MOLECULAR DECOMPOSITION MECHANISMS OF
ENERGETIC MATERIALS
C. Melius
To cite this version:
C. Melius.
MOLECULAR DECOMPOSITION MECHANISMS OF ENERGETIC
MATERIALS. Journal de Physique Colloques, 1987, 48 (C4), pp.C4-341-C4-352.
<10.1051/jphyscol:1987425>. <jpa-00226664>
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Submitted on 1 Jan 1987
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JOURNAL DE PHYSIQUE
Colloque C4, supplement au nb9,Tome
48, septembre 1987
C.F. MELIUS
Energetic Materials Division, Sandia National Laboratories,
Livermore, CA 94550, U.S.A.
La detonation des materiaux Qnergetiques implique la liberation de l'bnergie
chimique resultant du rearrangement des liaisons chimiques pour former des
mol6cules plus stables. Une comprBhension de ces processus chimiques au niveau
moleculaire demande une connaissance de la stabilite thermochimique des differents
intermediaires mol~culairesqui peuvent Btre formes. Elle necessite aussi une
connaissance des chemins de reaction possibles pour un rearrangement molbculaire. En
particulier, on a besoin de connaitre les hauteurs de barrisres d'energie (energies
d'activation) ainsi que les variations d'entropie (facteurs preexponentiels) au
goulot de la &action. Pour determiner la thermochimie et les chemins de &action au
cours de la d6composition, nous avons d6veloppk la nethode de chimie quantique
BAC-MP4. Utilisant les calculs de corrections d'additivitb de liaison a la theorie
des perturbations au 48me ordre Moller Plesset (Bond - Additivity - Corrections to
goller - glesset 4th Order), on peut determiner les energies des differentes
liaisons, les chaleurs de formation, les entropies, et les energies libres le long
des chemins possibles de reaction. Les reactions chimiques dominantes sont fortement
dependantes de la vitesse avec laquelle l'energie est portbe sur le front de choc.
La thermochimie et les mecanismes de decomposition sont discut6s en fonction de la
vitesse de chauffage et de la temperature. Nous distinguons dans le processus de
decomposition les etapes chimiques qui sont exothermiques, de celles qui sont, par
nature, endothermiques. Les resultats sont pr8senti.s pour une varietir de composes
nitres comprenant les nitro-aliphatiques et les nitramines HMX et RDX.
Abstract
The detonation of energetic materials involves the release of chemical energy resulting from the
rearrangement of the chemical bonds to form more stable molecules. An understanding of these
chemical processes at the molecular level requires a knowledge of the thermochemical stability of the
various molecular intermediates which can be formed. It also necessitates a knowledge of possible
reaction pathways for molecular rearrangement. In particular, one needs to know the heights of the
energy barriers (activation energies) as well as the changes in entropy (preexponential factors) at the
bottleneck to reaction. To determine the thermochemistry and reaction pathways occurring during
decomposition, we have developed the BAC-MP4 quantum chemical method. Using BondAdditivity-Cofiections to Moiler-Elesset sth-order pertubation theory calculations, the various bond
energies, heats of formation, entropies, and free energies along possible reaction pathways can be
calculated. The dominating chemical reactions are strongly dependent on the rate of energy deposited
at the shock front. The thermochemistry and decomposition mechanisms are discussed as a function of
heating rate and temperature. We distinguish those chemical steps in the decomposition process
which are exothermic from those which are inherently endothermic. Results are presented for various
nitro compounds including the nitro-aliphatics and the nitramines HMX and RDX.
