Volume 41 Number 3 7 February 2017 Pages 893–1392 NJC · 2017-10-19 · High energy density materials ... ideal HEDM should have high positive heat of formation (HoF), high density,

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PAPERNeeraj Kumbhakarna et al.Theoretical studies on the propulsive and explosive performance of strained polycyclic cage compounds

Volume 41 Number 3 7 February 2017 Pages 893–1392

920 | New J. Chem., 2017, 41, 920--930 This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017

Cite this: NewJ.Chem., 2017,

41, 920

Theoretical studies on the propulsive andexplosive performance of strained polycycliccage compounds

Lovely Mallick,a Sohan Lal,b Sasidharakurup Reshmi,c Irishi N. N. Namboothiri,b

Arindrajit Chowdhuryd and Neeraj Kumbhakarna*d

Compounds consisting of a carbon-based cage have a highly strained molecular structure and have

become the subject of interest in recent years because they possess high heat of formation and so are

highly energetic. In the present work, ab initio molecular modelling calculations have been used for

analysing 28 carbon-cage structures with the aim of identifying the best candidates for synthesis

particularly for use in propellant compositions. Density functional theory (B3LYP) was employed for the

geometry optimisation of the proposed molecules using the 6-311++G(d,p) basis set. Calculated heats

of formation and densities of the compounds have been used from the optimized structures to

compute their specific impulses and density specific impulses in various configurations (solid and liquid)

with an eye on propulsion applications. Detonation properties of the compounds have also been reported and

comments have been made correlating the properties of the cage compounds with their molecular structures.

Introduction

High energy density materials (HEDMs) are extensively used inrocket propulsion systems, gas generators, and explosives. Anideal HEDM should have high positive heat of formation (HoF),high density, low production cost and low sensitivity to impactand friction.1 Certain other requirements that an ideal HEDMshould fulfil include, low toxicity and low operational hazards.Cage hydrocarbons are a class HEDMs owing to their severelystrained molecular structure.2 Cubane (C8H8), which is one ofthe first cage compounds to be synthesised,3 has a density of1.29 g cm�3 and a heat of formation of 144 kcal mol�1,4 bothvalues being higher than those of normal straight-chain andunstrained hydrocarbons. Cubane being a solid can be usedeither as an energetic binder by possible conversion to polymericforms or as an additive to fuels in liquid propulsion systemsand in internal combustion engines.5 Hence, to exploit thestrained structure of compounds similar to cubane, efforts havebeen made to develop suitable variants and derivatives that canfunction as additives to fuels for systems which require highenergy in a compact volume.6–19 However, limited yield in theirsynthesis has restricted the quantitative and qualitative analysis of

their performance to droplet combustion experiments, thermo-gravimetric analysis, fast pyrolysis studies and other similartests.17–23 Computational analysis of several newly synthesisedcompounds based on the bishomocubane (BHC) ring, viz.nitromethyl-1,3-bishomocubane (NTMBHC), nitromethylene-1,3-bishomocubane (NMyBHC), and bis-nitromethyl-1,3-bishomo-cubane (DNTMBHC), at the density functional theory (DFT)/abinitio level in our previous work showed that they possess highpositive HoF and high density specific impulse18 renderingthem superior to hydrocarbon fuels such as RP1 with regardto propulsive performance in volume limited applicationsdespite being costlier to produce than RP1. While synthesisingBHC-based compounds, high nitrogen substituents are worthconsidering because high nitrogen groups tend to perform wellas HEDMs. Their HoFs are typically high and one of the majordecomposition products is molecular nitrogen.24 They havelower adiabatic flame temperatures as compared to conventionalpropellants and generally produce less smoke or soot. Some high-nitrogen materials show remarkable stability towards friction, heatand impact, and hence are easy to store and transport.24 Thushigh-nitrogen materials can be considered for use in applicationssuch as airbags and gun propellants. Tetrazoles19,25–29 andtriazoles19,30–33 are two prominent classes of high-nitrogenmaterials that have been studied significantly with regard totheir behaviour as energetic materials.

For the purpose of developing new HEDMs, compoundsneed to be chemically synthesised as per the envisioned molecularstructures. Synthesis is a tedious and often expensive process.

a Department of Aerospace Engineering, Indian Institute of Technology Bombay, Indiab Department of Chemistry, Indian Institute of Technology Bombay, Indiac Polymers and Special Chemicals Group, Vikram Sarabhai Space Centre,

Thiruvananthapuram, Indiad Department of Mechanical Engineering, Indian Institute of Technology Bombay,

India. E-mail: [email protected]

