Computational Design of Novel Energetic Materials: Dinitro-bis

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Computational Design of Novel Energetic Materials: Dinitro-Bis-Triazolo-Tetrazine (DNBTT) Roman V. Tsyshevsky, Philip F. Pagoria, and Maija M. Kuklja J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01086 • Publication Date (Web): 24 Mar 2015 Downloaded from http://pubs.acs.org on March 28, 2015

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The Journal of Physical Chemistry

Computational Design of Novel Energetic Materials: Dinitro-Bis-Triazolo-Tetrazine (DNBTT) Roman V. Tsyshevsky1, Philip Pagoria2, and Maija M. Kuklja1,* 1

Materials Science and Engineering Department, University of Maryland College Park, College Park, MD, 20742 2 Energetic Materials Center, Lawrence Livermore National Laboratory, 7000 East Ave., L-282, Livermore, CA 94550 Abstract The quantum-chemical computational methods are used to design a new highly energetic

heterocyclic molecule DNBTT, 2,7-dinitro-4H,9H-bis([1,2,4]triazolo)[1,5-b:1',5'-e][1,2,4,5] tetrazine. We analyze and predict the structure and a range of its properties. DNBTT has high energy content (i.e., high performance) and exhibits a high stability (i.e., low sensitivity) due to two triazole rings connected via a central tetrazine ring. A relatively high activation barrier needs to be overcame to trigger the thermal decomposition of DNBTT, indicating that the sensitivity of DNBTT is on par with (or better than) the benchmark stability of TATB. Additionally, decomposition chemistry can be controlled optically or electronically. Thus, it is expected that DNBTT is an excellent candidate energetic substance with low sensitivity and high performance and is attractive on its own or as a component in composite energetic formulations to improve their properties.

Key words: 2,7-dinitro-4H,9H-bis([1,2,4]triazolo)[1,5-b:1',5'-e][1,2,4,5]tetrazine, heterocyclic high explosives, sensitivity to detonation initiation, decomposition kinetics, density functional theory, BNFF, BNFF-1, and ANFF-1

*

Author to whom correspondence should be addressed, e-mail: [email protected] and [email protected];

phone: 703-292-4940.

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I. Introduction Search for new energetic materials that have a stunningly wide range of applications has been long a stubborn challenge for researchers and engineers1. New energetics must satisfy specific requirements, such as an increased performance, reliably high stability to external stimuli, costefficiency and ease of synthesis, be environmentally benign, and be safe for handling and transportation. Among the most important and probably the most challenging prerequisites is low sensitivity of energetic materials to detonation initiation, dictating that explosive decomposition chemistry will be initiated only on demand and not accidently in response to unintended, mild external stimuli. During last decade, the attention of researchers has drifted from widely used nitroether-, nitramine-, and nitroaromatic-based explosives to nitrogen-rich heterocyclic compounds2,3,4,5,6,7. Despite significant efforts and many suggested empirical correlations between properties and structural elements in energetic materials, clear understanding of the relationships between thermal stability of materials, their molecular and crystalline structures, and chemical properties remain unachieved8. The micro-scale theory of decomposition of energetic materials has yet to be developed. A typical approach to design novel high energy density materials is largely of Edisonian nature and involves sophisticated synthesis procedures combined with extensive sensitivity characterization tests. Such empirical explorations are time and effort consuming and often relatively expensive while the successful outcomes are never guaranteed. Thus, many novel energetic materials were attained only to be rejected because they turned out to be so dangerously sensitive that it was problematic to use and, sometimes, even to characterize them9. With tremendous progress in computer architectures and processors’ speed, computational modeling and simulation techniques gradually become a powerful and practical tool to analyze and even predict physicochemical properties of explosive materials including the geometry and electronic structure,10,11,12,13 thermal stability,14,15,16,17,18,19 and their response to mechanical impact20,21,22,23,24,25,26,27. Hence, a close collaboration between synthetic chemists, researchers who perform advanced characterization tests of materials, and those who analyze materials with theoretical and computational quantum-chemical methods prove to be particularly effective1,28. In this article, we report results of such a collaboration to design a potential energetic molecule 2,7-dinitro-4H,9H-bis([1,2,4]triazolo)[1,5-b:1',5'-e][1,2,4,5]tetrazine (DNBTT) with superior properties prior to its synthesis and experimental characterization. The molecular 1 ACS Paragon Plus Environment

