Theoretical study on the trans–cis isomerization and initial decomposition of energetic azofurazan and azoxyfurazan

In this work, we carried out the hybrid density functional theory (DFT) calculations in order to understand the thermal trans–cis isomerization and initial thermal decomposition of 3,3′-diamino-4,4′-azofurazan (DAAzF), 3,3′-diamino-4,4′-azoxyfurazan (DAAF), 3,3′-dinitro-4,4′-azofurazan (DAAF) and 3,3′-dinitro-4,4′-azoxyfurazan (DAAzF). The relative energy between the trans- and cis-isomer was also calculated at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level of theory. We found that a negative correlation existed between the relative energy and the sensitivity for these energetic azofurazan and azoxyfurazan compounds, where the higher relative energy means the lower the sensitivity. It was also found that the oxidation of azo-group could cause the decreasing in the relative energy between the trans- and cis-isomer, as well as the alteration of the isomerization mechanism. An inversion mechanism operates for azofurazan compounds (DAAzF and DNAF) while a rotation mechanism works for azoxyfurazan compounds (DAAF and DNOAF). Compared with the thermal trans–cis isomerization, the homolytic cleavage of C–N bond needs to overcome a much higher energy barrier, which indicates that the energy of the external stimulus should firstly trigger the trans–cis isomerization, rather than the breakage of C–N bond. A self-desensitization effect caused by the reversible thermal trans–cis isomerization process was firstly proposed to explain that the azofurazan and azoxyfurazan compounds are class of energetic materials with lower sensitivity. This new concept (self-desensitization effect) is expected to be useful to design the novel high density, insensitive energetic material.