Computational Study on Nitronium and Nitrosonium Oxalate: Potential

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Computational Study on Nitronium and Nitrosonium Oxalate: Potential Oxidizers for Solid Rocket Propulsion?† Elif Go¨kçınar,‡,§ Thomas M. Klapo¨tke,*,‡,| and Michael P. Kramer⊥ Ludwig-Maximilian UniVersity Munich, Energetic Materials Research, Department of Chemistry, Butenandtstr. 5-13, D-81377, Germany, Department of Chemistry, UniVersity of Ankara, Tandogan, Ankara, 06100, Turkey, Center for Energetic Concepts DeVelopment (CECD), UniVersity of Maryland, College Park, Maryland 20742, and ExplosiVes, Synthesis & Hazards Testing, ATK Launch Systems, UT40-244, P.O. Box 707, Brigham City, Utah 84302 ReceiVed: February 18, 2010; ReVised Manuscript ReceiVed: March 2, 2010

The enthalpies of formation for solid ionic nitrosonium oxalate, [NO]2[O2C-CO2], nitronium oxalate, [NO2]2[O2C-CO2], as well as covalent bis(nitroso)oxalic acid, ON-O2C-CO2-NO, and oxalic acid dinitrate ester, O2N-O2C-CO2-NO2, were calculated using the complete basis set (CBS-4M) method of Petersson and coworkers to obtain very accurate energies. For the nitrosonium species, the ionic form ([NO]2[O2C-CO2]) was identified as the more stable isomer, whereas for the nitrosonium compound, the covalently bound dinitrate ester (O2N-O2C-CO2-NO2) was found to be more stable. The combustion parameters with respect to possible use as ingredients in solid rocket motors for both stable species were calculated using the EXPLO5 and the ICT code. The performance of an aluminized formulation with covalently bound dinitrate ester (O2N-O2C-CO2-NO2) was shown to be comparable to that of ammonium perchlorate/aluminum. This makes oxalic acid dinitrate ester a potentially interesting perchlorate-free and environmentally benign oxidizer for solid rocket propulsion. 1. Introduction Solid propellants of essentially all solid rocket boosters are based on a mixture of aluminum (Al, fuel) and ammonium perchlorate (AP, oxidizer).1 Ammonium perchlorate (AP) has applications in munitions, primarily as an oxidizer for solid rocket and missile propellants. It is also used as an air-bag inflator in the automotive industry, in fireworks, and as a component of agricultural fertilizers. Because of these uses and AP’s high solubility, chemical stability, and persistence, it has become widely distributed in surface and groundwater systems. There is little information about the effects of perchlorate in these systems on the aquatic life that inhabits them. However, it is known that perchlorate is an endocrine-disrupting chemical that interferes with normal thyroid function and that, in vertebrates, thyroid dysfunction impacts both growth and development. Because perchlorate competes for iodine binding sites in the thyroid, the addition of iodine to culture water was examined to determine if perchlorate effects can be mitigated. Finally, perchlorate is known to affect normal pigmentation of amphibian embryos. In the US alone, the cost for remediation is estimated to be several billion dollars, money that is deeply needed in other defense areas.2-7 In the course of the global emerging interest in high-energetic, dense materials (HEDMs),4 we are currently developing new energetic materials preferentially with a positive oxygen balance (OB) value. OB (eq 1) is defined as the ratio of the oxygen content of a compound CaHbNcOd to the total oxygen required for the complete oxidation of all carbon, hydrogen, and other elements that can be oxidized to form CO (eq 1) or CO2 (eq 2), †

Part of the “Klaus Ruedenberg Festschrift”. * Corresponding author. E-mail: [email protected]. Ludwig-Maximilian University Munich. § University of Ankara, Tandogan. | University of Maryland, College Park. ⊥ ATK Launch Systems. ‡

H2O, and so on and is used to classify energetic materials as either oxygen deficient (negative OB) or oxygen rich (positive OB).1

[d - a - (b/2)] × 1600 M

(1)