Received (in Montpellier, France)5th August 2016,Accepted 10th October 2016

DOI: 10.1039/c6nj02444k

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Computational analysis of the molecular structures of con-ceptualised molecules can give an insight into how the envisionedcompounds will perform as HEDMs, particularly as propellants orexplosives. Molecular modelling ab initio methods can be usedalong with equilibrium calculations to theoretically gauge theperformance of these materials and generate valuable inputs forsynthetic chemists to help them optimise their time and resources.Such studies have been conducted in the past on various cagecompounds. Dilling calculated HoF and strain energy of variousBHC-based compounds and studied their isomerisation anddecomposition reactions.34 Wang and Law investigated cubane,benzvalene, and dihydrobenzvalene with regard to their thermo-chemical properties and adiabatic flame temperatures.35 Schmidtet al. analysed cubic molecules C4O4, N8, and Si8R8 as possiblehigh-energy propellants.36 Zhang et al. used various levels ofmolecular orbital theory to calculate HoF of 21 polynitrocubanesand discussed the relationship between HoF and molecularstructures.37 Novak elaborated on the relation between stabilityand properties such as HoF and strain energy which he calculatedusing the high level ab initio G3(MP2)/B3LYP method for a series ofhomocubanes.38 Fan et al. explained in detail how the strainenergy of a wide range of cubane derivatives is influenced bytheir structures.39 The substituent functional groups consideredby them were NF2, N3, NO2, and ONO2. Maslov et al. simulatedthe decomposition of cubane and evaluated the kinetic parametersassociated with its decomposition.40 Based on their results theyalso commented on the anomalously high thermal stability ofcubane. Chi et al. conceptualised a number of polydinitroamino-cubanes and computationally analysed them using DFT. Theycalculated HoF, detonation velocity (DV), detonation pressure(DP), and orbital energies of the compounds and discussedtheir structure–property correlations.41

The computational work carried out so far in the direction ofdeveloping new HEDMs, to the best of our knowledge, mainlyfocuses on determination of their potential as good explosivesand investigation of thermochemical and detonation properties ofthe envisioned compounds. In the present work, we additionallyexamine the potential of the compounds as propellants anddetermine their ballistic properties such as specific impulse (Isp)and density specific impulse (rIsp). The optimised structures ofa set of 28 cage compounds shown in Table 1 have beenreported. Based on their molecular structures the compoundshave been divided into three groups: (i) compounds containingoxygen-rich moieties, (ii) compounds containing nitrogen-richmoieties, and (iii) hydrocarbons. The optimised structures andrelated data have been used to calculate thermochemical,ballistic, and explosive properties. Plausible explanations tocorrelate the calculated properties to the structure of the moleculeshave been provided wherever necessary.

Computational methods

To compile a database of the thermochemical and ballisticproperties of the 28 compounds under consideration, quantummechanics based calculations were carried out using the Gaus-sian 09 suite of programs.44 The calculations involved ground

state optimisation of molecular structures of the compounds at theB3LYP/6-311++G(d,p) level of theory which consists of a triple splitvalence basis set with additional polarised functions.45 Diffusefunctions were also included since the molecules involved areconsiderably large.46 Our choice of the B3LYP/6-311++G(d,p)level of theory for our analysis was dictated by the endeavour tomaintain a balance between computational time and accuracy.47–49

HoFs for all the compounds under study were calculated usingthe method of atomisation.50 Densities of the compounds werecalculated by dividing their molecular weights by the respectivemolar volumes, determined on the basis of the 0.001 e Bohr�3

density envelope through the Monte-Carlo method.51 Althoughthis method of calculation accurately predicts the density in thecase of compounds such as cubane, the values of densitydetermined for the envisioned compounds tend to be over-predicted. Thus values obtained in this work can be taken tobe the upper limits of the densities of the respective compounds.

In the present work, NASA CEA (Chemical Equilibrium andApplications) utility52 was utilised to calculate the ballisticproperties such as vacuum specific impulse (Isp,vac) and itsderivative, density vacuum specific impulse (rIsp,vac). The per-formance of these compounds as (i) monopropellants, (ii) liquidbipropellants with liquid oxygen as the oxidiser, (iii) possiblesolid propellant binders with ammonium perchlorate (AP) asthe oxidiser, and (iv) additives to ammonium perchlorate–hydroxylterminated polybutadiene (AP-HTPB) solid propellant combinationwas analysed. In NASA CEA, the chamber pressure and exit-to-throatarea ratio were set to 1000 psi and 70 respectively. As per thecommonly followed procedure, while calculating Isp,vac of the liquidpropulsion systems, the mixture ratio (oxidiser to fuel ratio) forwhich the specific impulse is maximum was first calculatedfor expansion of the combustion products from 1000 psi toatmospheric pressure. This mixture ratio was then used todetermine the recorded Isp,vac by simulating expansion of thecombustion products through an area ratio of 70. Isp,vac for solidpropellant systems was calculated by considering a mixture of80% AP and 20% of the propellant compound as a monomericbinder. Finally, while analysing the compounds as additives themixture ratio of AP/HTPB/X was considered to be 80/15/5 bymass, where X is the compound being studied.

