m. b. talawar et al- novel ultrahigh-energy materials

8/3/2019 M. B. Talawar et al- Novel Ultrahigh-Energy Materials

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Combustion, Explosion, and Shock Waves, Vol. 41, No. 3, pp. 264277, 2005

Novel Ultrahigh-Energy Materials

UDC 536.46M. B. Talawar,

1

R. Sivabalan,

1

S. N. Asthana,1 and H. Singh1

Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 3, pp. 2945, MayJune, 2005.Original article submitted April 8, 2004.

This paper reviews the recent work carried out in the field of modern high-energymaterials (HEMs) with the emphasis on homoleptic polynitrogen compounds. A largevolume of quantum-chemical investigations have predicted the possibility of existenceof polynitrogen compounds not only as short-lived transient species but also in theform of isolable discrete molecules. Despite the theoretical speculations, only a fewpolynitrogen ions are known today in addition to well-entrenched N1

3discovered

almost 100 year ago. Extraordinary potential of these green molecules to deliverhigh amounts of energy in comparison with todays and tomorrows most powerfulHEMs, namely, hexanitrohexaazaisowurtzitane (CL-20) and octanitrocubane (ONC),has fuelled the imagination of propellant and explosive engineers and technologists.Research activities are in progress in many quantum-chemical schools to explore thepossibility of other promising polynitrogen compounds. After the recent discoveryof key synthons/building blocks Mg(N5)2, N

1+5

SbF16

, N1+5

SbF11, N1+5

, N1+5

SnF6, andN1+5

Sn(CF3)4, the wealth of polynitrogen compounds is just waiting to be harvestedby the HEMs community. There are ambitious plans all over the globe to realize N60,which only prove a eco-friendly dense powerhouse of energy.

Key words: polynitrogen, high-energy materials.

INTRODUCTION

With the increasing importance of satellites forcommunication and exploration of space as well as am-bitious space-shuttle programs, the sphere of applica-tions of propellants has extended tremendously from thedefense to the space sector. Explosives are also findingapplications in specific areas like metal cladding, min-ing, and demolition of dilapidated structures. Applica-tions of propellants and explosives as clean sources ofpower generation are also being envisaged. A quantum

jump in the usage of propellants has brought into focus

the issue of environment-friendly systems. The chal-lenge of increasing lethality of warheads has evincedinterest in High-Energy Dense Materials (HEDMs).Mammoth projects undertaken all over the globe havebrought into foray a totally new spectrum of HEMs dur-ing the last two decades. Therefore, a new era of HEMshas dawned on the sector of propellants, explosives, and

1High Energy Materials Research Laboratory,Pune-411021, India; [email protected].

pyrotechnics. The revolution in synthesis and manufac-turing technologies has opened the possibilities of realiz-ing HEDMs that were considered a theoretical curiositya few years ago.

GLOBAL SCENARIO

Composite ammonium perchlorate (AP)-based andcomposite modified double-base (CMDB) propellantsmay be dislodged from the premier position today withthe emergence of eco-friendly oxidizers: ammoniumdinitramide (ADN) [1, 2] and hydrazinium nitrofor-mate (HNF) [3, 4]. Their heat of formation (151 and71 kJ/mole, respectively) is higher than that of AP;therefore, these materials are capable of offering propel-lants with the performance level exceeding 260 sec de-spite their lower oxygen balance. Moreover, in contrastto nitramines, they undergo highly exothermic com-bustion reactions near the surface, leading to efficientheat feedback to the deflagrating surface, enhancing

264 0010-5082/05/4103-0264 c 2005 Springer Science + Business Media, Inc.


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Novel Ultrahigh-Energy Materials 265

TABLE 1Modern High-Energy Materials


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266 Talawar, Sivabalan, Asthana, and Singh

the burning rates. ADN was synthesized at the Zelin-sky Institute of Organic Chemistry (ZIOC, Moscow,Russia) during 1970s, and subsequently ADN propel-lants were productionized in Russia. The work on HNFwas carried out mainly at the Prins Maurits Labora-tory (the Netherlands). Today, it is manufactured byAerospace Propulsion Products (APP, the Netherlands)at the scale of 300 kg/year (Table 1).

Azide polymers, particularly, glycidyl azidepolymer (GAP) [5, 6], and co-polymers of bisazidomethyloxetane (BAMO) [7], have entered the do-main of advanced propulsion systems. Replacementof current workhorse binder [hydroxyl-terminatedpolybutadiene (HTPB)] by these energetic materialswill act as a force multiplier. Poly-3-nitratomethyl-3-methyloxetane (PNMMO) [8] and poly-glycidylnitrate (PGN) [9] have also made foray in the area ofadvanced systems. However, their stability problems

need to be tackled. The BAMO and nitratomethylmethyloxetane (NMMO) co-polymer is fast emerg-ing as an energetic thermoplastic elastomer [7,10]having wide applications in the area of extruded com-posite rocket propellants as well as pressed/sheetexplosives. Bis(2,2-dinitropropyl)formal/acetal(BDNPF/A) [11, 12] and metriol trinitrate (TMETN)[13] have also gained prominence as potential energeticplasticizers for rocket propellants. The structures ofmodern high-energy materials are presented in Table 1.

