Theoretical Analysis of the Solid-State Terahertz Spectrum of the High Explosive RDX

In press, in the journal Chemical Physics Letters. The RDX (cyclotrimethylenetrinitramine) solid-state simulation is the third from the original series of terahertz spectra that served as my ICPRFP research focus. This system is of interest (well, to me anyway) for three reasons.

1. This is the largest DMol3 calculation performed to date for a THz simulation (Z = 8). Between optimization, normal mode analysis, and vibrational mode displacements for the difference dipole intensity calculations, 1.5 solid months on a quad-core box.

2. Zeitler and Taday provided a 7 K THz spectrum of the RDX (in black) to complement the original room temperature spectrum (in blue), providing a far more resolved and feature-rich data set for theoretical comparisons.

3. As obvious from the high-resolution data, the RDX solid-state spectrum contains considerable vibrational structure. In fact, this THz spectrum contains more resolved peaks than there are vibrational modes in the isolated-molecule calculation in this region. While it has been demonstrated in several previous publications that solid-state THz spectra cannot be assigned using isolated-molecule calculations, all of the molecules in the previous studies contained enough vibrational structure in the THz region to potentially let the uninformed researcher attempt to get away with an isolated-molecule assignment. This is not the case for RDX, which contains only 6 isolated-molecule vibrational modes below 125 cm-1.

Damian G. Allis1,2, J. Axel Zeitler3, Philip F. Taday4, and Timothy M. Korter1

1. Syracuse University, Department of Chemistry, 1-014 CST, 111 College Place, Syracuse, NY 13244-4100 USA
2. Nanorex, Inc., P.O. Box 7188, Bloomfield Hills, MI 48302-7188 USA
3. Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK
4. TeraView Limited, Platinum Building, St. Johns Innovation Park, Cambridge CB4 0WS, UK

Abstract: The solid-state terahertz (THz) spectrum (2 – 120 cm-1) of α-form cyclotrimethylenetrinitramine (RDX) has been simulated using solid-state density functional calculations at a BP/DNP level of theory. BP/DNP features are in good agreement with both 298 K and a new 7 K polycrystalline RDX THz spectrum. The 7 K RDX spectrum is noteworthy for several mode shifts and spectral detail that greatly aids mode assignments. Previous RDX isolated-molecule calculations (with 6 calculated modes below 125 cm-1) are incapable of accurately predicting the numerous features in this region, highlighting the importance of solid-state theoretical methods for solid-state terahertz feature assignments.

Keywords: Cyclotrimethylenetrinitramine, RDX, solid-state density functional theory, terahertz (THz) spectroscopy

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