Thermal and Sensitiveness Determination of Cubanes: Towards Cubane-Based Fuels for Infrared Countermeasures

Article abstract of DOI:10.1002/chem.201901086

As infrared seeking technology evolves, threats are better able to distinguish defensive infrared (IR) flares from true targets. Spectrally matched flares, which generally employ carbon-based fuels, are better able to decoy some advanced missiles by more closely mimicking the IR emission of the target. Cubane is a high-energy carbon-based scaffold which may be suitable for use as a fuel in spectrally matched flares. The enthalpy of formation and strain energy of a series of cubanes was predicted in silico, and their thermal and impact stability examined. All were found to undergo highly exothermic decomposition in sealed cell differential scanning calorimetry, and two cubanes subsequently underwent quantitative sensitiveness testing. Despite their F of I values being in the secondary explosive range, cubane-1,4-dicarboxylic acid (F of I=70) and 4-carbamoylcubane-1-carboxylic acid (F of I=90) were identified as potentially useful fuels for pyrotechnic infrared countermeasure flare formulations.

Full text of DOI:10.1002/chem.201901086

DOI: 10.1002/chem.201901086
Full Paper
&
Energetic Materials
Thermal and Sensitiveness Determination of Cubanes:
Towards Cubane-Based Fuels for Infrared Countermeasures
Madeleine A. Dallaston,[a] Jason S. Brusnahan,*[b] Craig Wall,[b] and Craig M. Williams*[a]
Abstract: As infrared seeking technology evolves, threats
are better able to distinguish defensive infrared (IR) flares
from true targets. Spectrally matched flares, which generally
employ carbon-based fuels, are better able to decoy some
advanced missiles by more closely mimicking the IR emis-
sion of the target. Cubane is a high-energy carbon-based
scaffold which may be suitable for use as a fuel in spectrally
matched flares. The enthalpy of formation and strain energy
of a series of cubanes was predicted in silico, and their ther-
mal and impact stability examined. All were found to under-
go highly exothermic decomposition in sealed cell differen-
tial scanning calorimetry, and two cubanes subsequently un-
derwent quantitative sensitiveness testing. Despite their F of
I values being in the secondary explosive range, cubane-1,4-
dicarboxylic acid (F of I=70) and 4-carbamoylcubane-1-car-
boxylic acid (F of I=90) were identified as potentially useful
fuels for pyrotechnic infrared countermeasure flare formula-
tions.
Introduction
opment of spectrally matched flares and the theory behind
them have been extensively reviewed elsewhere.[2b,4]
High energy materials have a variety of uses in both military
and civilian life. They can be classified by their end use, which
include pyrotechnics, primary explosives, high (secondary) ex-
plosives, rocket fuels, fuel additives and propellants.[1] Each cat-
egory can be further broken down into more specific groups,
for example pyrotechnics include colourful fireworks, smoke
producing flares, and infrared countermeasure flares (IRCMs).
For military applications especially, constant innovation to-
wards new pyrotechnic fuels is imperative.[2] For IRCMs, the
technology in missile seekers is improving rapidly and there
are now missiles equipped with seekers which can distinguish
a true target (platform for example, aircraft) from the typical
grey-body IR-emitting flares most commonly used today.[3]
One way of overcoming these new seekers is to create flares
which more closely mimic the IR emission of the aircraft. These
new flares, called spectrally matched flares, require fuels which
are high in energy and which assist in the production of b-
band (3.6–4.8 mm) emitting combustion products such as
carbon dioxide, and limit the production of a-band (1.8–
2.5 mm) emitters such as water and carbon soot.[3b] The devel-
Since it was first synthesised by Eaton and Cole in 1964[5]
cubane (pentacyclo[4.2.0.02,5.03,8.04,7]octane, 1) has found uses
in many areas of research including medicinal chemistry, mate-
rials, and as an internal NMR standard.[6] Cubane itself, and de-
rivatives, are described as relatively stable despite the unusual
geometry and huge strain energy of the eight carbon atoms
configured in a cubic arrangement.[6g] Cubanes share the high
C:H ratio of aromatics, suggesting that using the cubane
carbon backbone would assist in minimising the production of
a-band emitters, whilst increasing the overall energy of the
formulation.
