MECHANISMS OF DETONATION IN MOLECULAR CRYSTALS : A REVIEW

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MECHANISMS OF DETONATION IN MOLECULAR CRYSTALS : A REVIEW

S. Odiot

To cite this version:

S. Odiot. MECHANISMS OF DETONATION IN MOLECULAR CRYSTALS : A REVIEW. Journal

de Physique Colloques, 1987, 48 (C4), pp.C4-225-C4-233. �10.1051/jphyscol:1987416�. �jpa-00226648�

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MECHANISMS OF DETONATION IN MOLECULAR CRYSTALS : A REVIEW

S. ODIOT

Departement de Recherches Physiques, U.A. au C.N.R.S. n o 04071, Universit4 Pierre et Marie Curie, 4 place Jussieu,

75252 Paris Cedex 05, France

RESUME

Le contrBle e t la maitrise d'un cristal .moleculaire Bnergetique demanded une connaissance detaillee des processus microscopiques de decomposition detonique des molBcules B I'intbrieur du cristal. Ce sont eux, B notre avis, qui sous tendent ies interactions plus compiexes que I'on devine au travers une etude thermodynamique macroscopique du phenombne. La molecule d'un compost. energetique doit porter en elle une ou plusieurs caractkristiques qui l a diffkrencient de sa voisine non explosive dans I'univers cornplexe de la Chirnie ou de la Chimie Physique. C'est dans c e t esprit de comparaison systematique que depuis 1976, nous avons men6 notre investigation. II apparait alors qu'une liaison dans un Btat molbculaire predissociatif "actif" plus ou moins excite, pourrait 6 t r e a I'origine du phknomene e t qu'une structure en chaine de ces liaisons dans le c r i s t a l jouerait le r g l e de support de transmlsslon au travers des interactions m o l ~ c u l a i r e s de type dipolaire. Deux questions principales sont soulevees :

Si un tel etat existe. quel en e s t le processus de peuplement ?

Une fois peuple cet Btat e t la molecule dissocibe, comment s'effectue le t r a n s f e r t energhtique explosif dans le c r i s t a l ?

ABSTRACT

The control o f an energetic molecular c r y s t a l requires a detailed knowledge o f the microscopic process o f the explosive decomposition o f the molecules Inside the c r y s t a l . From our point o f view, these microscopic processes underlie the more complex interactions which cannot be described through thermodynamic and macroscopic studies alone. It i s our belief that the molecules o f energetic materials are structurally different from those related but non-explosive materials. Since 1976, w e have conducted a systematic comparison o f some o f these molecular structures.

Our conclusion i s that a bond i n a predissociative "active", more o r less excited state, i s a t the origin o f the detonation phenomenon and that a chain structure o f these bonds i n the c r y s t a l would play the r o l e in supporting the transmission o f the detonation through molecular interactions o f a dipolar type. Two main questions are raised:

If such a state does exist, what i s the process o f populating such a state?

Once this state i s populated and the molecule dissociates, how does the explosive energy transfer occur In the c r y s t a l ?

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987416

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JOURNAL DE PHYSIQUE

I I

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If such a state does exist, then we would have to study the energy transfer induced by shock from one molecule to the neighbollring molecules through the intermolecular potential.

I I I - I f such a state does exist, we must also study the energy transfer frorn the shock wave to the internal modes through the lattice modes, following a'multi phonon process such as that involved i n the ultra sound absorption i n molecular crystals.

In conclusion, our aim i s to determine the mechanism for the propagation of the detonation wave.

By way of introducing the lectures to be given here today and tomorrow, let me briefly review what has been done i n the studies listed above.

