common trends in the relationship between chemical and
TRANSCRIPT
Common Trends in the Relationship Between Chemical and Physical Properties and Molecular Structure of Nitramines, Caged Nitramines,and Nitroaromatic Energetics
Mohammad Qasim1, Brett Moore1,2, Lyssa Taylor1,2, Leonid Gorb1,3, Jerzy Leszczynski1,3
1Environmental Laboratory, US Army Engineer Research and Development Center, Vicksburg, MS; 2Department of Chemistry and Biochemistry, Mississippi College, Clinton, MS; 3Computational Center for Molecular Structure and Interactions, Jackson State University,Jackson, MS
Because of its widespread presence and toxicity in the environment, as well as its mutagenicand potentially carcinogenic nature, the prediction of the chemical and physical properties oftrinitrotoluene (TNT) and the products of its environmental degradation are of high interest andintense research. The hypothesis the environmental fate and effects of energetic materials canbe predicted from their molecular structures (in regard to the nitroaromatic, TNT; cyclicnitramines, RDX, HMX; and cage nitramine, CL-20) was extended to include new emergingcompounds, such as trinitroanisole (TNA).
Computational chemistry (CC) in combination with experimental verification is useful in bothproving concepts and in ascertaining what is chemically possible. Our theoretical study involvedMOPAC quantum mechanical and classical force field mechanics to predict most likely bondlengths and angles, heat of formation, steric energy, dipole moments, solvent accessibility andelectrostatic potential surfaces, partial charges, and HOMO/LUMO energies. Correlationsbetween the compound’s chemical/physical properties and their molecular structure were thenelucidated to discover possible trends.
Comparison results in predicting potential or probable reactivity with media and prioritizingcompounds of greatest concern. The above hypothesis was proven, thus, enabling thediscovery of recalcitrant, toxic CL-20 intermediate. To develop a predictive framework and CCprotocols to assess energetic reactivity with environmental media, nitramine, cage nitramine,and nitroaromatic energetic compounds are being compared as to structure, sites and modes oftransformation and transformation products. Thus, study of CL-20 and its reactivityrelationships is providing the nucleus for transitioning to other MUCs. This study is expected tocome full circle in providing basic understanding of nitramine and nitroaromatic MUCs and theirreactivity in the environment, as well as the development of a predictive framework and CCprotocols practical for basic research into new compounds and for application specific toenvironmental sites.
PHYSICAL PROPERTY RESULTS
Log P• RDX, HMX, TNT, TNA, and CL-20 all have low LogP values, indicating that these explosives are water-soluble. These
compounds solubility in water makes them problematic and possible contaminants of ground water.• ONC and TNTAC have high LogP values, indication that these compounds are very hydrophobic and will not travel easily
in water.
Henry’s Law• TNT, TNA, and ONC have low Henry’s Law constants. This tells that these three molecules when in the gaseous phase
will not redissolve into solution.• RDX, HMX, TNTAC, and CL-20 have higher Henry’s Law constants and dissolve into solution more easily.
Density• RDX, HMX, TNT, and TNA have lower densities than the caged molecules ONC, TNTAC, and CL-20.
COMPARISONS BETWEEN CLASSES OF COMPOUNDSThree different comparisons were made within each class of compounds (nitramine, nitroaromatic, and caged nitramine).The first is between RDX and HMX; the second is between TNT and TNA; and the third is between TNTAC, CL-20, andONC.
• RDX / HMXBased on our computational study alone, HMX is a less reactive compound than RDX. This can be seen by viewing thelarger HOMO/LUMO gap of HMX. HMX’s larger size is represented by its higher heat of formation, larger molar volume,larger parachor, and larger surface tension. HMX contains one more nitrogen in the ring system and also an additionalnitro group attached to that nitrogen. This gives HMX a larger steric energy due to HMX collapsing back on itself. HMXis more polar than RDX as seen by it’s larger dipole charge and higher polarizability. This is also due to the increasednumber of nitrogens in the molecule. The two are very similar in their LogP, pKa, index of refraction, and density.
• TNT / TNATNA is more sterically hindered than TNT due to the oxygen of the methoxy group. This oxygen also gives TNA a largerdipole charge and causes a decrease in the heat of formation. TNA is slightly less reactive than TNT due to interactionbetween the hydrogen atoms of the methoxy group with the oxygens of the nitro groups. It is also a less reactivemolecule with environmental media due to more electrons in the π-system. The methoxy group of TNA is important to thestability of the molecule. It adds more resonance and thus increases stability compared to TNT. The methoxy group ofTNA also makes it a larger molecule than does the methyl group of TNT. This is evident by larger molar volume,parachor, and average mass data.