Previous high energy materials research into cubanes has
produced mixed results regarding their stability. For example,
1,4-diisocyanocubane (2) has been reported as being highly
thermally unstable, and detonates upon strong heating.[7] A va-
riety of iodinated cubanes (e.g., 3 and 4) have been described
as undergoing rapid thermo-decomposition and being less
stable than their non-iodinated counterparts.[8] Octanitro- (5)
and heptanitrocubane (6) are reported as insensitive to
hammer taps,[9] while 4-((nitrooxy)methyl)cubane-1-carboxylic
acid (7), dimethyl cubane-1,4-dicarboxylate (10), and cubane-
1,4-dicarboxylic acid (11) have been reported to exhibit sensi-
tiveness to impact.[10] Azidocubanes are extremely sensitive to
impact, with Eaton warning that the oils of cubyl azide (8) and
methylcubyl azide (9) are “exceedingly shock sensitive and can
(and have) detonated on touch”.[11]
[a] M. A. Dallaston, Prof. C. M. Williams
School of Chemistry and Molecular Biosciences
University of Queensland, Brisbane, 4072 (Australia)
E-mail: c.williams3@uq.edu.au
[b] Dr. J. S. Brusnahan, C. Wall
While cubanes (e.g., Scheme 1) have many potential applica-
tions as high-energy materials,[7,9,12] they have not been ex-
plored in the context of new fuels for spectrally matched
flares. Considering the characteristics of flares formulated with
oxygenated aromatic fuels[13] (i.e. favourable emission spectrum
but low spectral efficiency), and the higher in energy but un-
Defence Science and Technology Group, Edinburgh
South Australia 5111 (Australia)
E-mail: jasonstewart.brusnahan@dst.defence.gov.au
Supporting information containing full synthetic, computational, and sensi-
tiveness testing details as well as the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201901086.
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Table 1. Structure and substituents of cubanes 10–17.
Cubane
R1
R2
dimethyl cubane-1,4-dicarboxylate (diester, 10)
cubane-1,4-dicarboxylic acid (diacid, 11)
COOMe COOMe
COOH COOH
4-methoxycarbonylcubane-1-carboxylic acid (acid ester, COOH COOMe
12)
methyl cubane-1-carboxylate (ester, 13)
cubane-1-carboxylic acid (acid, 14)
cubane-1-carboxamide (amide, 15)
H
H
H
COOMe
COOH
CONH2
methyl 4-carbamoylcubane-1-carboxylate (amide ester, CONH2 COOMe
16)
4-carbamoylcubane-1-carboxylic acid (amide acid, 17)
COOH CONH2
Enthalpy of formation
Scheme 1. Cubane (1) and high energy derivatives (2–9).
Considering fuels with large, positive enthalpies of formation
(Df Hsoolid) assist in yielding sufficient flare spectral efficiency,[14]
high Df Hosolid values are a desirable characteristic. However, we
were unable to achieve complete combustion of diester 10 in
bomb calorimetry, and thus were unable to experimentally de-
termine the Df Hosolid value. Spiking with benzoic acid using a
stacked pellet approach, as described previously for this com-
pound,[29,39] did not reliably produce complete combustion,
and therefore computational methods were explored to pre-
dict the Df Hosolid values for 10–17. Df Hsoolid can be calculated by
Equation (1) in which Df Hgoas is the gas phase enthalpy of for-
mation and DsubHꢀ is the enthalpy of sublimation.[17]
stable carbonitriles[14] (i.e. good spectral efficiency but unfav-
ourable emission spectrum), we envisioned cubanes as having
the potential to satisfy both criteria. That is, cubanes share the
low carbon to hydrogen ratio of aromatic fuels, which should
assist in producing a favourable emission spectrum. Moreover,
they are high in energy, and thus should result in an increase
in the enthalpy of combustion which in turn should increase
the spectral efficiency.[2,4] Furthermore, in the view that cu-
banes have demonstrated a spectrum of stability through
pharmaceutical development, this situation suggested that
identifying optimal characteristics that would meet modern
day criteria for the development of new high energy fuels
could be achieved.
Df Hosolid ¼ Df Hogas ꢁ DsubHo
ð1Þ
Several predictive methods for DsubHꢀ are available including
group additivity,[18] quantitative structure-property relationships
(QSPR),[19] atom–atom potential,[20] and electrostatic surface po-
tential (ESP) methods.[21] All of these methods rely on experi-
mental DsubHꢀ values for the construction of a training set, and
the accuracy of each application is contingent upon both the
selection of the training library and the reliability of the experi-
mental values therein. Therefore, it is often necessary to utilise
a training set based on the targets of interest. That said, using
a training set to predict DsubHꢀ of a compound with functional
groups or characteristics which are not well represented in the
training set will often lead to large errors.[21a]
Reported herein are in silico prediction methods and subse-
quent validation using a series of simplified and synthetically
practicable cubanes. The thermal decomposition characteristics
of the cubanes were examined using thermogravimetric analy-
sis (TGA) and sealed cell differential scanning calorimetry
(scDSC), and the results compared to thresholds used within
the pharmaceutical industry to flag compounds and reagents
which require extra safety precautions.