1. PoPentlel hypersurfaces rsletlve Po ths C-# bond lsnflths. (THEORETICAL CHEHISTRV CALCULATIONS)

By these techniques, we have studied f i r s t :

- Nifrobenzene, mefa andpara dhnifroben~ne, 1,3,5- frinitrobenzene : groundand fti-sf 9 sing/ef excffede/ecfron?b sfafes as a funcfhn of the C-N symmefr ic stretch fig v?Brafion. ( 44)

C.N.D.O./S method with Configuration Interaction (CI ; single and double excitations) and ST03G method were used. The corresponding curves were calculated point by point i n CNDO/S, correlating the symmetry of each state, from the ground state calculated by the ST03G method corrected by CI.

then:

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Nffrmfhana. gmdandfiPsf exc/7eds1hg/ef andfrfp/ef stafes ( 5)

We have used 4 - 3 1G and CI process ( 5 classes of excitations), with a total geometry optimization i n Cs symmetry, with the NOz group lying i n the plane of symmetry. For each state, the potential surface section was plotted with respect to the C-N bond. The geometry of the CH3 and NO2 fragments was that deduced from an optimization of the SCF ground state of nitromethane.

A similar plot of the potential surface was studled with this CH3 geometry but with an NO2 geometry like that of the (excited) state i n which nitromethane dissociates. All the electronic states were labeled i n the C ~ V symmetry, since the electronic transltlons i n a l l the excited states studied are mainly i n the NO2 group.

1A2 appears as a predissociative and exothermic excited electronic state.

I n a same manner we have studied thed?ssoc1bfionofdifluuram~i7'e and we found that a predissociation following two pathes a) or b) would occur through the ground state

S0

due to the presence of a low dissociative triplet state T I .

This low predissociative ground state, electronic molecular result, compared to the predissociative excited electronic state of nitromethane could be interpreted, at a microscopic molecular level, as the origin of the difference i n sensitivity between primary (difluoramide) and secondary (nitromethane) explosives.

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JOURNAL DE PHYSIQUE

I

----*

1.50

2.00 2.50

C-N

Fig.2

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Potential surfaces of the ground state So, predissociative state 1A2, 1 6 I, the predlssoclative state 1 B2 and the two f i r s t triplets 3 6 1 and 3A2.

The curves indicated b y a solid line (-1 correspond t o the interpolation o f the value o f the energy i n assuming that the NO2 and CH3 fragments f o r each value o f RC-N are i n an optimized geometry. The curves indicated b y a broken line (---I correspond to the caiculation o f the variation i n the energy as a function o f RC-N; the CH3 and NO:! fragments are i n the equilibrium geometry o f the ground state o f nitromethane. The curves indicated by a dotted line (...I correspond to the calculation o f the variation i n the energy as a function o f RC-N; the CH3 fragment i s i n the equllibrlum geometry o f i t s ground state. The geometry o f the NO2 fragment i s adaptated to the geometry o f each o f i t s excited states.

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N-H

F i g . 3b

E ,

\ HNFE --c H N F + F

\

2.21 eV

t L

r NJ=

Fig.3

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Ground state and f l r s t triplet state of dlfluoramide following two dlssoclation pathes, a. r ~ - H b. rN-F

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C4-230 JOURNAL DE PHYSIQUE

2.

Cecpctretlve precess of energy trensfer In e crystal. CRaln structures

i n

mergrtlc crystal.

(MOLECULAR DYNAMICAL CALCUI-ATIONS)

By inspection ( 6 ) of the crystalline structure of nitromethane (71, we observed groups of equal C-C and equal N-N bonds whose lengths were shorter than the lattice periodicity. These bonds could be arranged i n zigzag chains as shown Fig.4.

Fig.4

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Nitromethane zig-zag C-N bond chains along the two crystallographic axis a. and b.

Here I thank G. Watelle (Dijon), who introduced me to her research laboratory " RhctivitE! des SolidesU(R.S.) and to Peyrard (O.R.C.), whose knowledge i n soliton propagation i n crystals has allowed us to progress i n the second part of the program.

With the crystallographic data from Hardy (Poitiers), and from literature, Bertrand and Mutin (R.S.) have been able to classify 36 molecular crystals into three classes : non explosives, weak explosives, and high explosives, according to the spatial grouping (piling up) of their atomic bond structures. (8) Meanwhile, with Peyrard, we have proposed some initial steps of a theoretical microscopic approach to detonation, correlated to the molecular structure through the intramolecular potential of the bond, and to the crystal structure through the intermolecular potential

.

