 6 The HOMO and LUMO energies (6-31G) of monovalent EGDN ions a
Neutral Cation Anion
b
HOMO
LUMO
HOMO
LUMO
K20.5425 (AU) K21.3188 (BG) K26.0498 (A 00 ) K22.7286 (A 00 ) K8.4752 (A) K8.4747 (A)
4.1813 (BG) 1.6610 (BG) K7.3563 (A 00 ) K9.3794 (A 00 ) 9.3872 (A) 9.4889 (A)
K20.8217 (AU) K21.3188 (BG) K25.8471 (A 0 ) K24.7602 (A 0 ) K8.4771 (A) K8.4763 (A)
4.1021 (BG) 1.6610 (BG) K10.1909 (A 00 ) K12.7122 (A 00 ) 9.4482 (A) 9.3721 (A)
Energies in 10K19 J. The second entries in each case are for the values including MP2 correlation. Symmetries in parenthesis.
calculated at the level of 6-31G method are shown in Table 4. Note that the viral (KV/T) in every case is very close to 2. The HOMO and LUMO of the species considered are tabulated in Tables 5 and 6. The AM1 data shown in Table 5 indicate presence of degenerate a- and b-types of HOMO/ LUMO levels for the neutral EGDN. The ab initio data with and without MP2 correlation are shown in Table 6.
4. Conclusion The chemistry of explosives materials at the molecular level needs to be studied by means of quantum chemical methods to understand better the underlying phenomena behind the explosion reaction(s) for the considered explosive agent. However, one should keep in mind that an explosion is a dynamic process and the static methods can describe only a part of it.
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