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structure and a range of properties (including the formation enthalpy, oxygen balance, decomposition kinetics, optical absorption spectrum, electronic affinity, and ionization potential) are obtained and carefully analyzed by means of quantum-chemical computational methods. Our study concludes that the DNBTT is an outstanding candidate new energetic molecule and the material composed out of these molecules will exhibit high performance and low sensitivity, the properties most desirable for high energy density solids.

II. Computational Details Decomposition reactions in the gas phase were studied within density functional theory (DFT)29,30 with M0631 functionals as implemented in the GAUSSIAN code32. All calculations were performed by using valence split double-zeta 6-31+G(2df,p) basis set. Minimal energy paths were investigated by conducting intrinsic reaction coordinate computations using the Hessian-based Predictor-Corrector integrator algorithm33,34 for each transition state. Reaction rates were calculated by applying transition state theory (TST)35 for reactions with well-defined transition states and variational TST36 for reactions with no barriers as detailed elsewhere15,16,17. It has been recently claimed that M06-based functionals show an outstanding accuracy in predictions of various physical and chemical parameters31. Hence, we carried out a series of test calculations that assess the ability of M06 functional with 6-31+G(2df,p) basis set to predict formation enthalpies (∆fH0298) and C-N bond dissociation enthalpies (BDE) of some C-nitro compounds as well as their ionization potentials (IP) and electronic affinities (EA). These tests served as a quantitative calibration of our approach. Table 1 shows that the calculated ∆fH0298 and BDE values are in good agreement with experiment, with absolute deviations not exceeding ~1-3 kcal/mol, which is within best accuracy attainable for advanced DFT methods. The calculated ionization potentials (IP) and electron affinities (EA) collected in Table 1 are also in respectful agreement with experiment and discrepancies do not exceed a fraction of eV. Table S1 of Supplementary Information shows that M06 predicts well formation enthalpies, dissociation enthalpies of C-NO2 bond of C1-C3 mono nitroalkanes, and activation barriers of the HONO elimination reaction from C2-C3 mononitroalkanes. For example, absolute deviations of calculated activation barriers from experimental data do not exceed 1 kcal/mol (Table S1). Therefore, one should conclude that the M06 functional with 6-31+G(2df,p) basis set can be

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reliably used to explore the decomposition mechanisms of DNBTT molecule and its thermodynamic properties and electronic properties. Table 1. Calculated formation enthalpies (∆fH0,298, kcal/mol), enthalpies of the C-N bond dissociation (BDE, kcal/mol), electronic affinities (EA, eV), ionization potentials (IP, eV), and vertical ionization potentials (IPvert, eV) for a series of nitro molecules. Molecule

∆fH0, 298

BDE (kcal/mol)

EA

IP

IPvert

NO2

5.8 [7.9]37a

--

2.26 [2.27]38,39

9.50 [9.59]40,41

11.49 [11.23]42,43

CH3NO2

-20.9 [-17.8]44,45

61.4 [60.8]46

0.16 [0.17]47,39

11.29 [11.28]48,49

11.70 [11.80]50,49

C2H5NO2

-27.1 [-24.4]51

61.8 [60.7]46

0.15

10.59 [11.92]52,53

11.24 [11.02]54,49

C6H5NO2

16.9 [16.4]55,45

73.9 [70.7]56

1.10 [1.0]57,39

9.78 [9.8]58,43

9.95 [9.93]59,49

TNT

5.1 [5.8]60,45

64.3 [58.961, 58.062]