EMERGING TRENDS

Polycyclic compounds containing interconnected,closely packed atoms referred to as cage compounds areof great interest to HEM technologists. Conversion ofpolycyclic materials to nitro/nitrato/nitramino deriva-tives results in realization of high-power packed store-houses of energy. However, straightforward nitration ofprecursor caged hydrocarbons is not viable. It resultsin formation of byproducts and often cleavage of thecarefully constructed cage structure. Such complexitiesled to a strong opinion that cage compounds, such ashexanitrohexaazaisowurtzitane (CL-20), will remain atthe most a laboratory curiosity. However, innovativenitration approaches and selection of highly specializednitrating agents as well as reaction conditions made itpossible to realize CL-20 at the pilot scale after lab-scalesynthesis reported by Nielsen [14] during late 1990s.CL-20 can outstrip tetranitro tetramethylene cyclooc-tane (HMX) in terms of velocity of detonation (VOD).It is expected to give 14% higher energy than HMX. Thetechnological breakthrough [15] in this direction gave animpetus to research in the area of another fascinatingHEM, octanitrocubane (ONC).

In the early 1980s, ONC was projected as a high-potential energetic material [16, 17]. Statistical andcomputational approaches predicted a density of 2.1 to2.2 g/cm3 of ONC having a perfect oxygen balance. Re-cently published reports [18] estimate its heat of forma-tion to be 594 kJ/mole, which is higher than the heatof formation of hexanitrobenzene (HNB, 200 kJ/mole).The energy content of HNB is lowered by the stabilizingresonance energy of the aromatic system, whereas theenergy content in ONC is augmented due to the strainenergy of the cubane. The theoretically computed VODof ONC is 10 km/sec.

As a matter of fact, synthesis of cubane andbishomocubane was reported [19] way back in 1966.The challenging task of nitration of cubanes to ni-trocubanes was undertaken, and several discoveries weremade, leading to the opening of new vistas of chem-istry. The innovative photochemical method developed

by Basher-Hashmi et al. [20, 21] was adopted to obtaincubane-2,4,6,8-tetracarboxy acid chloride. The well-known Curtius rearrangement was employed to con-vert the latter to its isocyanate, which, on oxidationby dimethyl dioxerane, yielded 2,4,6,8-tetranitrocubane(TNC). The highly acidic nature of atomic hydrogenpresent in the TNC was expected to facilitate the syn-thesis of higher nitrocubanes (Scheme 1).

Thus, straightforward nitration of TNC with ni-trating agents such as N2O4, NO2BF4, and N2O5yielded complex mixtures. A novel approach [22]called interfacial nitration led to the long-awaited break-through. It involved freezing of a solution of an anion

salt of TNC on a glass substrate and deposition of solidN2O4 onto its surface. The reactants came into produc-tive combination, and the glass was allowed to melt.

In the method applied in [23], TNC was treatedwith 1.5 equivalents of NaN(TMS)2 at 78

C in a 1/1tetrahydrofuran/-metyl tetrahydrofuran mixture, re-sulting in formation of the TNC monoanion. The solu-tion was cooled to a temperature between 125 and130C. N2O4 in cold isopentane was added to thehighly viscous liquid, followed by nitric acid in cold di-ethyl ether, yielding pentanitrocubane (PNC). An in-crease in the NaN(TMS)2 proportion yielded hexani-trocubane (HNC), and subsequently heptanitrocubane

(HpNC). But a further increase in the content ofNaN(TMS)2 did not yield ONC, and it remained elu-sive for a long time, probably due to the high stabilityof the HpNC anion (Scheme 2).

Zhang et al. [23] utilized more powerful oxidants,such as nitrosyl chloride, considering the high acidity ofHpNC. Addition of an excess of NOCl to a solution ofthe lithium salt of HpNC in dichloromethane at 78Cfollowed by ozonisation at 78C led to the synthesis


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Scheme 1.

Scheme 2.

of ONC (the most effective modern energetic material)with a yield of 4555%. Thus, ONC acquired the statusof the only nitrocubane synthesized in the last 18 years(Scheme 3).

ONC is a stable white solid soluble in hexane andpolar organic solvents. It sublimes without decomposi-tion at atmospheric pressure at 200C, and its sampleshave survived unchanged for 14 months in sealed glasstubes. The density of one polymorph of synthesizedONC is 1.979 g/cm3, which is still lower than the cal-culated value, and attempts are continued to obtain adenser material.

Currently, ONC is quite expensive to synthesizeowing to involvement of a large number of synthesissteps. The research is focused on economizing its syn-thesis. One of the projected inexpensive methods of

ONC synthesis is tetramerization of dinitroacetylene(Scheme 4).

The state-of-the art theoretical calculations [24] in-dicated that the free-energy change in dinitro-acetyleneto octanitrocubane conversion is thermodynamicallydownhill by 417 kJ/mole, i.e., this reaction is thermo-dynamically feasible.


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Scheme 3.

Scheme 4.

