dissociative electron attachment to the nitroamine hmx...

B American Society for Mass Spectrometry, 2013DOI: 10.1007/s13361-013-0588-y

J. Am. Soc. Mass Spectrom. (2013) 24:744Y752

RESEARCH ARTICLE

Dissociative Electron Attachment to the Nitroamine HMX(Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine)

Johannes Postler,1 Marcelo M. Goulart,1,3 Carolina Matias,1 Andreas Mauracher,1

Filipe Ferreira da Silva,2 Paul Scheier,1 Paulo Limão-Vieira,2 Stephan Denifl1

1Institut für Ionenphysik und Angewandte Physik, Technikerstr. 25 / 3, A-6020 Innsbruck, Austria2Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia,Universidade Nova de Lisboa, 2829-516 Caparica, Portugal3CAPES Foundation, Ministry of Education of Brazil, Brasilia, DF 70040-020 Brazil

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Abstract. In the present study, dissociative electron attachment (DEA) measure-ments with gas phase HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine,C4H8N8O

8, have been performed by means of a crossed electron-molecular

beam experiment. The most intense signals are observed at 46 and 176 u andassigned to NO

2

− and C3H6N5O

4

−, respectively. Anion efficiency curves for 15negatively charged fragments have been measured in the electron energy regionfrom about 0–20 eV with an energy resolution of ~0.7 eV. Product anions areobserved mainly in the low energy region, near 0 eV, arising from surprisinglycomplex reactions associated with multiple bond cleavages and structural andelectronic rearrangement. The remarkable instability of HMX towards electron

attachment with virtually zero kinetic energy reflects the highly explosive nature of this compound.Substantially different intensity ratios of resonances for common fragment anions allow distinguishing thenitroamines HMX and royal demolition explosive molecule (RDX) in negative ion mass spectrometry basedon free electron capture.Key words: Free electron attachment, Explosives, Dissociation, Nitroamines

Received: 10 December 2012/Revised: 10 January 2013/Accepted: 17 January 2013/Published online: 13 March 2013

Introduction

T he constant and actual need for detection of explosivesand the development of techniques to distinguish them

among several other similar yet unperilous substances hasincreased over the last years because of the high risk ofterrorist attacks. One of the key issues is the fast capabilityto distinguish explosives amongst a background of othernitrogen-containing compounds. A large variety of massspectrometric methods have been suggested for rapidexplosive detection, including laser photon ionization [1,2], ion mobility spectrometry [3, 4], chemical ionizationmass spectrometry [5, 6], proton transfer reaction massspectrometry [7, 8], and negative ion mass spectrometrybased on free electron capture [9–12] (see also the extensivereview by Moore [13]). The latter method benefits from theremarkably high electron attachment cross sections ofexplosive compounds. In recent years, a series of studies

on dissociative electron attachment (DEA) to several nitrocompounds used as explosives have been undertaken [14–24]. These investigations have been performed in crossedelectron-molecular beam experiments with high-energyresolution (~70 meV) [20] or high sensitivity (utilizing ~10μA of electron current) [25]. These experiments haveincluded free electron interactions with bare molecules inthe gas phase [14–24] and embedded in He droplets [26]. Inthe former, and generally speaking, capture of an excesselectron with virtually no kinetic energy leads to formationof a variety of DEA fragments reflecting therefore theexplosive nature of the compounds, whereas in the latter theultra-cold environment efficiently quenches all the gas-phasedissociation channels. DEA studies to explosives yieldingNO2

– formation, allowed considering this fragment to serve asa fingerprint for the identification of the neutral compound.However, no previous DEA study on octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, C4H8N8O8, commerciallyknown as HMX, exists in the literature to our best knowledge.HMX is a powerful and relatively insensitive nitroamine highexplosive and chemically related to Royal Demolition Explo-sive (RDX). HMX and RDX are nitroamine polymers

Correspondence to: Paulo Limão-Vieira; e-mail: [email protected],Stephan Denifl; e-mail: [email protected]

consisting of four and three CH2NNO2 units (see Figure 1),respectively. Compared with other explosives, HMX has aparticular low vapor pressure [27] representing a challengingcompound for detection in the context of true-to-life situationsas well as gas phase experiments in laboratory. Electrosprayionisation (ESI)/ion mobility spectrometry [28] and ESIcoupled with Fourier transform ion cyclotron resonance massspectrometry [29] were utilized for composition analysis ofexplosives mixture also containing RDX and HMX, whereasplasma desorption mass spectrometry with HMX revealed CN–

and NO2– as the most abundant species [30].