[d - (2a) - (b/2)] × 1600 M

(2)

ΩCO )

ΩCO2 )

The objective of our ongoing work is to explore the chemical synthesis of possible replacements for AP as oxidizer in tactical missile rocket motors.8 We investigate the synthesis, sensitivities, thermal stability, binder compatibility and decomposition pathways of these new high-oxygen materials. In the present study, we now theoretically evaluate the suitability of nitrosyl (NO+) and nitronium (NO2+) oxalate as potential ingredients for solid rocket propellants. 2. Methods All calculations were carried out using the Gaussian G03W (revision B.03) program package.9 The enthalpies (H) and free energies (G) were calculated using the complete basis set (CBS) method of Petersson and coworkers in order to obtain very accurate energies. The CBS models use the known asymptotic convergence of pair natural orbital expressions to extrapolate from calculations using a finite basis set to the estimated CBS limit. CBS-4 begins with an HF/ 3-21G(d) geometry optimization; the zero point energy is computed at the same level. It then uses a large basis set SCF calculation as a base energy, and a MP2/6-31+G calculation with a CBS extrapolation to correct the energy through second order. A MP4(SDQ)/6-31+(d,p) calculation is used to ap-

10.1021/jp101487h  2010 American Chemical Society Published on Web 03/19/2010

Study on Nitronium and Nitrosonium Oxalate

J. Phys. Chem. A, Vol. 114, No. 33, 2010 8681 TABLE 2: Literature Values for Atomic Enthalpies of Formation ∆fH°298 (kilocalories per mole) ref 12

NIST15

52.6 170.2 113.5 60.0

52.1 171.3 113.0 59.6

H C N O

Figure 1. Optimized molecular structures of (ON)O2C-CO2(NO) (left) and (O2N)O2C-CO2(NO2) (right).

TABLE 3: Enthalpies of Formation of the Gas-Phase Species M

proximate higher order contributions. In this study, we applied the modified CBS-4M method (M referring to the use of minimal population localization), which is a reparametrized version of the original CBS-4Method and also includes some additional empirical corrections.10,11 3. Results and Discussion Thermodynamic Aspects. The molecular structures of neutral (ON)O2C-CO2(NO) and (O2N)O2C-CO2(NO2) were fully optimized without symmetry constraints in both cases to Ci symmetry (Figure 1).

∆fH

(g,M)

) H(M) -

∑H

◦

atoms

+

∑ ∆fH

◦

∆fH°(g,M)/kcal mol-1

∆fH°(g,M)/kcal mol-1, lit. value

NO2+ NO+ O2C-CO22O2N-O2C-CO2-NO2 ON-O2C-CO2-NO CO2 NO2 NO

235.4 240.8 -135.5 -94.3 -77.5 -92.5 +7.5 +26.9

228.9715 a 236.416

-94.115 +7.915 +21.615

Estimated from ∆fH°(NO2,g) ) 7.9 kcal mol-1 15 and IE(NO2,g) ) 9.586 eV.15 IE: ionization energy. The value for ∆fH°(NO2+,g) was obtained according to: ∆fHo(NO2+,g) ) ∆fH°(NO2,g) + IE(NO2,g). a

The enthalpies of formation of the gas-phase species M were computed according to the atomization energy method (eq 3) (Tables 1-3).12-14 In eq 3, ∆fH°(g,M) stands for the gas-phase enthalpy of formation of the molecule, M, under investigation, H(M) represents the CBS-4M calculated enthalpy of the molecule M (H298 in Table 1), ∑atomsH° denotes the CBS-4M calculated enthalpies for the individual atoms (see bottom of Table 1), and ∑atoms,fH° stands for the experimentally reported literature values for the enthalpies of formation for the corresponding atoms (∆fH°298 in Table 2). The excellent agreement between the reported experimental enthalpies of formation for NO+ and NO2+ with the CBS4M calculated values (Table 3) also gives credence that the computed values for the oxalate dianion and the neutral species are quite accurate.

