A Combined Experimental and Theoretical Study of HMX(Octogen, Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine) in theGas Phase

Angelika H‰u˚ler and Thomas M. Klapˆtke*

Department of Chemistry, Ludwig-Maximilians University of Munich, D-81377 Munich (Germany)

Gerhard Holl and Manfred Kaiser

Bundeswehr Research Institute for Materials, Fuels and Lubricants, Swisttal-Heimertsheim, D-53913 Swisttal (Germany)

Summary

The structures of �- and �-HMX were fully optimized and thevibrational frequencies computed at the hybrid DFT B3LYP/6-31G(d, p) level of theory. The DCI�mass spectrum of HMX usingammonia (NH3) as a ™soft∫ ionising gas is reported. Fielddesorption mass spectrometry (FD) was used because of thehigh molecular intensities and low fragmentation. The FDspectrum shows only one significant peak at m/e 297 due to theprotonated molecular ion [M�H]�. These observations clearlyestablish that under the conditions of CI and FD mass spectrom-etry experiments HMX is present in the gas phase (withoutdecomposition) prior to the chemical ionisation.

1. Introduction

HMX is a conformationally flexible cyclicmoleculewhichexists in two primary (gasphase) conformations(1±9). In the �form, the HMX molecule has C2 symmetry as illustrated inFigure 1. In the � form the conformation of the eight-membered ring is a boat form in the sense that the NO2

groups are positioned on one side of the molecule. Themolecular conformation of the � form is quite different asillustrated in Figure 2. Here, the molecule has Ci symmetry.The ring in �-HMX is found to exist in more of a chair formgiving the entire molecule a center of symmetry.In the crystalline state HMX exists in four polymorphs,

labeled � through �. The � form corresponds to theC2 formof the isolatedmolecule (� form)with in the � and� form theconformations of HMX being quite similar to that found inthe � form but without precise 2-fold axial symmetry. �-HMX is the commonly encountered form but the otherpolymorphs have relevance to the thermal decomposition ofHMX.The ambient thermal stabilities of the four crystallineforms are ���� �� �.

2. Methods

All calculations were carried out using the programpackage Gaussian 98(10). Since the density functional theory(DFT) methods achieve significantly greater accuracy thanHartree-Fock (HF) methods, especially for the calculationof vibrational frequencies, at only a moderate increase incost (CPU time) we decided to do all calculations at DFTlevel of theory. The DFT methods include some of theeffects of electron correlation much less expensively thantraditional correlatedmethods (e.g.MP2). InDFT, the exactexchange (HF) for a single determinant is replaced by amore general expression, the exchange-correlation func-tional, which can include terms accounting for bothexchange energy and the electron correlation which isomitted from HF theory. (N.B. A functional is defined inmathematics as a function of a function. InDFT, functionalsare functions of the electron densitywhich itself is a functionof coordinates in space)(11, 12). In other words, DFT methodscompute electron correlation via general functionals of theelectron density. DFT functionals partition the electronicenergy into several components which are computedseparately:

(i) the kinetic energy(ii) the electron-nuclear interaction(iii) the Coulomb repulsion(iv) the exchange-correlation term for the remainder of the

electron-electron interaction which is usually itselfdivided into separate exchange and correlation com-ponents.

In this work, the so-called hybrid DFT method wasapplied in which a hybrid functional is used, which definesthe exchange functional as a linear combination ofHF, local,and gradient-corrected exchange terms. This exchangefunctional is then combined with a local and gradientcorrected correlation functional. In this studyBecke×s three-parameter functional (B3LYP) was applied where the non-

¹ WILEY-VCH Verlag GmbH, 69469 Weinheim, Germany, 2002 0721-3113/02/2701-0012 $ 17.50+.50/0

* Corresponding author; e-mail: tmk@cup.uni-muenchen.de

12 Propellants, Explosives, Pyrotechnics 27, 12 ± 15 (2002)

local correlation is provided by the LYP (Lee, Yang, Parr)correlation functional(13±16).All structures were fully optimized and the vibrational

frequencies computed at B3LYP/6-31G(d, p) level of theo-ry. For a simple definition of the terms ab initio, basis sets,Hartree-Fock and DFT were used, including the Gaussian(Pople) notation of basis sets see Ref. 16.

3. Results

The obtained computational results are summarized inTable 1. The fully optimized structures of�- and �-HMXareshown in Figures 1 and 2.

