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4. TITLE AND SUBTITLE

5a. CONTRACT NUMBER FA9300-06-C-0023

Thermal Decomposition Mechanism of HNNO2H Dissociated from Mononitrobiuret and 1,5-Dinitrobiuret

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6. AUTHOR(S)

5d. PROJECT NUMBER

H. Sun, G. L. Vaghjiani 5e. TASK NUMBER

5f. WORK UNIT NUMBER Q0RA

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Air Force Research Laboratory (AFMC) AFRL/RQRP 10 E. Saturn Blvd. Edwards AFB CA 93524-7680

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)Air Force Research Laboratory (AFMC) AFRL/RQR 5 Pollux Drive 11. SPONSOR/MONITOR’S REPORT Edwards AFB CA 93524-7048 NUMBER(S) AFRL-RQ-ED-VG-2014-058

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13. SUPPLEMENTARY NOTES Briefing Charts presented at WSS Meeting, Cal Tech, CA, 24-25 Mar, 2014. PA#14159

14. ABSTRACT Mononitrobiuret (MNB) and 1,5-Dinitrobiuret (DNB) are, tetrazole-free, nitrogen-rich compounds, which have been reported as powerful new explosives. The initiation of thermal decomposition of MNB and DNB was found to involve an intra-molecular transfer of the H-atom from the central NH group to one of the adjacent nitro oxygens to eliminate the unstable intermediate, HNNO2H, which undergoes further decomposition. In this work, we have investigated the thermal decomposition of HNNO2H using multi-reference second-order perturbation theory and coupled-cluster theory. The following HNNO2H decomposition pathways were found to be important. First, a direct N−OH bond fission occurs with a loose saddle point to form OH and cis-HNNO radicals. Second, an inversion of the aminylene H-atom elongates the N−OH bond due to repulsion between the aminylene H-atom and the hydroxyl H-atom, and this leads to N−OH bond fission to form OH and trans-HNNO radicals. Third, the thermodynamically stable products, N2O + H2O, are formed by a complex mechanism, which involves rotation of the N−OH bond, an H-atom shift from the hydroxyl H-atom to the nitric oxygen, and then migration of the aminylene H-atom to the hydroxyl O-atom, resulting in H2O elimination with 50.4 kcal/mol of exothermicity.

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1

Thermal Decomposition Mechanism of HNNO2H Dissociated from Mononitrobiuret and 1,5-Dinitrobiuret

Hongyan Sun Ghanshyam L. Vaghjiani

DISTRIBUTION A: Approved for public release; distribution unlimited

Spring 2014 Technical Meeting, Western States Section of the Combustion Institute, March 24-25, 2014

California Institute of Technology

DISTRIBUTION A: Approved for public release, distribution unlimited

Energetic Materials: MNB & DNB

(Klapötke et al., Propellants, Explosives, Pyrotechnics, 2004)

DNB detonates strongly without oxidizer in the steel sleeve test after 4 seconds • MNB & DNB: tetrazole-free, nitrogen-rich molecules

• High energy density: 1.859 g/cm3 (DNB)

• Thermal decomposition mechanism of MNB and DNB was proposed by Klapötke et al. (Combust. Flame 2004) as below:

• The decomposition is initiated by elimination of HNNO2H intermediate, which was confirmed by direct dynamics trajectory simulations as the only dominant channel below 1500 K (Liu, Chambreau, Vaghjiani,

J. Phys. Chem. A 2011)

2


New Ignition Mechanism of MNB & DNB

• The thermal decomposition of MNB and DNB occurs through a multistep reaction process

1. Initiated by an intramolecular H-atom transfer from the central NH group to an adjacent nitro oxygen of NO2 group to form the HNNO2H intermediate

1. The HNNO2H rapidly dissociates to OH, H, cis-HNNO, trans-HNNO, NO, NO2,

NH2, N2O, and H2O

2. The H-abstraction from unburned MNB and DNB by active radicals such as OH produces corresponding MNB and DNB radicals

3. MNB and DNB radicals subsequently decompose to low-molecular weight intermediates

4. Decomposition/oxidation of the intermediates form final stable products

(Liu, Chambreau, Vaghjiani J. Phys. Chem. A 2011)

3


HNNO2H Decomposition Paths

4


Rotation of N−N bond in HNNO2H


PES of HNNO2H Decomposition

6


14.1 15.1 16.1

17.1 18.1 19.1

7 Optimized at the CASPT2(12e,9o)/aug-cc-pVDZ level

HNNO2H → TS8 → H2NNO2

13.1 12.1 11.1

14.1 15.1 16.1

17.1 18.1 19.1

13.1 12.1 11.1

E

HNNO2H TS3 Active space was selected to describe the bond breaking/making process


High-Accuracy Potential Energy Surface

8

E(cis-HNNO) - E(trans-HNNO) 6.51 this work

6.41 K. A. Peterson & J. S. Francisco, J. Chem. Phys. (2011), calculated at the R/UCCSD(T)/CBS + ∆Rel + ∆CV + ∆T + ∆Q level of theory

E(OH) + E(trans-HNNO) - E(N2O) - E(H2O) 97.34 this work 97.58 determined from ΔfH° listed on NIST Gas phase thermochemistry database

E(NH2) + E(NO2) - E(N2O) - E(H2O) 91.47 this work 91.60 determined from ΔfH° listed on NIST Gas phase thermochemistry database