1,2,5-oxadiazoles (furazans) [25] and their 2-oxides

[26] are also highly-promising building blocks for cre-ation of high-performance materials owing to the highdensity of their molecular crystals and positive enthalpyof formation because of active oxygen atoms simulta-neously present inside the molecular ring. Introduc-tion of explosophoric groups to furazan and furoxanrings allows higher density and energetics. One inter-esting furoxan-based high-energy material obtained atZIOC is 4,4-dinitro-3,3-diazenofuroxan (DNAF) [26].The velocity of detonation in DNAF determined ex-perimentally by extrapolation of data on pressed sam-ples (85, 94, and 97% of the theoretical value) was re-ported to be 10 km/sec for a single-crystal density of

2.00 g/cm3, which is close to ONC density. DNAF wassuccessfully synthesized in three steps from a key syn-thon 3-azidocarbonyl-4-aminofuroxan (AzCAF). How-ever, DNAF worthiness in comparison to CL-20 andONC need to be proved in view of its low decompo-sition temperature (127128C) and particularly highimpact sensitivity.

High-nitrogen-content high-energy materials(HNC-HEMs) [2729] are becoming the focal point ofresearch in the area of advanced HEMs aimed at futur-istic defence- and space-sector needs. The high energycontent of HEMs stems from the presence of adjacentnitrogen atoms poised to form molecular nitrogen

(N N). Such transformations are accompanied byenormous energy release due to the large difference inthe average energies of single N N (160 kJ/mole) anddouble N N (418 kJ/mole) bonds, compared to themean energy of the triple bond N N (954 kJ/mole).As a natural consequence of their chemical structure,HNC-HEMs also generate a large volume of the gas(N2) per gram of HEM, projecting them as a possiblematerial for clean gas generators.

The research in this direction commenced with the

studies on relatively easily isolable materials from theclass of azotetrazolates and tetrazines. The high po-tential of nitrogen-rich compounds, such as diaminoazobis tetrazine (DAAT) [30, 31], dihydrazine tetrazine(DHTz) [31, 32], and some azotetrazolate salts [3335]was reported. DAAT having a high positive heat offormation (1032 kJ/mole) is considered as a potentialHEM to be used in rocket propellants and insensitivehigh-explosive (IHE) formulations. Among tetrazolates,the triaminoguanidinium salt with the heat of formationequal to 560 kJ/mole and reasonable sensitivity char-acteristics competes with DAAT. DHTz may find ap-plication as an eco-friendly pyrotechnic ingredient (see

Table 1).

NOVEL ENERGETIC MATERIALS

Homoleptic polynitrogen compounds are the realcontenders [36, 37] as HEMs of this class. Storageof the maximum amount of energy in a polynitrogenmolecule would mean having the largest number of sin-gle N N bonds in its molecular structure. To obtainisolable and manageable homoleptic polynitrogens, thecompound needs to possess a sufficiently high energybarrier to decomposition. Researchers at U.S. Air ForceOffice of Scientific Research (AFOSR) have exploredunusual polynitrogen molecules with the aim of find-ing new rocket fuels, which could surpass cryogenic sys-tems based on hydrogen and liquid oxygen. Quantum-mechanical calculations suggest that N4, N5, N8, andN10 are viable molecules.


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Scheme 5.

N4 and N5 Compounds

The WoodwardHoffman rules have been appliedto calculate the energy barrier of tetrahedral N4 todissociate into two N2 molecules [38, 39]. It requiresthat two electrons from the highest occupied molecu-lar orbital (HOMO) need to be promoted into the low-est unoccupied molecular orbital (LUMO). Theoreti-cal calculations demonstrate that the energy barrier forsuch transitions is around 3050 kcal/mole for the N4

molecule, which is higher than that for trinitrotoluene(TNT), suggesting the possibility of the N4 molecule toexist. However, it is an oversimplification to considersingle-step dissociation of the N4 molecule. In reality,several steps are expected to be involved in N4 dissoci-ation, and a number of geometric configurations otherthan tetrahedral symmetry are likely to exist for the N4molecule.

Most of these configurations are transition states.It is conceptualized that the building principle andunique resonance stabilization of the well-known andexceptionally stable azide anion, where each N N bondhas a full double-bond character, may be extended by

adding N1+ containing four valence electrons to con-struct the N4 molecule and the N

1+5 ion (see Scheme 5).

Several quantum-chemical calculations have indi-cated that the tetrahedral form of N4 should be bothstable and quite energetic. The production and identi-fication of the N4 molecule, however, have not yet beenconclusively demonstrated.

Although N4 (II) and N1+5 (III) are conceptual-

ized to contain (like azide ions) only cumulated linearN N bonds, the neighboring positive charges presentin their structure, in contrast to N13 (I), render bothN4 and N

1+5 energetically unfavorable. Ab initio self-

consistent-field (SCF), coupled-cluster, and many-bodyperturbation theory (MBPT) calculations also predictthat tetrahedral N4 is metastable [40].

Quantum-mechanical calculations of the N1+5 andN15 isomers within the framework of the MollerPlessetperturbation theory (FU)/6-31G(d) [41] suggest thatthe most stable open-chain structure of N1+5 has the C2vsymmetry. The natural bond orbital (NBO) analysissuggests that the stability of this cation is enhanced byhyperconjugation (see IV in Scheme 5). The limitations

of the neighboring positive charges can be remedied forN1+5 by considering resonance structures (with the C2vsymmetry) with a bond order of 1.5 for the central N Nbonds.