2.3

10.43 [10.59]63,49

10.70

a

Corresponding experimental data are in parentheses

III. Results and Discussion 3.1 The Structure Our approach to theoretically design and computationally model a novel energetic material with superior properties capitalizes on many years of experience coupled with interdisciplinary expertise in the field. In our recent study, we performed a detailed holistic analysis of the structure, stability, and decomposition mechanisms of oxadiazole-based BNFF28, BNFF-164 and ANFF-165 (Fig. 1), novel improved energetic materials that were recently synthesized and characterized in Lawrence Livermore National Laboratory. High density (~1.8−1.9 g/cm3) together with low sensitivity to mechanical impact and low melting point (85−110 °C) make these materials attractive melt castable explosives and an excellent energetic ingredient of modern explosive composites. It was established that these three relevant materials are comparatively thermally stable, as the dissociation of the critical C-NO2 chemical bond requires overcome a high energy barrier of ~60-70 kcal/mol, similar to the stability benchmark TATB. However, in a series of gas-phase64 and solid-state28 calculations, we also discovered another plausible decomposition channel, common for these materials, the cleavage of the oxadiazole rings, which requires a lower energy, ~45-50 kcal/mol, and potentially can prompt an increase of sensitivity to detonation initiation. 3 ACS Paragon Plus Environment

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In designing a new material, we formulate specific criteria that it has to meet, in particular: (i) the energetic molecule should be comprised of several nitrogen and/or oxygen fused hetero rings to ensure high energy content; (ii) the nitro group has to be attached to a carbon atom to ensure high thermal stability, as the C-NO2 bond has the highest dissociation energy as compared to O-NO2 and N-NO2 bonds; and (iii) the molecular structure should serve to additionally stabilize heterocyclic rings to prevent their dissociation as the primary decomposition step. Our search resulted in the following compound 2,7-dinitro-4H,9H-bis([1,2,4]triazolo)[1,5b:1',5'-e][1,2,4,5]tetrazine depicted in Fig. 2. The DNBTT molecule belongs to C2 symmetry point group and has an inversion as symmetry operation. It has a planar structure with two triazole rings connected via a central tetrazine ring. Two nitro groups are inversely symmetrically attached to the carbon atoms of the triazole rings (Fig. 2). This molecule possesses a zero dipole moment due to symmetry, which will additionally stabilize the molecule. Quantum-chemical calculations demonstrated that the existence of the molecular dipole moment may be a reason for a crucial drop in thermal stability of energetic materials due to polar surface effect66,67. For example, among chemically related C-nitrocompounds, polar DADNE is found to be more sensitive than nonpolar TATB20,21,68,69,70, and among stoichiometrically similar nitramines, polar δ-HMX is more sensitive than nonpolar βHMX16,17,66,67.

Figure 1. Sketched structures of BNFF, BNFF-1 and ANFF-1 molecules



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H O N N N N + N N N N O N H

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O + N O

Figure 2. The suggested molecular structure of DNBTT (bonds are in Å).

3.2 Formation Enthalpy and Oxygen Balance Our calculations demonstrate that the DNBTT molecule has a high positive formation enthalpy of 177.1 kcal/mol. The formation enthalpy of a nitro compound is directly related to enthalpy (heat) of combustion according to Hess’ law: ∆H0combustion = Σ∆H0f(P) - ∆H0f(NC) - ∆H0f(O2),

(1)

where Σ∆H0f(P) is a sum of formation enthalpies of products, which usually are H2O, CO2, CO, N2; ∆H0f(NC) is the formation enthalpy of the corresponding nitro compound; ∆H0f(O2) – enthalpy of formation of O2, which is equal to zero. Taking into consideration that Σ∆H0f(P) is in most cases a constant value, the higher the formation enthalpy of a nitro compound, the higher the heat released during its combustion, as it is seen from Eq. 1. In other words, the formation enthalpy is a measure of thermal energy stored in the material (and released in explosion), or the materials performance. The formation enthalpy is also indirectly related to the velocity of detonation of explosive materials71.


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The calculated formation enthalpy of DNBTT (∆H0f(DNBTT)=177.1 kcal/mol)

is

appreciably higher than formation enthalpy of widely used materials, HMX (∆H0f(HMX)=32.7 kcal/mol)72, TNT (∆H0f(TNT)=5.1 kcal/mol)72, and recently synthesized triazole-based explosives (for more details see, for example, ref.4). Another important parameter related to the molecular formula of a compound is an oxygen balance (OB) which indicates the degree to which an explosive can be oxidized:

OB = [16(Z-2X+Y/2)/Mw] × 100%,

(2)

where X, Y, and Z represent a number of atoms of carbon, hydrogen, and oxygen in the molecule, and Mw stands for the molecular weight. Widely used secondary explosives, such as PETN, HMX, TATB and TNT, are considered oxygen deficient, which means that they have negative oxygen balances of -10.1, -21.6, -55.8, and -74.0%, respectively. Negative OB values show that the oxygen content in all these compounds is insufficient to react with the available carbon and hydrogen. DNBTT is also oxygen deficient material with OB = -31.5%.