*This work supported by the U.S. Department of Energy and the U.S. Department of Army
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987425
JOURNAL DE PHYSIQUE
Introduction
The detonation of energetic materials involves the rapid release of heat and pressure due to chemical
reactions arising from the decomposition of the chemically energetic material. In particular, the
decomposition products under high heating rates representative of detonation can be significantly
different than the products formed during slow heating rates and low pressures. The energetics of the
chemical reactions and bond breaking occumng during ignition, deflagration and detonation require a
knowledge of the thermochemistry of the various molecular species that can occur. In particular, the
thermal stability of the short-lived, highly reactive radical species occurring during the decomposition
and subsequent chemical processes must be determined. The activation barriers of possible reaction
pathways involving these intermediates must be determined as well.
A theoretical means now exists for determining the thermochemistry and reaction mechanisms
occurring under these high temperature and pressure, short time scale conditions. We have developed
the theoretical quantum chemical approach known as the BAC-MP4 methodl-5 to calculate the
thermochemica1properties of the molecular species. In this paper we briefly describe the theoretical
method, present thermochemical results, and discuss possible decomposition mechanisms which are
consistent with these BAC-MP4 calculations. Applications of this technique have been presented
previously for the determination of corresponding reaction mechanisms in combustion6-7.
Theoretical Approach
Since the thermochemical stabilities of the transient decomposition species are generally not known, it
is important to have an accurate theoretical method for determining their heats of formation. The
-~
such a means of obtaining thermochemical energies accurate to
BAC-MP4 r n e t h ~ d lprovides
approximately 10 kJ-mol-l. This method can also be used to obtain estimates of activation barriers
along reaction pathways, although the error uncertainty is somewhat larger.
The BAC-MP4 approach begins with the electronic structure calculation of a given molecule using the
Hartree-Fock method. This technique is used to determine the optimum molecular geometry and
harmonic vibrational frequencies. Total electronic energies are then calculated at a higher level of
theory using M~ller-Plessetmany-body perturbation theory to fourth order (MP4). This method
extends Hartree-Fock theory to include electron correlation, which is important in evaluating bond
energies. We then include bond-additive corrections (BAC) to obtain heats of formation. Combining
this information with the moments of inertia of the molecule and the vibrational frequencies provides
the thermochemical entropies and free energies of the various decomposition and combustion
intermediates. For transition state structures, the same approach is used except that a saddle point
on the electronic potential energy surface is located. Comparison of BAC-MP4 and experimental
heats of formation for selected molecular species of relevance to energetic materials is given in Table I.
Note that the BAC-MP4 method works well for unstable and radical species as well as for the stable
molecules.
Results a n d Discussion
A. Bond Energies
The resulting bond energies of various C-nitro, N-nitro, and 0-nitro compounds are given in Table
11. One can see from Table I1 that the more insensitive energetic materials have the larger bond
energies, with NH2-CH=CH-NO2 (representative of the insensitive high explosive TATB) having
the largest bond energy. Also, one can see that the bond energies are relatively localized by the
Table I. Comparison of BAC-MP4 heats of formation
experimental values. (Energies in kJ-mol-1.)
Molecule
mf,298
Theor.
Exp.
CH3N02
CH30NO
CH30N02
CH3NHNH2
NH2CHO
HONO
HON02
CH3CHO
CO
co2
HC(0)OH
CH300H
Molecule
for various molecules with
AH!,298
Theor.
Exp.
HNCO
HN3
N20
NO
NO2
CH3
CH2
NH2
NH
OH
Table 11. Bond dissociation energies for various C-nitro, N-nitro, and 0-nitro compounds
calculated using the BAC-MP4 method. (Energies in Id-mol-1 at 298K.)
C-Nitro Compounds
CH3--NO2
C2H5--NO2
NH2CH2--NO2
02N-CH2--NO2
CH2=CH--NO2
CH3CH=CH--NO2
trans-NH2CH=CH--NO2
cis-NH2CH=CH--NO2
HOCH=CH--NO2 (no h.b.)
HOCHzCH--NO2 (h.b.)