The decomposition mechanism of pentanitrogen(N5, N

15 , and N

1+5 ), its clusters and ions was stud-

ied in [42] using ab initio molecular orbital (MO) cal-culations up to the Coupled Clusters Single Doubleand non-iterative Triple method [CCSD(T)] level with6-31 + G(3dp) and aug-cc-pVTZ basis sets, as well

as the Density Functional Theory (DFT)/Beckes gra-dient corrected exchange correlation density functions(B3LYP). These findings suggest that the cyclic anionN15 and the open-chain cation N

1+5 may be relatively

stable with respect to N2 elimination, while the neutralN5 radical does not exist at all as a discrete species.

A tremendous thrust to research in the area ofpolynitrogen compounds has been provided by thetechnological breakthrough achieved at the Air ForceResearch Laboratory (AFRL) on realizing the N1+5molecule. An important discovery was the synthesisof the N1+5 cation by the reaction of N2F and AsF

16

with HN3 in an anhydrous solution [43]. N1+5 AsF

16 is

a highly energetic white solid with a strong oxidizingnature:

N2F + AsF6 + HN3HF

78CN+5 AsF

6 + HF

HN3 + HF + AsF5HFH2N

+3 AsF

6

4N+5 AsF

6 + 2H2O 4HF + 4AsF5 + 10N2 + O2

N+5 AsF

6 + NaN3 Na+AsF6 + N

+5 N

3

Scheme 6.

It detonates violently with a detonation pressure fourtimes that of HMX, and its specific impulse (Isp) istwice as high as the value of Isp of todays mono-propellants. Christe et al. [43] have also succeededin preparing the N1+5 SbF

16 salt with high purity and

yield. This salt is surprisingly stable up to 70Cand exhibits low impact sensitivity, in contrast toN1+5 AsF

16 , which is reported marginally stable at

room temperature. Attempts to recrystallize the com-pound from the SO2/SO2ClF solution yielded anothernew N5F salt, namely, N

1+5 Sb2F

111 . The crystallo-

graphic studies confirmed the V-shaped configuration


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of these compounds. The bond lengths and bond an-gles for N1+5 [N1 N2 N3 N2 N1] calculated at theB3LYP level of theory are r1(N1 N2) = 1.11 A andr2(N2 N3) = 1.31 A with dihedral angles 166.6

(N1 N2 N3) and 110.3 (N2 N3 N2). Efforts arein progress in many schools to convert N1+5 AsF

16 to its

N8 adduct.The discovery of N1+5 has been widely acclaimed

internationally as the third all-nitrogen species to beisolated in the discrete form and the first in over 100years.

Exchange reactions (Scheme 7) were used to con-vert N1+5 SbF

16 into N

1+5 [B(CF3)4]

1 and (N5)1+2 SnF

16

[44]. The latter salt is noteworthy because it containstwo N1+5 ions per anion, thus demonstrating that saltswith contacting polynitrogen cations could be prepared.This constitutes an important milestone toward the ulti-mate goal of synthesizing a stable ionic nitrogen atmo-

sphere. Stepwise decomposition of (N5)2SnF6 yieldedN5SnF5. Attempts to isolate FN5 from (N5)2SnF6 ther-molysis were unsuccessful, yielding only the expecteddecomposition products, FN3, trans-N2F2, NF3, andN2:

N2FMF6 + HN3HFN5MF6 + HF,

N5SbF6 + MY N5Y + MSbF6 ,

2N5SbF6 + Cs2SnF6HF

78C(N5)SnF6 + 2CsSbF6 ,

(N5)2SnF6 N5SnF5 + FN5,

N5SnF5 SnF4 + FN5,

N5SbF6 + K[B(CF3)4]SO264C

N5[B(CF3)4]

+KSbF6 (M = Sb,As)

Scheme 7.

The presence of the delocalized electronic structureof N15 seems to offer stronger N N bonds. As a mat-ter of fact, N15 should have been the building blockfor polynitrogen molecules from an electronic-structureviewpoint. However, its extreme sensitivity combinedwith the fact that it cannot be stabilized in a structurewithout a large delocalized aromatic ring has made itsuse difficult.

N6 Compound

The N6 molecule appears to be another interest-ing system with an apparent benzene-type structure.However, high-precision theoretical investigations at theQuadratic Configuration Interaction including Singleand Double substituents (QCISD) level reveal that such

a structure does not correspond to a minimum on thepotential energy surface. To increase the stability ofN6 without compromising the energy-storage ability, itis envisaged to introduce other hetero-atoms into thestructure. Thus, repulsion between electron pairs of ad-

jacent nitrogen atoms can be overcome by introducingoxygen at the 1,3,5-positions of N6 resulting in forma-tion of N O co-ordinate bonds. The calculations of[4547] suggest reasonable stability of N6 with an acti-vation energy barrier of40 kcal/mole.