A modification of the ESP method was utilised to predict
DsubHꢀ values.[21d,22] The ESP method is widely used in high
energy material predictions,[23] and is especially attractive as it
has been suggested that electrostatic surface potential param-
eters may also be related to other physical properties such as
density and impact sensitiveness.[24] The modification, pro-
posed by Suntsova and Dorofeeva, introduces an additional
parameter (P) to improve predictions.[21a] The relationship be-
tween DsubHꢀ and ESP parameters is shown in Equation (2)
where SA is the surface area, s2tot is a measure of variability on
the surface potential, n is the degree of balance between the
Results and Discussion
A series of known mono and 1,4-disubstituted cubanes (10–
17; Table 1) were chosen to refine our knowledge of cubane
sensitiveness, most of which had no reported issues, except for
that of cubane-1,4-dicarboxylic acid (11) and dimethyl cubane-
1,4-dicarboxylate (10).[10] This selection was predicated on the
ready availability of dimethyl cubane-1,4-dicarboxylate (10),
which can be prepared on kilogram scale, as developed by Tsa-
naktsidis et al.,[15] and has since become commercially avail-
able.
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positive and negative areas on the surface, and P is a measure
of local polarity. Using Gaussian09[25] and Gaussian16[26] soft-
ware, all structures were first optimised at the B3LYP/ccpVTZ
level of theory, and then frequency calculations were per-
formed using the same level of theory. ESP parameters were
calculated on the 0.001 electronsbohrꢁ3 contour of the elec-
tron density,[27] and were obtained using the Multiwfn comput-
er program.[28] The coefficients a, b, c, and d were determined
from a least squares fitting of a training set with known DsubHꢀ
and their ESP parameters (Equation (2), Table 2).
tanedioic acid has revealed inconsistent values with other ex-
perimental methods, and so it is possible that the reported
value is unreliable.[33] The training set herein predicted a value
of 129.4 kJmolꢁ1, which was a difference of ꢁ30.8 kJmolꢁ1
from the literature value, and thus was outside the uncertainty
value of ꢂ21 kJmolꢁ1. This demonstrated that unreliable ex-
perimental values in the test set can reduce confidence in pre-
dictions.
In the test set, listed in Table 3, both malonamide (21) and
cubane diacid 11 are predicted outside the uncertainty range
at ꢁ23.0 and ꢁ22.7 kJmolꢁ1 difference, respectively. Consider-
ing other amides in the test set are well predicted, the malona-
mide discrepancy was unexpected, although the reported ex-
perimental value was determined over a narrow temperature
range (397.4–403.2 K), and then extrapolated to 298.15 K. It is
therefore possible that the experimental value for malonamide
is likely erroneous. The prediction for diacid 11 presented
more of an issue, however other highly strained compounds
[that is, dimethyl 1,4-cubane dicarboxylate (10), dimethyl 2,6-
cuneane dicarboxylate (19), 1,1-cyclobutanedicarboxylic acid
(27), and homocubane-4-carboxylic acid (29)] are predicted
within uncertainty, which provided sufficient confidence in the
predicted value for diacid 11. Unfortunately, these findings
suggested that for cubanes there is not yet enough experi-
mental data available to validate modern training or test set li-
braries, however the ESP method performed satisfactorily for
the majority of cases.
pffiffiffiffiffiffiffiffiffi
2
DsubHo ¼ aðSAÞ þ b st2otn þ c þ dP
ð2Þ
Table 2. Coefficients for Equation (2).
