Unraveling the Key Role of the Benzyl Group in the Synthesis of CL-20 Precursor HBIW

As one of the most important energetic material molecules, hexanitrohexaazaisowurtzitane (CL-20) can only be synthesized using an amine with a benzyl group. Moreover, the reaction mechanism remains unexplored and the special role of the benzyl group has not been revealed. To address these issues, we perform an extensive theoretical study to investigate the synthesis mechanism of CL-20 precursor HBIW by employing density functional theory. Our calculated results demonstrate that the benzyl group can reduce the energy of the intermediate and the energy barrier of the rate-determining step due to the π-π stack interaction between two benzene rings of the benzyl group. For the first time, we revealed that the reactions can produce 16 intermediates with different chiralities during the formation of the first two side five-membered rings and only two of which can further form the bottom six-membered ring and finally obtain the product HBIW. The steric hindrance effect of the benzyl group leads to the formation of a higher-energy intermediate first, thereby providing an opportunity to correct the wrong chirality. All of these factors make the diimine with the benzyl group the most suitable reactant for the synthesis of CL-20.