The Elusive Ketene (H2CCO) Channel in the Infrared Multiphoton Dissociation of Solid 1,3,5-Trinitro-1,3,5-Triazinane (RDX)

Article abstract of DOI:10.1002/cphc.201901202

Understanding of the fundamental mechanisms involved in the decomposition of 1,3,5-trinitro-1,3,5-triazinane (RDX) still represents a major challenge for the energetic materials and physical (organic) chemistry communities mainly because multiple competing dissociation channels are likely involved and previous detection methods of the products are not isomer selective. In this study we exploited a microsecond pulsed infrared laser to decompose thin RDX films at 5 K under mild conditions to limit the fragmentation channels. The subliming decomposition products during the temperature programed desorption phase are detected using isomer selective single photoionization time-of-flight mass spectrometry (PI-ReTOF-MS). This technique enables us to assign a product signal at m/z=42 to ketene (H₂CCO), but not to diazomethane (H₂CNN; 42 amu) as speculated previously. Electronic structure calculations support our experimental observations and unravel the decomposition mechanisms of RDX leading eventually to the elusive ketene (H₂CCO) via an exotic, four-membered ring intermediate. This study highlights the necessity to exploit isomer-selective detection schemes to probe the true decomposition products of nitramine-based energetic materials.

Full text of DOI:10.1002/cphc.201901202

Accepted Article
Title: The Elusive Ketene (H2CCO) Channel in the Infrared Multiphoton
Dissociation of Solid 1,3,5-Trinitro-1,3,5-Triazinane (RDX)
Authors: Santosh K. Singh, Jesse La Jeunesse, Vasant Vuppuluri,
Steven F. Son, Bing-Jian Sun, Agnes H. H Chang, Alexander
M. Mebel, and Ralf I. Kaiser
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10.1002/cphc.201901202
ChemPhysChem
The Elusive Ketene (H2CCO) Channel in the Infrared
Multiphoton Dissociation of Solid 1,3,5-Trinitro-1,3,5-
Triazinane (RDX)
Dr. Santosh K. Singha, Jesse La Jeunessea, Vasant Vuppulurib, Prof. Steven F. Sonb,
Dr. Bing-Jian Sunc, Prof. Agnes H. H. Chang c, Prof. Alexander M. Mebeld,
Prof. Dr. Ralf I. Kaisera*
a Department of Chemistry, University of Hawaii, Honolulu, HI 96822 HI, USA
W. M. Keck Research Laboratory in Astrochemistry, University of Hawaii, 96822 HI, USA
b Mechanical Engineering, Purdue Energetics Research Center, Purdue University, West
Lafayette, IN 47907, USA
c Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan
d Department of Chemistry and Biochemistry, Florida International University, Miami,
Florida 33199, USA
*Corresponding author. E-mail: ralfk@hawaii.edu
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Abstract
Understanding of the fundamental mechanisms involved in the decomposition of 1,3,5-trinitro-
1,3,5-triazinane (RDX) still represents a major challenge for the energetic materials and physical
(organic) chemistry communities mainly because multiple competing dissociation channels are
likely involved and previous detection methods of the products are not isomer selective. In this
study we exploited a microsecond pulsed infrared laser to decompose thin RDX films at 5 K
under mild conditions to limit the fragmentation channels. The subliming decomposition
products during the temperature programed desorption phase are detected using isomer selective
single photoionization time-of-flight mass spectrometry (PI-ReTOF-MS). This technique enables
us to assign a product signal at m/z = 42 to ketene (H2CCO), but not to diazomethane (H2CNN;
42 amu) as speculated previously. Electronic structure calculations support our experimental
observations and unravel the decomposition mechanisms of RDX leading eventually to the
elusive ketene (H2CCO) via an exotic, four-membered ring intermediate. This study highlights
the necessity to exploit isomer-selective detection schemes to probe the true decomposition
products of nitramine-based energetic materials.
