Bonding analysis of the effect of strain on trigger bonds in organic-cage energetic materials
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Bonding analysis of the effect of strain on trigger bonds in organic‑cage energetic materials Craig A. Bayse1 · Mohammad Jaffar1 Received: 14 January 2020 / Accepted: 6 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Wiberg bond index and natural bond orbital analyses were used to examine the effect of strain on trigger bonds for a series of organic-cage molecules. In substituted cage hydrocarbons, weakening of the interior C–C bonds and strengthening of exterior C–X bonds are explained in terms of the contributions of the carbon 2s and 2p atomic orbitals to the bonds. The interior C–C, rather than C–NO2, bonds are expected to break to initiate explosive decomposition in nitro-substituted tetrahedranes, prismanes, and cubanes. Activation of C–C bonds in cage molecules increases with nitro substitution, but persubstituted cages are not necessarily the most activated. For example, heptanitrocubane is predicted to be more sensitive than octanitrocubane, consistent with experimental studies. In contrast, for a series of hypothetical nitroester substituted prismanes, the strengthening of the exterior C–ONO2 bond weakens the O–NO2 bond more than the interior C–C bonds. In CL-20 and TEX, cage strain as determined by WBI analysis in the fused five- and six-membered rings is less significant than for the cage hydrocarbon. In these known energetic materials, the N–NO2 trigger bonds are activated by the orientation of the nitro groups. Keywords Density functional theory · Energetic materials · Wiberg bond indices · Cage compounds · Natural bond orbitals
1 Introduction Understanding the molecular features that lead to explosive decomposition is important for improving the performance of energetic materials (EMs) while lowering their potential environmental impact [1–3]. Explosophores, such as nitro or azide groups, are substituents which facilitate explosive decomposition by incorporating bonds readily broken under conditions of impact, shock, friction, or other stimulative events [4]. Within EMs, the trigger bond is defined as the linkage that breaks to initiate the explosion [5]. These bonds are typically associated with an explosophore and are weakened, or activated, toward cleavage. For example, in nitrobased EMs, a C–NO2 is often the trigger bond. In organic cage compounds, energetic properties are additionally Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00214-020-02604-0) contains supplementary material, which is available to authorized users. * Craig A. Bayse [email protected] 1
Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529, USA
influenced by strained bonds that lead to high heats of formation and compact structures [6]. Examples of known organic-cage EMs include hexaazaisowurtzitane-based CL-20 and the structurally related explosive TEX (Fig. 1). The primary application of CL-20, proposed in 1979 and synthesized for the first time at the China Lake weapons development faci
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