N8: The Most Widely StudiedPotential Polynitrogen Compoundof HEM Importance

Scientists are currently focusing their investiga-tions on developing a reaction system to combine N1+5

with N1

3 in an attempt to isolate a neutral N8 com-pound in the form of a ionic salt N1+5 N

13 or covalently

bonded azidopentazole. If such compounds could berealized in a stable form, they could be monopropel-lants with over 200% greater energy than hydrazinewidely used in upper-stage and spacecraft propulsion.Nguyen and Ha [48] carried out ab initio MO cal-culations at the quadratic CI [QCISD(T)/6-31G(d)],CCSD(T), and Double-z plus Polarization (DZP) levelsbased on MP2/6-31G(d) optimized geometries. Thesecalculations show that the azidopentazole structure islikely to be the global minimum of the N8 isomers, ly-ing 13 and 18 kcal/mole below the acyclic diazidyldi-

imide and cyclic pentazole (five-member ring consistingof nitrogen atoms) analog, respectively.

Azidopentazole is characterized by a significantenergy barrier to ring closure and is expected to bestable with respect to cycloreversion. Leininger etal. [49] have also applied ab initio molecular elec-tronic structure methods to examine three isomers ofN8: octaazacubane, cyclooctatetraene with D2d sym-metry, and a pentazole analog with a planar bicyclicform. Using a DZP basis set, geometries were optimizedwith the HartreeFock self-consistent-field (HF-SCF)method and second-order MP2 as well as configuration-interaction (CISD) and coupled-cluster (CCSD) meth-ods. Vibrational frequencies and IR intensities havebeen obtained at the SCF and MP2 levels of the-ory. Although cubane (C8H8) and cyclooctatetraene(C8H8) are known experimentally, unsubstituted pe-natazole has never been synthesized; nevertheless, thevibrational analysis indicates that all three nitrogenanalogs, including the pentazole analog, represent po-tential energy minimums. Metastability is attributed totwo additional p-electrons present in the first two com-


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pounds but not in pentazole. Further, since all threeminimums lie very high with respect to the energy offour N2 molecules, their potential application as high-energy-density materials (HEDMs) have been reported.

Calculations on the stability of N8 [50] indicatethat, if it could be synthesized, its dissociation intofour N2 molecules might result in energy release esti-mated as (530 50) kcal/mole. The potential energysurfaces for the synthesis and decomposition reactionsof N8(CS) N

+5 + N

3 and the isomerization reactionof N8(CS) to N8(C2h) were investigated in [51] by DFT.The barrier heights of N8(CS) N

+5 + N

3 in the for-ward and reverse directions were predicted to be 23.4and 2.0 kcal/mole, respectively. The barrier height forthe isomerization reaction of N8(CS) t o N8(C2h) waspredicted to be 4.0 kcal/mole at the QCISD/6-311+G

and B3LYP/6-311 + G levels. Fau and Bartlett [46]also reported transitional states for various interconver-

sions and dissociative reactions to assess the stabilityof N+5 and N

3 . Seven structures appear to be at theenergy minimums at the B3LYP/aug-cc-pVDZ (aug-mented double polarized valence) and MBPT(2)/aug-cc-pVDZ levels of theory, i.e., five diazidyldiazenes andtwo diazidylaminonitrenes or N3 N5 complexes. Gibbsfree energies based on CCSD(T)/aug-cc-pVDZ single-point calculations strongly suggest that only four of thediazidyldiazene structures have minimums at higher lev-els of theory. In diazidyldiazenes, the loss of N2 fromone of the azidyl end groups is as likely as the loss ofboth azidyl groups. Thus, isolation of covalently bondedN8 from N

+5 and N

3 will be difficult because the most

probable product has a decomposition barrier of only18 kcal/mole. Mutual neutralization is likely to followfragmentation.

A theoretical study was presented by Gagliardiet al. [52] on dissociation of N8 octaazapentalene(I) to four N2 molecules with the use of the Multi-Configurational Self-Consistent Field (MC-SCF) andMP2 methods. The reaction proceeds via isomeriza-tion of (I) to azidopentazole (II), which then disso-ciates directly into four N2 molecules. The calcula-tions determined the relative energies of configurationsI and II, as well as two transitional states involvedinto the dissociation reaction, and indicate that con-

figuration II was 13 kcal/mole more stable than con-figuration I. The barrier to dissociation of N8 intofour N2 molecules was computed as 19 kcal/mole. Anab initio study of fully optimized structures of thir-teen N8 isomers at the HF/6-31G(d) and MP2/6-31Glevels was reported in [53]. These isomers are ni-trogen analogs of various hydrocarbon skeletons: 1,2-dihydropentalene, cyclooctatetraene (tub form), bar-relene bicyclo[2,2,2]octa-2,5,7-triene, bicyclo[4,2,0]octa-

TABLE 2Homoleptic Polynitrogen Molecules

to be Synthesized Yet

2,4,7-triene (C2 and C2v), tricyclo[3,3,0,02.6]octa-3,7-

diene, tetracyclo[4,2,0,02.4,03.5]octa-7-ene, anti-tricyc-lo[4,2,0,02.5]octa-2,6-diene, syn-tricyclo [4,2,0,02.5]octa-2,6-diene, cuneane, octabisvalene, cyclooctatetraene(crown form), and cubane. Twelve local minimumswere found on the potential energy hypersurface at theHF/6-31(d) basis set via vibrational frequency calcula-tions at the same levels of theory. The effect of elec-


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TABLE 4Key Synthons Proposed for the Synthesis of Futuristic Unknown Polynitrogen Compounds

N12, N14, N16, N18, and N20: PotentialHigher Analogs of Polynitrogen-BasedHigh-Energy Materials

Klapotke and Harcourt [59] carried out ab initioHF/6-31 G(d) molecular orbital calculations for Nncomponents (n = 12, 10, and 8) and suggested a quali-tative valence bond (VB) mechanism for interconversionN12 N10 + N2 and N10 N8 + N2. Sun and Chen[53] studied the bond energy of small- and medium-sizenitrogen clusters (N8, N10, N12, N14, N16, N20, andN24) at different levels of quantum-chemical investiga-tions. All these methods show that the energies of N Nbonds in polynitrogen clusters are higher than the stan-dard energy of the N N single bond but lower than the

standard energy of the N N double bond. From theviewpoint of the total bond energy, the N20 compoundappears to be the most stable one among these clusters.