a
b
c
d
0.0002448164
ꢁ0.5052741293
4.3284151896
1.0895482769
The paucity of experimental DsubHꢀ data for cubanes, and
similarly highly strained caged hydrocarbons, unfortunately
placed limitations on training set selection. However, by com-
bining both strained systems and selecting carboxylic acid and
related amide functional groups, a training set of twelve mem-
bers was developed, which gave good prediction levels for the
test set (Table 3). The uncertainty of the predicted DsubHꢀ
values was defined as two times the root-mean-square devia-
tion of the training set, ꢂ21 kJmolꢁ1 (see Supporting Informa-
tion), which was comparable to the uncertainty of previously
reported training sets that are much larger.[21a]
To complete the requirements for Equation (1) values of
Df Hogas were determined for 10–17 using the isodesmic reac-
tion protocol.[34] Recently the CꢁC bonds of the cube have
been shown to be bent, with electron density sitting outside
CꢁC vectors which make up the edges of the cube.[35] The exo-
cyclic bonds are also shorter than equivalent bonds in un-
strained molecules.[36] To maintain this unusual bonding, and
the resulting strain energy, the cubane core was preserved in
the designed reactions, as outlined in Scheme 2 for ester 13.
The Df Hogas of cubane (1), isobutane (30), and methyl pivalate
(31) are known, facilitating the prediction of Df Hogas of ester 13
through Equations (3) and (4). From Equation (3) the enthalpy
of the reaction (DrHꢀ) was calculated from the sum of the elec-
tronic and thermal enthalpies (Sðe0 þ HcorrÞ) obtained from fre-
quency calculations. Then, by rearranging Equation (4), the
Df Hogas of the unknown compound can be predicted.
Table 3. Literature and calculated DsubHo values for the test set. All values
are in kJmolꢁ1
.
Compound
Literature Calculated
D
dimethyl 1,4-cubane dicarboxylate
cubane-1,4-dicarboxylic acid
dimethyl 2,6-cuneane dicarboxylate
pentacyclo[5.4.0.02,6.03,10.05,9]undecane (20) 55.85[31] 60.1
(10) 117.19[29] 121.8
(11) 126.50[30] 103.8
(19) 106.80[29] 119.7
4.6
ꢁ22.7
12.9
4.3
malonamide
propanamide
(21) 126.40[32] 103.4
(22) 75.00[32] 81.4
(23) 108.00[32] 91.3
(24) 100.20[32] 91.5
(25) 86.19[32] 83.2
(26) 111.40[32] 116.1
(27) 111.29[32] 95.6
(28) 111.71[32] 100.9
(29) 82.01[32] 81.7
Root Mean Square
Deviation
ꢁ23.0
6.4
ꢁ16.7
ꢁ8.7
ꢁ3.0
4.7
ꢁ15.7
ꢁ10.8
ꢁ0.3
12.5
adamantane carboxamide
piperidine-1-carboxamide
2-methyl propanamide
malonic acid
1,1-cyclobutanedicarboxylic acid
dimethyl malonate
DrHꢀ ð298KÞ ¼ Sðe0 þ HcorrÞproducts ꢁ Sðe0 þ HcorrÞreactants
DrHo ¼ SDf Hporoducts ꢁ SDf Hporoducts
ð3Þ
ð4Þ
homocubane-4-carboxylic acid
The predicted Df Hogas, DsubHꢀ, and resulting Df Hosolid values are
presented in Table 4, and a comparison of predictions to litera-
ture values for diester 10 and diacid 11 are given Table 5. The
To validate the accuracy of the experimentally determined
DsubHꢀ values presented in Table 3, as well as demonstrate the
importance of accurate and reliable experimental training/test
sets,
3-oxopentanedioic
acid
(literature
DsubHꢀ =
160.21 kJmolꢁ1[32]) was chosen as a test substrate. The method
used in the literature measurement of the DsubHꢀ of 3-oxopen-
Scheme 2. Designed reaction for the prediction of the Df Hogas of 13.
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et al., but 46 kJmolꢁ1 higher than the value measured by Roux
et al. Overall, the scarcity and variety of experimental values
for enthalpy of formation for cubanes creates a considerable
challenge for developing predictions in this area, which in-
spired a search for alternative determinants.
Table 4. Predicted enthalpies of formation and strain energies for cu-
banes 10–17. All values are in kJmolꢁ1
.