Oxygen balance, defined as percentage of oxygen inherentlyavailable for combustion, of a compound having the molecularformula CaHbNcOd and molecular weight MW was calculatedusing eqn (1):

OB ð%Þ ¼�32 aþ 1

4b� 1

2d

� �MW

� 100 (1)

Detonation properties of the compounds under considerationwere calculated using the Kamlet–Jacobs empirical correlation53

which is based on the maximum exothermicities of the detonationreactions. According to the maximum exothermicity principle, inany detonation reaction, the nitrogen present in the compoundforms N2 and the intrinsic oxygen present in the compound formswater reacting with hydrogen first and the remaining reacts withcarbon to form CO2. The rest of carbon and hydrogen are

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Table 1 Molecular structures of the compounds studied in this work

Entry Compounda Formula Trivial name Abbreviationb

1 C10H11NO2 6-Nitro-1,3-bishomocubane NTBHC

2 C10H10N2O4 2,5-Dinitro-1,3-bishomocubane DNTBHC2

3 C10H10N2O4 6,6-Dinitro-1,3-bishomocubane DNTBHC1

4 C11H13NO2 6-Nitromethyl-1,3-bishomocubane NTMBHC18

5 C12H14N2O4 2,5-Dinitro-dimethyl-1,3-bishomocubane DNTDMBHC3



8 C11H11N3O6 6,6-Dinitro-5-nitromethyl-1,3-bishomocubane DNTNTMBHC

9 C10H10N2O6 2,5-Dinitrato-1,3-bishomocubane DNBHC

10 C12H14N2O6 2,5-Dinitrato-dimethyl-1,3-bishomocubane DNDMBHC17

11 C10H12N4O4 2,5-(N,N0-Dinitramido)-1,3-bishomocubane DNitramBHC1

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Table 1 (continued )


12 C10H12N4O4 6,6-(N,N0-Dinitramido)-1,3-bishomocubane DNitramBHC2

13 C12H14N4O2 5-Azidomethyl-6-nitromethyl-1,3-bishomocubane AMNTMBHC

14 C11H11N5O4 5-Azidomethyl-6,6-dinitro-1,3-bishomocubane AMDNTBHC

15 C10H10N6 2,5-Diazido-1,3-bishomocubane DABHC

16 C14H14N6 2,5-Ditriazolo-1,3-bishomocubane DTrizBHC

17 C12H12N8 2,5-Ditetrazolo-1,3-bishomocubane DTetzBHC

18 C12H14N6 2,5-Diazido-dimethyl-1,3-bishomocubane DADMBHC19

19 C16H18N6 2,5-Ditriazolo-dimethyl-1,3-bishomocubane DTrizDMBHC

20 C14H16N8 2,5-Ditetrazolo-dimethyl-1,3-bishomocubane DTetzDMBHC

21 C8H8 Cubane CB3

22 C9H10 Homocubane HCB13,38

23 C10H12 1,1-Bishomocubane 1,1-BHC13,38

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converted to solid carbon and molecular hydrogen. To computethe detonation pressure (in kbar) and detonation velocity(in km s�1), the following equations were used:

P = 15.58r2NMav1/2Q1/2 (2)

D ¼ 1:01

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiNMav

1=2Q1=2

q1þ 1:13rð Þ (3)

where r (g cm�3) is the density, Mav (g mol�1) is the averagemolecular weight of the gaseous products, N (mol g�1) is thenumber of moles of gaseous products per gram of explosive andQ (cal g�1) is the mass specific enthalpy of detonation.

Results and discussion

BHC-based compounds that have been analysed in this workcontain substituent groups such as nitro (–NO2), nitrato (–O–NO2), nitramine (–NH–NO2), azide (–N3), tetrazole, and triazole.Dimers of cubane and bishomocubane have also been consid-ered. Because half of these compounds have not yet beensynthesized, their physical state (whether solid or liquid) isnot known. Hence their propulsive properties were studied withregard to both liquid and solid propulsion systems. Whilestudying them as liquid propellants three cases were consideredfor each compound. In the first case the compound was assumedto be a liquid under normal conditions and its combustion was

simulated along with liquid oxygen. In the second case thecompound was assumed to be a solid which forms a 30% solution(by mass) with RP1 and burns along with liquid oxygen. And finallythe compounds were studied as liquid monopropellants.

Compounds containing oxygen-rich moieties

Compounds with oxygen-rich groups (1–14 in Table 1) consideredhere primarily contain the nitro, nitrato, and nitramine groupsattached to the cage. Their calculated HoFs (molar as well asspecific) and densities are given in Table 2 and propulsiveproperties in various configurations are listed in Tables 3 and 4.In all the tables the properties of conventional binders andpropellants such as glycidyl azide polymer (GAP), cyclotrimethylenetrinitramine (RDX), HTPB, and rocket propellant 1 (RP1) are alsolisted for the sake of comparison at relevant locations. It can beseen in Table 2 that all the cage compounds have densitieshigher than those of conventional binders GAP and HTPB aswell as the liquid propellant RP1. Their HOFs are also significantlyhigher which can be attributed to their strained cage structure. Bytheir very nature, larger molecules have higher energy content andhence specific HoFs can be considered for comparison of cagecompounds having different molecular weights and functionalgroups, instead of molar HoFs.

Except for the compounds containing nitrato groups, therest of the combustion bearing nitro groups were found to have

Table 1 (continued )





27 C16H14 Cubane dimer CBD42

28 C20H20 1,3-Bishomocubane dimer 1,3-BHC dimer43

a Among the numerous possibilities, the choice of the molecular structures depended on considerations of ease of synthesis, availability of startingmaterials for synthesis and possible energetic characteristics of the material. Some of these compounds were previously synthesized as indicatedby the references. b Relevant references are given for compounds that are previously known.