◦

M

4) according to the equations provided by Jenkins et al. (eqs 4 and 5)17-19 and are summarized in Table 5.

(

UL ) |z+ ||z- |ν

R 3

√VM

)

+β

(4)

where R and β are constants, VM is molecular volume, z+ and z- are charges of the cation and anion, and ν is the sum of the number of cations and anions per “molecular” unit (M2X salt: R ) 165.3 kJ mol-1, β ) -29.8 kJ mol-1; V is in cubic nanometers).

[(

∆HL(MpXq) ) UL + p

(3)

) (

)]

nm nx -2 +q - 2 RT 2 2

atoms

(5)

where UL is the lattice energy (see eq 4) and p and q are the number of cations and anions per “molecular” unit (nm, nx: three monatomic ions, five linear polyatomic ions, six nonlinear ions).

The lattice energies (UL) and lattice enthalpies (∆HL) were calculated from the corresponding molecular volumes (Table

TABLE 1: CBS-4M Results p.g.a NO2+ NO+ O2C-CO22-, C2O42O2N-O2C-CO2-NO2, C2N2O8 ON-O2C-CO2-NO, C2N2O6 TS: C2N2O8 f 2 CO2 + 2 NO2 CO2 NO2 NO H C N O a

Point group. frequencies.

b

Electronic state.

D∞h C∞V D2h Ci Ci D∞h C2V C∞V

c

-H298/auc

-G298/aud

NIMAGe

ν1/cm-1

204.500147 129.405266 376.678701 786.364570 636.164958 786.305417 188.379390 204.863140

204.524572 129.427729 376.712587 786.416952 636.214229 786.366165 188.403736 204.890443

0 0 0 0 0 -1 0 0

604 2618 71 32 42 -817 659 748

0.500991 37.786156 54.522462 74.991202

0.514005 37.803062 54.539858 75.008515

0 0 0 0

stateb 1

Σg Sg 1 Ag 1 Ag 1 Ag 1 A 1 Σg 2 A1 1

CBS-4M calculated enthalpy.

d

CBS-4M calculated free energy.

e

NIMAG: number of imaginary

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TABLE 4: Molecular Volumes (VM)

NO2+ NO+ O2C-CO22-

VM/Å3

VM/nm3

22 10 90.220 a

0.02217-19 0.01017-19 0.090220 a

TABLE 9: Detonation Parameters for O2N-O2C-CO2-NO2 and [NO]2[O2C-CO2]a

a Molecular volume of the oxalate dianion was calculated from the volume of the ammonium oxalate monohydrate, V([NH4]2[O2C-CO2] · H2O) ) 157.2 Å3,20 and the volumes of V([NH4]+) ) 21 Å3 17,18 and the volume for a hydrate water of VM(H2O, hydrate) ) 25 Å3.18

TABLE 5: Lattice Energies (UL) and Lattice Enthalpies (∆HL)

[NO2]2[O2C-CO2] [NO]2[O2C-CO2]

VM/nm3

UL/kJ mol-1

∆HL/kJ mol-1

∆HL/kcal mol-1

0.1342 0.1102

1758.4 1889.9

1763.4 1894.9

421.5 452.9

TABLE 6: Enthalpies of Sublimation (∆Hsub)21 O2N-O2C-CO2-NO2 ON-O2C-CO2-NO

O2N-O2C-CO2-NO2

[NO]2[O2C-CO2]

b

1.6b +21.62 -2326 2728 122 5767 605

-3

Tm/Ka

∆Hsub/kcal mol-1

423 423

19.0 19.0

a Melting points, Tm, were taken equal to the estimated (from experience) decomposition temperatures.