Compound
Df Hogas
DsubHo
Df Hosolid
Es
diester
diacid
acid ester
ester
acid
amide
amide ester
amide acid
10
11
12
13
14
15
16
17
ꢁ141.0
ꢁ206.0
ꢁ118.3
+269.5
+222.0
+411.6
+69.3
121.8
103.8
134.3
99.6
95.3
86.1
ꢁ262.8
ꢁ309.8
ꢁ252.7
169.9
126.7
325.5
ꢁ59.4
ꢁ103.6
645.3
602.1
680.1
699.1
663.0
661.7
676.8
641.0
Strain energy
Cubane (1) is by definition
a highly strained molecule
(681.18ꢂ9.8 kJmolꢁ1),[29] and it is this strain which is the major
contributor to its high heat of formation. Therefore the strain
energy (ES) of cubanes 10–17 were estimated by the method
of Fan et al. using the reaction described by Equation (5).[37,41]
128.7
125.8
+22.2
literature Df Hogas values for 10 and 11 are both derived from ex-
perimental Df Hosolid and DsubHꢀ values,[29,30] however, various
Df Hosolid values are reported, and therefore different Df Hogas
values can be derived. Using the experimental DsubHꢀ values
from Roux et al. (117.2 kJmolꢁ1 for diester 10[29] and
126.5 kJmolꢁ1 for diacid 11[30]) and adding them to the various
Df Hosolid values available in the literature gave the derived
Df Hogas values seen in Table 5.
C8H8ꢁmXm þ 12C2H6 ! ð8 ꢁ mÞðCH3Þ3CH þ mðCH3Þ3CX
ð5Þ
Taking the total energy and the zero-point vibrational
energy (ZPE) from the frequency calculations, the change in
energy (DEn) between the reactants and the products was cal-
culated using Equation (6). Due to the strain free nature of the
other components in Equation (5), DEn is taken as the ES of the
cubane target.[37b,41]
While the values are not particularly close, they demonstrate
that no consensus on enthalpies of formation could be deter-
mined for 10 and 11 making it difficult to validate the predic-
tions. Isodesmic reactions have been used to predict gas
phase enthalpies of formation in many studies and have been
found to be one of the most accurate methods.[37] Elioff et al.
state that the goal for any method of Df Hogas prediction should
be errors no larger than ꢂ2 kcalmolꢁ1 (ꢂ8.37 kJmolꢁ1),[38] and
the predictions for diester 10 and diacid 11 are clear outliers.
When comparing the predicted Df Hosolid values of 10 and 11
there was a spread of values available as seen in Table 5. Three
literature values are available for diester 10; Roux et al. give
DEn ¼ SEproducts ꢁ SEreactants þ ZPE
ð6Þ
In order to validate this ES predictive method for cubane de-
rivatives the ES for cubane (1) was evaluated using the reaction
C8H8 +12C2H6!8(CH3)3CH and Equation (6) (Table 6). The pre-
Table 6. Comparison of predicted and literature strain energies. All
values are in kJmolꢁ1
.
Compound
Es
D from cubane prediction
ꢁ232.62 kJmolꢁ1 [29]
,
Avdonin et al. give ꢁ231.2 kJmolꢁ1,[39] and
this work
685.1
cubane 1
Fan et al.[41] 707.64
Kirklin et al. give ꢁ218.99 kJmolꢁ1.[40] Our prediction
(ꢁ262.8 kJmolꢁ1) was closer to the first two values, however
this also highlighted little literature consensus. Roux et al. were
unable to achieve complete combustion without using an aux-
iliary substance, whereas Avdonin et al. accomplished this goal,
and we were unable to achieve complete combustion even
with an auxiliary substance. Although the value determined by
Roux et al. agreed with that determined by Avdonin et al. they
used the same procedure as Kirklin et al. to achieve complete
combustion. For diacid 11 there are two different values;
ꢁ355.9 kJmolꢁ1 from Roux et al.[30] and ꢁ318.5 kJmolꢁ1 from
Avdonin et al.[39] The prediction determined herein
(ꢁ309.8 kJmolꢁ1) was closer to that provided by Avdonin
this work
692.0
6.9
2.01
ꢁ10.5
1,4-dinitrocubane 32
azidocubane 7
Fan et al.[41] 709.65
this work
Fan et al.[41] 696.22 ꢁ11.42
674.6
dicted value was determined to be 685.1 kJmolꢁ1, which was
close to the value calculated by Roux et al. of 681.0 kJmolꢁ1 [29]
although it is 22.5 kJmolꢁ1 lower than the value predicted by
Fan et al. (707.64 kJmolꢁ1).[41] Additionally, the ES calculated for
1,4-dinitrocubane (32, 692.0 kJmolꢁ1) is 17.6 kJmolꢁ1 lower
than the value predicted by Fan et al. (709.65 kJmolꢁ1).[41] Im-
portantly, both predictions herein and those by Fan et al. indi-
,
Table 5. Comparison of predicted values of 10 and 11 to values derived from literature. All values are in kJmolꢁ1
.