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similar specific HoFs. However, AMNTMBHC and AMDNTBHChave relatively high specific HoFs than other compounds. Thisshows that the azide group (–N3) is instrumental in impartingenergetic characteristics to the cage skeleton. Nitro and nitratogroups are typically electron withdrawing groups and theirmutual repulsion when located close to each other on the ringsincreases the cage strain resulting in higher HOFs.39 HenceDNTBHC1 has a higher HoF than DNTBHC2, and a similarobservation can be made about DNTDMBHC1, DNTDMBHC2and DNTDMBHC3. The HoF values in Table 2 also show that theeffect of nitro groups in destabilising the cage is more pronouncedthan that of the nitrato groups (DNTBHC2 vs. DNBHC andDNTDMBHC3 vs. DNDMBHC).39 The density of di-substitutedcompounds is higher than that of the corresponding mono-substituted compounds but HoF may not be necessarily higher.

The data in Table 2 make it clear that the nitramine group ismore effective in enhancing the HoF than nitro, nitrato andnitromethyl groups but not as effective as the azide group.

Propulsive properties of oxygen containing cage compoundsin Table 3 suggest that in the liquid propulsion mode alongwith liquid oxygen, the specific impulse of all these compoundsis slightly lower than that of RP1 with the exception of NTBHC.However, the density of RP1 is considerably lower than thedensity of these compounds, resulting in a lower density specificimpulse. Thus in spite of their lower specific impulse, cagecompounds would be able to give a longer range of propulsionsystems in volume constrained applications. NTBHC has thehighest specific impulse whereas AMDNTBHC has the highestdensity specific impulse. It is interesting to note that theoxidiser–fuel (O/F) ratios for the combustion of these compoundsare considerably lower than that for RP1 because the fuel moleculesare rich in oxygen. If the oxygen containing cage compounds aresolids then there is a possibility of using them as additives toRP1. Table 3 also provides propulsive properties with liquidoxygen, assuming a 30% solution in RP1 to be achievable. Asexpected, the specific impulses of the propellant–RP1 mixturesare marginally lower than those of RP1 alone but the densityspecific impulses show an increase of approximately 14 s on anaverage. Note that the HoF of a compound was not observed tobe a reliable indicator of its specific impulse because the HOFsof the nitramine and azide compounds are considerably higherthan those of the rest of the compounds but their specificimpulses with liquid oxygen as the oxidiser do not show thesame trend. This is true even if the compound is used as anadditive to RP1. But the ratio of adiabatic flame temperature tomolecular weight of the products at the throat appears to be avery reliable indicator of a compound’s specific impulse fromthe data displayed in Table 3. As monopropellants the compoundsappear to perform better with regard to specific impulse than

Table 2 Heats of formation and densities of compounds containingoxygen-rich groups calculated using the B3LYP/6-311++G(d,p) level oftheory

Entry CompoundDensity(g cm�3)

HOF(kcal mol�1)

Specific HoF(kcal g�1)

1 NTBHC 1.46 81.2 0.462 DNTBHC2 1.49 81.4 0.373 DNTBHC1 1.65 89.9 0.404 NTMBHC 1.39 78.6 0.415 DNTDMBHC3 1.54 75.3 0.306 DNTDMBHC1 1.50 76.7 0.317 DNTDMBHC2 1.58 84.4 0.348 DNTNTMBHC 1.58 103.9 0.379 DNBHC 1.58 62.1 0.2410 DNDMBHC 1.59 55.3 0.2011 DNitramBHC1 1.39 113.5 0.4512 DNitramBHC2 1.57 112.4 0.4513 AMNTMBHC 1.55 157.6 0.6414 AMDNTBHC 1.66 170.1 0.61— GAP 1.30 38.0 —— HTPB 0.93 �12.3 —

Table 3 Predicted propulsive properties of compounds containing oxygen-rich groups as liquid propellants

Compound

Compound + liquid oxygen Compound (30%) + RP1 (70%) + liquid oxygen Compound as monopropellant

O/Fratio

CCTa

(K)

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiTad

MWprod

s b

Isp,vac

(s)rIsp,vac

(s)O/Fratio CCTa (K)


MWprod

s b

Isp,vac (s)rIsp,vac

(s)CCTa

(K)


MWprod

s b

Isp,vac

(s)rIsp,vac

(s)

