Article pubs.acs.org/JPCA
Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
Thermochemistry, Tautomerism, and Thermal Decomposition of 1,5Diaminotetrazole: A High-Level ab Initio Study Margarita V. Shakhova,†,‡ Nikita V. Muravyev,§ Nina P. Gritsan,†,‡ and Vitaly G. Kiselev*,†,‡,§ †
Novosibirsk State University, 2 Pirogova Street, Novosibirsk, 630090 Russia Institute of Chemical Kinetics and Combustion SB RAS, 3 Institutskaya Street, Novosibirsk, 630090 Russia § Semenov Institute of Chemical Physics RAS, 4 Kosygina Street, Moscow, 119991 Russia ‡
S Supporting Information *
ABSTRACT: Thermochemistry, kinetics, and mechanism of thermal decomposition of 1,5-diaminotetrazole (DAT), a widely used “building block” of nitrogen-rich energetic compounds, were studied theoretically at a high and reliable level of theory (viz., using the explicitly correlated CCSD(T)-F12/aug-cc-pVTZ procedure). Quantum chemical calculations provided detailed insight into the thermolysis mechanism of DAT missing in the existing literature. Moreover, several contradictory assumptions on the mechanism and key intermediates of thermolysis were resolved. The unimolecular primary decomposition reactions of the seven isomers of DAT were studied in the gas phase and in the melt using a simplified model of the latter. The two-step reaction of N2 elimination from the diamino tautomer was found to be the primary decomposition process of DAT in the gas phase and melt. The effective Arrhenius parameters of this process were calculated to be Ea = 43.4 kcal mol−1 and log(A/s‑1) = 15.2 in a good agreement with the experimental values. Contrary to the existing literature data, all other decomposition channels of DAT isomers turned out to be kinetically unimportant. Apart from this, a new primary decomposition channel yielding N2, cyanamide, and 1,1-diazene was found for some H-bonded dimers of DAT. We also determined a reliable and mutually consistent set of thermochemical values for DAT (Δf H0solid = 74.5 ± 1.5 kcal· mol−1) by combining theoretically calculated (W1 multilevel procedure along with an isodesmic reaction) gas phase enthalpy of formation (Δf H0gas = 100.7 ± 1.0 kcal·mol−1) and experimentally measured sublimation enthalpy (ΔsubH0 = 26.2 ± 0.5 kcal· mol−1).
1. INTRODUCTION Nitrogen-rich heterocyclic compounds are promising components of environmental friendly energetic compositions.1−9 Among them, 1,5-diaminotetrazole (DAT, 1) is prominent due to its good thermal stability and high nitrogen content (∼84 wt %).10−12 Besides that, DAT is widely used as a building block for a huge variety of novel high-energy compounds.13−21 Similarly to other tetrazoles, tautomerism is very typical of DAT (Scheme 1).22 However, despite notable experimental and theoretical efforts,22,23 the details of tautomeric equilibria in DAT yet remain unclear. Furthermore, tautomeric transformations are known to play an important role in the mechanism of thermolysis of tetrazole (TZ) and 5-aminotetrazole (5-ATZ).24,25 It is therefore reasonable to expect the same issues in the case of DAT.