• TNTAC / CL-20 / ONCOf the caged nitroaromatics, TNTAC is much more sterically hindered than either CL-20 or ONC. ONC is the leastreactive of the three as seen by the HOMO/LUMO gap. However, ONC is almost impossible to form as seen by its heatof formation. ONC has the smallest dipole when compared to the other caged explosives. This is due to the symmetry ofthe molecule. The symmetry of the molecule makes it susceptible to free radical attack as opposed to electrophilicattack. ONC allows for a much larger polar surface area, however, CL-20 has more molar volume and a larger parachorthan the others. TNTAC has much more surface tension and is far more dense than the other molecules. All threemolecules possess a high MM2 and heat of formation. This is due to the cage-effect and the crowdedness of the nitrogroups branching from the main ring system.
Hypothesis: Environmental fate and effects of energetic materials can be predicted from theirmolecular structures.
The purpose of this study is to relate chemical and physical properties with molecular structureof known explosive compounds such as RDX, HMX, CL-20, TNT, TNA, and other new emergingcompounds such as ONC and TNTAC. This information allows for the prediction of sites andease of reactivity, transformation products, stability, and toxicity.
ABSTRACT
PURPOSE AND HYPOTHESIS
DATA RESULTS
METHODS AND MATERIALS
Three different methods were used to calculate the physical properties for relevant explosivecompounds.
1. Molecular Mechanics (MM2)1. Calculate the total steric energy of each compound2. Visually depict the lowest energy for each of the listed compounds
2. Semi-empirical Molecular Orbital PACkage (MOPAC) via AM1 methods Followed MM2 and used to calculate the following:
1. heat of formation2. dipole moment3. highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energies and levels
3. Advanced Chemistry Development (ACD/LABS) Software Calculate the following physical properties:
1. pKa2. LogP3. Henry’s Law4. tPSA5. CMR6. Molar Refractivity7. Molar Volume
8. Parachor9. Index of Refraction10. Surface Tension11. Density12. Polarizability13. Average Mass
O(19) -0.084054O(20) -0.431937N(21) 0.550508O(22) -0.141984O(23) -0.406882N(24) 0.563279O(25) -0.10594O(26) -0.445196N(27) 0.532598O(28) -0.096438O(29) -0.433393N(30) -0.304503H(31) 0.281611H(32) 0.280943H(33) 0.278883H(34) 0.28448H(35) 0.285002H(36) 0.28058
C(1) 0.007753N(2) -0.307302C(3) 0.0383N(4) -0.305227N(5) -0.301951C(6) -0.010497C(7) 0.025965N(8) -0.265921C(9) 0.014133N(10) -0.302346C(11) 0.026164N(12) 0.533127O(13) -0.096433O(14) -0.452825N(15) 0.522396O(16) -0.125288O(17) -0.41063N(18) 0.523026
ChargesC(1) -0.098743C(2) -0.098716C(3) -0.098749N(4) -0.273797N(5) 0.520277O(6) -0.13379O(7) -0.426994N(8) -0.273675N(9) 0.520212O(10) -0.133807O(11) -0.426966
N(12) -0.273813N(13) 0.520328O(14) -0.133773O(15) -0.427047H(16) 0.223689H(17) 0.189333H(18) 0.223695H(19) 0.189302H(20) 0.223694H(21) 0.189339
ChargesC(1) -0.090899C(2) -0.019884C(3) -0.145813C(4) -0.072788C(5) -0.072567C(6) 0.021178N(7) 0.424743H(8) 0.308317O(9) -0.136048H(10) 0.293289N(11) 0.420792
C(12) -0.353371H(13) 0.169445H(14) 0.175546H(15) 0.17555O(16) -0.407179N(17) 0.45493O(18) -0.171951O(19) -0.423056O(20) -0.149051O(21) -0.401184
Charges
C(1) -0.126448C(2) 0.008931C(3) -0.17043C(4) -0.05622C(5) -0.122032C(6) 0.201015N(7) 0.430948H(8) 0.31299O(9) -0.140617H(10) 0.296087N(11) 0.410167
O(12) -0.223049C(13) -0.218623H(14) 0.195847H(15) 0.14264O(16) -0.405971N(17) 0.459052O(18) -0.174894O(19) -0.425199O(20) -0.125604O(21) -0.410648H(22) 0.142058
ChargesO(15) -0.141251O(16) -0.431611N(17) -0.267992N(18) 0.510831O(19) -0.135014O(20) -0.439975H(21) 0.184673H(22) 0.224611H(23) 0.252714H(24) 0.168442H(25) 0.224595H(26) 0.184644H(27) 0.252729H(28) 0.168459
C(1) -0.089097C(2) -0.08296C(3) -0.089066C(4) -0.08294N(5) -0.268012N(6) 0.51081O(7) -0.134962O(8) -0.439973N(9) -0.27649N(10) 0.523064O(11) -0.141248O(12) -0.431609N(13) -0.276489N(14) 0.523117
ChargesO(17) -0.099188C(18) 0.088113N(19) 0.428401O(20) -0.396891O(21) -0.11678C(22) 0.025803N(23) 0.454225O(24) -0.401226O(25) -0.121794C(26) 0.037842N(27) 0.402502O(28) -0.31092O(29) -0.085319N(30) 0.434812O(31) -0.120012O(32) -0.389552
C(1) 0.017668C(2) -0.00108N(3) 0.470947O(4) -0.388332O(5) -0.098348C(6) 0.109429N(7) 0.425679O(8) -0.404102O(9) -0.113937C(10) 0.052291N(11) 0.402861O(12) -0.320175O(13) -0.074456C(14) 0.005789N(15) 0.482335O(16) -0.396584
Charges
RDX HMX TNT TNA TNTAC CL-20 ONC
C(1) 0.256271N(2) -0.176634C(3) 0.167268N(4) -0.214654N(5) 0.294123O(6) -0.196939O(7) -0.205963N(8) 0.302513O(9) -0.139576N(10) -0.185377
C(11) 0.167639N(12) -0.17491C(13) 0.105258O(14) -0.114329N(15) 0.401195O(16) -0.222752O(17) -0.211522N(18) 0.301544O(19) -0.078706O(20) -0.074449
Charges
Partners in Environmental Technology Technical Symposium & WorkshopSERDP December 4-6, 2007 Washington, D.C.