NTBHC 1.45 3869.3 12.54 366.9 458.9 2.30 3716.8 12.47 366.7 391.5 1723.6 12.29 251.7DNTBHC2 0.96 3845.4 12.42 360.5 467.2 2.09 3692.4 12.49 364.7 388.3 2044.0 12.06 268.4 400.3DNTBHC1 0.95 3856.9 12.48 361.7 489.4 2.08 3692.7 12.50 364.8 390.9 2107.3 12.25 272.2 447.9NTMBHC 1.45 3829.0 12.64 365.5 449.3 2.24 3698.6 12.54 365.7 388.6 1607.5 12.02 247.1 342.8DNTDMBHC3 1.14 3811.6 12.46 362.4 470.1 2.14 3689.0 12.50 364.9 389.8 1751.1 11.69 258.4 397.6DNTDMBHC1 1.14 3813.4 12.47 362.6 465.2 2.14 3689.3 12.50 364.9 389.1 1758.8 11.72 258.9 387.6DNTDMBHC2 1.12 3817.3 12.52 363.0 476.2 2.14 3691.3 12.51 365.2 390.7 1803.5 11.88 261.9 413.3DNTNTMBHC 0.80 3860.3 12.35 361.9 488.6 2.02 3688.1 12.49 364.6 389.3 2308.3 12.07 283.9 449.7DNBHC 0.74 3826.6 12.31 358.7 486.9 2.02 3686.0 12.48 364.4 388.9 2207.9 11.72 279.7 442.1DNDMBHC 0.91 3811.8 12.28 362.4 484.9 2.07 3683.1 12.49 364.6 389.7 1887.5 11.48 269.0 427.7DNitramBHC1 0.95 3829.5 12.43 363.5 455.9 2.15 3701.0 12.45 366.1 388.3 2074.3 12.09 275.6 366.4DNitramBHC2 0.95 3827.9 12.43 363.3 481.6 2.04 3719.8 12.54 368.6 393.5 2067.1 12.06 275.1 381.9AMNTMBHC 1.24 3853.1 12.64 365.4 472.7 2.17 3696.1 12.53 365.5 390.9 1903.8 12.63 261.9 406.9AMDNTBHC 0.85 3879.6 12.53 362.0 496.4 2.05 3692.1 12.51 364.8 390.9 2352.2 12.59 282.3 467.9RP1 2.58 3666.4 12.50 366.2 374.3 — — — — — — — — —N2H4 — — — — — — — — — — 644.1 6.85 234.1 235.3IPN — — — — — — — — — — 1350.9 8.96 251.6 261.7

a Combustion chamber temperature. b Ratio of adiabatic flame temperature to molecular weight.

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the commonly used hydrazine (N2H4) and another hydrocarbon-based compound, isopropyl nitrate (IPN), as shown. Althoughthe compounds are envisioned as monopropellants, their oxygenbalance renders them unsuitable as practical monopropellants.Additionally, no known catalysts exist for these compounds. If theoxygen containing cage compounds are solids then there is scopefor them to be used in solid propellant combinations as bindersor as additives provided that their molecular structures andother properties permit them to polymerise easily. They may beconsidered as replacements for HTPB in AP-HTPB compositepropellant combinations. If polymerisation proves impossible,then the cage compounds can be examined with respect to theirability to play the role of energetic additives in AP-based compositepropellants with HTPB as the binder. Results of analysis in both ofthese modes are shown in Table 4. All the oxygen containing cagecompounds show more than 25 s increase in specific impulse ascompared to HTPB. As energetic additives the compounds werecompared with the widely used nitramines, RDX and HMX. Thecomposition considered was 5% additive, 15% HTPB binder and80% AP. It is clear from Table 4 that RDX and HMX perform betterthan cage compounds giving specific impulse which is higher by atleast 4 s. Thus in composite solid propellants the use of thesecompounds rests on their ability to polymerise which is a topic offurther study.

As the cage compounds are high energy compounds theirdetonation properties were also calculated to assess their perfor-mance in explosive compositions. The data obtained are shown inTable 5. Although referred to as oxygen containing compounds forthe purpose of classification in the present work, from the point ofview of combustion, all of them are highly deficient in oxygen. Thisresults in their low detonation velocities and detonation pressures.Their performance is nowhere close to that of RDX which is acommonly used explosive material and has a significantly higheroxygen balance.

Compounds containing nitrogen-rich moieties

Nitrogen containing compounds (15–20 in Table 1) consideredin this work have high-nitrogen substituents such as azide,tetrazole, and triazole attached to the cage. These are commonfunctional groups in high-nitrogen energetic materials. Theirpresence in addition to the strained cage structure results in anacute rise in HoF as seen in Table 6. All compounds were foundto possess similar specific HoFs, which were higher than thoseof the compounds containing oxygen-rich moieties.

The HoFs are significantly higher than those of the oxygencontaining compounds, and as a result, adiabatic flametemperatures in general are also higher. Their calculated densitiesare also substantial, making them good candidates for use inpropulsion applications. Considering the listed HoF values, theability of high-nitrogen groups to impart energetic character to thecage follows the order triazole o azide o tetrazole, irrespective ofthe presence of the methyl group.

The values of density do not follow this trend. From theirpropulsive properties listed in Table 7 it can be concluded thatbecause of no oxygen content, the oxidizer–fuel (O/F) ratios thatgive the maximum Isp are higher as compared to the oxygencontaining compounds when combusted with liquid oxygen.

