On the Release of Stored Energy from Energetic Materials

Elliot R. Bernstein 1    Department of Chemistry, Colorado State University, Fort Collins, Colorado, USA
1 Corresponding author: email address: erb@lamar.colostate.edu

1 Introduction

Energetic materials are employed for the storage of energy in molecular systems to be utilized as fuels, explosives, and pyrotechnics at different times and places. These are unique systems characterized by high heats of formation (stored energy) and particular electronic structure. One can usefully compare energetic molecules with apparently similar species that are not classified generally as energetic; that is, energetic molecules, when in the condensed phase, will explode under appropriate conditions while “model” energetic species will not. Perhaps, the best example for such direct comparison is RDX [(CH2NNO2)3, energetic] and N-nitropiperidine and (o, m, p) N,N-dinitropiperazine (nonenergetic model systems). These molecules are displayed in Fig. 2.1 along with many other such molecular species couples. The comparison between energetic and model or nonenergetic species is an important one because the difference is obviously essential in terms of mechanisms for energy release and for any theoretical model developed to explain energetic behavior: not only must a proposed quantum mechanical mechanism demonstrate energetic behavior as found experimentally, but it must also demonstrate that model molecules, with similar chemical structures that are not energetic, will possess a different decomposition mechanism that will not follow an energetic pathway. These are stringent requirements and constraints to place on a theoretical molecular mechanism, especially for large organic molecules with many and complex electronic potential energy surfaces (PESs) and potential reaction coordinates accessible in the decomposition/energy release process.

Release of the stored molecular energy is certainly phase dependent; that is, an energetic molecule isolated in the gas phase, in a thin film of pure material, or dissolved in a solvent, will not explore. True energetic behavior is typically displayed only in crystalline or concentrated solids. Nonetheless, the fundamental kinetics and dynamics of the decomposition, energy release process must be molecular in nature prior to any intermolecular chain reaction that can occur in the crystalline or condensed phase. Thus, studying the molecular behavior of the identified, isolated energetic molecule (in the gas phase) is an important pursuit both experimentally and theoretically. In fact, only under such conditions do the experiment and quantum mechanical theory directly corresponds: the study of energetic materials at a molecular level is thereby an essential undertaking, if we are to understand and design them at a fundamental level.

Moreover, the goal of such studies is to present to synthetic chemists, who actually make the molecules of interest, a fundamental molecular mechanistic understanding of what is required to generate better (more energy storage, less sensitive, more environmentally benign, …) and new fuels, explosives, and pyrotechnics. Thus, we study energetic molecules and nonenergetic model molecules to learn about their detailed mechanistic, decomposition channels, and the relation of such channels to the physical and chemical properties of the individual molecules.

Since nearly all practical, secondary explosives are organic molecules, we must concern ourselves with the behavior associated with rather large systems with many heavy first and second row atoms (e.g., C, N, O, F, S, P, Cl, …). So what do we know about such systems, in general? Under typical initiation events, such as shocks, arcs, sparks, heat, pressure waves, laser pulses, …, energetic materials decompose. In fact, using a mortar and a pestle, many such compounds will emit light flashes. These flashes come from many molecular and atomic sources: O±,0, N±,0, N2±,0, O2±,0, C2±,0, the subject molecules themselves, small radicals, and reaction intermediates. Even for systems from which triboluminescence1–10 is not obvious, excited electronic states can still be anticipated. Many mechanisms for triboluminescence have been suggested11–19 but, in general, as crystal planes fracture, large electric fields can ionize and fragment even such tightly bound systems as N2 (ionization energy ~ 15.6 eV and bond energy ~ 10 eV). Thus, even shock waves from gentle hand grinding can generate excited electronic valence, Rydberg, ionic, and atomic fragment states of organic molecules and small molecule inclusions. These excitations are a fact and cannot be ignored in consideration of the decomposition of energetic molecules and material. Of course, not all molecules that look similar to energetic ones are indeed energetic (e.g., RDX vs. a dinitropiperazine, Fig. 2.1) and we need to explain (theoretically) the energetic behavior for RDX and the nonenergetic behavior for its model systems. We have thus set ourselves the task of understanding the importance of and mechanisms for the decomposition of energetic and nonenergetic model molecules following excitation to higher electronic and eventually ionic states. Even if these excited state species are not the majority systems in the condensed phase, they can still play an essential role in the energy release process, as their products (e.g., NO, N2, NO2, …) can be highly vibrationally and translationally excited due to the eventual conversion of electronic excitation energy (5–10 eV) into vibrational and translational energy of small fragment molecules. Such “hot” fragments become very reactive in the condensed phase and can generate a chain of subsequent reactions to initiate explosive decomposition. Understanding the kinetics and dynamics of these mechanisms for energetic and model molecules will enable a fundamental explication of energetic behavior and its difference with respect to that of model systems. Thus, our program encompasses the experimental study of photoexcited, gas phase, cold, energetic, and nonenergetic model molecules to determine the initial steps in their dissociation: we identify the product molecule or radical and its rotational, vibrational, translational, electronic degrees of excitation, and its time evolution. From this information we determine, theoretically, a mechanistic pathway through the various PESs of the parent molecule that will generate such kinetics and dynamics. The essential theoretical effort is based on ab initio quantum chemistry calculations at a very high level (complete active space self-consistent field, CASSCF), and often with added second order perturbation theory (CAS PT2).

Our program is thereby both experimental and theoretical: experiments show us what is necessary to calculate and theory determines the kinetic mechanisms, dynamics, and reaction pathways and open channels based on the various PESs that must be accessed for the chemistry to occur. The initial reactant molecule is an isolated, gas phase, laser photoexcited (Sn ← So) parent molecule...