Compound
Literature Df Hsoolid
Literature DsubHo
117.2[29]
Derived Df Hogas
Predicted Df Hogas
D (PredictedꢁDerived)
diester 10
ꢁ232.62ꢂ5.84[29]
ꢁ231.2ꢂ2.5[39]
ꢁ218.99ꢂ2.12[40]
ꢁ355.9ꢂ11.7[30]
ꢁ318.5ꢂ12.6[39]
ꢁ115.4ꢂ7.0[29]
ꢁ114.0
ꢁ25.6
ꢁ27.0
ꢁ39.2
23.4
ꢁ141.0
ꢁ101.8
diacid 11
ꢁ229.4ꢂ14.8[30]
ꢁ192.0
126.5[30]
ꢁ206.0
ꢁ14.0
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cate an increase in ES between cubane (1) and 1,4-dinitrocu-
bane (32). Our prediction for azidocubane (7, 674.4 kJmolꢁ1) is
also lower than the literature value (696.22 kJmolꢁ1),[41] but the
simultaneous thermogravimetric analysis/differential scanning
calorimetry (TGA/DSC). Pleasingly, all cubanes were observed
to be stable to above 1008C except ester 13 which melted at
51.488C.[42a] Cubanes diester 10 and ester 13 effectively lost
100% mass in one step while all others displayed multi-step
mass loss (Figure 1). The simultaneous DSC data recorded
during the TGA analysis showed that both 10 and 13
(Figure 2) only underwent endothermic transitions while the
main mass loss events for 11, 12, and 14–17 were accompa-
nied by exothermic decomposition. Amide acid 17 displayed a
particularly steep TGA slope, and the accompanying heat flow
curve showed a sharp and narrow peak, suggesting that this
derivative underwent particularly rapid thermal decom-
position.[8a]
differences between 1 and 7 from this work (ꢁ10.5 kJmolꢁ1
)
and from Fan et al. (ꢁ11.42 kJmolꢁ1) are comparable. The pre-
dictions herein are consistently around 20 kJmolꢁ1 lower than
those of Fan et al., however the change in ES between unsub-
stituted cubane (1) and the substituted derivatives azido 7 and
dinitro 32 are comparable. As the prediction for cubane itself
was very close to the value calculated by Roux et al.,[29] and
the change in strain energy between cubane and the deriva-
tives was comparable to those reported by Fan et al., this
method was validated with confidence.
Synthesis
In brief the synthesis of the selected cubanes were achieved as
follows: the half hydrolysis of diester 10 (e.g., acid ester 12)
was performed using one equivalent of hydroxide in tetrahy-
drofuran (THF) and methanol, whereas the fully hydrolysed
diacid (11) was obtained with excess base in methanol. Mono-
substituted cubanes can be accessed from acid ester 12, by
Barton decarboxylation, to give methyl cubane-1-carboxylate
(13).[42] Hydrolysis of ester 13 gave carboxylic acid 14 in excel-
lent yield. Subsequent conversion to the acid chloride and re-
action with aqueous ammonia in situ afforded cubane-1-car-
boxamide (15) in 63% yield over four steps. Treatment of acid
ester 12 with ethyl chloroformate followed by ammonia in THF
gave amide ester 16, while reacting the acid ester 12 with
aqueous ammonia provided amide acid 17 (Scheme 3).
Figure 1. Thermogravimetric analysis (TGA) mass loss curves for cubanes
10–17.
Thermal characterisation
Impact sensitiveness testing
The melting and decomposition temperatures of pyrotechnic
ingredients play a key role in the pyrotechnic reaction.[43] Thus
the thermal properties of the cubanes were investigated using
As mentioned above, the ESP method has been applied to pre-
dicting impact sensitiveness of high energy compounds. The
prediction of impact sensitiveness is an especially attractive
goal, both for safety reasons and to direct synthetic resources
away from targets which are likely to be unsuitable for their in-
tended application. The ESP method has seen some success in
predicting impact sensitiveness, however much like predicting
DsubHꢀ each application can only be used for compounds
which are well represented in the training set.[24a,44] Unfortu-
nately, a suitable training set could not be constructed from
the literature for cubanes 10–17, and thus empirical testing
was investigated.