TABLE 7: Enthalpies of Formation (∆fH°) of the Condensed Species, M

F/g cm Ω/% Qv/kJ kg-1 Tex/K P/kbar D/m s-1 V0/L kg-1

1.6 +35.6 -1785 2314 102 5341 622

a F: density, Ω: oxygen balance, Qv: heat of detonation, Tex: detonation temperature, P: detonation pressure, D: detonation velocity, V0: volume of detonation gases. b Note: The density of 1.6 g cm-3 is a “worst case” estimate for the lowest possible density based on the density of ethyleneglycol dinitrate 1.5 g cm-3 22 and the PM3 estimate of 1.77 g cm-3.27

([NO2]+) species, the covalently bound form is favored over the ionic salt by 26.9 kcal mol-1, whereas for the nitrosonium species ([NO]+), the salt is favored over the covalent isomer by 10.5 kcal mol-1. This change from the preferred covalent form for the -NO2 compound (actually a nitrato ester) to the ionic nitronium salt can be attributed almost exclusively to the increased lattice enthalpy of the (smaller) NO+ species [∆HL(NO+ - NO2+ salt) ) 31.4 kcal mol-1]. (See Table 5.) (N.B. The difference in the ionization potentials of NO (215 kcal mol-1) and NO2 (221 kcal mol-1) is only marginal.)15,16

Hm ) Um + ∆nRT

∆fH°(s,M)/kcal mol-1 -86.6 -107.0 -113.5 -96.5

[NO2]2[O2C-CO2] [NO]2[O2C-CO2] O2N-O2C-CO2-NO2 ON-O2C-CO2-NO

The enthalpies of sublimation for the neutral species O2N-O2C-CO2-NO2 and ON-O2C-CO2-NO were estimated according to Trouton’s rule (eq 6, Table 6)21 with estimated melting points of 150 °C. The validity of Trouton’s rule reflects the fact that the entropy of vaporization is approximately constant for many compounds and that ∆Hsub ≈ ∆Hvap + ∆Hfusion, with ∆Hvap . ∆Hfusion so that ∆Hsub ≈ ∆Hvap.

∆Hsub ) 188Tm J mol-1

(6)

The molar enthalpies of formation for the ionic compounds (salts) were obtained by subtracting the lattice enthalpy ∆HL (Table 5) from the sum of the enthalpies of formation for the cation and anion (Table 3). The calculated molar enthalpies of formation for the ionic (salts) and neutral compounds are summarized in Table 7. Table 8 presents the calculated energies of formation for the solid neutral species and salts. The ∆fH°(s) values (Table 7) were converted into the ∆fU°(s) values using the correlation shown in eq 7.22 From Table 8, we further see that for the nitronium

(7)

Detonation Parameters. The calculation of the combustion and detonation parameters was performed with the program package EXPLO5 (version 5.03).23 The detonation parameters were calculated using the EXPLO5 computer program.23 The program is based on the chemical equilibrium, steady-state model of detonation. It uses the Becker-Kistiakowsky-Wilson’s equation of state (BKW EOS) for gaseous detonation products and Cowan-Fickett’s equation of state for solid carbon.24-26 The calculation of the equilibrium composition of the detonation products is done by applying modified White, Johnson, and Dantzig’s free energy minimization technique. The program is designed to enable the calculation of detonation parameters at the CJ point. The BKW eq 8 in the following form was used with the BKWN set of parameters (R, β, κ, θ), as stated below (the equations and Xi being the mol fraction of ith gaseous product; ki is the molar covolume of the ith gaseous product)24-26

pV/RT ) 1 + xeβx x ) (k

∑ Xiki)/[V(T + θ)]R

(8)

R ) 0.5, β ) 0.176, k ) 14.71, θ ) 6620 The detonation parameters calculated with the EXPLO5 program using the estimated determined densities are summarized in Table 9.

TABLE 8: Solid-State Energies of Formation (∆fU°)22 [NO2]2[O2C-CO2] [NO]2[O2C-CO2] O2N-O2C-CO2-NO2 ON-O2C-CO2-NO a

∆fH°(s)/kcal mol-1

∆na

∆fU°(s)/kcal mol-1

M/g mol-1

∆fU°(s)/kJ kg-1

-86.6 -107.0 -113.5 -96.5

-5 -4 -5 -4

-83.6 -104.6 -110.5 -94.1

180.0 148.0 180.0 148.0

-1943.2 -2957.1 -2568.5 -2660.2

Change of molar number of gaseous species in the formation process of M from the elements (in their standard states).