In the present work, we investigate the negative ionformation from neutral HMX in the gas phase upon freeelectron capture at low electron energies (0–20 eV) byrecording the ion yield as a function of the electron energywith a modest electron energy resolution of ~0.7 eV. By far,the two most dominant signals are due to formation ofC3H6N5O4

− and NO2–. It is shown that at the threshold of

~0 eV a variety of intense DEA products are formed.

Experimental andComputational DetailsElectron attachment to HMX was investigated by means of acrossed electron-molecular beam set-up utilizing a doublefocusing two-sector field mass spectrometer equipped with astandard Nier-type ion source [25]. The electron energy spreadclose to 0 eV is about 0.7 eV full width at half maximum(FWHM), but the source is characterized by the high sensitivityand the rapid extraction of the anions from the ion source (lessthan 1 μs). The electron current is regulated to 50 μA, which isreached for electron energies higher than 15 eV. Below that, theelectron current varies linearly. A voltage drop of 6 kVaccelerates the ions from the ion source towards the sector fields.Negative ion yields are obtained as a function of the electronenergy. HMX is solid at room temperature and, therefore, has tobe heated in order to increase its vapour pressure so that atmoderately elevated temperatures an effusive molecular beamcan be generated. The effusive molecular beam emerges from aheated oven through an orifice of 3 mm diameter operated at atemperature of around 370 K, which is well below the meltingpoint of HMX. Thermal decomposition of HMX starts attemperatures above the melting point of 481 K. The HMXsample was obtained from defusing section of the Austrian

ministry of interior and contained an unknown amount ofhydrocarbons. These impurities however led to no contaminationof anion yields from HMX. The electron energy scale and theelectron energy resolution are calibrated using the well-knownSF6

−/SF6 signal near 0 eV and the resonances of the F−/SF6 andF2

−/SF6 anions at higher electron energies [31]. Metastabledissociation of anions in the field free region between themagnetic and electric sector are investigated by the massanalyzed ion kinetic energy (MIKE) technique. Thereby ananion with mass m0 is selected by the magnet and the electricsector field voltage is scanned.While a stable anionwithmassm0

passes the electric sector at U0, the anion (with mass m1) formedin a metastable decay from m0 passes the electric sector at thereduced sector field voltage U10U0m1/m0. From the MIKE scanthe kinetic energy release distribution (KERD) can be derived.

To complement the experimental results, we haveperformed quantum chemical calculations utilizing Møller-Plesset perturbation theory truncated at the second order(MP2) for geometry optimizations, visualization of themolecular orbitals, and calculation of energetics togetherwith the 6-311++G(2d,p) basis set. The uncertainty of theenergies derived at this level of theory and basis set isapproximately at ±0.20 eV and was derived from a series oftest calculations for nitro-organic compounds, where wecompared the energetics derived from MP2/6-311++G(2d,p)with values from G4(MP2) calculations, which have aknown uncertainty of approximately ±0.1 eV [32].

Results and DiscussionNegative Ion Mass Spectra

Figure 2 (upper diagram) shows the negative ion massspectrum of HMX obtained by summation of individual massspectra measured at several different electron energies (i.e., fromthe energy close to 0 up to 10 eV in eight steps). This figureprovides an overview of all fragment anions formed, despite thefact that for a mass spectrum recorded at one electron energyonly those anions appear which are produced in a resonancenear the chosen electron energy. The most significant anions arelisted in Table 1 together with the position of the correspondingresonances and the relative contribution to the total anion yield(integrated over the whole electron energy range studied).