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For the last decades, the elucidation of the fundamental decomposition mechanisms of cyclic
nitramines such as 1,3,5-trinitro-1,3,5-triazinane (RDX, Scheme 1) have received considerable
attention from the material sciences, physical (organic), and theoretical chemistry communities
due to their high energy content and vital applications as explosives and rocket propellants.[1] An
investigation of the central pathways involved in the fragmentation of RDX in the condensed
phase and an identification of the primary and higher order reaction products, among them car-
bon-, nitrogen-, and oxygen-centered free radicals, which are formed in these processes, still
represents a major challenge. These data are very much required by the physical (organic) and
energetic material communities to unravel elementary mechanisms and bond breaking processes,
which trigger the decomposition of energetic molecules. Multiple experimental and theoretical
studies have been performed to elucidate the complex decomposition mechanism(s) of RDX.[2]
Ultraviolet (UV) photodissociation of RDX was explored in the gas phase and in the condensed
phase at discrete wavelengths of 226, 248, 236, and 229 nm. The majority of these studies
revealed that the N-NO2 bond fission represents the primary pathway.[2a, 2b, 2n-p] Wight et al.
studied the thermal decomposition of solid RDX employing a pulsed carbon dioxide infrared (IR)
laser and confirmed that the N-NO2 bond dissociation represents the initial decomposition step.[2j,
2k] However, infrared multiphoton dissociation (IRMPD) of RDX in a molecular beam studied by
Lee et al. suggested that the concerted triple C-N scission of RDX leading to three methylene
nitramine (H2CNNO2) molecules represents the dominant pathway.[2c] Follow up studies
exploiting a pyrolysis of solid RDX at temperatures above and below the melting point (478 K)
reveal yet another decomposition pathway: the formation of a nitroso intermediate (1-nitroso-
3,5-dinitrohexahydro-s-triazine; ONDNTA) that subsequently decomposes to dinitrogen oxide
(N2O) and formaldehyde (H2CO).[2e] On the other hand, thermal decomposition studies by Farber
et al. in a high vacuum favor the ring fragmentation pathway to methylene nitramine.[2y]
Scheme 1. Molecular structure of 1,3,5-trinitro-1,3,5-triazinane (RDX).
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The perceptible controversies of the initial decomposition step and the products can be
reconciled by suggesting that RDX decomposes through multiple competing channels with one
favored over the other at distinct experimental conditions (temperature, pressure, decomposition
methods). The primary decomposition channels are i) elimination of dinitrogen oxide (N2O) and
formaldehyde (H2CO), ii) formation of nitrogen dioxide (NO2), nitrogen monoxide (NO) and
higher order molecular weight products via N-NO2 fission, iii) a concerted elimination of nitrous
acid (HONO), and iv) ring fragmentation via C-N scission.[2e, 2q, 3] 6, [2e, 2j, 2k] Therefore, from the
experimental point of view, a challenging task is to avoid the presence of multiple channels by
selectively opening a single decomposition pathway through tuning the experimental conditions.
This approach will gain a better understanding of the initial decomposition pathway under well-
defined experimental conditions as presented here.
A hitherto overlooked reason of the aforementioned discrepancies of the experimental results
might be more subtle. The decomposition products of RDX were principally probed and
ultimately assigned through the identification of their molecular masses after electron impact
ionization of the neutral reaction products. In principle, for a given molecular mass, multiple
structural isomers do exist, and traditional mass spectrometry coupled with electron impact
ionization cannot always discriminate the structural isomer(s) formed.[4] These complications
might be partially overcome by exploiting isotopically labeled reactants. Beheren et al. studied
the thermal decomposition of isotopically labeled RDX (RDX-d6, RDX-13C, RDX-15N6) to trace
the products based on the mass shifts.[2f] However, postulated products observed at, for instance,
m/z = 42 were not confirmed by studying decomposition of RDX-18O. Therefore, an alternative
experimental approach to identify the nature of the structural isomers formed is required. Soft
photoionization (PI) represents an ideal tool to differentiate structural isomers selectively based
on their distinct adiabatic ionization energies (IE).[5] This technique has been exploited
successfully to determine the structural isomers of the decomposition products of nitromethane -
a model compound of nitramine-based energetic materials,[6] but, has not been utilized to date to
elucidate the initial bond rupture processes and actual products isomer selectively in ‘real’
energetic materials such as for RDX under well-defined experimental conditions.