3.3 Decomposition Mechanisms Now we will explore fragmentation mechanisms of the DNBTT molecule to establish its stability against thermal excitation. In modeling the thermal decomposition of the DNBTT molecule, we studied five most plausible decomposition channels (Fig. 3, 4): I) a cyclization of the nitro-triazole ring into the oxazete N-oxide isomer (CNON cyclization), II) a homolytic cleavage of the C-NO2 bond, III) an opening of the outer triazole ring (OR cleavage) via a scission of the N-N bond accompanied by the C-NO2 bond homolysis and the detachment of NO2, IV) a nitro-nitrite isomerization (CONO) followed by the NO loss, and V) an opening of the central tetrazine ring (CR cleavage). Structures of some reaction products and intermediates are collected in Fig. 5a-c, and Cartesian coordinates of all intermediates, products and transitions states are presented in Supplementary materials.



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I

H N

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N

N N+ N+ O N O H H N

II H N N H

N N N

DNBTT

N

N N N H

O N O

III

H N N H

IV1

H N

C

+

NO2

N C N

+

N

N

N N N H

NO2

N O IV 2 O

H N

N

N N N H

O

+

NO

Figure 3. Schematic representation of decomposition mechanisms of DNBTT.


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3.2.1 Isomerization into oxazete N-oxide isomer (path I, Fig. 3) The decomposition mechanism through a formation of the 1,2-oxazete 2-oxide structure, initially proposed for high-temperature pyrolysis of β-nitrostyrenes,73 has been recently reported for nitroalkenes74, DADNE75, and BNFF28and claimed to be the primary initiation reaction75. The calculated activation barriers of this isomerization channel, although are fairly low in energy, differ significantly; and they are 48.1 kcal/mol for nitroethylene74, 27.1 kcal/mol for DADNE75, and 57.4 kcal/mol for BNFF28. The similar isomerization of DNBTT requires significantly higher energy (~90 kcal/mol, Table 2), which suggests to rule out such a reorganization of the molecule as a primary or the most favorable initiation reaction. The structure of oxazete N-oxide isomer of DNBTT is depicted in Figure 5a.

3.2.2 Homolysis of C-NO2 bond (path II, Fig. 3) The C-NO2 bond dissociation is considered a canonical initiation reaction of nitro compounds. The calculated enthalpy of the C-N bond cleavage in the DNBTT molecule (69.6 kcal/mol, Table 2) is in good agreement with theoretical and experimental estimates previously reported for C-nitro derivatives of triazole (~67 kcal/mol)76, dinitro isomers of diazole (~65-70 kcal/mol)77, some nitroaromatic compounds, including nitrobenzene (~70 kcal/mol)78, TATB (60-70 kcal/mol)61,78,79,80 and DADNE (67.0-72 kcal/mol)20,68,81. We note that the cleavage of CNO2 in DNBTT requires a higher energy (by ~6-8 kcal/mol) than the activation of this channel in the oxadiazole-based molecules BNFF-1 (63.1 kcal/mol)64, ANFF-1 (61.5 kcal/mol)64, and BNFF (62.3 kcal/mol)28, alluding to a higher stability of DNBTT.