In general, capture of a free electron by a polyatomicmolecule (represented as ABC) generates a transientnegative ion (TNI), (ABC)#–, which may further decomposevia the following processes:

e� þ ABC ! ABCð Þ# � ! ABþ C� ð1aÞ

e� þ ABC ! ABCð Þ# � ! ACþ B� ð1bÞHMX RDX

Figure 1. Chemical structures of HMX and RDX

J. Postler et al.: Dissociative Electron Attachment to HMX 745

The TNI is seen as a quasi-bound state embedded in theautodetachment continuum and unstable towards the loss of theextra charge. The negative ion mass spectra of HMX indicatethat autodetachment or fragmentation occurs in a time windowshorter than the detection time, resulting in the absence of anobservable parent negative ion. This result is analogous toRDX previously studied in our laboratory [20] and is alsoreinforced by the considerable change in geometry from theneutral to the anion, resulting in the delocalization of the extracharge over one of the NO2-groups (Figure 3), leading to

fragmentation. Generally speaking, the formation of fragmentions in HMX through dissociative electron attachment (DEA)is most intense in features close to 0 eV.

The ion yields can be classified into four different groupsaccording to their magnitudes: (1) C3H6N5O4

− fragment (176 u)and NO2

– fragment (46 u) are the most dominant anions; (2) thesecond group comprises OH– (17 u) and the species at 102 and129 u where the former can be formed by a loss of an HCN fromthe anion at 129 u and the latter may result from the metastableparent anionM#– via loss of NO2HNO2HNO2HCN; (3) the thirdcomprises the masses 16, 26, 28, 60, 82, 93, 156, and 160 u,which have been identified as O–, CN–, CH2N

–, NNO2–,

[M – CNNO2H2NO2H2NO2NO2]– , [NO2HNO2]

– ,[M – 3NO2 – 2H]–, and [C2H2N5O4]

–, respectively; (4) masses203 and 250 u are assigned as [M –NO2HNO2]

– and [M –NO2]–,

respectively. The bottom diagram in Figure 2 shows the massspectrum of HMX obtained at 0 eV. The mass spectrum obtainedclose to 0 eV shows a rich chemistry driven upon low-energyelectron attachment to HMXwhich was also found for RDX (seeFigure 2 in Reference [20]) lending evidence to their moreexplosive character when compared to other explosives such asTNT [18] and the other nitrocompounds [14–17, 21, 22, 24].

In Figure 3, we show the fully optimized geometry obtainedat MP2/6-311++G(2d,p) level of theory and basis set for theneutral molecule (upper left part) and the highest occupiedmolecular orbital (HOMO, MO 76) (upper right part) derivedfrom the generalized density at MP2 level. Additionally, theoptimized geometry of the anionic structure (lower left part) isshown. It can be seen that the geometry shows noticeablechanges when an excess charge is added to the molecularsystem, with a clearly extended N–NO2 bond (from 1.38 to2.30 Å). The singly occupied molecular orbital (SOMO, MO77) is also shown in Figure 3 (lower right part). The excesselectron is localized in one of the four NO2-groups; particularlyin that with the extended N – NO2 bond. Two nodes are at leastalong the ring C – N bonds adjacent to the extended N – NO2

bond, lowering therefore the stability of the ring structure. Theadiabatic electron affinity of HMX is 1.35 eV (Table 2).

103

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156

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Figure 2. Negative ion mass spectrum of HMX obtained bysummation of individual mass spectra at several differentelectron energies (i.e., from the electron energy close to 0 eV upto 10 eV in eight steps (upper panel)). The mass spectrum in thelower panel is measured at the electron energy close to 0 eV.Please note that both mass spectra show peaks at 127 u/129 u(SF5

–) and 146/148 (SF6–) formed by electron attachment to the

calibration gas SF6. However, the anion yield at 129 u originatesboth from the sample (corresponding to [M – NO2HNO2

HNO2HCN]–) and the isotope of SF5

–. The peak at 130 u visible

in the zero eV spectrum can be ascribed to one of the isotopesof [M – NO2HNO2HNO2HCN]

–

Table 1. Peak Positions for the Fragment Ions of HMX Obtained in the Present Experiment

Mass (u) Anionic species assignment Relative intensity (%) Peak position (eV)