Whereas for RDX IR absorption spectra have recentlybeen reported for the vapor phase(17) in a combinedexperimental and theoretical study (HF/6-31G(d)) to ourknowledge there has been no report on a gas-phase IR studyof HMX. Our attempt to obtain a gas-phase IR spectrum ofHMX using a heated Ventacon IR gas cell (Perkin ElmerSpectrum One FT-IR spectrometer) was not successful andonly showed gas phase species due to the decomposition ofHMX.Whereas Electron Impact Ionisation (EI) mass spectra of

HMX have been reported by two independent groups(18±20),no peaks corresponding to the molecular ion were observedleading to the conclusion that nitroaliphatics and polynitrocompounds do not yield amolecular ion on electron-impact.There have also been two reports on Field Desorption (FD)and Direct Chemical Ionisation (DCI� ) mass spectra ofHMX(21, 22).In the present study we again recorded the DCI�mass

spectrumofHMXusing ammonia (NH3) as a ™soft∫ ionisinggas (Fig. 3). The spectrum clearly shows the basis peak atm/e �314 which corresponds to the molecular ion adductwith NH�

4 , i.e. [M�H�NH3]�. This observation clearlyestablishes that under the conditions of the mass spectrom-etry experiment HMX does go into the gas phase (withoutdecomposition) prior to the chemical fragmentation. Othersignificant peaks in theDCI� spectrumofHMXarem/e 250[M�NO2]� and m/e 176 [3/4M�NO2]�. This result corre-sponds to the CI (NH3) mass spectrum of HMX reported byVouros et al.(22). Molecular ion adducts are also formed withhydrogen(23) and methane(22) as reagent gases. When iso-butene(24) and water(25) were used as CI reagent gases nomolecular ions were observed. The highest peak of thenegative ion chemical ionisation mass spectrum of HMXusing isobutene as a reagent gas corresponds to [M�CH2

NNO2](26).FD mass spectrometry was used because of the high

molecular intensities and low fragmentation. During theexperiment with HMX the desorption of the moleculeseemed to be more statistically than continuously. With

Table 1. Computational results for HMX in its � and � form;B3LYP/6-31G(d, p).

�-HMX �-HMX

�E/a.u. 1196.553477 1196.556784Erel/kcal mol�1 2.1 0.0NIMAG 0 0zpe/kcal mol�1 120.1 120.4Symmetry C2 Cicharacteristic (strong) IRvibrations/cm�1 (rel. int.)wag (CH2) 905 (1.00) 948 (0.28)wag (CH2) 932 (0.14)wag (CH2) 932 (0.26)wag (CH2) 964 (0.25)wag (CH2) (�); �(CN�NN) (�) 1019 (0.18) 1087 (0.14)wag (CH2) 1091 (0.11)�(NN) 1164 (0.19)�(CN) 1241 (0.2)�(NN) (�); �(CN) (�) 1252 (0.10) 1249 (0.18)�(CN) 1286 (0.61)� sym (NO2) 1316 (0.65) 1320 (1.00)� sym (NO2) 1336 (0.10) 1336 (0.46)� sym (NO2) 1370 (0.51)� (CH2) 1436 (0.10)� (CH2) (�); � sym (NO2) (�) 1489 (0.22) 1502 (0.12)� asym (NO2) 1693 (0.84) 1673 (0.55)� asym (NO2) 1696 (0.80) 1685 (0.58)

Figure 1. Molecular structure of �-HMX, fully optimized atB3LYP/6-31G(d, p) level of theory (in the bottom structure the Hatoms have been omitted for clarity).

Figure 2. Molecular structure of �-HMX, fully optimized atB3LYP/6-31G(d, p) level of theory (in the bottom structure the Hatoms have been omitted for clarity).

A Combined Experimental and Theoretical Study of HMX 13Propellants, Explosives, Pyrotechnics 27, 12 ± 15 (2002)

raising temperature of the emitter the fragmentationincreased. The spectrum shows only one significant peakat m/e 297 due to the protonated molecular ion [M�H]�

(Fig. 4). The reported FD mass spectrum of HMX(27) alsoshows the peak of the protonated molecular ion but alsopeaks of fragment ions, which partly are more intense.In the fast atomic bombardment (FAB, Xe) mass spec-

trum of HMX no peaks due to HMX were observed.These observations clearly establish that under the

conditions of CI and FD mass spectrometry experimentsHMX is present in the gas phase (without decomposition)prior to the chemical ionisation.

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Figure 3. DCI�mass spectrum of HMX.

Figure 4. FD mass spectrum of HMX.

14 H‰u˚ler, Klapˆtke, Holl, Kaiser Propellants, Explosives, Pyrotechnics 27, 12 ± 15 (2002)

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Acknowledgements

Financial support of this work by the University of Munich(LMU), the Fonds der Chemischen Industrie and the GermanFederal Office of Defense Technology and Procurement (BWB) isgratefully acknowledged. We also thank Dr. G. Fischer for record-ing the mass spectra.

(Received September 24, 2001; Ms 2001/059)

A Combined Experimental and Theoretical Study of HMX 15Propellants, Explosives, Pyrotechnics 27, 12 ± 15 (2002)