Introduction
Hexanitrohexaazaisowurtzitane (HNIW), well-known as CL-20, is one of the most powerful high-energy molecules.\cite{Nair_2005} Like octogen (HMX) and hexogen (RDX), CL-20 belongs to the class of the nitroamine explosives. Due to its strained carbon-nitrogen skeleton with six attached nitro groups, CL-20 possesses much higher performance than HMX.\cite{Simpson_1997} CL-20 was firstly synthesized from benzylamine and glyoxal in 1987 and began to be produced industrially from the 1990-s.\cite{Sysolyatin_2005} CL-20 can be obtained in both gaseous and condensed phases. Five different phases of solid CL-20, known as α, β, γ, ε, and ζ modifications, are possible.\cite{Russell_1993,Tan_2011} All of them are the molecular crystals with the weak van-der-Waals bonds between the CL-20 molecules. However, ε modification is the most stable under normal conditions.\cite{Foltz_1994}
With regard to the practical importance of CL-20, there were many efforts to improve its properties (density, power, detonation velocity, sensitivity, etc.). The most suitable way to do this is co-crystallization of CL-20 with the other high-energy molecules that can fill hollows between isolated CL-20s. Co-crystals of CL-20 with HMX,\cite{Bolton_2012} caprolactam,\cite{Guo_2013} MDNT,\cite{Anderson_2016} DNT,\cite{Liu_2016} TNT,\cite{Li_2013} TATB,\cite{Xu_2015} DNDAP,\cite{Liu_2018} and other compounds were recently prepared and characterized. All of them demonstrate some advantages over the pristine CL-20. Liu et.al. presented systematic consideration of different co-crystals based on CL-20.\cite{Liu_2018a}
There are some alternative ways to improve the CL-20 properties. One of them is the use of substitutional derivatives of CL-20, in which one or more carbon atoms are replaced by silicon.\cite{Tan_2014} Silicon is placed in the same column of the periodic table as carbon and, therefore, possesses similar properties. Many carbon compounds have their silicon-substituted analogous. For example, silicon analogs of the high-energy pentaerythrityl tetraazide (PETN) were successfully synthesized.\cite{Klap_tke_2007} They possess better sensitivity than the pristine carbon PETN.\cite{Liu_2009,Murray_2010} Another possibility for improving the CL-20 energy characteristics is to replace the NO2 groups by NF2 since fluorine demonstrates the higher oxidation activity than the oxygen. In addition, possible detonation product SiF4 has high negative formation enthalpy in comparison with SiO2.\cite{Tan_2014} Tan et.al. presented an extensive computational study of all possible CL-20 derivatives containing silicon and fluorine.\cite{Tan_2014} According to these calculations, some derivatives possess higher crystalline densities, decomposition reaction heats, detonation velocities, detonation pressures and explosion temperatures, than the pristine CL-20 (for example, heat of decomposition of unsubstituted CL-20 and its silicon derivative CSi5H6N12O12 at constant volume are equal to -3241.6 and -5045.3 kJ/mol, respectively). Both pristine and substituted CL-20s do not have dangling bonds and, therefore, can not form covalent bonds with each other. However, they presumably can form covalent complexes via the methylene “molecular bridges” after detaching several nitro groups.\cite{Degtyarenko_2014} Ab initio calculations confirm that dimers and larger covalent complexes based on CL-20 molecules are stable, and their stability increases with the effective size of the system.\cite{Katin_2017,Gimaldinova_2018} Such complexes have higher densities in comparison with the molecular crystals due to the shorter distances between the CL-20 cages.
Here we present the computational study of CL-20 derivatives containing silicon and fluorine atoms. The mechanisms of their pyrolysis are studied using the ab initio molecular dynamics and potential energy surfaces investigations. In addition, we study in detail the structure, reactivity and optical spectra of the corresponding dimers in which two cages are attached to each other through the methylene molecular bridges.