J. Phys. Chem. A, Vol. 114, No. 33, 2010 8683

∆U)Qp - p∆V

Figure 2. Schematic presentation of a rocket combustion chamber and nozzle.

TABLE 10: Combustion Properties (Solid Rocket Motor) of Neat O2N-O2C-CO2-NO2 and [NO]2[O2C-CO2] (Frozen Expansion)a O2N-O2C-CO2-NO2

[NO]2[O2C-CO2]

isobaric 70 1.6 +35.6 -1727 1804 159

isobaric 70 1.6 +21.62 -2269 2198 175

condition p/bar F/g cm-3 Ω/% Qp/kJ kg-1 Tcomb/K Isp/s

a Ω ) oxygen balance, Qp ) heat of isobaric combustion, Isp ) specific impulse.

TABLE 11: Combustion Properties (Solid Rocket Motor) of Al Formulations with Essentially Zero Oxygen Balance (Frozen Expansion) O2N-O2C-CO2-NO2/ [NO]2[O2C-CO2]/ AP/Al Al 0.70:0.30 Al 80:20 0.70:0.30 condition p/bar F/g cm-3 Ω/% Qp/kJ kg-1 Tcomb/K Isp/s

isobaric 70 1.93 -2.0 -6473 4642 223

isobaric 70 1.82 -0.6 -5347 4039 220

isobaric 70 2.18 -2.8 -6787 4290 243

As we can see from Table 9, neither the nitrosonium ([NO]+) salt nor the covalently bound nitrosonium (nitrato ester) compound possess desirable detonation parameters in terms of their detonation pressure and velocity. This is not unexpected because both compounds have a significant positive OB and may only be used as explosives in heterogeneous metallized formulations. For this reason, no further detonation parameters were studied. Combustion Parameters. For a solid rocket propellant, one can assume free expansion of the combustion products into space (or atmosphere) with p ) const.; therefore, eq 9 is a good approximation, that is, one can consider the combustion process as isobaric

(9)

In this study, we assumed firing the rocket motor against ambient atmosphere (p ) 1 bar) because it is commonly the case for tactical missiles. The following combustion calculations were carried out under isobaric conditions based on the assumptions that the combustion of the fuel proceeds without heat loss to the surrounding (i.e., adiabatically) and that in the combustion products the state of chemical equilibrium establishes. The calculation of the theoretical rocket performances was based on the following assumptions: (i) the pressure in the combustion chamber and the chamber crosssection area are constant, (ii) the energy and momentum conservation equations are applicable, (iii) the velocity of the combustion products at the combustion chamber is equal to zero, (iv) there is no temperature and velocity lag between condensed and gaseous species, and (v) the expansion in the nozzle is isentropic. (N.B. In thermodynamics, an isentropic process or isoentropic process is one during which the entropy of the system remains constant: ∆S ) 0.) The theoretical characteristics of the rocket motor propellant may be derived from the analysis of the expansion of the combustion products through the nozzle. The first step in the calculation of the theoretical rocket performance is to calculate the parameters in the combustion chamber, and the next step is to calculate expansion through the nozzle (Figure 2). The EXPLO5 and ICT program code28 provide the following options: (i) frozen flow (composition of combustion products remains unchanged, frozen, during the expansion through the nozzle, i.e., equal to composition in the combustion chamber) and (ii) equilibrium flow (composition of combustion products at any station in the nozzle is defined by chemical equilibrium). The frozen performance is based on the assumption that the composition of combustion products remains constant (“frozen”), whereas equilibrium performance is based on the assumption of instantaneous chemical equilibrium during the expansion in the nozzle. The specific impulse Isp* is the change of the impulse (impulse ) mass x velocity or force x time) per propellant mass unit. The specific impulse is an important parameter for the characterization of rocket propellants and can be interpreted as the effective exhaust velocity of the combustion gases when exiting the expansion nozzle (eq 10).

