Ab initio calculation of ionization potential and electron affinity of six common explosive compounds

Jason K Cooper,¹ Christian D Grant,² Jin Z Zhang<sup>1

1</sup>Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA, ²Physical and Life Sciences, Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA, USA

Abstract: The vertical and adiabatic ionization potential (IP_V and IP_A) and vertical electron affinity (EA_V) for six explosives, hexogen (RDX), octogen (HMX), triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD), 2,4,6-trinitrotoluene (TNT), and pentaerythritol tetranitrate (PETN), have been studied using ab initio computational methods. The IP_V was calculated using MP2, CBS-QB3, and Koopmans’ theory, while the IP_A was calculated with B3LYP, CAM-B3LYP, ψB97XD, B2PLYP, and MP2 using the ΔE method for the ground state B3LYP optimized geometries. IP_As of RDX and TNT were also calculated using CBS-QB3 with relaxed geometries of the ions. Of the methods tested, B3LYP and B2PLYPD provided superior and more consistent results for calculating the IP compared to CBS-QB3 level IP_A calculations and experimental data (where available). CBS-QB3 was used as a benchmark for calculating the EA_V as experimental data has not been reported. For calculations of the EA_V, B3LYP performed the worst while MP2 and B2PLYPD predicted values closest to those made by CBS-QB3. Basis set effects were evaluated using 6-31+G(d,p), 6-311+G(d,p), and 6-311+G(3df,2p) for both IP and EA. 6-31+G(d,p) gave satisfactory results for calculating IP while 6-311+G(3df,2p) had improved results for calculating the EA. The four nitro-containing compounds have exothermic reduction potentials while the peroxides are endothermic. In addition, it was determined that RDX, HMX, TATP, and HMTD had unstable geometries in their reduced forms. The results should be useful in developing detection and screening methods including ionization methods for mass spectroscopy and fluorescence quenching methods of detection.

Keywords: ionization, affinity, explosive, TNT, ab initio, DFT