Simulations ofPhysical & Chemical Processes in
Gas, Liquid, and Solid Phases
Donald L. ThompsonUniversity of Missouri-Columbia
MURI ReviewOctober 27, 2004
Collaborators
Dr. Paras M. Agrawal (OSU)Prof. Saman Alavi (MU)Prof. Rod Bartlett (U.Fla.)Prof. Carol Deakyne (MU)Prof. Yin Guo (OSU)Dr. Larry Harding (ANL)Mr. Josh McClellan (U.Fla.)Mr. Michael McNatt (MU)Dr. Betsy M. Rice (ARL)Dr. Igor Takmakov (MU)Dr. Gustavo F. Velardez (MU)Dr. Al Wagner (ANL)
Overview
Physical Properties and Processes Melting
Gas-Phase ReactionsUsing Ab Initio PESs: Fitting Surfaces and Direct Dynamics
Chemistry in the Condensed Phases Rate Calculations in Liquids Simulating Impact
A report on work in progress…
Physical Properties and Processes: Condensed Phases
Pre-MURI: Sorescu-Rice-Thompson
Crystal ModelsLiquid Nitromethane
MURI: (with Dr. Betsy Rice, ARL)Practical methods for simulating melting Applications: Rare gases, Nitromethane, TNAZ, RDX, PETN
Approaches: Melting SimulationsDirect heating of the solid
Straightforward to performSuperheating effect due to G s- interface formationOver-estimation of MP ~ 5%-25%, many cases ~20%
Extrapolation of the free energy of solid and liquid phasesAccurate determination of melting point (no superheating)Difficult to set up for ionic and complex molecular solids
Constant energy two phase solid-liquid simulationAccurate determination of melting pointTime consuming, difficult for complex systems
Heating of the solid with voidsStraightforward to performSuperheating effect eliminated
Melting of Nitromethane (molecular solid)
•Direct heating •Void nucleated •S- in contact Melting Point: Expt: 244.7 K Calc’d: 255±10 K
P. M. Agrawal, B. M. Rice, and D. L. Thompson, J. Chem. Phys. 119, 9617 (2003).
Solid Liquid
Nitromethane Potential
Sorescu, Rice, and Thompson, J. Phys. Chem. B 104, 8406 (2000).Agrawal, Rice, and Thompson, J. Chem. Phys. 119, 9617 (2003).
intrainter VVV
r
r
C – e A ii
6ijrB-
ijinterij V
torsionbendstretchintra VVVV
112
)(stretch
0
iii rri eDV 20
21
bend )( iiikV
)cos(1torsion iiimVVi
TNAZ, RDX, PETN Melting
A work in progress…
A paper is being written on the lessons we have learned
Force fields are inadequate: All predict too high MP
Ultimately, we want to use the MURI potential.
But, it would be useful to have simple FF models
Gas Phase Chemistry
The Challenges:
Develop methods for simulating the sequential, branching decomposition of large molecules to form small stable product molecules.
Predict rates of elementary reactions involving many atoms(e.g., RDX, TNAZ, DMNA …)
What we are doing
Reactions TNAZ dissociation DMNA dissociationH2CN dissociationH2CN + OH and other small molecules and radicals
Using quantum chemistry to explore potential energy surfaces TST calculations using the quantum chemistry results Developing analytical PESs for MD simulations Developing better fitting methods Direct dynamics (forces-on-the-fly)
Methods for Fitting Ab Initio PESs Methods that allow facile, accurate local fitting of ab initio points to give global fits and for use in direct dynamics simulations.
Partial Support from DOECollaborator: Larry Harding & Al Wagner
* J. Phys. Chem. A 107, 7118 (2003). J. Chem. Phys. 119, 10002 (2003). J. Chem. Phys. 120, 6414 (2004). J. Chem. Phys. 121, 5091 (2004). Approximate –Fits at
critical points
Real
Fitted
Interpolating Moving Least Squares (IMLS) methods* Application: H2CN dissociation and reactions Methods for coupling PESs for molecules, radicals, & intemediates via TSs**
**J. Chem. Phys. 118, 1673-1678 (2003).
H2CN Dissociation
Direct Dynamics Simulations of theUnimolecular Decomposition of CH3NO2
CH3- NO2 → CH 3 + NO2
(UCCSD/ TZP)
-80
-40
0
40
80
120
160
200
0.5 1.5 2.5 3.5 4.5
r(C-N), Ǻ
V(r
), k
cal/m
ol
CH3NO2 → CH3 + NO2
• Objectives: • Apply Bartlett’s transfer hamiltonian in simulations• Calculate ∂V/∂r on-the-fly using a simple
HamiltonianPotential and ForcesNDDO-SRP semi-empirical MO theory• Specific Reaction Parameters (SRP)Starting with AM1 values, optimize SRP to reproduce:i) equilibrium bondsii) UCCSD/TZP forces along the C-N bond fission
coordinate• Work in ProgressNext: SRP for additional reaction channels (e.g., CH3NO2 → CH3ONO → CH3O + NO) Igor Tokmakov (MU)
Josh McClellan (U.Fla.)Rod Bartlett (U.Fla.)