3.2.3 Cleavage of outer triazole ring (path III, Fig. 3) The cleavage of oxadiazole rings was found to require an appreciably lower energy (~50 kcal/mol) than the dissociation of C-NO2 bonds (~60-65 kcal/mol) and therefore it was suggested as a dominating decomposition mechanism of the gas-phase decomposition of BNFF-164, ANFF164 and BNFF28. Another theoretical study19 proposed that the cleavage of the tetrazole ring and a subsequent elimination of N2, with the activation energy of 37-45 kcal/mol, will be the dominant process in the thermal decomposition of bistetrazole. According to our calculations, an opening of the outer triazole ring in DNBTT proceeds via a rupture of the N-N bond (Fig. 3) and subsequent homolysis of the C-NO2 bond. The calculated 8 ACS Paragon Plus Environment


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activation barrier of the outer triazole ring opening, 63.6 kcal/mol (Table 2), is noticeably higher than that in previously reported data for the decomposition of BNFF (49.5 kcal/mol)28, its derivatives (BNFF-1 (50.7 kcal/mol)64 and ANFF-1 (51.8 kcal/mol)64), and bistetrazole (45.1 kcal/mol)19.

O O N

V2

N N

N

N N

O

+

N O

N

NH

HN H N

-

O O N N N + + N N N N N O O N H DNBTT

O O N

V3

N N

N

N N

+

O N O

N

HN

HN

V1 V4

O O N N HN

N N

N N NH

N

O

N N

N

N N NH

N O

N

HN

N O

O

+

NO2

O O N

V5

N N

N N

N

+

NO2

N

NH HN O N O

V6

N N

N N

N

NH

N

HN

O N O

V7

... +

NO

... +

NO

O O N

V8

N

N N

N O V9

N N O

NH N HN Figure 4. Schematic representation of decomposition mechanisms of DNBTT proceeding via opening of central tetrazine ring. 9 ACS Paragon Plus Environment

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Figure 5. The structures of a) oxazete N-oxide, b) CONO isomers of DNBTT, and c) intermediate, formed in the course of central tetrazine ring opening.

3.2.4 CONO isomerization and subsequent NO loss (path IV, Fig. 3) The CONO isomerization is often suggested as a competing or a slow background initiation reaction in nitro compounds. The nitro-nitrite isomerization proceeds via a pseudo rotation of a nitro group accompanied by breaking the C-NO2 bond and creating the C-ONO bond (path IV1, Fig. 3). The formed nitrite isomers of DNBTT are shown in Fig. 5b. The CONO-isomerization in DNBTT requires 55.6 kcal/mol, which is consistent with the barrier of the CONO formation in 10 ACS Paragon Plus Environment


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C-nitro isomers of triazole (60–65 kcal/mol)76, BNFF (54.2 kcal/mol)28, BNFF-1 (53.2 kcal/mol)64, ANFF-1 (60.3 kcal/mol)64, DADNE (66.4 kcal/mol81, 59 kcal/mol82), nitroethylene (57.9 kcal/mol)83, and TNT (54.9 kcal/mol)62. The CONO isomerization in DNBTT is an exothermic reaction, releasing 7.4 kcal/mol of heat (Table 2). The following NO loss from the CONO isomer of DNBTT (path IV2, Fig. 3) requires only 16.7 kcal/mol (Table 2). 3.2.5 Fragmentation of DNBTT via opening of central tetrazine ring (path V, Fig. 4) The final mechanism of the unimolecular DNBTT decomposition, we consider here, involves opening of the central tetrazine ring during the first step, followed by subsequent fragmentation mechanisms. The opening of the central tetrazine ring (path V1, Fig. 4) proceeds via a cleavage of one N-N bond, accompanied by a rotation of the tetrazine ring about the second N-N bond. The resulting intermediate with the opened tetrazine ring (Fig. 5c) rests 31.0 kcal/mol above the DNBTT equilibrium state. The tetrazine ring opening has the lowest activation barrier (53.9 kcal/mol, Table 2) among all studied mechanisms. Further, we simulated several secondary fragmentation channels, including a cleavage of the central C-N (path V2, Fig. 4) and N-N (path V3, Fig. 4) bonds, C-NO2 bond homolysis (paths V4, V5, Fig. 4), and CONO isomerization mechanism (paths V6, V8, Fig. 4) together with subsequent NO loss reactions (paths V7, V9, Fig. 4). The secondary fragmentation channels were explored to reveal whether or not the primary tetrazine ring opening reaction can be considered as the rate limiting step. An analysis of the secondary reactions, illustrated in Fig. 4 and Table 2, shows that the dissociation of the central C-N bond (path V2) and homolytic loss of NO2 (paths V4, V5) require noticeably higher energies (~63-73 kcal/mol, Table 2) than the primary reaction of the central ring opening (53.9 kcal/mol). The cleavage of the central N-N bond (path V3, Fig. 4) has the lowest activation barrier, 34.7 kcal/mol (Table 2). The CONO isomerization of the DNBTT isomer with the opened tetrazine ring (paths V6, V8, Fig. 4) requires much lower energies (45.9 and 37.5 kcal/mol, Table 2, Fig. 6), as compared to the primary CONO isomerization (55.7 kcal/mol, Table 2, path IV1, Fig. 6). The subsequent NO loss (path V9, Fig. 4) from the CONO isomer requires 16.0 kcal/mol (Table 2)84.