Isp* )

j tb 1 F ) m m

∫0t F(t) dt b

(10)

The force, F, is the time-dependent thrust, F(t), or the average j , tb is the burning time of the motor, and m is the mass thrust, F

TABLE 12: Specific Impulses (Solid Rocket Motor) of Al Formulations Calculated Using Different Codes (EXPLO5 and ICT) O2N-O2C-CO2-NO2/Al 0.70:0.30

[NO]2[O2C-CO2]/Al 80:20

I composition (Figure 3)

EXPLO5

condition p/bar F/g cm-3 Ω/% Isp/s frozen equilibrium

isobaric 70 1.82 -1.8 223 226

AP/Al 0.70:0.30

II ICT28

EXPLO5

III ICT28

isobaric 70 1.74 -0.5 206 245

220 225

EXPLO5

ICT28

isobaric 70 2.13 -2.85 215 230

243 247

229 257

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Figure 3. Graphical representation of the calculated equilibrium specific impulses for the optimized formulations: I: covalent O2N-O2CCO2-NO2/Al, II: ionic [NO]2[O2C-CO2]/Al, and III: AP/Al. (See Table 12.)

of the propellant. Therefore, the unit of the specific impulse Isp* is N s kg-1 or m s-1. It is convention to divide the specific impulse Isp* by g0 (standard gravity, g0 ) 9.81 m s-2) so that the resulting specific impulse, Isp, has the unit s (seconds) (eq 11).

Isp )

I*sp g0

(11)

The specific impulse Isp can also be defined according to eq 12 with γ ) Cp/CV. (For details, see ref 1.)

Isp* )

1 g

2γRTc Cp γ) (γ - 1)M CV

(12)

Table 10 summarizes the calculated rocket propellant performance parameters for an assumed chamber pressure of 70 bar for the neat propellants covalent O2N-O2C-CO2-NO2 and ionic [NO]2[O2C-CO2]. Table 11 summarizes the calculated propulsion parameters for aluminized formulations in which the Al content has been varied to achieve an almost zero OB (with respect to CO2, see eq 2). Table 11 also contains the corre-

sponding values for a AP/Al formulation for comparison. Finally, Table 12 shows the calculated specific impulses for equilibrium expansion for the three optimized formulations (covalent O2N-O2C-CO2-NO2/Al, ionic [NO]2[O2C-CO2]/ Al and AP/Al). The results of Table 12 are graphically summarized in Figure 3. From Table 12 and Figure 3, we can conclude that in general the agreement between the EXPLO5 and ICT calculated equilibrium specific impulses is reasonably good, with the ICT code always predicting slightly better performance. It is further apparent that the formulation with covalent O2N-O2C-CO2-NO2 and Al gives better performance than the formulation using ionic [NO]2[O2C-CO2] and Al. The specific impulse of the chlorine and perchlorate-free formulation with covalent O2N-O2C-CO2-NO2 and Al (I) is just slightly lower than that of the AP/Al (III) formulation. Therefore, it can be concluded that bis(nitronium) oxalate (or oxalic acid dinitrato ester), O2N-O2C-CO2-NO2, may be a promising new perchlorate-free and environmentally benign oxidizer for formulations to be used in solid rocket motors. Stability of Covalent O2N-O2C-CO2-NO2. The above discussion (section combustion parameters) clearly revealed that the covalently bound oxalic acid dinitrate ester is the most promising candidate in this series as a high oxidizer and potential replacement for AP. To evaluate its thermodynamic and kinetic stability, we calculated the decomposition into CO2 and NO2 according to eq 13.(See also Table 3.) The reaction enthalpy of ∆H(13) ) -56.5 kcal mol-1 clearly indicates that oxalic acid dinitrate ester is (as expected) thermodynamically unstable with respect to its decomposition into CO2 and NO2.