250 [M – NO2]– 0.002 ~0 – – –

203 [M – NO2HNO2]– 0.013 ~0 – 4.4 –

176 C3H6N5O4– 24.5 ~0 – 4.7 –

160 C2H2N5O4– 0.83 ~0 – 4.4 –

156 [M – NO2HNO2HNO2]– 0.14 ~0 – 4.4 –

129 [M – NO2HNO2HNO2HCN]– 0.81 0.1 – 4.5 –

102 CH2NCH2NNO2– 24 ~0 1.9 a 4.8 9.8

93 NO2HNO2– 0.13 ~0 – 5.0 9.7

82 C3H4N3– 1.51 ~0 – 5.0 9.8

60 NNO2– 3.83 0.3 – 5.8 9.8

46 NO2– 34.1 ~0 – 5.3 10

28 CH2N– 3.05 – 1.2 5.5 9.8

26 CN– 6.16 0.2 1.8 5.4 9.917 OH– 1.18 0.1 – 4.6 9.716 O– 2.58 – 2.2 4.9 –

aMeans shoulder structure

746 J. Postler et al.: Dissociative Electron Attachment to HMX

Ion Yield Curves

The anion efficiency curves1 of the most intense fragmentanions observed in the negative ion mass spectra are shownin Figures 4 and 5. The most intense DEA signals can befound close to 0 eV. Several fragment anions are alsoformed in an extended electron energy range showingresonance features at around 2, 5, and 10 eV. However,the relative abundance of these high-energy resonances is formost of the fragment anions at least one order of magnitudelower than the feature close to 0 eV. For heavier fragmentanions the low energy resonance is more dominant (i.e.,these anions decay into lower mass fragments if they areformed at high electron energies). Four out of 15 fragmentsare not predominantly formed close to 0 eV. Thesefragments were identified to be O–, CN–, CH2N

–, andNNO2

– at 16, 26, 28, and 60 u, respectively. While the TNIsgenerated at low energies may be assigned as shape

resonances involving the π* antibonding orbitals, it is likelythat the resonance features at higher energies can becharacterized as core excited resonances with possiblecontributions of high-energy shape resonances.

Assuming the general case of a polyatomic molecule(ABC), the threshold energy (Eth) of the DEA reaction 1a isgiven by (regarding energy conservation):

Eth ¼ D AB � Cð Þ � EAðCÞ ð2aÞwhere D(AB−C) is the dissociation energy and EA(C) is theelectron affinity of the (neutral) fragment carrying the extracharge after the capture event. In terms of the standard heatsof formation (ΔHf

0) Equation (2a) can be written as:

Eth ¼ ΔHR0 ¼ ΔHf

0 ABð Þ þΔHf0 C�ð Þ �ΔHf

0 ABCð Þ ð2bÞwhere ΔHR

0 represents the standard reaction enthalpy ofreaction (1a), and ΔHf

0(C-) 0ΔHf0(C)+EA(C). From the

energy balance, the threshold energy for reaction 1a is typicallybelow 4 eV if, generally speaking, the negative ion is formedby simple bond-cleavages and no rearrangement processes in aneutral fragment take place. This is due to the fact that theelectron affinity for most radicals is below the bond dissoci-ation energy. However, for more complicated reactionsinvolving rearrangement (represented in reaction 1b) theenergy gain by formation of the highly stable neutral productAC can shift the threshold energy to very low values.

NO2–(46 u), [M–NO2]

– (250 u) and [M – NO2HNO2HCN]–

(176 u)

The anions [M – NO2]– and NO2

– are formed via the cleavageof one of the four N – N bonds leading to the complementaryDEA reactions with respect to the extra charge:

e� þHMX ! HMXð Þ# � ! HMX � NO2½ � þNO2� ð3aÞ

e� þHMX ! HMXð Þ# � ! HMX � NO2½ �� þNO2 ð3bÞ

Table 2. Gas Phase Standard Heats of Formation (ΔHf°) and ElectronAffinities Relevant in Dissociative Electron Attachment to HMX (Takenfrom Reference [33], Otherwise Presently Calculated Value)

Compound ΔHf° (kJ mol–1)

C4H8N8O8 (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, HMX)

159

C2H4N4O4 (1,1-diamino-2,2-dinitroethylene) −133.9aNO2 33.1HNO2 (nitrous acid) −76.73NO2HNO2

