Elsevier

Thermochimica Acta

Volume 333, Issue 2, 3 August 1999, Pages 121-129
Thermochimica Acta

Analysis and prediction of the Arrhenius parameters of low-temperature thermolysis of nitramines by means of the 15N NMR spectroscopy

https://doi.org/10.1016/S0040-6031(99)00104-5Get rights and content

Abstract

The paper presents 15N NMR chemical shifts δ of 24 nitramines out of which 1-nitro-1-azaethylene, 1,3-dinitro-1,3-diazacyclobutane and 1,3,5,7,9-pentanitro-1,3,5,7,9-pentaazacyclodecane have not been synthesized yet. The relationships between the Arrhenius parameters Ea and log A of the low-temperature thermolysis under conditions of Russian manometric method and δ values of nitrogen atoms in nitro groups are specified for substances studied. The δ values correspond to nitro groups in nitramino groupings which are the first to undergo thermolysis. On the basis of the relationships specified the Ea and log A values are predicted for 13 compounds from the nitramines studied, the parameters of homolytic course of thermolysis are differentiated from those of bimolecular or another heterolytic thermal decomposition, some aspects of the thermolysis of nitramines, particularly with geometrical constraints and/or with strongly-withdrawing groups in their molecules, are interpreted and evaluation of effect of solid–liquid phase transition on the kinetics of initial stage of HMX thermolysis are made.

Introduction

There is a great interest in the thermal reactivity of nitramines due at least in part to their importance as energetic materials. (e.g. see 1, 2). The homolysis of the N–NO2 bond was proved as a primary step of thermolysis of the secondary nitramines in a condensed state 3, 4, 5. In the case of primary nitramines the homolysis is limiting step in their thermal reactivity in gaseous state, whereas their thermolysis in the condensed state is a bimolecular autoprotolytic reaction [7]. As the valence states of nitro compounds in generally (including ground state) have a large component of biradical character [6]a bimolecular course of the primary thermolysis of the compounds, including some nitramines, cannot be fully excluded from (about the pseudomonomolecular course of their thermolysis, see [8]). All the said types of reactivity of nitramine nitro group depend upon the electron density on its nitrogen atom. The density is predominantly a function of the extend to which the amino nitrogen lone pair is involved in π-bonding with this nitro group, i.e. both the homolysis of N–NO2 bond and hydrogen abstraction by an oxygen of the nitro group should be depended upon this N–N bond strength.

It is a well-known fact that 15N NMR chemical shifts may be taken to indicate the degree of shielding of the 15N atoms which affect the adjacent N–N bond strengths. A relationship corresponding to this has the following general form [9]Ea=a×δN+b,where Ea is activation energy of non-autocatalyzed thermolysis of nitramines and δN is the 15N NMR chemical shift of nitro group nitrogen. With respect to the kinetic compensation effect in this thermolysis [8]also this relationship was found [9]logA=a1×δN+b1,where A means the Arrhenius preexponent of the above-mentioned decomposition. As the nitro groups play a key role in thermal reactivity of nitro compounds in general a less close correlation results from the application of the 15N NMR chemical shifts δA of amino nitrogens of nitramino groups in the both relationships [9].

The study of thermal reactivity of nitramines is the important starting point for selection and exploitation of these nitro compounds. However, some published conclusions of the study are contradictory, which is due to both unsuitable choice of experimental conditions and a wrong interpretation of results 4, 8, 9. Therefore a method is needed for mutual comparison and evaluation of results obtained in various laboratories. From the point of view of development of the new energetic materials also predicted characteristics of thermal reactivity of nitramines are significant. The relationships 1 and 2 signalize a possibility of their application not only in a prediction but also in analysis of the Arrhenius parameters resulted from the low-temperature thermolysis of nitramines. These problems are discussed in the present paper which extends the findings of the recent study [9].

Section snippets

15N NMR spectroscopy

Survey of the nitramines studied and their code designation is given in Table 1. The 15N NMR chemical shifts δ of the nitramines were obtained with the help of an AMX-360 Bruker apparatus using the INEPT method. The samples were dissolved in hexadeuteriodimethyl sulfoxide at a concentration of 0.2 mol nitramine per 1 dm3 solution. For some of the substances the values of these shifts were taken from literature 11, 12; those of the substances not yet prepared were predicted. These all δA and δN

Discussion

Fig. 1Fig. 2 are graphic representation of , , respectively. In these figures the dependence A corresponds to N–N bond homolysis in the solid state and the dependence B to the same fission in the liquid state. The Arrhenius parameters have been predicted by means of these dependencies (calibration curves) for nitramines whose thermolysis data have not been experimentally obtained yet (see Table 1). Predicted Ea value for liquid phase thermolysis of DIGEN agrees well with the N–N bond energy

Conclusion

The earlier-described relationship [9]between the Arrhenius parameters of low-temperature thermolysis obtained at the conditions of the Russian manometric method (SMM) and the 15N NMR chemical shifts of nitrogen atoms in nitro groups of their nitramino groups possesses a broader validity. The said relationship makes it possible to:

  • 1.

    predict the Arrhenius parameters for the nitramines whose thermal decomposition has not been studied yet,

  • 2.

    assess which nitramino group in the molecule is the first to

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