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Article

Ab Initio Molecular Dynamics Study on the Initial Chemical Events in Nitramines: Thermal Decomposition of CL-20

Supporting Info

Olexandr Isayev^†, Leonid Gorb^†^‡, Mo Qasim^‡ and Jerzy Leszczynski*^†^‡

Computational Center of Molecular Structure and Interactions, Jackson State University, Jackson, Mississippi 39217, and U.S. Army Engineer Research and Development Center, Vicksburg, Mississippi 39180

J. Phys. Chem. B, 2008, 112 (35), pp 11005–11013

DOI: 10.1021/jp804765m

Publication Date (Web): August 8, 2008

†

Jackson State University.

‡

U.S. Army Engineer Research and Development Center.

* Corresponding author. E-mail: jerzy@ccmsi.us.

Abstract

CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane or HNIW) is a high-energy nitramine explosive. To improve atomistic understanding of the thermal decomposition of CL-20 gas and solid phases, we performed a series of ab initio molecular dynamics simulations. We found that during unimolecular decomposition, unlike other nitramines (e.g., RDX, HMX), CL-20 has only one distinct initial reaction channelhomolysis of the N−NO₂ bond. We did not observe any HONO elimination reaction during unimolecular decomposition, whereas the ring-breaking reaction was followed by NO₂ fission. Therefore, in spite of limited sampling, that provides a mostly qualitative picture, we proposed here a scheme of unimolecular decomposition of CL-20. The averaged product population over all trajectories was estimated at four HCN, two to four NO₂, two to four NO, one CO, and one OH molecule per one CL-20 molecule. Our simulations provide a detailed description of the chemical processes in the initial stages of thermal decomposition of condensed CL-20, allowing elucidation of key features of such processes as composition of primary reaction products, reaction timing, and Arrhenius behavior of the system. The primary reactions leading to NO₂, NO, N₂O, and N₂ occur at very early stages. We also estimated potential activation barriers for the formation of NO₂, which essentially determines overall decomposition kinetics and effective rate constants for NO₂ and N₂. The calculated solid-phase decomposition pathways correlate with available condensed-phase experimental data.