Most physical properties increase in value as the compound increases in size.
This study is an attempt to show how molecular structure is related to physical and chemical properties and therefore relatedto reactivities and environmental effects. Our data shows that small changes in molecular structure can largely affect thechemical/physical properties of a molecule.
This computational summary is a combination of our calculations and known established literature values; thereforecomputational chemistry, which is shown to correspond to textbook values, is proven to be an instrumental tool in theprediction of environmental fate and effects of energetic materials from their molecular structure.
CONCLUSIONS
OrbitalLevels
GapDipoleCharge(debye)
Heat ofFormation(kcal/mole)
Log P pKaHenry's
LawtPSA CMR
MolarRefractivity
(cm3)
MolarVolume
(cm3)
Parachor(cm3)
Index ofRefraction
SurfaceTension
(dyne/cm3)
Density(g/cm3)
Polarizability( x 10-24 cm3)
AverageMass(Da)
N=42 HOMO -11.655
N=43 LUMO -2.115
N=56 HOMO -11.739
N=57 LUMO -1.140
N=42 HOMO -11.705
N=43 LUMO -2.432
N=45 HOMO -11.797
N=46 LUMO -2.498
N=52 HOMO -13.105
N=53 LUMO -2.782
N=81 HOMO -11.992
N=82 LUMO -2.662
N=84 HOMO -12.706
N=85 LUMO -1.163
-2.19
0.03
20.85
NA1.68
38.38
1.04 NA
-18.23
NA
50.714.9869155.430.53 71.91.637411.3141.2
3.68 19.84 288.0916
227.131120.11.608
78.2 368.4 2.517 492.515.6 220.2
52.565.14164.660.53
5.0664 50.06
69.61.623430.3148.9
167.3 651.1 1.953 229-18.95 3.9 330.3
77.257.5608414.480.35
8.1318 81.04
2291.903644165.5
296.1551
464.129630.622.8
2.61 30.62 464.1296
243.130520.831.632
1.693 119 1.95 23.07
11.54 1.309
Advanced Chemistry Development (ACD/LABS)
Table 1 : Chemical Properties of Studied Explosive Compounds
9.44 220.2 5.776 58.2 151.7 501.2-2.92 -16.32
10.32 1.515 367.03873
10.60 10.315 142.6701
TNT 5.8556
Compound
MM2Total Steric
Energy(kcal/mole)
AM1 Minimal Energy
HOMO / LUMOEnergies
(eV)
ONC 45.5870
HMX 39.9815
TNTAC 228.8171
948.99853
CL-20 46.8229 9.33 2.89 277.94661
RDX 19.8541 9.54 2.092 104.88387
9.30 2.146 12.30487
9.27 1.391 40.98045
TNA 14.529
7.08-15.06 165.15 4.332 43.65 117 376 1.668 106.6 17.31.89 222.1163
N
N NN N
N+
O
O-
N+
OO-
N+ O-O
N+O
-O
N+ O
O-
N+
O
O-
N
N N+
O
O-N
N+O-O
N
N+
O O-
NN+
O
-O
N+
O–
O
N+–O
O
N+
–O
O
N+
–O O
N+
–O
O
N+ O–O
N+
O–
O
NO2
NN+
O
O-N
N
N+
O-O
N+
O
O-
N+
O
N+
O
O-
N+
O-O
O
-O
N+
O
N+
O-
N+
O-O
O
-O
N
N
N
N
N+
–O
O
N+
O–
O
N+
–O
O
N+
–O
O