Theoretical Chemistry: Applications in Energetic Materials Research Betsy M. Rice U. S. Army Research Laboratory

1. Theoretical Chemistry: Applications in Energetic Materials Research Betsy M. Rice U. S. Army Research Laboratory Aberdeen Proving Ground, Maryland 21005-5066

2. Acknowledgements • Donald L. Thompson, Oklahoma State University • Samuel F. Trevino, ARL • William Mattson, U. Illinois Urbana Champaign and ARL • Dan C. Sorescu, National Energy Technology Laboratory • John Grosh and Jen Hare, formerly of ARL • Herman Ammon, University of Maryland

3. OBJECTIVE Use standard theoretical chemical approaches to • Screen proposed materials—eliminate poor candidates before expending resources on synthesis, formulation and tests • Identify and understand the individual fundamental chemical and physical steps that control the conversion of the material to final products

4. METHODS • Quantum Mechanics – First principles • Solution of HY=EY for collection of atoms - characterizes system • Provides information for parameterization of classical models • Molecular Dynamics – A classical simulation method • Integration in time of F=ma for every atom – requires model • Provides molecular-level details of chemical and physical processes through computer simulation of dynamic events • How a material responds to set of initial conditions at the atomic level. • What mechanisms control the response—will provide guidance on how to manipulate system such that desired response obtained. • Molecular Packing – A classical simulation method for “ab initio” crystal prediction • Evaluates lattice energy of molecule in a variety of possible crystalline environments – requires model • Ranks possible crystal structures (usually in order of increasing energy) • Provides density and details of structure of crystal (size, shape and position of atoms in it)

5. Prediction of Energetic Materials Properties from correlations with charge distribution Electrostatic Potential Mapping out e- Density + 1 Nanometer (electron poor) (electron rich) CL20

6. COMPLETELY PREDICTED! Correlations of Quantum mechanical predictions with bulk properties B. M. Rice, S. V. Pai and Jennifer Hare, “Predicting Heats of Formation of Energetic Materials Using Quantum Mechanical Calculations”, Combustion and Flame, Vol. 118, p. 445 (1999). Condensed Phase Heats of Formation: • DHGas from quantum mechanics • DHSub and DH Vapestimated from correlation between bulk properties and electrostatic potential of a molecule. J. S. Murray and P. Politzer, “A General Interaction Property Function (GIPF): An Approach to Understanding and Predicting Molecular Interactions” in “Quantitative Treatments of Solute/Solvent Interactions”, ed. P. Politzer and J. S. Murray, (Elsevier Pub. Co., New York, 1994). B. M. Rice, S. V. Pai and J. Hare, “Predicting Heats of Detonation Using Quantum Mechanical Calculations”, Thermochemica Acta, Vol. 38, p. 377 (2002).

7. Measured Impact Sensitivity Weight Drop Height (cm) Explosive Sample Impact machine • Explosives (in mg) placed in between on flat tool steel anvil and flat surface of tool striker. • 2.5 kg drop weight is dropped from predetermined height onto the striker plate. • Result of the event (explosion or otherwise) is determined by sound, smell and visual inspection of the sample. • Drop height is varied, with height increased or decreased depending on result of previous event. • Sequence of tests carried out, with result quoted at h50, the height at which 50% of tests result in explosions.

8. 22 cm 47 cm 141 cm 320 cm 490 cm B. M. Rice and J. J. Hare, “A Quantum Mechanical Investigation of the Relation Between Impact Sensitivity and the Charge Distribution in Energetic Molecules”, B. M. Rice and J. J. Hare, Journal of Physical Chemistry, Vol. 106, 1770 (2002). 11 cm 11 cm 28 cm 71 cm

9. b-HMX a-HMX d-HMX Evaluating the Model: Predicting Crystal Structures using molecular packing • Place single molecule in variety of crystalline environments • Using classical force field, minimize energy with respect to crystal parameters • Rank various crystal structures (usually lattice energy)

10. Potential Energy Functions for classical molecular simulation of energetic molecular crystals • 8 Papers published in J. Physical Chemistry • Transferability (4) • Limitations of Rigid Body Approximation (1) • Inclusion of Flexible Motion (1) • Behavior in Liquid State(1) • Current investigation: prediction of crystal structure using molecule packing D. C. Sorescu, B. M. Rice and D. L. Thompson, “Intermolecular Potential for the Hexahydro-1,3,5-trinitro-1,3,5-triazine Crystal (RDX): A Crystal Packing, Monte Carlo and Molecular Dynamics Study,” the Journal of Physical Chemistry B, vol. 101, pp-798-808, 1997.