The cage-like structure of the N18 molecule withC2v symmetry was optimized at the Restricted HartreeFock (RHF/4-31G), RHF/6-31G, DFT(B3LYP,

B3P86, BHLYP)/6-31G

, and MP2(full)/6-31G

levelsof theory. Harmonic vibration frequencies and their IRand Raman intensities were also reported at the RHF/4-31G, RHF/6-31G, and B3P86/6-31G levels. The ex-amined cage-like structure corresponds to a local mini-mum on the potential energy hypersurface, and its sta-bility is commensurable with the stability of the dodec-ahedral N20 molecule. The calculations show that theenergy of the N18 configuration with C2v symmetry is


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TABLE 5Other Stabilized Polynitrogen Molecules

with a High Nitrogen Content

50 kcal/mole higher than that of nine N2 molecules.The resultant structure also suggests aromaticity of con-

jugated pentagons, which can stabilize cage-like polyni-trogen clusters. Both the ab initio (STO-3G) and mod-ified semi-empirical (INDO) methods were applied tomolecular orbital calculations of the N20 molecule. Thestability of the examined cage-like structure turned outto be commensurable with the stability of the dodec-ahedral N20 molecule. The optimized bond distancesbetween the nearest nitrogen atoms (N N) and mostof the calculated thermodynamic data approximated by

Scheme 8.

both methods were close to each other. Possible valuesof the enthalpy of formation Ha and Gibbs energyGa for the atomization reaction prove that N20 is sta-ble.

N60: Futuristic Celebrity of HEMsCommunity as an Ultrapowerful HEM

The C60 molecule (fullerene) exists, why not N60?N10 is currently projected as a key polynitrogen

that can serve as a building block for clustering into anitrogen analog of buckminster fullerene comprising 32to 600 carbon atoms. Because nitrogen atoms are nor-mally triple-bonded, no one had previously examinedthe possibility of creating a nitrogen fullerene, whichappears to be an oddity.

According to Manaa [57], it might be possible

to join six N10 molecules into a soccer-ball-shaped60-atoms molecule, on the lines of carbon-based buck-minster fullerene, by subjecting N10 to ultrahigh pres-sures. The synthesis of N60 proposed by Manaa [57]is presented in Scheme 8. Huge amounts of energy arebound to be released on cleavage of tight bonds of theN60 molecule.

Three absorption bands in the IR spectrum ofthe N60 molecule are predicted at 608 (56), 701 (94),and 1153 (4) cm1. Engelke and Stine [50] estimatedthe density of N60 between 2.25 and 2.67 g/cm

3, andthe predicted enthalpy of formation (Hf) is approx-imately 546 kcal/mole. The predicted energy releasedue to N60 decomposition into 30 molecules of N2 is2400 kcal/mole. The ChapmanJouguet (CJ) perfor-mance with nitrogen considered as the basic decompo-sition product was calculated by applying the BeckerKistiakowskyWilsion (BKW) and Effective Spheri-cal Potential (ESP) equations of state for N60 andsuggest its high potential as a high-energy material.The ball-and-stick models of potential polynitrogencompounds [drawn by the software named Advanced


12/14


13/14


5. M. B. Frenkel, L. R. Grant, and J. E. Flanagan, Histor-

ical development of glycidyl azide polymer, J. Propuls.

Power, 18, 560563 (1992).6. J. K. Chen and T. B. Brill, Thermal decomposition of

energetic materials 54: Kinetics and near surface prod-

ucts of azide polymers AMMO, BAM,O and GAP in

simulated combustion, Combust. Flame, 87, 157168

(1991).7. J. K. Nair, R. S. Satpute, T. Mukundan, et al., Syn-

thesis and characterization of bis-azido methyl oxetane

(BAMO), its precursors, polymer and copolymer with

THF, Defence Sci. J., 52, 147156 (2002).8. E. Kimura and Y. Oyumi, Effects of polymerization

ratio of BAMO/NMMO and catalyst on sensitivity and

burning rate of HMX propellant, Propellants, Explos.,

Pyrotech., 20, No. 4, 215221 (1995).9. A. Arber, G. Bagg, E. Colcough, et al., Novel en-

ergetic polymers prepared using dinitrogen pentoxide

chemistry, in: Proc. of the 21st Int. Annual Conf. of

ICT, Germany (1990), pp. 3/13/11.10. R. A. Earl and R. Douglas, Synthesis of energetic pre-

polymers, US ONR Report ADA 13770 (1985).11. B. Stogkovie and N. Stojanovic, Synthesis of ni-

troacetal plasticizers, Nancno-Tech. Pvegl., 40, 4146

(1990).12. K. G. Shipp and M. E. Hill, Acetal preparation in sul-

furic acid, J. Org. Chem., 31, 853859 (1996).13. M. D. Coburn, Synthesis of 2-nitroalkyl vinyl ethers,

Synthesis, 570572 (1977).14. A. T. Nielsen, A. P. Chafin, S. L. Christian, et al., Syn-

thesis of caged nitramine explosives, Tetrahedron, 54,

1179311812 (1998).