The standard state of DAT is crystalline with a melting point of 460 K.22 Several tautomeric forms of DAT have been discussed in the literature. In the crystalline state, X-ray diffraction data indicate that DAT exists as an amino-form (1 or 1,5-DAT, Scheme 1).26 He et al.23 performed CCSD(T)/6-311G(d,p) calculations and considered 1 along with its isomer 2,5diaminotetrazole (2,5-DAT, Scheme 1), which was predicted to lie close to 1 on the enthalpic scale. The authors also calculated activation barriers of the monomolecular interconversion between hypothetical isomers 1, 2, and 2,5-DAT. The H-transfer reaction 1 → 2 has a thermally inaccessible activation barrier of ∼62 kcal mol−1, moreover, even higher activation barrier (∼75 kcal mol−1)23 was predicted for the sigmatropic shift of amino group in 1 yielding 2,5-DAT (Scheme 1). Thus, monomolecular reactions cannot lead to equilibribration of isomers. Brill et al.27 studied thermolysis of four amino derivatives of tetrazole, including DAT and 2,5-DAT. The authors detected the gas decomposition products using rapid-scan FTIR/temperature profiling technique. For both DAT and 2,5-DAT, the two competitive global decomposition pathways were proposed: the one leads to the formation of HCN, NH3, and N2, and another
Scheme 1. Isomers of Diaminotetrazole Considered in the Literature
Received: February 15, 2018 Revised: March 26, 2018 Published: March 27, 2018 © XXXX American Chemical Society
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DOI: 10.1021/acs.jpca.8b01608 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
NH2CN and predicted the activation energy in the temperature range 200−2500 K to be 48.0 kcal mol−1 at the CCSD(T)/6311G(d,p)//MP2/6-311G(d,p) level of theory. To the best of our knowledge, other elementary reactions of the DAT decomposition have never been scrutinized, and tautomeric transformations have not been studied in detail as well. Moreover, the reactions in the H-bonded dimers of DAT have not been considered at all. The main goals of our paper are to investigate the thermodynamic properties of different isomers of DAT and their mutual interconversion, to identify the intermediates of their primary reactions, and to provide insight into the overall mechanism of the DAT thermal decomposition. To this end, we measured thermal properties of DAT including its sublimation enthalpy and calculated the activation barriers of the primary reactions in the gas phase and in the melt using the simplified model of the latter. For the purpose of comparison, the decomposition reactions of 2,5-DAT were considered as well. We also revealed previously unexplored reactions proceeding in the H-bonded dimers of DAT, and we demonstrated their importance for equilibration of isomers in the gas phase and melt.
yields NH2CN, NH3, and N2. On the basis of the measured ratio [NH3]/[HCN], the authors estimated the relative contributions of these competitive channels to be approximately 1:1 and 2:1 for DAT and 2,5-DAT, respectively. Levchik et al.28 emphasized an important role of tautomeric transformations (e.g., 1 ↔ 2, Scheme 1) in the mechanism of thermal decomposition of amino-substituted tetrazoles. DAT was proposed to coexist in amino 1 and imino 2 forms in both the solid state and melt, while evaporation shifts the equilibrium to the amino form 1.28 The authors proposed a reversible interconversion 1 ↔ 2 upon evaporation, condensation, and crystallization. Differential scanning calorimetry (DSC) and termogravimetric analysis (TGA) demonstrated that the decomposition of DAT occurred in the melt in the temperature range of 470−540 K. The two main channels of decomposition were proposed for DAT, viz., the imino form 2 decomposes via elimination of HN3 (Scheme 2), which is the predominant Scheme 2. Dominating Decomposition Channel of 1-Imino5-aminoteterazole (2) Proposed in the Literature
2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Quantum Chemical Calculations. Electronic structure calculations were carried out using the Gaussian 0930 and Molpro 201031 program packages. The geometries of each structure corresponding to the stationary point on the potential energy surface (PES) for the DAT decomposition were fully optimized using density functional theory (DFT) at the M06-2X/6-311+ +G(2df,p) level.32 Zero-point energies and thermal corrections to enthalpy and Gibbs free energy were computed at the same level of theory. Single-point electronic energies were afterward refined using an explicitly correlated coupled-cluster formalism CCSD(T)-F12b in conjunction with the correlation consistent basis set aug-cc-pVTZ.33,34 In the case of dimers of DAT and its isomers, the cc-pVDZ-F12 basis set was employed.35 For the sake of brevity, these basis sets are denoted hereafter as aVTZ and VDZ-F12, respectively. Note that the explicitly correlated F12 procedure accelerates the slow basis set convergence of conventional CCSD(T) techniques.36 The moderate size of DAT (7 non-H atoms) renders various coupled-cluster methods feasible in the present case. The extensive benchmarking on a series of related heterocyclic species revealed a perfect agreement (within 0.5 kcal mol−1) between the activation barriers calculated using CCSD(T)-F12b/aVTZ and a multilevel procedure W137 (Figure S1, Supporting Information). Apart from this, the earlier benchmarks also demonstrated a good agreement between complete basis set extrapolated CCSD(T) and CCSD(T)-F12 values calculated with various correlation consistent basis sets of a double- and triple-ζ quality.38,39 On the basis of these facts, we infer that the CCSD(T)-F12b/aVTZ procedure provides a good balance between computational cost and accuracy. The multireference character of the wave functions of the reagents, intermediates, and transition states considered in the present work was estimated using the T1 diagnostic for the CCSD calculations.40 The modest T1 values obtained in all cases (