– −400.0±4.2NO2

– 82.84Compound Electron Affinity (eV)HMX (calculated) 1.35HMX – NO2 (calculated) 3.41NO2 2.27CN 3.862±0.005

aFrom Reference [34]

1Please note that numerical values of the measured anionyields for all fragments presented in this study will be alsoavailable in the Virtual Atomic and Molecular Data Centre(VAMDC) -compliant Innsbruck Dissociative ElectronAttachment Database (IDEADB: http://ideadb.uibk.ac.at).

optimized neutral structure MO 76 – HOMO

optimized anionic structure MO 77 – SOMO

Figure 3. Optimized structures of HMX as neutral (upper leftpanel) and negatively charged (lower left panel) molecule.Highest occupied molecular orbital of the neutral molecule(upper right panel) and singly occupied molecular orbital ofthe anion (lower right panel). All results obtained at MP2/6-311++G(2d,p)


http://ideadb.uibk.ac.at

Figure 4a shows the ion yield curve for [HMX – NO2]–/

C4H8N7O6– and Figure 4b for NO2

–. We derived the energeticthresholds for reactions 3a and 3b at MP2/6-311++G(2d,p) levelof theory and basis set. Reaction 3a is endothermic by 0.48 eVand 3b is exothermic by 0.91 eV. It should be noted that thesevalues do not describe the actual path along the correspondingreaction coordinate but instead the final states. A quick look atFigure 4 reveals that the signal of NO2

– extends to higherenergies. This is in contrast to its complementary fragment anion[M – NO2]

–, which is restricted to the ~0 eV resonance only. Ifwe assume that the excess energy of the transient negative ionM#– is statistically distributed over the vibrational degrees offreedom, the large fragment should carry away about 95%. In thecase of NO2

– the neutral counterpart will become increasinglyunstable towards dissociation which does not influence the anion

efficiency curve of NO2–. In contrast, if the charge localizes on

the heavy fragment, the excess energy will drive furtherdecomposition into lower mass fragments and thus the anionyield for 250 u at higher electron energies will be suppressed.Remarkably, heavier fragment anions (between 129 and 203 u)show the resonance at medium energies with suppression of thehigh energy feature while all lighter fragment anions ≤102 uexhibit the high energy resonance as well (see Figures 4 and 5).This indicates that the subsequent decomposition of [M –NO2]

#–

may contribute to the formation of these anions.The 176 u anion efficiency curve shown in Figure 4e is

assigned to [M – NO2HNO2HCN]– (i.e., C3H6N5O4

–).Another possible fragment anion with mass 176 u may bethe ammonium 5-nitrotetrazolate anion, C2H4N6O4

–. How-ever, the detailed analysis of the isotope pattern leads to usto the assignment of C3H6N5O4

–. The resonance close to0 eV indicates that no activation energy is required for thisanion. The high energy resonance of this anion is barelydiscernible at ~4.7 eV.

The Complementary Ions [M – NO2HNO2]–/ C4H7N6O4

–

(203 u) and [NO2HNO2]– (93 u)

The signal at 203 u (Figure 4c) can be identified as the ionarising from the loss of neutral NO2 and HNO2 (nitrous acid)units. The loss of these neutral units has been recently reportedfor RDX [20]. The resulting stoichiometric composition of thecorresponding anion, C4H7N6O4

–, is only possible by rear-rangement including hydrogen transfer. The ion yield of thisanion shows the main feature close to zero energy and a weakresonance at 4.4 eV. It is particularly interesting to note that theinitial loss of a NO2 radical from the TNI followed by the lossof a HNO2 radical from [M – NO2]

– can actually be triggeredby an excess electron at just 0 eV. With the thermochemicaldata of Table 2 for the reaction:

e� þ C4H8N8O8 ! C4H8N8O8ð Þ#�

! C4H7N6O4� þ NO2 þ HNO2 ð4Þ

the ΔHfo (C4H7N6O4

–) ≥2.1 eV, where the exact value holdsfor the case when reaction 4 proceeds without excess energy(i.e., at the threshold energy).