15. M. Geetha, U. R. Nair, D. B. Sarwade, et al., Studieson CL-20: The most powerful high energy material,

J. Therm. Anal. Calorim., 73, 913922 (2003).16. G. P. Sollott, J. Alster, E. E. Gilbert, and N. Slagg,

Research towards novel energetic materials, J. Energ.

Mater., 4, 528 (1986).17. J. Alster, O. Sandus, R. Gentner, et. al., Calculation of

molecular properties for polynitrohedranes molecules,

in: Working Group Meeting on High Energy Com-

pounds, Hilton Head, SC (1981).18. A. M. Astakhov, R. S. Stepanov, and A. Yu. Babushkin,

On the detonation parameters of octanitrocubane,

Combust., Explos., Shock Waves, 34, No. 1, 8587

(1998).19. C. G. Chin, H. W. Cuts, and S. J. Masamune, Strained

systems: Cubane, J. Chem. Soc. Chem. Commun.,

880882 (1966).20. A. Basher-Hashmi, Photochemical carboxylation of

cubanes, Angew. Chem., Int. Ed. Engl., 32, 612613

(1993).21. A. Basher-Hashmi, J. Li, N. Gelber, and H. Ammon,

Photochemical functionalization of cubanes, J. Org.

Chem., 60, 698702 (1995).

22. K. Tani, K. Lukin, and P. E. Eaton, Nitration of

organolithiums and grignards with dinitrogen tetraox-

ide, success at melting interfaces, J. Amer. Chem. Soc.,

119, 14761477 (1997).

23. M. X. Zhang, P. E. Eaton, and R. Gilardi, Hepta-

octanitrocubanes, Angew. Chem., Int. Ed. Engl., 39,

No. 2, 401404 (2000).

24. P. Politzer, P. Lane, and J. J. M. Wiener, Carbocyclic

and Heterocyclic Cage Compounds and Their Building

Blocks, JAI Press, Stamford, CN (1999), pp. 7385.

25. A. K. Zelenin and M. L. Trudell, A two step synthesis

of diamino furazan and synthesis of N-monoarylmethyl

and N, N-diarylmethyl derivatives, J. Heterocycl.

Chem., 34, 10571060 (1997).

26. I. V. Ovchinnikov, M. N. Makhova, L. I. Khemelniskii,

et al., Dinitrodiazenofuroxan as a new energetic explo-

sive, Dokl. USSR Acad. of Sci., 359, 6770 (1998).

27. D. E. Chavez and M. A. Hiskey, 1,2,4,5-tetrazine-based

energetic materials, J. Energ. Mater., 17, 357377

(1999).

28. D. E. Chavez, M. A. Hiskey, and R. D. Gilardi, 3,3-

azobis (6-amino-1,2,4,5-trerazine): A novel high nitro-

gen energetic material, Angew. Chem., Int. Ed. Engl.,

39, 17911793 (2000).

29. M. A. Hiskey, D. E. Chavez, D. L. Naud, et al.,

Progress in high nitrogen chemistry in explosives, pro-

pellants and pyrotechnics, in: Proc. of the 27th Int. Py-

rotechnic Seminar, Grand Junction, USA (2000), pp. 3

14.

30. J. Kerth and S. Lobbecke, Synthesis and character-

ization of 3,3-azobis(6-amino-1,2,4,5-trerazine) DAAT:

A new promising nitrogen rich compound, Propellants,

Explos., Pyrotech., 27, 111118 (2002).

31. R. Sivabalan, M. B. Talawar, S. N. Asthana, et al., Syn-

thesis and characterization of high nitrogen content high

energy materials, in: Proc. of the 4th Int. Symp. on

High Energy Materials and Exhibits, November 1719,

2003, Pune, India (2003), pp. 184191.

32. D. V. Chavez and M. A. Hiskey, High nitrogen

pyrotechnic compositions, J. Pyrotechnics, 7, 1113

(1998).

33. M. A. Hiskey, N. Goldman, and J. R. Stine, High ni-

trogen energetic materials derived from azotetrazolate,

J. Energ. Mater., 16, 119127 (1998).

34. Y. L. Peng and C. W. Wong, Preparation of guani-

dinium azotetrazolate, U.S. Patent No. 5,877,300,

March 2, 1999.

35. A. Hammerl, T. M. Klapotke, H. Noth, et al., [N2H5]2+

[N4C N N CN4]2: A new high-nitrogen high-

energetic materials, Inorg. Chem., 40, 35703675

(2001).

36. R. Engelke, Ab initio calculations of ten car-

bon/nitrogen cubanoids, J. Amer. Chem. Soc., 115,

29612967 (1993).