11. MOLPAK (MOLecular PAcKing)J. R. Holden, Z. Du and H. L. Ammon, J. Comp. Chem. 14, 422 (1993) • Uses rigid-body molecular structure to provide packing arrangements in 13 space groups. • Triniclinic: P1, P-1 • Monoclinic: P21, P21/c, Cc, C2, C2/c • Z=4 Orthorhombic: P21212, P212121, Pca21, Pna21 • Z=8 Orthorhombic: Pbcn, Pbca • MOLPAK search produces “initial guesses” --- needed to energy refinement. For each space group ~7000 “Possible structures” are generated. • 25 most dense structures are further refined using WMIN

12. How good is the force field? • Applied to 39 nitramine and non-nitramines • From Nitramine and non-Nitramine paper, nitrocubane series • Predicted experimental structure for 38 of 39 (1 catastrosphic failure, believed numeric) – max. deviation no more than 4% in edge length, largest deviation of cell angle is 7º. • Low-energy structure is experimental structure for 28 • For remaining 10 cases, all within 1.5 kcal/mol of low-energy structure; 7 were within 0.4 kcal/mol.

13. O N 2 N N O 2 O N 2 N N N O 2 N N N N O O N 2 2 Modeling Results for candidate materials from ARDEC Rapid Assessment Heat of Formation (solid): 113.1 kcal/mol Heat of Detonation: 1.41 (kcal/g) h50%: 9 cm Density of low-energy structure: 1.77 g/cc Tetradecanitrobicubane Heat of Formation (solid): 242 kcal/mol Heat of Detonation: 1.81 (kcal/g) h50%: 68 cm Density of low-energy structure: 1.81 g/cc

14. What now? • Ab initio crystal prediction of chemical families of explosives: • Nitrocubane series (6 have been resolved) • Nitramines (71) • Nitrate Esters (32) • Notified October 4 that team consisting of Rice, Mattson, (ARL),Ammon (U MD),Singh (NRL) and Kim (U Miss) awarded 2003 DOD High Performance Computing Modernization Plan CHSSI grant to parallelize MOLPAK and incorporate DOD Planewave

15. MOLECULAR DYNAMICS SIMULATION OF DETONATION Model Explosive: A-B. Reactions that can occur: 2 A-B  A2 + B2 A-B  A + B Initial crystal at 10 K, molecules arranged in equilibrium configuration Left side of plate hit with flyer plate of molecules moving at a very high speed. The impact compresses the quiescent crystal, and a shock wave propagates through the material. Reactions begin, and heat released from the reaction drives the shock-wave, resulting in a self-sustained detonation

17. MOLECULAR SIMULATION OF DETONATION B. M. Rice, W. Mattson, J. Grosh and S. F. Trevino, “A Molecular Dynamics Study of Detonation: II. The Reaction Mechanism”, Physical Review E, Vol. 53, 623 (1996). B. M. Rice, W. Mattson, J. Grosh and S. F. Trevino, “A Molecular Dynamics Study of Detonation: I. A Comparison with Hydrodynamic Predictions”, Physical Review E, Vol. 53, 611 (1996). Reaction Mechanism: Pressure-induced atomization, little thermal excitation

18. +   The Bij term is a short-range function that introduces many-body effects into the interaction between two atoms that are within a range typically associated with a covalent bond. Bij = 1 corresponds to isolated molecule Bij REBO Potentials Intramolecular bonds (covalent) Intermolecular bonds

19. DESENSITIZATION OF DETONABLE MATERIAL B. M. Rice, W. Mattson and S. F. Trevino, “Molecular Dynamics Investigation of the Desensitization of Detonable Material”, Physical Review E, Vol. 57, 5106 (1998).

20. Reactive Potentials • Reactive Force Fields (ReaxFF) (Goddard et al., Center for Simulation of Dynamic Response of Materials, California Institute of Technology) • Uses QM calculations to parameterize a function • Applied to RDX and HMX • Flyer-plate shock simulations show: • Initiation threshold exists • Large fraction of products have been observed in experiment • Some unlikely fragments • Improvements will include products of secondary reaction channels Chakraborty, D.; Muller, R. P.; Dasgupta, S.; Goddard, W. A., J. Phys. Chem. A 2000, 104, 226.

21. SUMMARY • Theoretical chemistry calculations will provide information necessary to tailor explosives – BUT OFTEN RESULTS ARE COMPLETELY DEPENDENT ON QUALITY OF THE MODEL • Realistic classical molecules exist for non-reactive events for CHNO explosives -- are not bad • Reactive potentials—basic concepts there, but need additional and better information for parameterization • Direct Ab initio MD simulations progressing, not quite there, but extremely promising