Dual Functions of Water in Stabilizing Metal-Pentazolate Hydrates [M (N5) 2 (H2O) 4]· 4H2O (M= Mn, Fe, Co, and Zn) High-Energy-Density Materials
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High-energy-density materials (HEDMs) containing the cyclo-pentazole anion (cyclo-N5) are highly desirable due to the release of more energy and being environmentally more friendly than conventional HEDMs. However, the synthesis of stable cyclo-N5-containing HEDMs has been a challenge. In this study, quantum mechanical calculations were employed to elucidate the stability of [M(N5)2(H2O)4]·4H2O (M = Mn, Fe, Co, and Zn), one of the few recently reported cyclo-N5-contained HEDMs, under ambient conditions. The results from our study indicate that the stability is due to the presence of two types of water (coordinated H2O (c-H2O) and hydrogen-bonded H2O (h-H2O)). Each type uses a unique mode to stabilize the highly reactive M(N5)2 cores. c-H2O binds with M to reduce the M ↔ cyclo-N5 interaction, leading to a less activated cyclo-N5 and higher kinetic barriers (Eas) for its decomposition. In contrast, h-H2O takes advantage of its permanent electrostatic interactions with cyclo-N5 to inhibit the decomposition. The stabilizing effects of the two types of water on M(N5)2 are similar. On the basis of the lower energy cost to remove h-H2O from the materials and the subsequent large decrease in the Ea due to this removal, we propose that h-H2O acts as a “safety device” that prevents the materials from becoming kinetically unstable. For future design of cyclo-N5-contained HEDMs, we proposed the use of various molecular building blocks, such as NH3, H2S, and PH3, which can tightly bind to M to reduce the M ↔ cyclo-N5 interaction and impose permanent electrostatic interactions with cyclo-N5 to provide the similar dual functions of H2O to suppress cyclo-N5 decomposition.
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