In this Letter, we exploit for the first time soft photoionization (PI) to elucidate the underlying
reaction mechanism(s) and to assign the reaction products isomer selectively in the infrared
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multiphoton dissociation (IRMPD) of thin films of RDX in the condensed phase at 5 K. These
processes are induced by a pulsed carbon dioxide (CO2) IR laser (10.6 µm, 6.0 ± 0.6 mJ cm-2 per
pulse) in an ultrahigh vacuum (UHV) surface science machine operating at base pressures of 2 ×
10-10 torr (Figure S1).[5a, 7] These conditions allow the decomposition of RDX under mild
energetic conditions opening up limited reaction channels. After the radiation exposure, infrared
spectroscopy was exploited to monitor the changes in the RDX film (Figure S2, Table S1). The
exposed samples were then annealed at a rate of 1 K min-1 to 320 K (temperature programmed
desorption; TPD), and the subliming neutral products were ionized using tunable vacuum
ultraviolet (VUV) photoionization (PI). The ions were separated in a reflectron time-of-flight
mass spectrometer (PI-ReTOF-MS) and identified based on their mass-to-charge ratio (m/z) by a
microchannel plate detector (MCP). Fascinatingly, exploiting a photon energy of 10.49 eV, only
a single mass-to-charge ratio was observed at m/z = 42. This ion was detected in previous
decomposition studies of RDX utilizing electron impact ionization of the neutral decomposition
products (Table 1) [2c, 2e, 2l, 2m, 2y-ab] and was assigned based on chemical intuition as diazomethane
(H2CNN)[2c, 2l, 2m, 2ab] and/or the radical(s) C2H4N[2c, 2y, 2aa] or CNO[2e, 2z]. In the present work,
based on tunable photoionization at 10.49 eV and 9.30 eV combined with electronic structure
calculations, we provide compelling evidence that m/z = 42 is linked solely to ketene (H2CCO;
IE = 9.6 eV) - a molecule with the molecular mass as diazomethane (H2CNN; IE = 8.9 eV). The
explicit identification of ketene changes our perception how we think about the thermal
decomposition mechanisms of RDX involving triple dissociation to methylene nitramine
(H2CNNO2) (reaction (1)), consecutive decomposition to formaldehyde (H2CO) and dinitrogen
monoxide (N2O) (reaction (2)), and reaction of two formaldehyde molecules leading to ketene
(H2CCO) plus water (H2O) (reaction (3)). This key result highlights the necessity to elucidate the
identity of the true decomposition products exploiting tunable vacuum ultraviolet
photoionization.
(1)
(2)
(3)
C3H6N6O6 (RDX)  3 H2CNNO2
H2CNNO2  H2CO + N2O
2 H2CO  H2CCO + H2O
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Table 1. Previous assignments of signal observed at mass-to-charge (m/z) of 42.
References
Method of decomposition Method of ionization Method of detection
Assignment
X. Zhao et al. [2c]
IR multiphoton
Electron impact
ionization
Time-of-flight mass
spectrometer
CH2N2/
C2H4N
dissociation using CO2
laser in molecular beam
(10.6 µm, 6-20 J cm-2 per
pulse)
J. T. Dickinson et
KrF excimer laser radiation Electron impact
Quadrupole mass
spectrometer
CH2N2/
C2H4N
al.[2l]
of RDX crystals (248 nm,
ionization
< 5 mJ cm-2)
R. Behrens et al.[2f] Thermal decomposition at
Electron impact
ionization
Quadrupole mass
spectrometer
CNO
465 K
T. B. Tang et
Photolysis at 266 nm laser
(~500 µJ)
Multiphoton
ionization
Time-of-flight mass
spectrometer
CH2N2
C2H4N
al.[2m]
M. Farber et al.[2y] Thermal decomposition at
Electron impact
ionization
Quadrupole mass
spectrometer
473 K
L. Zhou et al.[2z]
Rapid heating with a
Electron impact
ionization
Time-of-flight mass
spectrometer
C2H4N,
CH2N2, CNO
heating rate of ~ 105 K s-1
P. A. Snyder et
Pyrolysis at 633 K
Atmospheric pressure Quadrupole mass
C2H4N
al.[2aa]
chemical ionization
spectrometer
H. Ostmark et
CO2 Laser pyrolysis (10.6
µm, 180 W)
Electron impact
ionization
Quadrupole mass
spectrometer