14/14


37. M. N. Glukhovtsev, H. Jiao, and P. V. R. Schleyer, Be-

sides N2, what is the most stable molecule composed

only of nitrogen atoms? Inorg. Chem., 35, No. 24,

71247133 (1996).38. A. A. Korkin, A. Balkova, R. J. Bartlett, et al., The

28 electron tetra atomic molecules N4, CN2O, BFN2,

C2O2, B2F2, CBFO, C2FN and BNO2 challenges for

computational and experimental chemistry, J. Phys.

Chem., 100, 57025714 (1996).39. D. R. Yarkony, Theoretical studies of spin-forbidden

relationless decay in polyatomic systems: insights from

recently developed computation methods, J. Amer.

Chem. Soc., 114, 54065611 (1992).40. W. J. Lauderdale, J. F. Stanton, and R. J. Bart-

lett, Stability and energetics of metastable molecules:

tetraazatetrahedrane (N4), hexaazabenzene (N6), and

octaazacubane (N8), J. Phys. Chem., 96, No. 3,

11731178 (1992).41. X. Wang, H. R. Hu, A. Tian, et al., An isomeric study

of N+5 , N5, and N

5 : A Gaussian-3 investigation, Chem.

Phys. Lett., 329, Nos. 56, 483489 (2000).42. M. T. Nguyen and T. K. Ha, Decomposition mecha-

nism of the polynitrogen N5 and N6 clusters and their

ions, Chem. Phys. Lett., 335, Nos. 34, 311320 (2001).43. K. O. Christe, W. W. Wilson, J. A. Sheehy, and

J. A. Boatz, N+5 : A novel homoleptic polynitrogen ion

as a high energy density material, Angew. Chem., Int.

Ed. Engl., 38, Nos. 13/14, 20042009 (1999).44. W. W. Wilson, A. Vij, V. Vij, et al., Polynitro-

gen chemistry: Preparation and characterization of

(N5)2SnF6, N5SnF5 and N5B(CF3)4, Chem. Eur. J.,

9, 28402844 (2003).

45. R. J. Bartlett, Exploding the mysteries of nitrogen,

Chem. Ind., 4, 140143 (2000).46. S. Fau and R. J. Bartlett, Possible products of the end-

on addition of N3 to N+5 and their stability, J. Phys.

Chem. A, 105, No. 16, 40964106 (2001).47. J. F. Stanton and R. J. Bartlett, in: D. Boyd and K. Lip-

kowitz (eds.), Reviews in Computational Chemistry,

Vol. 5, Chapter 2, VCH, New York (1994), pp. 65169.48. M. T. Nguyen and T. K. Ha, Azidopentazole is proba-

bly the lowest-energy N8 species. A theoretical study,

Chem. Ber., 129, No. 10, 11571159 (1996).

49. M. L. Leininger, C. D. Sherrill, and H. F. Schaefer, N8:

A structure analogous to pentalene, and other high-

energy density materials, J. Phys. Chem., 99, No. 8,

23242328 (1995).50. R. Engelke and J. R. Stine, Is N8 cubane stable?

J. Phys. Chem., 94, No. 15, 56895694 (1990).

51. Q. S. Li and L. J. Wang, Theoretical studies on thepotential energy surfaces of N8 clusters, J. Phys. Chem.

A, 105, No. 10, 19791982 (2001).52. L. Gagliardi, S. Evangelisti, A. Bernhardsson, et al.,

Dissociation reaction of N8 azapentalene to 4N2:

A theoretical study, Int. J. Quant. Chem., 77, No. 1,

311315 (2000).53. K. C. Sun and C. Chen, Ab initio study of various

structures of N8, Huoyao Jishu, 13, No. 1, 123 (1997).54. A. Tian, F. Ding, L. Zhang, et al., New isomers of N8

without double bonds, J. Phys. Chem. A, 101, No. 10,

19461950 (1997).55. S. Evangelisti and L. Gagliardi, A complete active-

space self-consistent-field study on cubic N8, Nuovo Ci-mento Soc. Ital. Fis. D, 18D, No. 12, 13951405 (1996).

56. L. Gagliardi, S. Evangelisti, P. O. Widmark, and

B. O. Roos, A theoretical study of the N8 cubane to N8pentalene isomerization reaction, Theor. Chem. Acc.,

97, Nos. 14, 136142 (1997).57. M. R. Manaa, Toward new energy-rich molecular sys-

tems: from N10 to N60, Chem. Phys. Lett., 331,

262268 (2000).58. T. Matsunaga, T. Ohana, T. Nakamura, et al., Polyni-

trogen compounds and their manufacture, Jpn. Kokai

Tokyo koho JP 110 43315 A2 1999; Chem. Abstr.

No. 130: 198519.

59. T. M. Klapotke and R. D. Harcourt, The interconver-sion of N12 to N8 and two equivalents of N2, J. Molecul.

Struct., Theochem., 541, 237242 (2001).60. P. J. Haskins, J. Fellows, M. D. Cook, and

A. Wood, Molecular level studies of polynitrogen

explosives, in: Proc. 12th Int. Detonation Symp. (Au-

gust 1116, 2002); www.intdetsymp.org/detsymp2002/

PaperSubmit/FinalManuscript/pdf/Haskins-102.PDF.

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