C6H5--NO2
HC(0)--NO2
BDE
H-Nitro Com~ounds
BDE
N-Nitro Compounds
0-Nitro Compounds
nature of the group, ie., nitramine (NH2N02), methyl nitramine (CH3NWN02), and dimethylnitramine
((CH3)2NN02) have similar N-NO2 bond energies (- 200 Id-mol-l). Also, when the carbon
becomes unsaturated (e.g. CH2=CHNO2), the C-NO2 bond energy increases, but when the nitrogen
becomes unsaturated (e.g. CH2=NN02), the bond energy decreases. As has been indicated in
previous papers, the nitro-group bond energy is the weakest bond in typical energetic materials.
JOURNAL DE PHYSIQUE
B. Decomposition of Nitromethane, CH-jNOz
Nitromethane, CH3NO2, is an energetic material which can decompose to form more stable
molecules. In particular, N-0 bonds are weaker than C-0 bonds; C-H bonds are weaker than N-H
or 0-H bonds. This is illustrated in Fig. 1 where the heats of formations of various isomers of
CH3NO2 are shown. One can see that CH3N02 is relatively unstable compared to its other
isomers. However, the aci- form of nitromethane, H;?CN(O)OH, is less stable while methyl nitrite,
CH30N0, has a comparable stability. Before energy can be released (as can be seen from Fig. I),
multiple rearrangements must occur, involving the breaking of many old bonds and the forming of
many new bonds. In Fig. 2 we present the decomposition pathways for CH3N02. One can see from
Fig. 2 that rearrangement involves large activation barriers, comparable to the NO2 bond breaking
energy. While the calculations indicate that there is a tight transition state structure with a high
barrier for rearrangement to form CH30N0, it is possible at large bond distances for the NO2 group
to flip around and recombine as ONO. Having formed the nitrite (which has a weaker CH30-NO
bond energy), it can now decompose to form CH3O + NO or CH20 + HNO. This process should
become more important in the high pressure regime of detonation. However, it should be noted from
Fig. 2 that these initial stages of decomposition are endothermic.
C. HONO Elimination from Nitro-Compounds
In Fig.3 we compare the energies of decomposition for 0-, N-, and C-nitro compounds. In each
case, the molecule can undergo simple bond scission to form NO2 or can undergo a five-centered
elimination to form HONO. The trend from left to right is an increase in the endothermicity of the
decomposition process, as was previously mentioned for Table 11. This trend is consistent with the
trend toward more insensitive expiosives to the right. In particular, having an unsaturated C
attached to the nitro group, as in the nitroaromatics, greatly increases the endothermicity of the
initial decomposition step.
The BAC-MP4 calculations provide not only bond energies but also geometries and frequencies
which can be used to obtain entropies, A S's, and free energies, AG's. These thermochemical
properties can be used in molecular dynamics, such as transition state theories, to determine rate
constants for decomposition. As an example, Fig. 4 shows a comparison between theory and
expeement for the rate constant of the five-centered elimination of HONO from nitroethane
CH3CH2N02 + CH2CH2 + HONO
= 68 kJ-mol-l
The theoretically determined unimolecular rate constant (with an effective energy barrier of 172 kJmol-I and a pre-exponential A factor of 1011.9 sec-I at 600K) is consistent with the experimental
datag. At high temperatures, the experimental data differs from the theoretical curve due to the
simple bond fissioning of the NO2 group,
which is not included in the theoretical calculation. The high temperature experimental slope (250
kJ-mol-1) is consistent with the calculated CH3CH2-NO2bond energy of 244 Id-mol-1 and a preexponential A factor of 1016.6 at 800K. It is important to note that the significantly larger A factor
(by more than four orders of magnitude) for simple bond scission suggests that under the rapid
heating rates occurring at the shock front during detonation (corresponding to very high effective
temperatures) breaking apart of the molecule to form NO2 should be the fist step in the
decomposition process. While this step may be immediately followed by abstraction of a hydrogen
Heats of Formation of Nitromethane
and Its Isomers
H~C'ANOH
+47
H~CN
(O
0
HzCO
+ HNO
+1
HEON=O
-1 29
-189
Fig. 1. Heats of formation of various isomers and decomposition products of
nitromethane, CH3N02, based on BAC-MP4 heats of formation at 298K.