CH2N2/
C2H4N
al.[2ab]
W. Zhang et al.[9]
Laser irradiation (1064 nm, Multiphoton
800 mJ) ionization
Time-of-flight mass
spectrometer
C2H4N/CNO
Infrared spectroscopy was employed to analyze the 15.5 ± 1.0 µm thick RDX samples before and
after the exposure to the carbon dioxide laser (Experimental Methods). The infrared spectrum of
RDX recorded at 5 K prior to the irradiation reveals prominent vibrational bands in the region
4000-600 cm-1 (Fig. S2; Table S1) characteristic of amorphous RDX as evident from the band
broadening. The crystalline phases of RDX exhibit sharp infrared absorptions (Fig. S3). In the
amorphous sample, absorption features at 3079 and 2994 cm-1 correspond to CH2 asymmetric
and symmetric stretching modes respectively. A broad absorption band observed from 1590 to
1531 cm-1 can be attributed to the asymmetric vibrations of nitro group (-NO2). The N-NO2
stretching vibration of RDX appears at 1319 cm-1, while ring vibrations dominate the spectral
regions of 1217-1173 cm-1, 1080-908 cm-1 and 848-667 cm-1. The infrared spectrum measured
after the irradiation at high dose display only a small minor (8 ± 1 %) decrease in the intensity of
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the RDX vibrational bands (Figure S2). In addition, a new broad absorption feature appears in
the region 3513-3203 cm-1, which is linked to O-H stretching vibration of water (H2O).
Considering the small decomposition fraction of RDX and the sole detection of the strong
infrared absorber water (H2O), prominent absorptions of less infrared sensitive products might be
masked by the absorption of the RDX reactant. Therefore, to detect additional degradation
products, a highly sensitive analytical technique such as soft photoionization (PU) coupled with
reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) is applied.
Figure 1. PI-ReTOF-MS data recorded at high dose (a) and low dose (b, c) experiments at
photoionization energies of 10.49 eV (a, b) and 9.30 eV (c).
The temperature-dependent mass spectra of the decomposition products subliming from the
irradiated RDX samples are depicted in Fig. 1. The mass spectra were collected at a
photoionization energy of 10.49 eV and revealed sole signal at m/z = 42. The corresponding
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Figure 2. TPD profiles recorded at mass-to-charge ratio of 42 in (a) high dose and (b) low dose
experiments at a photoionization energy of 10.49 eV. (c) TPD profile recorded in the high dose study at a
photoionization energy of 9.30 eV.
TPD profiles for the high dose and low dose experiments are depicted in Figures 2(a) and (b)
respectively. The control experiment, i.e. an experiment performed under identical conditions but
without exposing the RDX samples to the infrared laser, does not reveal any ion counts at all at
m/z = 42 (Fig. S4). This finding suggests that signal observed at m/z = 42 is the result of the
exposure of RDX to the infrared laser. Ion counts at m/z = 42 can be ascribed to two molecules:
diazomethane (H2CNN) and ketene (H2CCO). To identify which isomer(s) is(are) actually
formed in the experiment, we selectively photoionize the isomer(s) based on their ionization
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energies. The adiabatic ionization energies of ketene (H2CCO; IE = 9.6 eV) and diazomethane
(H2CNN; IE = 8.9 eV) are separated by 0.7 eV. At a photoionization energy of 10.49 eV both the
isomers can be ionized. However, a photoionization energy of 9.30 eV permits only the
ionization of diazomethane (H2CNN). The mass spectra collected at a photoionization energy of
9.30 eV after the irradiation of RDX sample do not reveal any ion counts at m/z = 42 (Figs. 1c
and 2c). Therefore, we can conclude that diazomethane (H2CNN) is not observed, and the ion
signal at m/z = 42 is solely linked to ketene (H2CCO).
