Theoretical insights into the synthesis reaction mechanism of HMX based on TAT nitration reaction

Improving the yield and reducing the production cost of high energy density energetic material 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) has been a problem for the past few decades. The synthesis of HMX by multi-step nitration of 1,3,5,7-tetraacyl-1,3,5,7-tetrazacyclooctane (TAT) is an efficient synthetic strategy, which has the advantages of mild reaction and high product recovery. However, there is a lack of sufficient theoretical research on the difficulty of TAT nitration and the difference in nitration rate at each step. To address these issues, the mechanism of TAT nitration to synthesize HMX was investigated by the density functional theory (DFT) calculations in this work. The combination of nitroxyl cation with the oxygen atom on the acetyl group hinders the nitration of the nitrogen atom on the ring skeleton. The second step of nitration of the nitrogen atom tends to occur in the para position. The rate difference between the nitration steps is determined by the driving force of the reverse reaction of the unstable intermediate and the energy barrier of the forward reaction of the stable intermediate. Through this work, the synthesis mechanism of HMX from TAT could be known more clearly.