O2N-O2C-CO2-NO2(s) f 2 CO2(g) + 2 NO2(g) ∆H(13) ) -56.5 kcal mol-1

(13) To evaluate the kinetic stability of covalently bound O2N-O2C-CO2-NO2, it was decided to compute a 2D potential energy hypersurface (Figure 4) at the B3LYP/6-31G*

Figure 4. Potential energy hypersurface for the simultaneous dissociation of O2N-O2C-CO2-NO2 into CO2 and NO2 at the B3LYP/6-31G* level of theory.


J. Phys. Chem. A, Vol. 114, No. 33, 2010 8685 can be related to the sensitivity of the bulk material. The ESP at any point r is given by eq 14 in which ZA is the charge on nucleus A, located at RA.

V(r) )

Figure 5. Transition-state structure for the simultaneous dissociation of O2N-O2C-CO2-NO2 into CO2 and NO2 at the CBS-4M level of theory (NIMAG ) 1, ν1 ) -817 cm-1, d(C-C) ) 2.33 Å, d(O-NO2) ) 1.91 Å).

level of theory. As one can see, the simultaneous dissociation of O2N-O2C-CO2-NO2 into CO2 and NO2 has a relatively high activation barrier (Figure 4). The transition state (Figure 5) was located 95.9 kcal mol-1 (HF/3-21G(d)) and 37.1 kcal mol-1 (CBS-4M) above the dinitrate ester. It is interesting to mention that in agreement with Hammond’s postulates,29,30 the transition state lies more toward the higher-energy starting material. Electrostatic Potential of Covalent O2N-O2C-CO2-NO2. The electrostatic potential (ESP) of covalent O2N-O2C-CO2NO2 was computed at the optimized structure at the B3LYP/ 6-31G(d) level of theory. Figure 6 shows the ESP for the 0.001 electron/bohr3 isosurface of electron density evaluated at the B3LYP level of theory. The colors range from -0.06 to +0.06 hartree with green denoting extremely electron-deficient regions (V(r) > 0.07 hartree) and red denoting electron-rich regions (V(r) < -0.07 hartree). It has recently been found by Politzer, Murray et al.,31 and extensively used by Rice et al.32 that the patterns of the computed ESP on the surface of molecules in general

Z

F(r′) dr′ ∑ |RA -A r| - ∫ |r′-r|

(14)

Politzer et al. were able to show31 that impact sensitivity can be expressed as a function of the extent of this anomalous reversal of the strengths of the positive and negative surface potentials. In most nitro (-NO2) and nitrato (-O-NO2) systems, the regions of positive potential are stronger than the negative, contrary to the usual situation. This atypical imbalance between stronger positive regions and weaker negative ones can be related to the impact sensitivities. The calculated ESP of O2N-O2C-CO2-NO2 (Figure 6) shows strong positive regions over the nitro (-NO2) groups with the positive areas extending into the O-NO2 region (oxygen-NO2 bond). Moreover, there is also a strong positive region over the relatively weak C-C bond. This is in good accord with the labile O-NO2 and C-C bonds and also accounts for the easy bond cleavage. In comparison, free oxalic acid (Figure 6) does not show any positive regions over the bonds in the molecule. 4. Conclusions From this computational study, the following conclusions can be drawn: (i) Covalently bound and ionic nitronium and nitrosonium oxalate were investigated with respect to their potential use as energetic materials or oxidizers for solid rocket motors. None of these compounds can be expected to be a good high explosive. However, the covalent molecule oxalic acid dinitrate ester, O2N-O2C-CO2-NO2, was identified to be a potentially interesting oxidizer. (ii) The computed specific impulse of a O2N-O2C-CO2-NO2/Al formulation (80:20) is

Figure 6. Electrostatic potential of O2N-O2C-CO2-NO2 and oxalic acid (B3LYP/6-31G(d), 0.001 e bohr-3 isosurface, energy values -0.06 to +0.06 H); color coding: red (very negative), orange (negative), yellow (slightly negative), green (neutral), turquoise (slightly positive), light blue (positive), dark blue (very positive).