We assign the complementary ion with mass 93 u to[NO2HNO2]

– (see Figure 4d) showing ion yield similar to the[M – NO2HNO2]

– ion despite its intensity being higher byalmost an order of magnitude. The dissociative electronattachment studies with RDX by Sulzer et al. [20] have reportedthe complementary ions [RDX – NO2 – HNO2]

– and[NO2HNO2]

–; both formed within a narrow resonance close to0 eV. They have observed that the extra electron has a strongertendency to get localised on the heavier complement than onNO2HNO2, which is in clear contrast to HMX. Though, anotherreason for such difference may reside on the fact that the [M –

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m250 m46

m203m203 x20

m93

m176m176 x100

m160

m156m156 x50

m129m129 x40

Figure 4. Anion efficiency curves of selected anions mea-sured with a commercial sector field mass spectrometerequipped with a Nier-type ion source. The width of theelectron energy distribution is about 0.7 eV and the electroncurrent was set to 50 μA


NO2HNO2]– ion is more unstable towards further dissociation in

HMX than in RDX.

The Anions C2H2N5O4– (160 u), [M – NO2HNO2HNO2]

–

(156 u) and [M – NO2HNO2HNO2HCN]– (129 u)

The 160 u anion in Figure 4f shows a particularly strong lowenergy resonance close to 0 eV and a second low intensityfeature at 4.4 eV. This fragment anion is tentatively assignedto C2H2N5O4

–, which results from the cleavage of severalbonds and a series of intramolecular rearrangements. The156 and 129 u (Figure 4 g and h, respectively) are almostexclusively formed via the low-energy resonance at ~0 eV,whereas the contribution at 4.5 eV is about two orders ofmagnitude lower. The only possible composition of thesefragments requires the loss of three nitro groups togetherwith two hydrogen atoms for the former and three hydrogenatoms and a CN for the latter.

The Anions C2H4N3O2– (102 u) and [M – NO2HNO2

HNO2HNO2HCN]¯ (82 u)

With the exception of the NO2– ion discussed above, the

102 and 82 u anions (including those discussed furtherbelow) are formed via at least three resonances as listed inTable 1. The appearance of a fragment ion with 102 u hasbeen reported in plasma desorption mass spectrometry ofHMX by Hakansson et al. [30], who tentatively assigned itto C4H6O3

–. In recent DEA experiments with RDX [20], a102 u anion was detected and identified as C2H4N3O2

–.Due to the structural similarity of RDX and HMX, wetherefore assign the fragment anion at 102 u rather toC2H4N3O2

–. In the present experiment, this anion is thethird strongest one at ~0 eV. Figure 5a, shows the ionicyield for this anion which is formed via the followingreaction:

e� þ C4H8N8O8 ! C4H8N8O8ð Þ#�

! C2H4N3O2� þ NO2 þ C2H4N4O4 ð5Þ

With the thermochemica l da ta of Table 2 ,ΔHf

o(C4H8N8O8)01.65 eV, ΔHfo(C2H4N4O4)0−1.39 eV

and ΔHfo(NO2)00.34 eV, we get ΔHf

o(C2H4N3O2–)9

2.69 eV. Further resonances are observed at 4.8 and9.8 eV (Table 1). C2H4N3O2

– is the heaviest fragmentanion, which is stable at resonance energies close to 10 eV.

The 82 u anion in Figure 5b shows a low energyresonance close to 0 eV followed by two more at 5.0 and9.8 eV. The formation of this anion may involve thecleavage of several bonds and a series of intramolecularrearrangements. The resulting chemical composition of thisfragment anion may be C3H4N3

–.

Other Anionic Yields: NNO2– (60 u), CH2N

– (28 u), CN –

(26 u), OH – (17 u) and O – (16 u)

In Figure 5c and d, the relative cross-sections for NNO2−,

CH2N− are shown, where these two anions are mostly

formed through two resonances at around 5.5 and 9.8 eVwith a relative ratio of 2:1 and 2.5:1, respectively. The 60and 28 u anions show a weak contribution at low electronenergies amounting only 6 % and 3 % of the strongestresonance. The DEA yield for the 26 u fragment can arisefrom the isobaric fragments CN¯ and/or C2H2¯. However,in previous experiments with RDX [20] and DNB and itsdeuterated analogues [15], the formation of the vinyldeneanion (CH20C)¯ in a complex rearrangement reaction wasexcluded. In the light of these findings we also assign thepresent signal at 26 u to the cyanide anion CN¯ (seeFigure 5e for the anion efficiency curve and Table 1 for the