In high energy materials research initial impact sensitiveness
testing is carried out by striking a small amount of material
with a hammer.[45] When this preliminary test was carried out
on 10–17, diacid 11 was the only substrate to result in a “go-
event”, which mirrored results reported by Nesterenko et al.
who disclosed that cubane-1,4-dicarboxylic acid (11) reacted to
18 out of 25 hammer strikes.[10] The authors also reported that
diester 10 reacted to one hammer strike out of 25, however
we were unable to elicit any go-events for this compound.
This was perhaps of no surprise as sensitiveness to impact and
other stimuli (friction, electrostatic discharge [ESD]) is not only
Scheme 3. Synthetic Scheme for compounds 11–17. i) NaOH, MeOH;
ii) NaOH, MeOH/THF; iii) (COCl)2, DMF, CH2Cl2; iv) 18, DMAP, CHCl3, hn, D;
v) NH3, CH2Cl2, vi) TEA, ethyl chloroformate, THF; vii) NH3, THF; viii) NH3,
MeOH. TEA=triethylamine; DMAP=4-N,N-dimethylaminopyridine;
DMF=N,N-dimethyformamide].
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uals performing the test varying.[45] The amount of material re-
quired for standardised quantitative testing is much larger
than that for a hammer test, and therefore diacid 11 and
amide acid 17 were selected to undergo further sensitiveness
testing (Table 7) due to a demonstrated positive hammer test
Table 7. Sensitiveness data for 11, 17, secondary explosive RDX, and pri-
mary explosive TATP.
Diacid 11
F of I[a] 70
Amide acid 17 RDX (47)
TATP (48)
90
80
<10
friction >360 N
ESD
>360 N
120 N
0.1 N
ignition at 4.5 J ignition at 4.5 J ignition at 4.5 J ignition at
but not at
0.45 J
but not at
0.45 J
but not at
0.45 J
<0.045 J
[a] Figure of Insensitiveness (F of I) standardised to RDX (F of I=80); a
smaller number is more sensitive to impact.
and a steep TGA curve, respectively. Impact sensitiveness for
both 11 and 17 were found to be in the range of secondary
explosives, although they are insensitive to friction, and only
slightly sensitive to ESD. While these data confirmed that care
should be taken when handling cubanes 11 and 17, it was not
sufficient to classify either as explosive. Sensitiveness data for
secondary explosive RDX and primary explosive TATP are
shown for comparison.
Calculations made little suggestion that 11 and 17 should
be more or less impact sensitive than the other cubanes.
Diacid 11 registered the lowest enthalpy of formation and the
lowest strain energy yet displayed highly exothermic decom-
position under DSC analysis with a relatively sharp curve.
Whereas amide acid 17 displayed the second lowest strain
energy and fourth lowest enthalpy of formation but produced
an almost vertical TGA curve and very sharp DSC curve, indi-
cating rapid thermal decomposition.
Inspired by these results, an investigation to determine a
correlation between energy of decomposition, onset tempera-
ture, and impact sensitiveness was undertaken. In the pharma-
ceutical industry Yoshida correlations are utilised to predict po-
tentially problematic chemical reagents in scaled up reactions,
and thus were deemed appropriate in this instance. Yoshida
correlations are simple mathematical equations which relate
the DSC measured onset temperature (TDSC), and energy of the
exotherm (QDSC), to the potential for a molecule to be impact
sensitive and/or to propagate an explosion.[48] To quantitatively
measure QDSC mass loss must be prevented during measure-
ment, and therefore high-pressure rated capsules were used to
carry out sealed cell DSC (scDSC), as opposed to the open
pans used for TGA/DSC measurements. Using the lower energy
threshold recommended by Sperry et al.,[48b] compounds falling
above the green line are potentially impact sensitive and those
above the red line could potentially propagate an explosion
(Table 8 and Figure 3).
Figure 2. Simultaneous TGA/DSC plots of A) diester 10; B) ester 13; C) amide
acid 17.
affected by the molecular structure, but by particle size, crys-
tallinity, and environmental factors (e.g., humidity, and temper-
ature).[1,43,46] Despite this report, cubane-1,4-dicarboxylic acid
(11) has been used in many laboratories worldwide, and there
have been no reported incidents of impact initiated accidents.
While the hammer test is routinely used as an evaluation
method for high energy materials it has been shown to be
highly subjective, with the amount of force applied by individ-
Cubanes 10–17 were all flagged as potentially impact sensi-
tive and explosive according to the Yoshida analysis. Despite
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training set and it should only be used for predictions of struc-
turally similar compounds.
Table 8. Sealed cell DSC (scDSC) results for cubanes 10–17, and explo-
sives TATP (48) and TNT (49).