Decomposition of Nitromethane
Fig. 2. Calcualted unimolecular decomposition pathways for the reaction of
CH3N02, based on BAC-MP4 heats of formation at 298K for stable species and
transition state activated complexes.
JOURNAL DE PHYSIQUE
CH2CHN02
+ CH2CH
+ NO,
7
283
CH30N02
+ CH30
4
163
-85
CH30N02
+ CH,O
+ HONO
CH3CH2N02
+ CH2CH2
+ HONO
CH3NHN0,
+ CH2NH
+ HONO
J
CH,CHNO,
+ HCCH
+ HONO
Fig. 3. Comparison of decomposition pathway energies for CH30N02, CH3NHN02, CH3CH2N02,
and CH2=CHN02 calculated using the BAC-MP4 heats of formation at 298K. For each molecule,
the top energy value represents the bond dissociation energy for N02,the middle energy value
represents to barrier height for the five-centered elimination to form HON0,and the bottom energy
value represents the heat of dissociation for formation of HONO. (Energy in kJ-mol-1.)
1000 I T
(K)
Fig. 4. Comparison of experimental and theoretical decomposition rate constants for nitroethane,
CH3CHzNOz. Theoretical curve represents the five-centered HONO elimination, calculated using
the BAC-MP4 method. Experimental data is taken from Ref. 8.
atom or the formation of a nitrite and subsequent formation of NO of HNO (due to the very high
pressures), these initial steps are endothermic and require additional chemical reactions to provide
heat feedback to the shock front.
D. Autocatalyzed Decomposition By Radicals
An alternative to unimolecular bond breaking is radical-assisted attack on the molecule. This is
indicated in Fig. 5 for H atom attack on nitromethane, CH3N02. One can see from Fig. 5 that the net
bond breaking energies for this reaction is small, on the order of 20 k~-mol-1,corresponding to the
activation energy for hydrogen atom addition. The overall bond breaking process is exothermic,
given the presence of the H atoms. From Fig. 5 one can see that H atom attack on the energetic
molecule produces new radicals, i. e., OH and CH3, which can further attack the energetic molecule,
producing a possibly rapid chain of exothermic chemical reactions.
Reaction of CH3N02
+
H
240 7
HONO
-120
1
+ CH3
- CH3OH +
NO
Reaction Coordinate Diagram
Fig. 5. Calculated reaction pathways for the reaction of CH3N02 + H + products, based on BACMP4 heats of formation at 298K for stable species and transition state activated complexes.
E. Changes in Bond Energies Due to Radical Formation
The bond energies within a molecule change when a radical is formed from the original molecule.
This was illustrated in the previous section in which the addition of a hydrogen atom greatly
weakened the C-N bond and the N-0 bonds. In general, for radical species, one must also consider
a barrier to dissociation in addition to the bond energy. However, the net bond breaking energy can
be significantly smaller than in the stable molecule, as was discussed in ref. 5. This is illustrated in
Fig. 6 for HMX where the bond breaking energies have been estimated from smaller prototype
molecular species4. In the HMX molecule, the weakest bond is N-N02, which is -200 kJ-mol-1.
However, once the NO2 group has left, the resulting second-nearest-neighbor bond energies
become very weak (e.g., the C-N bond breaking energy is -75 kJ-mol-1). Similarly, if a H atom has
been removed, the second-nearest-neighbor bond energies also become very weak (e.g., the NNO2 bond energy is negative with only a 8 M-mol-I barrier for leaving).
JOURNAL DE PHYSIQUE
C4-348
Bond Breaking Energies of HMX
Initially
I
110
"2C
/N
'8 - CH.