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3.4 Decomposition Kinetics The homolytic NO2 loss (path II, Fig. 3) is to be considered the predominant fragmentation channel (Fig. 7a) at the temperature above ~650 K (Fig. 7a) due to the highest value of preexponential factor log(A, s-1) = 17.9 (Table 2). The CNNO cyclization mechanism (path I, Fig. 3) exhibits the highest barrier among all studied mechanisms (Table 2, Fig. 6) and, hence, would proceed at the lowest rate (Fig. 7a). Although the cleavage of the outer triazole ring (path III, Fig. 3) has a noticeably lower activation barrier (63.6 kcal/mol, Table 2, Fig. 6) than the energy of the C-NO2 bond homolysis (69.6 kcal/mol, Table 2, Fig. 6), this fragmentation channel will unlikely be able to compete with the NO2 loss due to its lower pre-exponential factor (log(A, s-1) = 15.0, Table 2). The CONO isomerization (path IV1, Fig. 3) has even lower pre-exponential factor (log(A, s-1) = 13.5, Table 2) than the outer triazole ring cleavage. Nevertheless, the CONO isomerization will be favored over the C-NO2 bond cleavage at low temperatures (Fig. 7a) due to the relatively low activation barrier height (55.6 kcal/mol, Table 2, Fig. 6). The cleavage of the central tetrazine ring (path V1, Fig. 4) has the lowest activation barrier (53.9 kcal/mol, Table 2, Fig. 6). As a result, this mechanism has the highest rate (Fig. 7a) in the temperature range below ~650 K in spite of quite moderate value of pre-exponential factor log(A, s-1) = 13.7 (Table 2). However, Fig. 6 shows that all studied secondary reactions have higher overall activation barriers than the primary cleavage of the central tetrazine ring. For example, the overall energy of the central C-N bond cleavage (path V2, Fig. 4) and homolytic NO2 loss reaction (paths V4, V5, Fig. 4) exceeds 90 kcal/mol (Fig. 6, Table 2). Such high activation barriers dictate to eliminate these mechanisms from further consideration. The lowest overall activation barriers of 65.7 and 68.5 kcal/mol (Fig. 6, Table 2) were obtained for the secondary steps that involve the central N-N bond rupture (paths V3, Fig. 4) and CONO isomerization (paths V8, Fig. 4), respectively. Figure 7b reveals that the latter two mechanisms have lower rates than the primary cleavage of tetrazine rings and hence they should be considered as the rate limited steps for the stepwise channels V1-V3 and V1-V8- V9 (Fig. 4). Summing up, the primary C-NO2 homolysis (paths II, Fig. 3) and NO loss via CONO isomerization (paths IV1, Fig. 3) will dominate other possible decomposition channels with the NO2 loss being most favorable at higher temperatures (Fig. 7b).



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Figure 6. Energy diagram showing main decomposition channels of DNBTT.