The large strain energy of the cube scaffold resulted in large
exothermic decomposition, despite some substituted cubanes
having negative Df Hosolid values. Such energetic decomposition
may assist in achieving sufficient spectral efficiency to decoy
missiles away from platforms. It should be noted that the fuel
is only one part of a pyrotechnic formulation, and there are
many factors which contribute to spectral efficiency, combus-
tion behaviour, and overall performance.[2b,3a,4a,14,49]
Compound
QDSC [Jgꢁ1
]
TDSC [8C]
diester[a]
diacid
acid ester
ester[a]
acid
amide
amide ester
amide acid
TATP[47]
TNT[47]
10
11
12
13
14
15
16
17
48
49
ꢁ3005
ꢁ2689
ꢁ2298
ꢁ4078
ꢁ4081
ꢁ3839
ꢁ2712
ꢁ3247
ꢁ3173
ꢁ3099
179.5
217.4
176.8
124.3
135.3
126.6
173.0
162.5
167.5
281.4
Although quantitative sensitiveness testing has indicated
that diacid 11 and amide acid 17 are impact sensitive, it is not
as high as predicted by Yoshida correlations. The investigated
cubanes were found to undergo highly exothermic decomposi-
tion in scDSC measurements yet exhibit reduced sensitiveness
to various ignition stimuli. Hammer test results herein when
compared to previous reports highlight the subjectiveness of
hammer testing, for example, diacid 11 reacted to hammer
strikes both in our hands and as reported in the literature,
whereas we were unable to illicit any reaction from diester 10
as was reported by Nesterenko et al.[10]
[a] Cubanes 10 and 13 did not display exothermic transitions in open
pan TGA/DSC due to complete mass loss, but did decompose exothermi-
cally in sealed crucibles.
Despite their F of I values being in the secondary explosive
range, diacid 11 and amide acid 17 are still potentially useful
fuels for pyrotechnic IRCM formulations. RDX, which is slightly
more impact sensitive than cubanes 11 and 17, is used as a
fuel in similar formulations.
In summary, utilisation of the cubane framework presents a
promising approach to high energy molecule design, which
can offer aspects of stability for use as high energy pyrotechnic
fuels.
Acknowledgements
Figure 3. Yoshida Correlation results for cubanes 10–17, and explosives TATP
and TNT.[47] Data for TATP and TNT are literature values while all other values
were collected as part of this work.
The authors thank the University of Queensland (UQ) and the
Defence Science and Technology (DST) Group for financial sup-
port. The Australian Research Council are gratefully acknowl-
edged for
a Future Fellowship award (grant number
FT110100851) to C.M.W. M.A.D gratefully acknowledges the
School of Chemistry and Molecular Biosciences and DST Group
for Honours scholarships, and the Australian Government for a
Research Training Program Scholarship.
these predictions 10–17 have not displayed untoward han-
dling issues in our laboratories, barring the hammer test result
for diacid 11. It should be kept in mind that the Yoshida corre-
lation thresholds are designed for pilot plant and large-scale
reactions, far beyond what is found in a normal research labo-
ratory.
Conflict of interest
M.A.D. and C.M.W. were recipients of scholarship and research
funding respectively from Defence Science and Technology
Group.
Conclusion
The cubane motif was investigated to determine whether it
displayed promising scaffold attributes for the development of
high energy pyrotechnic fuels. A model for predicting DsubHꢀ
values for cubanes substituted with a variety of functional
groups (e.g., methyl ester, carboxylic acids, and amides) has
been developed. An error of ꢂ21 kJmolꢁ1 for DsubHꢀ predic-
tions is acceptable and comparable to other applications of
the ESP method. However, application of the model developed
herein is limited by the compound classes found within the
Keywords: cubane
·
energetic materials
·
infrared
·
pyrotechnics · Yoshida
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Manuscript received: March 8, 2019
Revised manuscript received: April 17, 2019
Version of record online: && &&, 0000
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FULL PAPER
&
Energetic Materials
M. A. Dallaston, J. S. Brusnahan,* C. Wall,
C. M. Williams*
&& – &&
Feeling the strain: A series of cubanes
were tested for thermal, impact, and
electrostatic discharge sensitiveness.
The results are discussed in the context
of pyrotechnic fuels for spectrally
matched flares.
Thermal and Sensitiveness
Determination of Cubanes: Towards
Cubane-Based Fuels for Infrared
Countermeasures
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