Modeling thermal decomposition mechanisms in gaseous and crystalline molecular materials: application to β-HMX.

Exploration of initiation of chemistry in materials is especially challenging when several coexisting chemical mechanisms are possible and many reactions' products are produced. It is even more difficult for complex materials, such as molecular, supramolecular, and hierarchical materials and systems. A strategy to draw a complete picture of the earliest stages of rapid decomposition reactions in molecular materials is presented in this study. The strategy is based on theoretical and computational modeling of chemical decomposition reactions in the gaseous and crystalline molecular material that has been performed by means of combined density functional theory and transition state theory. This study reveals how a crystalline field affects materials chemical degradation. We also demonstrate how incomplete results, which are often used due to difficulties in obtaining comprehensive data, can lead to erroneous conclusions and predictions. We discuss our approach in the context of the obtained reaction energies, activation barriers, structures of transition states, and reaction rates with the example of a representative molecular material, β-HMX, which tends to decompose violently with large energy release upon an external perturbation. The performed analysis helps to provide a consistent interpretation of available experimental data. The article illustrates that the complete picture of decomposition reactions of complex molecular materials, while theoretically challenging and computationally demanding, is possible and even practical at this point in time.