Table 2. Calculated activation barriers (E, kcal/mol), reaction energies (given in parentheses in kcal/mol) zero-point energy corrected barriers (EZPE, kcal/mol), activation enthalpies (∆H≠298, kcal/mol), and pre-exponential factors (log(A, s−1)) of DNBTT decomposition reactions. Reaction I CNNO cyclization II NO2 loss III OR cleavage 1 CONO IV 2 NO loss 1 CR cleavage 2 C-N bond cleavage 3 N-N bond cleavage 4 NO2 loss 5 NO2 loss V 6 CONO 7 NO loss 8 CONO 9 NO loss

E 92.5 (87.1) 72.7 (72.7) 67.0 (32.4) 57.9 (-6.4) 19.0 (19.0) 57.2 (32.9) 76.5 (76.5) 36.8 (24.2) 69.6 (69.6) 66.3 (66.3) 48.0 (-10.8) 39.3 (-18.4) 18.6 (18.6)

EZPE 90.1 (85.2) 68.6 (68.6) 62.8 (27.1) 55.6 (-7.8) 15.9 (15.9) 53.9 (30.4) 72.8 (72.8) 34.6 (20.8) 65.2 (65.2) 62.3 (62.3) 45.9 (-12.3) 38.2 (-19.8) 15.3 (15.3)

∆H≠298 89.8 (85.2) 69.6 (69.6) 63.6 (28.5) 55.6 (-7.4) 16.7 (16.7) 53.9 (31.0) 73.1 (73.1) 34.7 (21.3) 65.9 (65.9) 62.9 (62.9) 45.9 (-11.9) 37.5 (-19.3) 16.0 (16.0)

log A 12.7 17.9 15.0 13.5 13.7 15.6 14.9 14.2 -


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Figure 7. Reaction rates of the simulated decomposition channels of DNBTT.

3.5 Electronic Properties and Optical Absorption Electronic properties serve as a measure of electronic stability of the material against photo stimulated decomposition85,86,87,88,89,90 or decomposition induced by electromagnetic field91. The calculated HOMO-LUMO gap of the DNBTT molecule is 4.97 (Table 3) eV, which is close to that of BNFF (4.87 eV)28 and TATB (5.0 eV)28. The energy gap represents a typical value for organic molecular high explosive materials and indicates that DNBTT crystals will likely to be wide-gap dielectrics. The highest occupied molecular orbital (HOMO) orbital is mainly formed from 2pz atomic functions of tetrazine nitrogen atoms and an additional small 14 ACS Paragon Plus Environment


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contribution from 2pz atomic functions of nitrogen and carbon atoms of triazole rings and oxygen atoms (Fig. 8a). The lowest unoccupied molecular orbital (LUMO) orbital is mainly formed from 2pz atomic functions of oxygen and nitrogen atoms of nitro groups and carbon atoms of triazole rings with an additional relatively moderate contribution from 2pz atomic functions of triazole nitrogen atoms (Fig. 8a). The energies of optical transitions of DNBTT were calculated using time dependent TDM06/6-31+G(2df,p) approximation to predict the absorption spectrum. The energy of the vertical spin-forbidden transition from the ground state to the lowest triplet state (S0→T1) requires 3.15 eV. The energy of the lowest singlet-singlet (S0→S1) transition is 3.85 eV (Table 3, Fig. 8b). The large difference (1.8 eV) between the HOMO-LUMO gap and S0→T1 energy indicates the existence of tightly bound excitons in DNBTT. Our calculations also show that the UV-VIS absorption spectrum of DNBTT (Fig. 8b) has two absorption bands corresponding to a series of symmetry- and spin- allowed singlet-singlet transitions in the range of 3-7 eV with distinct maximums at 4.25 and 6.60 eV. These features are consistent with the optical spectra of many nitro-compounds11,13. In a practical sense, the obtained absorption spectrum means that once the DNBTT molecule is irradiated by photons with the predicted energies, the molecule will likely to suffer rapid bond dissociation. On the other hand, the chemical bonds can be stimulated selectively, for example, by laser irradiation, implying highly controllable initiation reactions. Such a precision control is not achievable in thermally stimulated decomposition. The ionization potential of DNBTT (9.19 eV, Table 3) is comparable to that of BNFF, RDX, HMX, PETN, TNT (~ 10 eV), and TATB (8.7 eV)28. The high IP indicates that the removal of an electron from those energetic molecules is an energetically costly process. Another useful characteristic relevant to molecule and materials stability is an electronic affinity, which tells whether the molecule is electron-withdrawing or electron-donating. The calculated EA of DNBTT is positive and equal to 2.03 eV (Table 3), which is close to EA of BNFF (2.3 eV) and TNT (2.0 eV) and somewhat higher than EA of PETN (1.7 eV), HMX (1.4 eV), and TATB (1.3)28. The high EA of DNBTT suggests that in an electron-rich environment, the DNBTT will easily trap an electron, gain energy by doing so, and the ion radicals may exhibit different activation barriers for dissociating chemical bonds. For example, a dramatic effect of charged states on decomposition chemistry, illustrated on DADNE69, HMX8,66,67 and PETN92, was found to be manifested in altered dominating chemistry with pronounced 15 ACS Paragon Plus Environment