What we have done (a work in progress):
• Performed DFT (B3LYP): 6-31G(d,p) calculations to map out PES
• Identified and characterized reactants, intermediates, transition states, and products of TNAZ decomposition reactions, providing a general map of the potential energy surface.
• The geometries, energies and vibrational frequencies of all species are calculated at a uniform level of theory.
Sequential, branching decompositionof large molecules
TNAZGood prototype for our purposesExperimental data (although not necessarily definitive)Proposed sequentially branching mechanisms
TNAZZhang-Bauer Mechanism
++
+ NO2+ NO2
C3H4 + NO2+ HONO + HONO
Y.-X. Zhang & S. H. Bauer,J. Phys. Chem. A102, 5846 (1998).
Lee & Coworkers: TNAZ Mechanism
N2O2 + C3H4
+ NO2 + NO2
+ NO2
+ NO2
D. S. Anex, J. C. Allman, and Y. T. Lee, in Chemistry of Energetic Materials, ed. by G. A. Olah and D. R. Squire (Academic Press, New York, 1991), pp.27-54.
E –
E(T
NA
Z)
(kc
al/m
ol)
TNAZ
TSs
HONO +
*
+
The initial energy barriers to reaction are approximately the same for the different pathways.
Initial Steps
HONO elimination
38 kcal/mol
44 kcal/mol
45 kcal/mol
41 kcal/mol
TNAZ: Barriers toInitial Reactions
NO2 elimination
E –
E(T
NA
Z)
(kc
al/m
ol)
TNAZ
ONNO + C3H4
+ 2NO2
Fig. 4
+ NO2 +
NO2 + ONNO +
*
+ NO2
NO2 +
TS
NO2 +
2NO2 +
2NO2 +
+ 2NO2
2NO2 +
NO2 +
Steps following C-NO2 bond fission
triplet
?
?
?
TNAZ
E –
E(T
NA
Z)
(kc
al/m
ol)
NO2 +
NO2 +
2NO2 +
NO2 +
+NO2 +
*
TS
2NO2 + triplet
singlet
Steps following N-NO2 bond fission
?
?
?
E –
E(T
NA
Z)
(kc
al/m
ol)
TNAZ
TS
Fig. 6
HONO + 2NO2 +
HCN + HCCH +HONO +2NO2TS
HONO +C≡CNO2 +C=NNO2
HONO +
*
+
TS
+
HCN + HONO +
HONO + NO2 +
Steps following HONO elimination
TNAZ
• We now have the information needed to compute RRKM rates for unimolecular steps
• Need to calculate IRCs• Develop analytical PESs
• Higher level calculations desirable, but low-level results sufficient for developing methods
DMNA Decomposition
Quantum Chemistry Calculations to determine decompositionPathways: barrier, intermediates
Calculate IRCs
Calculate rate
DMNA N-N Bond Fission
(CH3)2N NO2
Reaction Coordinate
-12 -10 -8 -6 -4 -2 0 2 4 6 8
Ene
rgy
(kca
l/m
ol)
0
10
20
30
40
50
60
= 1407i cm-1
DMNA
CH3CH2N + HONO
DMNAHONO Elimination
DMNA Nitro-Nitrite Isomerization
=318.9i cm-1
cis-(CH3)2NONO
DMNA
Bimolecular Reactions
Nizamov and Dagdigian: (J. Phys. Chem. A 2003, 107, 2256.) • Reported the room-temperature rate constant for the H2CN + OH reaction
• Concluded that H-atom abstraction giving HCN + H2O is
the predominant reaction channel.
• We have performed B3LYP/6-31G(d,p) & G2 calculations • Identified likely products • Eventually – Calculate rate constants
Reaction channels considered
H2CN + OH HCN + H2O (1)
H2CNOH (2)
H2CONH (3)
H2CN(H)O (4)
CH3NO (5)
CH3ON (6)
HCON + H2 (7)
HCNO + H2 (8)
HNCO + H2 (9)
H2CNH + O (10)G2 predicts that (9) is thermodynamically the most favorable.It is more exoergic than reaction (1) by 10 kJ/mol and at least 150 kJ/mol more exoergic than the remaining 8 reactions.