Considering the molecular structure of RDX (Scheme 1) and of the detected reaction products
(H2O, H2CCO), it is intriguing to unravel the underlying reaction mechanism(s) through ab initio
calculations at the B3LYP/cc-pVTZ//CCSD(T)/cc-pVTZ level of theory (Figs. 3-4; Compu-
tational Methods). The experimental results can be reconciled by an initially decomposition of
the RDX molecule through ring fragmentation via a triple concerted C-N bond scission to form
three molecules of methylene nitramine (H2CNNO2). The electronic structure calculations
suggest that the concerted ring fragmentation pathway connects via a transition state located 264
kJ mol-1 above the separated products formed in an overall endoergic reaction (+194 kJ mol-1)
(Fig. S5). These energetics are in close agreement with those of Goddard et al. computed at the
B3LYP/6-31G(d) level of theory.21 Our calculations predict that methylene nitramine (H2CNNO2)
can further undergo infrared-assisted decomposition (Fig. 3). This pathway involves an atomic
oxygen shift from the –NO2 moiety to the methylene group (CH2) resulting in an exotic cyclic
intermediate I1 (c-OCH2NNO) through a barrier of 123 kJ mol-1 in an endoergic reaction (+32 kJ
mol-1). This intermediate already holds key functional groups and decomposes easily through a
transition state located 81 kJ mol-1 above c-OCH2NNO to dinitrogen oxide (N2O) and
formaldehyde (H2CO). The structure of exotic intermediate I1 needs some attention here. It has a
distorted four membered ring structure where the N-O bond of the ring is elongated holding a
bond length of 145 pm; this indicates a strong single bond character; the N-N bond length in the
ring is 128 pm thus representing a partial double bond character. The C-O and C-N bond lengths
in the ring are very similar (146 pm). Selected molecular orbitals of intermediate I1 are provided
in Fig. S9 of the Supporting Information, which depicts electron density over the ring and
heteroatoms. Note that although the decomposition of methylene nitramine (H2CNNO2) is
overall exoergic (-165 kJ mol-1), formation of I1 requires a barrier of 123 kJ mol-1. Considering
that a barrier of 264 kJ mol-1 has to be overcome in the aforementioned concerted triple
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dissociation of RDX, the transition state linking methylene nitramine (H2CNNO2) and
intermediate I1 through a barrier of only 123 kJ mol-1 can be easily overcome under our
experimental conditions.
Figure 3. Computed decomposition mechanism of methylene nitramine (H2CNNO2). The pathway
leading to diazomethane (H2CNN) and singlet oxygen (O2; a1∆g) is shown in black while the route to
formaldehyde (H2CO) and dinitrogen monoxide (N2O) is represented in red. Bond lengths are in pm and
bond angles are in degree. Geometrical parameters of reactant and products are provided in Figure S7 of
the supporting information.
This finding implies that this dissociation pathway is highly favorable compared to the
decomposition leading to the non-detected diazomethane (H2CNN) and singlet oxygen (O2; a1∆g)
channel. The latter would require passing multiply high-energy transition states ranging up to
394 kJ mol-1 above the methylene nitramine reactant. In detail, methylene nitramine may
undergo unimolecular decomposition via a cyclic intermediate I2 (c-H2CNNO2) and an acyclic
isomer I2 (H2CNNOO) yielding of diazomethane and oxygen (Fig. 3). To access intermediate I2
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an energy of 394 kJ mol-1 is required. Intermediate I2 may dissociate in one step to
diazomethane (H2CNN) and oxygen (O2) after overcoming an energy barrier of 391 kJ mol-1;
alternatively, intermediate I2 may ring-open to I3, which requires a barrier of 81 kJ mol-1 to
overcome. Then, intermediate I3 can decompose to H2CNN (diazomethane) plus O2 (oxygen) in
an overall exoergic reaction of 257 kJ mol-1 with respect to the reactant. Therefore, these
considerations reveal that the unimolecular decomposition of methylene nitramine (H2CNNO2)
into diazomethane (H2CNN) and oxygen (O2) is much less favorable compared to the
decomposition into dinitrogen monoxide (N2O) and formaldehyde (H2CO).
Figure 4. Computed formation pathways of ketene (H2CCO) and water (H2O) from formaldehyde.
Geometrical parameters of reactants, products and intermediates are provided in Figure S8 of the
supporting information.