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exothermicity as well as in significantly reduced activation barriers of bond dissociation reactions. The observation indicates that DNBTT decomposition can be efficiently controlled by an electric current, donor impurity dopants, or electromagnetic fields. More so, DNBTT is expected to serve as an attractive additive ingredient of modern energetic formulations to improve their stability and achieve highly controllable decomposition chemistry.

Table 3. Electronic properties of DNBTT energetic molecules, optical energy gap (HOMOLUMO), optical transitions, electronic affinity, and ionization potential (eV). Property

Energy (eV)

HOMO-LUMO

4.97

S0→T1

3.15

S0→S1

3.85

EAad

2.03

IPad

9.19



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Figure 8. a) Isosurfaces of HOMO and LUMO orbitals of DNBTT and b) schematic of the predicted UV-Vis absorption spectrum of DNBTT.

IV. Summary and Conclusions

The molecular structure of the theoretically designed novel energetic heterocyclic molecule 2,7-dinitro-4H,9H-bis([1,2,4]triazolo)[1,5-b:1',5'-e][1,2,4,5] tetrazine, DNBTT, is proposed. This molecule was constructed based on a detailed analysis of available data on structures and thermal stability of several classes of energetic heterocycles. Potential decomposition mechanisms and kinetics were computationally simulated by using density functional theory combined with transition state theory.


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DNBTT is a fused heterocyclic molecule, which contains two triazole rings connected via a central tetrazine ring. The fused structure provides an additional stabilization of the hetero rings, which improves the overall thermal stability of the molecule, as evidenced by the modeled decomposition mechanisms. The DNBTT material has yet to be synthesized, and, thus, no data on the crystalline structure are available. Speculating on intermolecular interactions or cohesive binding in DNBTT crystals we suggest that the potential material will be organic molecular crystal in which molecules are held together by van der Waals forces and the electronic density is largely localized on individual molecules. The thermal decomposition of DNBTT is predicted to proceed via two possible channels, the fast NO2 loss (at high T) and slow NO loss via CONO isomerization (at low T). In contrast to the behavior observed on BNFF28, BNFF-164, ANFF-165 and bistetrazole19 based energetic materials, the cleavage of the hetero rings can barely compete with the two dominating dissociation mechanisms in DNBTT. DFT calculations suggest that the activation energy of thermal decomposition of DNBTT is relatively high, comparable to stability benchmark TATB (57-70 kcal/mol) and noticeably higher than the activation energy (~45-50 kcal/mol) of relevant materials of BNFF28, BNFF-164, ANFF-165, recently reported in literature. A high positive electronic affinity and optical absorption energies suggest specific ways to control decomposition chemistry of DNBTT and to increase sensitivity of energetic composite formulations. Overall, this study predicts that DNBTT is an excellent candidate low sensitivity energetic material individually and as an ingredient in composite formulations. This study provides a solid background for further design of new energetic materials and targeted improvements of existing composites.

Supporting Information Supporting Information Available: Details of the structure of a DBNTT molecule, the structures of considered molecules in their transition states during decomposition, and atomic coordinates of the molecule in its ground state and in the transition states for each decomposition reaction. This material is available free of charge via the Internet at http://pubs.acs.org.



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ACKNOWLEDGMENT Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This research is also supported in part by ONR (Grant N00014-12-1-0529) and NSF. We used NSF XSEDE resources (Grant DMR-130077) and DOE NERSC resources (Contract DE-AC02-05CH11231). MMK is grateful to the Office of the Director of National Science Foundation for support under the IRD program. Any appearance of findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF.


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Computational Design of Novel Energetic Materials: Dinitro-bis

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