Covalently Bonded 1D Chains and 2D Networks From Si-Doped CL-20: Computational Study

Abstract

To discover high-energy-density materials with characteristics superior to current models, it is necessary to study a wide range of potential structures. A promising representative of new derivatives of the class of high-energy compounds is silicon-substituted molecules CL-20, which have a reactivity and kinetic stability close to pure CL-20 but have a higher density and energy release. Low-dimensional covalent SiCL-20 nanostructures based on silicon analogue of the classical CL-20 high-energy molecule are considered in this work. Covalent nanostructures may have advantages over molecular crystals due to their special properties, such as higher packing density and kinetic stability. It has been established that silicon-substituted CL-20 molecules can connect through CH₂ molecular bridges into covalent structures. Geometrical parameters, energy characteristics, electronic properties, and quantum chemical reactivity descriptors for several representatives of 1D and 2D systems based on Si₅CL-20 have been calculated using density functional theory. The skeleton of each silicon fragment of the CL-20 system undergoes small changes when combined into covalent chains and networks. Still, the systems retain their consistency, and the effective diameter of the silicon frameworks in the nanostructure takes average values from 4.300 to 4.462 Å. The binding energy of nanostructures increases with the number of silicon CL-20 fragments in the system. The binding energies for a single silicon molecule CL-20 and a double chain SiCL-20 consisting of 12 fragments are 3.846 and 4.077 eV/atom, respectively. Thus, the silicon nanostructures become more thermodynamically stable with increasing the size and dimension of the compound. The study of electronic characteristics made it possible to establish that the value of the HOMO-LUMO gap decreases with an increasing number of fragments in the system, and the considered SiCL-20 covalent molecules can be classified as wide-gap semiconductors, like their classical CL-20 analogues. For example, the values of the HOMO-LUMO gaps for silicon derivatives of CL-20 with dimensions 1 × 1, 6 × 1, 6 × 2, and 4 × 3L are 5.601, 4.378, 4.004, and 3.882 eV respectively. Despite their highly stressed skeleton, they are stable enough to be considered for energy applications and are promising candidates for building blocks of high-energy materials and fuels.