The effect of external forces on the initial dissociation of RDX (1,3,5-trinitro-1,3,5-triazine): A mechanochemical study

Abstract

Experimental and theoretical studies have proposed different initiation reactions for the decomposition of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Three primary reactions are considered to start RDX decomposition: homolytic NAN bond fission, HONO elimination, and concerted fission of CAN bonds. The focus of this article is to study the effect of external forces on the energy barrier and reaction energies of all three mechanisms. We used the Nudged Elastic Band method along with ab initio Density Functional Theory within the framework of a generalized force-modified potential energy surface (G-FMPES) to calculate the minimum energy paths at different compressive (corresponding to pressure between approximately 6 and 294 MPa) andexpansive force values (between 10 and 264 pN). For all three reactions, the application of an expansive force increases the exothermicity and lowers the energy barriers to different extents, while a compressive force decreases the exothermicity and raises the energy barrier to different extents. K E Y W O R D SDFT, G-FMPES, mechanochemistry, RDX, shock wave | I N T R O D U C T I O NHexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is an important chemical compound widely used as a propellant and an explosive. It is also the smallest compound with qualities very similar to more powerful explosives like HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazocine) and CL20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane). RDX has been investigated for over 60 years on the nature of its structure [1][2][3][4][5][6][7] and on the nature of its decomposition mechanism by both experimental [8][9][10][11][12][13][14][15][16][17][18][19][20][21] and theoretical [3,[22][23][24][25][26][27][28][29] studies. Structural studies have shown that several conformers of RDX can coexist [7,30,31] and due to the low energy barrier, the various RDX forms can easily interconvert into one another. With regard to the decomposition mechanism of RDX, different possibilities have been investigated as initial dissociation reactions (see Figure 1). The lowest activation energies have been found for the homolytic NAN fission and the HONO elimination [23][24][25]29,32] ; rendering the concerted fission of CAN bonds (previously thought to be the primary decomposition path [13] ) as a secondary initiation path. However, experimental studies have shown evidence for all three mechanisms. The concerted mechanism was observed only in gas phase experiments, [13,33] while the other two mechanisms, NAN fission and HONO elimination, were observed in crystalline RDX. [20,34] For example, Patterson et al. [20] used time-resolved spectroscopy to study the decomposition of RDX induced by shock wave, and they found evidence that supports the NAN fission and the HONO elimination mechanisms.Theoretical studies that calculate energy barriers [25] for the decomposition mechanisms have focused on calculations in the gas phase. However, most scenarios that lead to RDX decomposition involve the passage of a shock wave or other exte...

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