-Nee
8 0
C-N = 75
C-H = 138
\L/cH~-~
\ J
After NO,
Removal
?HZ
>N-N
I
H2C
\ry
- CH,
co
/N -NeO
d&o
N-N =
After H
Removal
* \f
O? 0
/'CH -N\/
GN-N
0.-
I
H2C
'y-cn2
C-N = 117
y"-'
/N
,o
- Ne0
Fig. 6. Bond breaking energies of (a) HMX, (b) HMX after a nitro group has been removed, and
(c) HMX after an H atom has been removed. Energies in k~-mol-1.Note that when a radical center
is present (indicated by a dot), the second-nearest-neighbor bonds become significantly weaker.
F. Decomposition Pathways
In Fig. 7 we show the high heating rate, gas phase decomposition mechanism for HMX based on
BAC-MP4 bond energies and activation baniers5. The order of bond breaking and the bond breaking
energies are indicated in the figure. The net products are HCN, NO;?, and H atoms. The initial step
of breaking the N-N bond can occur either by direct bond scissioning (which dominates at high
heating rates) or by autocatalyzation of the nitramine by the H atoms formed from the decomposition
of the H2CN in step #7. The H atoms, NO2 and HONO can react rapidly to form N02, NO, and HzO.
Decomposition of HMX
At High Temperatures
+ #n ( A E) : Position, Order, and Energy
of Chain Bond Breaking (Energy in kJ-mol-1)
Fig. 7. Decomposition mechanism of HMX at high temperatures. Final products are HCN, N02,
and H atoms.
G. Ignition of HMX
In this section we presents results of modelling the ignition process for nitramines. We solve for
the time-dependent coupled chemical reactions. Thirty-two chemical species and over one hundred
chemical reactions were used. The chemical reactions were a combination of the mechanisms
presented above along with those from flame codes that modelled HCN and C2N2 combustion9.l0.
First, we consider ignition at a constant pressure of 1 atmosphere. The temperature profile, given in
Fig. 8, indicates a three stage ignition process. The resulting species concentration profiles are
given in Fig. 9. The fist stage, which is endothermic (the temperature decreases slightly)
represents the decomposition of the gaseous RDX. The first ignition step represents the conversion
of NO2 to NO. The second ignition step corresponds to the major heat release, forming the final
products of combustion. Next, we consider ignition at constant volume with pressures
representative of solid densities. The resulting temperature profile, given in Fig. 10, also indicates a
three-stage ignition. However, the species concentration profiles, given in Fig. 11, indicate a change
in mechanisms. The two ignition stages observed at low pressures have combined into the second
ignition step while a new step, involving the pressure stabilized HONO and HNO species, now
occurs as the f i s t ignition step. Thus, under the high pressure conditions representative of
detonation, many-body reactions (which are not treated adequately under usual flame conditions)
must be considered. However, it should be noted that the initial stage of decomposition is still
endothermic.
Conclusions
Using the BAC-MP4 method, we have calculated the thermochemical properties of simple energetic
compounds and their possible decomposition pathways. We have shown that the decomposition
mechanisms are typically endothermic. Also, at high temperatures, bond fissioning is favored over
rearrangement. Thus, a series of chemical reactions are required to produce the energy needed to
JOURNAL DE PHYSIQUE
1ii
Ignition of RDX
I
LI
at 1 Atmosphere
r
Decomposition
1st Ignition
I
2nd lgnition
1
I
Fig. 9. Species concentration profiles for the ignition of gaseous RDX at constant
pressure for a starting condition of lOOOK and 1 atmosphere. Major chemical reactions
are given at each ignition step.
Decomposition
2nd lgnition
1st Ignition
Fig. 10. Temperature profile for the-ignition of gaseous RDX at constant volume for
a starting condition of lOOOK and 1000 atmospheres. Profile indicates multistage
ignition.