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300(g)


Ion

Yie

ld (

Hz)

m102

m82 m60

m28 m26

m17m17 x15

m16

Figure 5. Anion efficiency curves of selected anions mea-sured with a commercial sector field mass spectrometerequipped with a Nier-type ion source. The width of theelectron energy distribution is about 0.7 eV and the electroncurrent was set to 50 μA


estimated positions). The cyanide anion may be formedeither by the excision of this unit from the target moleculeor a complex reaction pathway via an intermediate anion.Since the cyano radical has an appreciable electronaffinity (3.862 eV, Table 2) which exceeds even that ofthe halogen atoms, its formation via complex DEAreactions has been reported and is well-known for aminoacids and other large molecules containing C and N [35–37]. Analogously to NO2¯ the resonant features for CN¯ inDEA to HMX can be compared with other nitrocom-pounds in order to distinguish chemical compounds withsimilar structures.

Figure 5f and g show the yields of the fragment anionsfound at 17 and 16 u as a function of the electron energy,respectively. These are assigned to OH– and O–, with theformer showing a considerable strong resonance close to 0 eVelectron energy and two weak contributions at 4.6 and 9.7 eV(Table 1). The formation of OH– requires cleavage of N0O andC –H bonds followed by rearrangement, where such concertedmechanism is remarkable at low incident electron energies. Asfar as O– is concerned, the high energy resonances between 6and 12 eV are contributions from the background signalformed upon DEA to water, whilst the lowest resonance atabout 2.2 eV is due to background signal from an unknowncontamination present in the vacuum chamber.

Detection of HMX versus RDX and OtherNitroaromatic Compounds

Figure 6 shows a comparison of the ion yields of 102 u(Figure 6a), 60 u (Figure 6b) as well as NO2

– (Figure 6c) formedupon DEA to RDX [20] and HMX. An inspection of the figurereveals that the resonance positions for these common fragmentsanions are very similar in RDX and HMX. Thus, a determina-tion of HMX versus RDX based on the resonance positions ofcommon fragment anions like NO2

– seems not possible in ananalytical application utilizing DEA. This problem arises due tothe polymeric relation of these nitroamines, which for examplealso leads to a very similar electron ionization mass spectrum at70 eV for these molecules [33]. In contrast, comparing withother explosives like TNT [18] and nonexplosive compoundslikeMusk ketone [19] different resonance positions in ion yieldslike for example NO2

– shown in Figure 6c and d may be wellused as a fingerprint for the detection of these chemicalcompounds. A detection of HMX verus RDX in a DEA basedapplication may be nevertheless also possible by measuring theanion yield of common fragment anions and subsequentdetermination of the ratio of resonance intensities at differentelectron energies. C2H4N3O2¯ of RDX shows a significanthigher intensity of the zero eV contribution relative to theresonances located above ~0 eV (see Figure 6a). Taking alsointo account the different intensity ratios of common anions (e.g.C3H6N5O4

–/NO2– or [M –NO2HNO2]

–/[NO2HNO2]– discussed

above), a differentiation of these explosives is possible. Thelatter is also important in view of possible health risks due to a

supposed considerably higher toxicity and carcinogenicity ofRDX compared with HMX [38].

0

500

1000

(a)

0

2

4

(b)

0

200

400

600

800

(c)

0 5 10 15 200

50

100

(d)


Ion

Yie

ld (

Hz)

m102 / HMXm102 / RDX x3.66m102 / RDX x256

m60 / HMXm60 / RDX x1/77.5

m46 / HMXm46 / RDX x11.5

m46 / TNTm46 / Musk ketone

Figure 6. Comparison of the ion yield curves for 102, 60 ufrom HMX and RDX and 46 u for HMX, RDX, TNT, and thenonexplosive musk ketone (see text)

0 50 100 180 190

1

10

100

1000

Ion

Yie

ld (

Hz)