Currently – searching for TSs and calculating IRCs for each reaction \
Impact Studies: Nitromethane• Objectives & Topics
–Impact dynamics• Study sub-detonation-strength shock simulations of solids, liquids, and gases:
–Develop general codes to make and monitor sound, shock, and heat waves through systems–Study sound speeds through various mediums–Study energy transfer rates via sound and supersonic waves (where applicable)–Examine wave front shapes–Study energy transfer mechanisms, i.e. lattice vibrations (phonons) exciting intramolecular bonds (up-pumping), etc.
•Detonation strength (reactive) shock simulations of molecular systems
–Heat Shocks
• Current Work in Progress–Code development for above objectives–Shock simulations on prototype atomic systems (i.e. Lennard-Jones, ...)–Simulations carried out on prototype energetic molecular condensed phase nitromethane
• Simulates an ~85 Å crystal layer by imposing 2D periodic boundary conditions in the x & y directions. • Uses the potential developed by Sorescu, Rice, Agrawal, and Thompson for nonreactive solid, liquid, and gas phases
• Impacts of varying strengths are initiated by accelerating in the +Z direction a “flyer-plate” of ~1 unit cell (about 80 molecules in the X-Y plane)
• MD NVE simulations done using DL_POLY
~6.2 Å
~5.2 Å
~85 Å
Shock Wave YZ
X
Nitromethane 5x4x10 Supercell (800 molecules)
Simulations Supercell
-2
-1
0
1
2
0.5000
1.0000
1.5000
2.0000
2.5000
-40 -30 -20 -10 0 10 20 30 40
Velocity Shock Wave Through a Thin Layer (~86 ang.) of Nitromethane Perfect Crystal
-200 m /s
-100 m /s 0 m /s 100 m /s 200 m /s • Time step 0.75 fs
• This plot gives ~3.3 km/s as the speed of sound through the solid at 50 K.
(Å)
(Å /
ps)
Å
Shock front
Methods for reaction rate calculations in liquids
The approach allows for the computation of reaction rates by using a relatively inexpensive stochastic method that is calibrated with the results a few full-dimensional MD simulations. Application to date: HONO in liquid Kr
cis-trans isomerizationchemical decomposition
Next: Large moleculese.g., DMNA, TNAZ, RDX
• Y. Guo and D. L. Thompson, J. Chem. Phys. 120, 898-902 (2004). • Y. Guo and D. L. Thompson, “On Combining Molecular Dynamics and Stochastic Dynamics Simulations to Compute Reaction Rates in Liquids: Bond Fission in HONO in Liquid Kr,”J. Chem. Phys., in press.
Plans Continue studies of melting of energetic materials Studies of RDX, PETN,… with improved force fieldsStudies of RDX, PETN,… with MURI potential
Methods for fitting ab initio PESs for reactionsContinue developing IMLS fitting methods.Apply Bartlett’s transfer hamiltonian approach.Methods for coupling PESs for molecules, radicals, & intemediates via TSs
Rate calculations and dynamics calculations for decomposition reactions Perform TST calculations using quantum chemistry resultsDevelop an analytical PES and perform MD simulations for conditions corresponding to the various gas-phase experiments.
Plans, continued Simulations of shocked solids and liquidsImpact studies of nitromethaneAlso: PETN & hydrazine (With Rice & Brenner)
Develop PESs and perform rate calculations for energetic molecules and radicalsWe plan to perform quantum chemistry exploratory studies of DMNA decomposition channels.Develop global PESs and perform MD simulations of the initial steps of nitramine decompositionPerform direct dynamics
Methods for rate calculations for condensed-phase reactionsApplications to RDX (?)
Methods for simulating evaporation/sublimation
Publications & Preprints Paras M. Agrawal, Betsy M. Rice, and Donald L. Thompson, “Molecular Dynamics Study on the Effects of Voids and Pressure in Defect-Nucleated Melting Simulations,” J. Chem. Phys. 118, 9680-9688 (2003). Paras M. Agrawal, Betsy M. Rice, and Donald L. Thompson, “Molecular Dynamics Study of the Melting of Nitromethane,” J. Chem. Phys., in press. Saman Alavi, Lisa M. Reilly, and Donald L. Thompson, “Theoretical Predictions of the Decomposition Pathways of 1,3,3‑Trinitroazetidine (TNAZ)” J. Chem. Phys., in press. Yin Guo and Donald L. Thompson, “On Combining Molecular Dynamics and Stochastic Dynamics Simulations to Compute Reaction Rates in Liquids,”J. Chem. Phys., in press.
Preprints available upon request or at: http://www.chem.missouri.edu/thompson
The End
http://www.chem.missouri.edu/thompson/MURI