Chemistry of RDX lgnition
At 1000 Atmospheres
4 N2 + HID+ 3 CO
3 HONO
+
HNO
+
Hz+ CO2
+
Fig. 11. Species concentration profiles for the ignition of gaseous RDX at constant
volume for a starting condition of lOOOK and 1OOO atmosphere. Major chemical
reactions are given at each ignition step.
JOURNAL DE PHYSIQUE
propagate the detonation. Radicals can play an important role in autocatalyzing this process. The
modelling of the chemical reactions is presently being used to study the ignition of energetic
materials which can lead to detonation. The applicability of these reactions to the actual propagation
of a shock wave, however, is still not understood due to the short time scales and high pressures
involved . Much research is still needed in this area.
References
'c. F. Melius and J. S. Binkley, Twentieth Symp. (Internat.) on Comb., p. 575, The Comb. Inst.
(1984).
2 ~F.. Melius and J. S. Binkley, ACS Combustion Symposium, 249. p. 103 (1984).
3 ~Ho,
. M. E. Coltrin, J. S. Binkley, and C. F. Melius, J. Am. Chem. Soc.,
p. 4647 (1985).
4 ~ F.. Melius and J. S. Binkley, Twenty-first Symp.(Internat.) on Comb., in press.
5 ~F.. Melius and J. S. Binkley, Proceedings of the 23rd JANNAF Combustion Meeting, October
1986.
6 ~A.. Perry and C. F. Melius, Twentieth Symp. (Internat.) on Comb., p. 639, The Comb. Inst.
(1984).
'J. A. Miller and C. F. Melius, Twenty-first Symp.(Internat.) on Comb., in press.
8 ~M.
. Nazin, G. B. Manelis, and F. I. Dubovitskii, Russ. Chem. Rev. 37. p. 603 (1968).
'J. A. Miller M. C. Branch, W. J. McLean, and D. W. Chandler, Twentieth Symp. (Internat.) on
Comb., p. 673, The Comb. Inst. (1984).
'0. I. Smith and L. R. Thorne, Western States Section1 The Comb. Inst., Oct., 1986.
a,
-
Suggestion - J. BOILEAU
On p o u r r a i t envisager pour confirmer l e mecanisme, de c a l c u l e r ou d'essayer
l ' e f f e t de l ' a d d i t i o n de divers produits s o i t generateurs de radicaux
l i b r e s s o i t bloqueurs de radicaux l i b r e s t e l s que, dans ce d e r n i e r c a s ,
des ~ o u d r e su l t r a f i n e s de materiaux i n e r t e s , s i l i c e ou a u t r e .
Commentaires
- M.
SAMIRANT
Au cours d'etudes s u r l e s d e f l a g r a t i o n s du RDX on peut d e t e c t e r l e f r o n t
d ' i n i t i a t i o n ( h o t spots) e t l e f r o n t de combustion intense correspondant
1 'onde de pression. Pour une pression i n i t i a l e de quelques Kilobars on
observe un d e l a i de 150-300 u s e n t r e l e s deux ondes, en bon accord avec l e
d e l a i c a l c u l e e n t r e l e s premieres d i s s o c i a t i o n s e t l a reaction complete.
-
N. MANSON
1) La temperature que vous considerez dans vos c a l c u l s e s t - e l l e d e f i n i e en
admettant :
a . l ' e q u i l i b r e thermodynamique local ? e t
b. l ' e q u i l i b r e intramoleculaire ( e q u i o a r t i t i o n des formes d ' e n e r g i e
( t r a n s l a t i o n , r o t a t i o n vibration .. .) ?
2 ) Dans l e s c a l c u l s a haute pression i l sera necessaire de t e n i r compte des
i n t e r a c t i o n s e n t r e l e s molecules (Van der llaals e t c . . ) . Comment pensez-voUS
y parvenir (emploi de fuyantes ? equations d ' e t a t ? e t dans ce d e r n i e r cas,
lesquel l e s ? )