We present i n figure 5 a v e r y simplified one dimensional model which, i n spite of i t s simplicity, has allowed us to percieve the molecular features which control a detonation at a microscopic level. ( 9)

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Fig.5

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Monodimensional Model. P: Shock wave; H3 C

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^N02 : ( A

-

B ) ; e : the molecule

I

correlated to detonation threshold : P.

De correlated to detonation velocity.

These two parameters are independent.

This model revealed the importance of the energy dissipation introduced as a damping term i n the dynamic equations. Later we have shown ( l o ) , that the dissipation of energy was not transferred to the nitromethane fragments i n vibrational o r rotational modes during the 1A2 state dissociatior: . The main source of dissipation was to be found i n transverse modes i n the crystal .This kind of dissipation i s now included i n a two dimensional model.( 1 1

1

Moreover, a theoretical estimation of the relative weakness of the bonds i n a molecule (Fliszar and Vauthier) w i l l give information on the radicals involved i n the chemistry of detonation that is necessary for the development of a dynamical approach to detonation wave propagations. This w i l l be discussed tomorrow.

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3. How to populate a molscular exctted el@ctronlc state by a shock wnve i n a cryst01 ? This question has been reexamined i n the laboratory through the experimental and theoretical results obtained by P e r r i n and Zarembowitch from their studies of ultrasound absorption by molecular crystals (10). Thus, this morning, it w i l l be very important to discuss their results with the american school which i s so well represented here to day. I await w i t h a great deal of interest to the discussions which w i l l shed light on this important question.

I n conclusion, i f one succeeds i n developing further understanding of these last two points, i.e.

energy transfer from the shock wave through the lattice modes to the internal modes of a molecule, and energy transfer from one molecule to the neighbouring molecules, then we might be i n a better position to control the role of the molecules i n an energetic material and the role of the electronic structure of the crystal i n detonation. Extended to the macroscopic studies, perhaps our aim should be the mastery of the large chemical energy contained i n such a material and the development of techntques for controlling the release of this energy.

I specially thank Monique Blain for her collaboration during a l l t.hese years of our association.

We are indebted to D.R.E.T. for a financial support.

References

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S. Odiot, Rapports de synthkse DRET, N077/03 1 ; Ne78/0 15 ( 2 )

A. Delpuech. Paper presented i n this conference and references therein.

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S. Odiot, Rapport DRET, 10/ 12/ 1979. Transfert d'energie dans les cristaux moleculaires energet iques.

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a. J. Boileau. M. Blain. S. Odiot. J.M. Leclercq. S. Fliszar. 6. Fleury, 6. Vergoten

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Propel, and Explosives, 6, 1 26 ( 1 98 1

b. S. Odiot. M. Blain. J.M. Leclercq. J. Leclercq. S. Fliszar. J. Boileau. 6. Fleury - Propel, and Explosives, 9, 82 ( 1984)

c. J.M. Leclercq. J. Leclercq. J. Boileau. S. Odiot, E. Lavenir. M. Blain

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Propel. and Explosives,

9,

20 1 ( 1984)

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C. Mijoule. S. Odiot. S. Fliszar. J.M. Schnur

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Molstruc. (Theochem)

1 ,

31 1 ( 1987) ( 6 )

Thanks to C. Kappenstein. F. Gerard for thelr program.

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S. Trevlno. E. Prince, C.R. Hubbard - J. Chem. Phys. 73,2996 ( 1980) ( 8 )

6, Bertrand. J.C. Mutin

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Private communication.

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P. Pernot. 0. Atabek. A. Beswick. Ph. M i l l i e , S. Odiot

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Annalesde P h y s i q u e 1 2 0 9 ( 1 986).

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Paper presented by PI. Peyrard i n this conference and references (7,8) therein.

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A. Zarembowitch, Paper presented in thfs conference and references therein.