This difference is not significant if RP1 is also present alongwith liquid oxygen. The density specific impulse of high-nitrogen

Table 4 Predicted propulsive properties of compounds containingoxygen-rich groups as solid propellants

Entry Compound

Isp,vac (s)

X a Y b

1 NTBHC 304.3 281.82 DNTBHC2 312.5 284.83 DNTBHC1 313.2 285.04 NTMBHC 302.2 281.25 DNTDMBHC3 309.5 283.56 DNTDMBHC1 309.6 283.57 DNTDMBHC2 310.2 283.78 DNTNTMBHC 308.4 286.69 DNBHC 306.5 286.610 DNDMBHC 313.0 285.311 DNitramBHC1 313.6 285.412 DNitramBHC2 312.8 285.413 AMNTMBHC 308.1 282.914 AMDNTBHC 313.7 285.8— HTPB 274.6 —— RDX — 290.2— HMX — 290.3

a X = compound (20%) + AP (80%). b Y = compound (5%) + HTPB (15%) +AP (80%).

Table 5 Predicted detonation properties of compounds containingoxygen-rich groups

Entry Compound O.B.a (%) D.P.b (GPa) D.V.c (km s�1)

1 NTBHC �212.40 13.30 5.862 DNTBHC2 �151.35 27.50 7.653 DNTBHC1 �151.35 17.76 6.584 NTMBHC �221.99 10.76 5.365 DNTDMBHC3 �172.80 21.61 7.276 DNTDMBHC1 �172.80 16.53 6.397 DNTDMBHC2 �172.80 16.29 6.378 DNTNTMBHC �122.42 22.25 7.229 DNBHC �119.69 25.82 7.7910 DNDMBHC �141.84 12.61 5.9411 DNitramBHC1 �139.68 15.12 6.3612 DNitramBHC2 �139.68 19.25 6.8913 AMNTMBHC �188.62 15.95 6.2914 AMDNTBHC �135.74 18.19 6.73— RDX �21.62 35.10 8.93

a O.B. = oxygen balance. b D.P. = detonation pressure. c D.V. = detona-tion velocity.

Table 6 Heats of formation and densities of compounds with high-nitrogen substituents calculated using the B3LYP/6-311++G(d,p) level oftheory


HOF(kcal mol�1)


15 DABHC 1.48 241.0 1.1316 DTrizBHC 1.37 232.6 0.8717 DTetzBHC 1.57 265.0 0.9918 DADMBHC 1.44 239.7 0.9919 DTrizDMBHC 1.52 233.1 0.7920 DTetzDMBHC 1.53 260.9 0.88— GAP 1.30 38.0 —— HTPB 0.93 �12.3 —

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cage compounds is higher by at least 70 s than that of RP1. 30%solution of the compounds in RP1 also performs significantlybetter than pure RP1. As monopropellants, the high-nitrogencompounds other than the triazole derivatives appear to be betterthan hydrazine and IPN, but lack of knowledge of their stabilityand storability prevents us from making any concrete inferenceabout them at this point. Among the high-nitrogen cage compoundsconsidered in this work, the overall performance of DABHC appearsto be the most superior.

Solid propellant formulations obtained by mixing high-nitrogen cage compounds with AP show major improvementover HTPB in propulsion performance as shown in Table 8 butthe feasibility again depends on the ability of these compoundsto polymerise. If they cannot be compacted with AP then theycan be considered as solid propellant additives to form acomposite in which AP is the oxidiser and HTPB is the binder.In this composition their performance falls short whencompared to that of RDX and HMX. Even in comparison tooxygen containing compounds studied in the previous section,these compounds have lower Isp. This occurs since they aremore deficient in oxygen. The detonation properties of nitrogencontaining cage compounds are listed in Table 9. They are alsonot suitable in explosive applications as they are unable toproduce satisfactory detonation pressures and velocities incomparison to RDX.

Hydrocarbon compounds

Hydrocarbon compounds (Table 1, 21–28) studied in this workare homocubane, bishomocubane isomers and dimers of cubaneand bishomocubane. These compounds exhibit high-energycontent on account of cage strain, occurrence of thermo-dynamically favoured chemical reactions and an elementalcomposition which results in stable gaseous compounds suchas CO2 and H2O on combustion. The HoF values in Table 10show that cage hydrocarbons are high energy materials.

The HoF decreases from cubane to 1,4-BHC because cubanehas six four-membered rings, homocubane and 1,1-BHC havefour four-membered rings, 1,2-BHC has three of them whereas1,3-BHC and 1,4-BHC have only two. Five-membered rings areless strained compared to four-membered ones because theirC–C–C bond angles have a lower deviation from the unstrained

Table 7 Predicted propulsive properties of compounds with high-nitrogen substituents as liquid propellants

Compound

Compound + liquid oxygen Compound (30%) + RP1 (70%) + liquid oxygen Compound as monopropellant

O/Fratio CCTa (K)


MWprod

s b

Isp,vac

(s)rIsp,vac

(s)O/Fratio

CCTa

(K)


MWprod

s b

Isp,vac

(s)rIsp,vac

(s)CCTa

(K)


MWprod

s b

Isp,vac (s)rIsp,vac

(s)

