Nonadiabatic Reaction of Energetic Molecules

Abstract

Energetic materials store a large amount of chemical energy that can be readily converted into mechanical energy via decomposition. A number of different ignition processes such as sparks, shocks, heat, or arcs can initiate the excited electronic state decomposition of energetic materials. Experiments have demonstrated the essential role of excited electronic state decomposition in the energy conversion process. A full understanding of the mechanisms for the decomposition of energetic materials from excited electronic states will require the investigation and analysis of the specific topography of the excited electronic potential energy surfaces (PESs) of these molecules. The crossing of multidimensional electronic PESs creates a funnel-like topography, known as conical intersections (CIs). CIs are well established as a controlling factor in the excited electronic state decomposition of polyatomic molecules. This Account summarizes our current understanding of the nonadiabatic unimolecular chemistry of energetic materials through CIs and presents the essential role of CIs in the determination of decomposition pathways of these energetic systems. Because of the involvement of more than one PES, a decomposition process involving CIs is an electronically nonadiabatic mechanism. Based on our experimental observations and theoretical calculations, we find that a nonadiabatic reaction through CIs dominates the initial decomposition process of energetic materials from excited electronic states. Although the nonadiabatic behavior of some polyatomic molecules has been well studied, the role of nonadiabatic reactions in the excited electronic state decomposition of energetic molecules has not been well investigated. We use both nanosecond energy-resolved and femtosecond time-resolved spectroscopic techniques to determine the decomposition mechanism and dynamics of energetic species experimentally. Subsequently, we employ multiconfigurational methodologies (such as, CASSCF, CASMP2) to model nonadiabatic molecular processes of energetic molecules computationally. Synergism between experiment and theory establishes a coherent description of the nonadiabatic reactivity of energetic materials at a molecular level. Energetic systems discussed in this Account are nitramine- and furazan-based species. Both energetic species and their respective model systems, which are not energetic, are studied and discussed in detail. The model systems have similar molecular structures to those of the energetic species and help significantly in understanding the decomposition behavior of the larger and more complex energetic molecules. Our results for the above systems of interest confirm the existence of CIs and energy barriers on the PESs of interest. The presence of the CIs and barriers along the various reaction coordinates control the nonadiabatic behavior of the decomposition process. The detailed nature of the PESs and their CIs consequently differentiate the energetic systems from model systems. Energy barriers to the chemically relevant low-lying CIs of a molecule determine whether that molecule is more or less "energetic".