Mass per charge (Th)

m176 / HMX @ 0eV

m102

Figure 7. Mass-analyzed ion kinetic energy spectra of HMXrecorded with an electron energy of 0 eV, which shows themetastable dissociation of C3H6N5O4

– in C2H4N3O2–+neutral

fragment


Metastable Decay of C3H6N5O�4

Within the detection limit of the apparatus, we were able todetect a metastable decay of the most abundant fragmentanion, C3H6N5O4

– (176 u). The metastability allows insightinto the decomposition mechanism of this negative ion. TheMIKE scan in Figure 7 recorded at the electron energy of ~0 eVshows the metastable decay of C3H6N5O4

– leading to afragment anion at 102 u,

C3H6N5O4#� ! C2H4N3O2

� þ NO2H2CN ð6Þ

The time window in which we can observe the decay isbetween 21 and 39.4 μs, which corresponds to the flighttime of C3H6N5O4

– through the second field free region. Themetastable peak has Gaussian peak shape indicating a rathersmall kinetic energy release (KER) in contrast to some of themetastable decays found in other nitroaromatic compoundslike dinitrotoluene [16] and dinitrobenzene isomers (withKERs up to 1 eV) [39]. Indeed, the kinetic energy releasedistribution derived from the MIKE peak is of Maxwell–Boltzmann-type (we assume on the analysis that only oneneutral is produced in the reaction) and from the width theaverage kinetic energy release is obtained as 10.3 meV.Moreover, the KER value may provide information onfragmentation process: simple bond cleavage reactions andstepwise rearrangement reactions should lead to a lowkinetic energy release, concerted rearrangement reactionswill lead to a high kinetic energy release [40]. Themetastable decay peak would indicate the former inagreement with the proposed decay reaction 6 representinga simple bond cleavage reaction.

ConclusionCapture of an excess electron with virtually no kineticenergy by HMX leads to the formation of a variety of DEAfragments produced in a resonance near zero eV. The mostdominant signal from DEA reactions to HMX is theformation of C3H6N5O4

− (176 u) at electron energies below5 eV, whereas the second most intense signal assigned toNO2¯ shows also another contribution at about 10 eV. Otherand more complex reactions like the loss of several otherneutral units with an onset of the resonant ionic yields atzero energy are observed as well. In comparison with otheraromatic nitrocompounds, the absence of the nondecom-posed anion and the rich and intense fragmentation alreadyat electron energies close to 0 eV confers the explosivecharacter of HMX.

Previously it was proposed that the low-energy electronattachment ion yields obtained in DEA experiments can beused as unique characteristics for every molecule, makingthese types of powerful experiments attuned to identifyingsmall traces of chemically similar compounds. We note thatthe present yield at mass 120 u is about 4 kHz at the usedpressure of 10–5 Pa. Thus, concentrations up to ~1 ppmv in

air will be traceable with a total measurement time of 10 s.One issue to be kept in mind for applications concerns thedevelopment of suitable gas inlets (see also [8]) to becombined with electron attachment mass spectrometry.However, this would go beyond the scope of the fundamen-tal DEA study present here and will be a topic of futureprojects. Separation of the polymers RDX and HMX uponDEA seems not to be straight-forward by the measurementsof the ion yields but possible by determining the intensityratios of highly abundant anions (e.g., C3H6N5O4

–/NO2–). In

this case, DEA coupled with mass spectrometry can be usedto act as a fingerprint in sensing and field explosivedetection instrumentation.

AcknowledgmentsThis work was supported by the Fonds zur Förderung derwissenschaftlichen Forschung (FWF), P22665 and I978,Wien, the European Commission, Brussels, via COSTAction CM0805 programme “The Chemical Cosmos”. FFSacknowledges the Portuguese Foundation for Science andTechnology (FCT-MEC) for post-doctoral scholarshipsSFRH/BPD/68979/2010 and together with PL-V acknowl-edge the PEst-OE/FIS/UI0068/2011 grant. M.M.G.acknowledges the National Council for the Improvement ofHigher Education (CAPES), process no. 4752/11-2, theFoundation for Research Support of Minas Gerais State(FAPEMIG), and the National Council for Scientific andTechnological Development (CNPq). The authors gratefullyacknowledge the defusing section of the ministry of interiorthat provided us with HMX samples.

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