DABHC 1.21 3953.2 12.81 367.7 467.6 2.16 3709.1 12.56 365.9 390.0 2348.4 13.91 276.1 408.0DTrizBHC 1.42 3894.7 12.59 363.7 445.8 2.23 3705.4 12.52 365.2 387.7 1919.1 13.04 247.6 339.9DTetzBHC 1.21 3910.9 12.60 363.4 472.9 2.16 3701.2 12.52 365.0 390.6 2184.0 13.28 263.1 413.1DADMBHC 1.36 3899.3 12.79 368.6 460.8 2.20 3703.6 12.57 366.1 389.7 2051.7 13.54 266.4 383.3DTrizDMBHC 1.51 3862.8 12.63 364.8 461.8 2.25 3700.4 12.53 365.3 390.7 1743.8 12.69 242.6 368.6DTetzDMBHC 1.32 3867.6 12.62 364.1 466.4 2.20 3867.6 12.62 365.3 390.5 1936.4 12.92 254.7 389.9RP1 2.58 3666.4 12.50 366.2 374.3 — — — — — — — — —N2H4 — — — — — — — — — — 644.1 6.85 234.1 235.3IPN — — — — — — — — — — 1350.9 8.96 251.6 261.7


Table 8 Predicted propulsive properties of compounds with high-nitrogen substituents as solid propellants

Entry Compound

Isp,vac (s)

X a Y b

15 DABHC 309.7 283.316 DTrizBHC 301.8 281.117 DTetzBHC 307.6 282.818 DADMBHC 306.3 282.319 DTrizDMBHC 299.6 280.520 DTetzDMBHC 304.8 281.9— HTPB 274.6 —— RDX — 290.2— HMX — 290.3


Table 9 Predicted detonation properties of compounds with high-nitrogen substituents


15 DABHC �86.92 14.68 6.1316 DTrizBHC �10.53 12.53 5.6117 DTetzBHC �79.10 12.45 5.7618 DADMBHC �04.96 7.82 4.9819 DTrizDMBHC �23.13 12.07 5.5020 DTetzDMBHC �94.59 14.35 5.99— RDX �21.62 35.1 8.93

a O.B. = oxygen balance. b D.P. = detonation pressure. c D.V. = detonationvelocity.

Table 10 Heats of formation and densities of hydrocarbon compoundscalculated using the B3LYP/6-311++G(d,p) level of theory


HOF(kcal mol�1)


21 Cubane 1.29 144.0 1.3822 Homocubane 1.34 122.3 1.0423 1,1-BHC 1.19 117.3 0.8924 1,2-BHC 1.52 98.3 0.7426 1,3-BHC 1.07 81.9 0.6226 1,4-BHC 1.37 80.7 0.6127 Cubane dimer 1.45 343.2 1.6728 1,3-BHC dimer 1.34 204.9 0.79— GAP 1.30 38.0 —— HTPB 0.93 �12.3 —

Paper NJC


tetrahedral value which is 109.51.38 Four-membered rings areconverted into five-membered ones by insertion of a methylene(–CH2–) group. In the case of bishomocubanes if these insertionsare remote from each other, then the resulting molecule is morestable and has a lower HoF.38 Thus, as evident from Table 10,1,1-BHC has the highest HoF whereas 1,4-BHC has the lowestamong the bishomocubanes studied in this work. Dimers ofcubane and 1,3-BHC have noticeably higher HoFs than the rest ofthe hydrocarbons on account of their larger molecular structuresas expected. The specific HoFs of the dimers do not varyconsiderably from those of their corresponding monomers. FromTable 11, it can be observed that the propulsive performance ofall the cage hydrocarbons is better than that of RP1 whencombined with liquid oxygen provided that they are originallyin the liquid phase. Cubane is a solid and hence cannot be usedin this manner. As stated earlier, solids can possibly be used inliquid propellant configuration by dissolving them in RP1.

Specific impulse values calculated for mixtures of RP1 andcage hydrocarbons reacting with liquid oxygen are also given inTable 11 and it can be seen that the rise in specific impulse overRP1 is marginal. Density specific impulses are similar to thoseof oxygen and nitrogen cage compounds.

It is worth noting that the BHC compounds have higher Isp

values than all the corresponding substituted BHCs. The mono-propellant operation mode was not considered for hydrocarboncompounds because, on decomposition, they tend to form asolid carbon residue as opposed to gas and hence cannot act aseffective monopropellants.

Propulsive properties of solid propellant formulations obtainedby using hydrocarbon cage compounds along with AP are given inTable 12. From the specific impulse values in the table it can beconcluded that other than cubane and its dimer, rest of thecompounds are inferior to the cage compounds carrying nitrogenand oxygen containing groups. Testing of the cubane dimer as asolid propellant binder ingredient if successfully synthesized insufficient quantities could prove useful. Table 12 also makes itclear that cage hydrocarbons, when used as solid propellantadditives by relying on HTPB as the binder, are inferior to RDXand HMX. Detonation properties of cage hydrocarbons are given

in Table 13. Since hydrocarbons do not contain any oxygen, theyare unable to show good detonation characteristics. Theirperformance as explosives is far below that of the other twosets of compounds.

