Ab initio calculations of the effects of H + and NH 4+ on the initial decomposition of HMX

Abstract

In this work, the effects of H + and NH 4 + 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 N–NO 2 bond fission, HONO elimination and C–N bond dissociation were considered for the complexes formed by HMX with H + (PHMX1 and PHMX2) or with NH 4 + (AHMX). We found that H + and NH 4 + did not evidently induce the HMX to trigger the N–NO 2 heterolysis because the energy barrier of N–NO 2 heterolysis was found to be higher than the bond dissociation energy of N–NO 2 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 NH 4 +. As for the ring-opening reaction of HMX due to the C–N bond dissociation, the calculated potential energy profile showed that the energy of the complex (AHMX) went uphill along the C–N bond length and no transition state existed on the curve. However, the transition state energy barriers of C–N bond dissociation were calculated to be only 5.0 kcal/mol and 5.5 kcal/mol for the PHMX1 and PHMX2 complexes, respectively, which were much lower than the C–N bond dissociation energy of isolated HMX. Moreover, among the three initial decomposition reactions, the C–N 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 C–N bond of HMX, which, however, is influenced by NH 4 + slightly.