Initiation reactivity of cyclic nitramines mixed crystals from the perspective of XPS application

Abstract

The nitramines ε- and β-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitanes (ε-CL-20 and β-CL-20), cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5-d]imidazole (BCHMX), six β-CL20/BCHMX co-agglomerates (CACs) and 1,3,5-trinitro-1,3,5-triazepane (TTAZ) have been subjected to XPS analysis to determine their N 1s (N-NO2) and O 1s (NO2) binding energies (Ebind). The Ebind values for N 1s have been found to be closely related to the molecular structure of the nitramines studied; their growth logically corresponds to a decrease in sensitivity to both impact and electric spark. The Ebind of O1s is mostly inversely related to the energy of impact and directly proportional to that of sparks. For detonation rates, the Ebind values for pure nitramines are directly proportional; for CACs, however, this relationship is inversely proportional. These findings are supported by analogous Ebind relations with crystal density, especially in the case of the O 1s. In terms of co-crystal identification among CACs, there are auxiliary relations exist between Ebind values on the one hand and the values of Raman ring deformation vibration (wrt O 1s Ebind) and FTIR symmetric N-O stretching (wrt N 1s Ebind) on the other. All the relationships found are discussed. The Hirshfeld analysis of CL20 shows more stable intermolecular fingerprint plots for its β-isomer than for its ɛ-analog, which might be a reason for the stabilization of this nitramine in co-crystals.