Conclusions

With the intention of identifying new compounds as potentialpropellants this work focused on carrying out quantum

Table 11 Predicted propulsive properties of hydrocarbon compounds as liquid propellants

Compound

Compound + liquid oxygen Compound (30%) + RP1 (70%) + liquid oxygen

O/F ratio CCTa (K)


MWprod

s b

Isp,vac (s) rIsp,vac (s) O/F ratio CCTa (K)


MWprod

s b

Isp,vac rIsp,vac (s)

Cubane 1.74 4069.5 13.66 387.1 460.7 2.35 3766.9 12.77 371.5 393.6Homocubane 1.95 3946.5 13.14 377.0 435.7 2.39 3736.3 12.67 368.7 392.21,1-BHC 2.00 3904.8 13.08 375.5 434.5 2.41 3728.7 12.66 368.5 388.61,2-BHC 2.06 3886.4 12.94 373.0 463.2 2.42 3722.3 12.63 367.7 394.51,3-BHC 2.09 3865.2 12.83 370.6 413.2 2.43 3717.2 12.60 367.0 383.81,4-BHC 2.10 3865.4 12.82 370.5 446.4 2.44 3718.7 12.59 367.1 391.3Cubane dimer 1.70 4112.1 13.62 385.5 477.5 2.32 3768.0 12.76 370.7 395.91,3-BHC dimer 2.00 3922.3 12.83 369.8 444.1 2.40 3727.7 12.59 366.7 390.2RP1 2.58 3666.4 12.50 366.2 374.3 — — — — —N2H4 — — — — — — — — — —IPN — — — — — — — — — —


Table 12 Predicted propulsive properties of hydrocarbon compounds assolid propellants

Entry Compound

Isp,vac (s)

X a Y b

21 Cubane 306.2 282.422 Homocubane 293.9 279.223 1,1-BHC 291.2 278.524 1,2-BHC 287.9 277.725 1,3-BHC 285.1 277.026 1,4-BHC 284.9 276.927 Cubane dimer 306.1 282.328 1,3-BHC dimer 287.2 277.5— HTPB 274.6 —— RDX — 290.2— HMX — 290.3


Table 13 Predicted detonation properties of hydrocarbon compounds


21 Cubane �307.69 12.70 5.9922 Homocubane �311.86 9.07 4.9823 1,1-BHC �315.15 6.94 4.5624 1,2-BHC �315.15 10.33 5.0925 1,3-BHC �315.15 4.63 3.9026 1,4-BHC �315.15 7.57 4.5227 Cubane dimer �302.91 9.75 5.0228 1,3-BHC dimer �307.69 4.48 3.41— RDX �21.62 35.1 8.93

a O.B. = oxygen balance. b D.P. = detonation pressure. c D.V. = detonationvelocity.

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mechanics based calculations on a set of cage hydrocarboncompounds. The data thus obtained were used to calculatethermodynamic, propulsive, and detonation properties of thecompounds under a certain set of assumptions. The followingconclusions can be drawn from the analysis of results obtained.

1. The nitrogen and oxygen containing compounds havelower specific impulses than the widely used liquid propellantRP1 but have higher density specific impulses when combinedwith liquid oxygen as the oxidiser. This makes them attractive foruse in volume limited conditions. The cage hydrocarbon com-pounds are better than RP1 in terms of specific impulse as well.

2. If cage compounds are solids under normal conditions,one method of ascertaining how they perform as propellants isby considering their solution in RP1 combusting with liquidoxygen. The enhancement in density specific impulse is not ashigh as it is in the case of pure compounds and liquid oxygenbut is still competitive.

3. For any compound to be used as a propellant a high HOFdoes not necessarily result in a high specific impulse.

4. In the monopropellant mode, barring NTMBHC, DTrizBHC,and DTrizDMBHC, the calculated specific impulses of all thesubstituted cage compounds considered in this work are higherthan those of the widely used monopropellant IPN.

5. If HTPB in the HTPB-AP composite solid is replaced by anyof the compounds from the present work, there is considerableenhancement in specific impulse. Such an arrangement would workprovided the compounds can be polymerised to bind with AP.

6. If any of these cage compounds are used as additives whileretaining HTPB as binder in the composite solid propellant, it isunable to perform as well as RDX or HMX. Thus the potential ofcage compounds in solid propellant applications rests on theirability to polymerise and it demands further inquiry throughadditional research.

7. None of the cage compounds which are included in thepresent investigation have good explosive properties. All of them fallshort of matching the detonation pressure and velocity of RDX.

8. Data compiled in the present work provide significantinputs to synthetic chemists in the process of formulating newpropellants with the aim of improving the performance ofpropulsion systems. Synthesis aspects such as steps involvedin obtaining the final product and the costs involved are crucialfactors to be considered while choosing a final candidate.

Acknowledgements

The authors acknowledge the financial support provided by theIndian Space Research Organization (ISRO) and ArmamentResearch and Development Board (ARDB) as well as the Indus-trial Research and Consultancy Centre (IRCC) at the IndianInstitute of Technology Bombay.

Notes and references

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Volume 41 Number 3 7 February 2017 Pages 893–1392 NJC · 2017-10-19 · High energy density materials ... ideal HEDM should have high positive heat of formation (HoF), high density,