Comparison of Calculated Energy Difference with Literature Data

Theoretical values were determined at 0K with zero-point energy corrections

They are in excellent

agreement with recent data of Peterson & Francisco

They are in excellent agreement with those determined from experimental enthalpy of formation data


RN-O (Angstrom)

2 3 4 5 6 7 8 9 2010

Pot

entia

l ene

rgy

(kca

l/mol

)

-50

-40

-30

-20

-10

0

HNNO2H-1

HNNO2H → OH + cis-HNNO

9

15.1

16.1

17.1

E

Computed at the CASPT2/aug-cc-pVDZ level, state-averaged active space (4e,3o) consisting of two degenerated p orbitals of the OH and the p orbitals of N atoms


OH + cis-HNNO → HNNO2H

10

• Inner TS

– Covalent bond formation – Energy barriers: QCISD(T)/CBS

• Outer TS – Phase space theory

– Long range isotropic potential (Georgievskii & Klippenstein, J. Chem. Phys. 2005)

• Effective TS

1000/T(K)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

k (c

m3 m

olec

ule-1

s-1

)

2e-12

3e-12

4e-12

5e-12

6e-12

7e-128e-129e-12

2e-11

10-11

OH + cis-HNNO

High pressure limit

Reactions with submerged energy barriers, high-pressure limit k∞ was determined by two transition state theory at E, J resolved level The low-lying state 2Π3/2 and 2Π1/2 of the OH radical was included in the electronic partition function


1000/T(K)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

k (c

m3 m

olec

ule-1

s-1

)

1e-17

1e-16

1e-15

1e-14

1e-13

1e-12

1e-11

N2O + H2OOH + trans-HNNONH2 + NO2

HNNOH-1

p = 10atm

1000/T(K)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

k (c

m3 m

olec

ule-1

s-1

)

1e-19

1e-18

1e-17

1e-16

1e-15

1e-14

1e-13

1e-12

1e-11

N2O + H2OOH + trans-HNNONH2 + NO2

HNNOH-1

p = 1atm

Decomposition to bimolecular Products

11

Pressure dependent rate coefficients were determined by Rice–Ramsperger–Kassel–Marcus (RRKM) theory with Multi-Well Master Equation simulations at E, J resolved level Exponential down energy transfer model: ΔEdown = 200×(T/300)0.85

cm-1 Lennard-Jones parameters: σ = 4.45 Å and ε = 379.3 cm-1 Tunneling correction with asymmetric Eckart potentials


Unimolecular Decomposition of HNNO2H

12

Pressure dependent k(E,J) was determined by Rice–Ramsperger–Kassel–Marcus (RRKM) theory with Multi-Well Master Equation simulations

p = 10 atm

1000/T(K)

0.4 0.6 0.8 1.0 1.2 1.4

k (s

-1)

1e-4

1e-3

1e-2

1e-1

1e+0

1e+1

1e+2

1e+3

1e+4

1e+5

1e+6

1e+7

1e+8

1e+9

1e+10

HNNO2H-1 => OH + cis-HNNOHNNO2H-1 => OH + trans-HNNOHNNO2H-3 => N2O + H2OHNNO2H-3 => OH + trans-HNNONH2NO2 => NH2 + NO2

HNNO2H-1 => HNNO2H-3HNNO2H-3 => NH2NO2

1000/T(K)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

k (s

-1)

1e-161e-151e-141e-131e-121e-111e-101e-91e-81e-71e-61e-51e-41e-31e-21e-11e+01e+11e+21e+31e+41e+51e+61e+71e+81e+91e+10

HNNO2H-1 => OH + trans-HNNO, 1 atm HNNO2H-3 => N2O + H2O, 1 atm HNNO2H-3 => OH + trans-HNNO, 1 atmHNNO2H-1 => OH + trans-HNNO, 10 atm HNNO2H-3 => N2O + H2O, 10 atm HNNO2H-3 => OH + trans-HNNO, 10atm


Decomposition of HNNO Radical

13

Laursen et al., J. Phys. Chem. A 2000, 104, 3681-3692


H-Abstraction from MNB by OH

H-abstraction Pathways:

14

B3LYP/6-311++G(d,p)


H-Abstraction from DNB by OH

H-abstraction Pathways: B3LYP/6-311++G(d,p)

15


16

New ignition chemistry mechanism for MNB and DNB has been proposed/verified The PES for thermal decomposition of HNNO2H intermediate was characterized by high-accuracy ab initio CASPT2 and CCSD(T) theories N2O, H2O, OH and trans-HNNO radicals are predicted to be major species from HNNO2H decomposition Temperature and pressure-dependent rate coefficients for HNNO2H decomposition were determined by two-transition state theory, RRKM theory and Multi-Well Master equation analysis at E,J resolved level Transition states for H-abstraction from MNB and DNB by OH radical were located with approximate energy barriers of 10 kcal/mol. H-abstraction reactions by active radicals generated from HNNO2H decomposition can further induce ignition of MNB and DNB

Concluding Remarks


Computational Resources

National Energy Research Scientific Computing Center (NERSC) supported by the Office of Science of the U.S. Department of Energy

William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL)

DoD High Performance Computing Modernization Program at the Air Force Research Laboratory, Army Research Laboratory, Engineer Research and Development Center, and Navy Department of Defense Supercomputing Resource Centers (DSRCs)

Acknowledgements

17

http://www.nationalacademies.org/

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