What is the ultimate fate of formaldehyde (H2CO)? Two formaldehyde (H2CO) molecules
may react to form eventually ketene (H2CCO) and water (H2O); the energetically most favorable
section of the potential energy surfaces is revealed in Fig. 4. In principal, two molecules of
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formaldehyde (H2CO) can react to form methanol (CH3OH) and carbon monoxide (CO) via a
transition state located 99 kJ mol-1 above the reactants. Carbon monoxide and methanol can react
further to ketene (H2CCO) and water (H2O) in a concerted reaction; this requires a transition
state of 246 kJ mol-1 to be overcome. Alternatively, two molecules of formaldehyde may react to
form 2-hydroxyacetaldehyde ((OH)H2CCHO), which can dissociate into water (H2O) and ketene
(H2CCO). However, this reaction requires an energy of at least 309 kJ mol-1 to overcome the
entrance transition state. Once again, considering that 264 kJ mol-1 have to be incorporated to
pass the inherent barrier for the concerted triple dissociation of RDX, the highest barrier in the
reaction of two formaldehyde molecules to water and ketene only ranges at 174 kJ mol-1.
Therefore, formation of ketene from two formaldehyde molecules via methanol (CH3OH) and
carbon monoxide (CO) is energetically more viable that the mechanism involving 2-hydroxy-
acetaldehyde ((OH)H2CCHO). It is important to recall that previous studies reported dinitrogen
oxide (N2O) and formaldehyde (H2CO) at dominating decomposition products at relatively low
temperatures less than 450 K.[8] The 200 µs pulse width carbon dioxide laser operating at low
laser fluence of 6.0 ± 0.6 mJ cm-2 employed in the present study suggests a low heating rate
(Table S2).
In conclusion, the infrared multiphoton dissociation of RDX to eventually ketene (H2CCO)
and water (H2O) accompanied with dinitrogen monoxide (N2O) opens up a hitherto elusive
decomposition pathway under experimentally mild fragmentation conditions. The proposed
decomposition is initiated by the triple dissociation of RDX to three molecules of methylene
nitramine (H2CNNO2), which subsequently undergoes decomposition to formaldehyde (H2CO)
and dinitrogen monoxide (N2O) via an exotic acyclic reaction intermediate I1. Two
formaldehyde molecules then eventually undergo multi photon induced reaction to ketene
(H2CCO) and water (H2O). The ultimate evidence on the formation of water and ketene during
RDX decomposition was obtained by exploiting infrared spectroscopy (water) and isomer
selective single photoionization reflectron time-of-flight mass spectrometry (ketene). Based on
tunable photoionization at 10.49 eV and 9.30 eV, the experiments revealed signal at m/z = 42 -
previously incorrectly associated to diazomethane (H2CNN) – solely linked to ketene (H2CCO)
thus highlighting the necessity to exploit isomer-selective detection tools to probe the
decomposition of RDX via overall equations (1) – (3) (Figs. 3 and 4). Operating under the
premise that reaction intermediate I1 is unstable and methanol, carbon monoxide, and
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formaldehyde convert to ketene plus water, at least dinitrogen monoxide (N2O) should represent
a co-product. Considering its ionization energy of 12.89 eV, dinitrogen monoxide cannot be
photo ionized at 10.49 eV; also, it was not observable infrared spectroscopically. Considering the
infrared modes and integrated absorption coefficients of dinitrogen monoxide and the
experimentally detected water production, signal calculations reveal that dinitrogen monoxide
abundances are below the detection limit of the infrared setup (Fig. S2, Quantitative Analysis).
Considering that the elementary mechanisms involved in the decomposition of RDX are very
complex, our approach technique could be useful in prospective studies determining the structure
and hence nature of the isomer(s) of primary and higher order decomposition products of RDX
eventually aid in resolving the prevailing discrepancies associated with the decomposition
pathway of nitramine-based energetic materials.
Acknowledgments
This project was supported by the U.S. Army Research Office (ARO) (W911NF1810438).
BJS and AHHC appreciate the National Center for High-performance Computer in Taiwan for
providing the computer resources in the calculations.
Conflict of interest
There are no conflicts to declare.
Keywords: ab initio calculations, infrared multiphoton dissociation, mass-spectrometry,
photoionization, RDX
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10.1002/cphc.201901202
ChemPhysChem
Entry for the Table of Contents
Fundamental understanding of the elementary reactions involved in the decomposition of 1,3,5-
trinitro-1,3,5-triazinane (RDX) is still a major challenge due to lack of identification of accurate
structure of the decomposition products. For the first time, we have employed isomer-selective
photoionization mass-spectrometry technique to probe the true structure of a decomposition
product of RDX, which is observed at mass-to-charge ratio of 42.
15
This article is protected by copyright. All rights reserved.