Ab initio calculations of the effects of H+ and NH4+ on the initial decomposition of HMX
In this work, the effects of H+ and NH4+ on the initial decomposition of HMX were investigated on the basis of the B3P86/6-31G** and B3LYP/6-31G* calculations. Three initial decomposition pathways including the NNO2 bond fission, HONO elimination and CN bond dissociation were considered for the complexes formed by HMX with H+ (PHMX1 and PHMX2) or with NH4+ (AHMX). We found that H+ and NH4+ did not evidently induce the HMX to trigger the NNO2 heterolysis because the energy barrier of NNO2 heterolysis was found to be higher than the bond dissociation energy of NNO2 homolytic cleavage. Meanwhile, the transition state barriers of the HONO elimination from the complexes were found to be similar to that from the isolated HMX, which means that the HONO elimination reaction of HMX was not affected by the H+ and NH4+. As for the ring-opening reaction of HMX due to the CN bond dissociation, the calculated potential energy profile showed that the energy of the complex (AHMX) went uphill along the CN bond length and no transition state existed on the curve. However, the transition state energy barriers of CN bond dissociation were calculated to be only 5.0kcal/mol and 5.5kcal/mol for the PHMX1 and PHMX2 complexes, respectively, which were much lower than the CN bond dissociation energy of isolated HMX. Moreover, among the three initial decomposition reactions, the CN bond dissociation was also the most energetically favorable pathway for the PHMX1 and PHMX2. Our calculation results showed that the H+ can significantly promote the initial thermal decomposition of CN bond of HMX, which, however, is influenced by NH4+ slightly.
ab initio calculation initial decomposition hmx complex bond dissociation energy