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comparable to that of a conventional AP/Al (70:30) formulation, however, free of toxic perchlorate or any halogen. (iii) Oxalic acid dinitrate ester, O2N-O2C-CO2-NO2, is metastable with respect to decomposition into CO2 and NO2. The reaction barrier (transition state) for the monomolecular dissociation was calculated to be 37 kcal mol-1 at CBS-4M level of theory. (iv) The computed ESP of O2N-O2C-CO2-NO2 shows strong positive areas at the 0.001 s bohr-3 isosurface over the O-NO2 and C-C bonds indicating the relative weakness of there particular bonds. The results obtained in this study should encourage synthetic work to prepare O2N-O2C-CO2-NO2 on a laboratory scale and to evaluate its properties experimentally, first and foremost its thermal stability. Such work is underway in our laboratories. Acknowledgment. Financial support of this work by the Ludwig-Maximilian University of Munich (LMU), the U.S. Army Research Laboratory (ARL), and the Strategic Environmental Research and Development Program (SERDP) under contract nos. W911NF-09-2-0018 (ARL) and 10 WPSEED01002/WP-1765 (SERDP) is gratefully acknowledged. We acknowledge collaboration with Dr. Muhamed Sucesca (Brodarski Institute, Croatia) in the development of new computational codes to predict the detonation and propulsion parameters of novel explosives. We are indebted to and thank Drs. Betsy M. Rice and Brad Forch (ARL, Aberdeen, Proving Ground, MD) for many helpful and inspired discussions and support of our work. One of us (E.G.) acknowledges financial support by ¨ BI˙TAK (Ph.D. studies in Ankara) and the EU ERASMUS TU program (study leave in Munich). We also thank Mr. Norbert Mayr for his invaluable help and support with many computational problems. References and Notes (1) Klapo¨tke, T. M. Chemie der hochenergetischen Materialien; de Gruyter: New York, 2009 (2) SERDP Information: Cleanup CU-1164, 2003. http://www.p2pays.org/ ref/19/18164.pdf, accessed 3/17/10. (3) The official DoD source for perchlorate information: http:// www.epa.gov/fedfac/documents/perchlorate_links.htm, accessed 3/17/10. (4) SERDP & ESTCP Annual Symposium 2007. http://www.serdpestcp.org/symposium2007/, accessed 3/17/10. (5) Urbansky, E. T. EnViron. Sci. Pollut. Res. 2002, 9, 187. (6) Brown, G. M.; Gu, B. The Chemistry of Perchlorate in the EnVironment; Springer: New York, 2006. (7) Stroo, H. F.; Ward, C. H. In Situ Bioremediation of Perchlorate in Groundwater; Springer: New York, 2009 (8) (a) Klapo¨tke, T. M.; Sabate´, C. M. Chem. Mater. 2008, 20, 3629. (b) Darwich, C.; Klapo¨tke, T. M.; Sabate´, C. M. Chem.sEur. J. 2008, 14, 5756. (c) Stierstorfer, J.; Klapo¨tke, T. M.; Hammerl, A.; Chapman, B. Z. Anorg. Allg. Chem. 2008, 634, 1051. (d) Go¨bel, M.; Klapo¨tke, T. M. Z. Anorg. Allg. Chem. 2007, 633, 1006. (e) Guo, Y.; Gao, H.; Twamley, B.; Shreeve, J. M. AdV. Mater. 2007, 19, 2884. (f) Go¨bel, M.; Karaghiosoff, K.; Klapo¨tke, T. M. Angew. Chem., Int. Ed. 2006, 45, 6037. (g) Hiskey, M.; Hammerl, A.; Holl, G.; Klapo¨tke, T. M.; Polborn, K.; Stierstorfer, J.; Weigand, J. J. Chem. Mater. 2005, 17, 3784. (h) Xue, H.; Shreeve, J. M. AdV. Mater. 2005, 17, 2142. (i) Go¨bel, M.; Klapo¨tke, T. M. AdV. Funct. Mater. 2009, 19, 347–365. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;

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Computational Study on Nitronium and Nitrosonium Oxalate: Potential

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