Formamidinium Nitroformate: An Insensitive RDX Alternative

Article abstract of DOI:10.1021/jacs.8b10200

Five nitroformate (trinitromethanide) salts featuring nitrogen-containing cations were prepared. The salts were characterized by multinuclear NMR, IR, and Raman spectroscopy, single-crystal X-ray analysis, differential thermal analysis, and friction and impact sensitivity testing. These experimental data are supplemented with thermochemical calculations using the Gaussian-4 composite method, and the performance of these energetic materials was calculated based on the Chapman-Jouguet thermodynamic detonation theory. Out of the five compounds studied by us, the formamidinium salt, [CH(NH₂)₂]⁺[C(NO₂)₃]^-, is most interesting. Its performance matches that of RDX (research department explosive, cyclotrimethylenetrinitramine), while it is much less sensitive to impact and friction and, therefore, might be an excellent, less sensitive replacement for RDX.

Full text of DOI:10.1021/jacs.8b10200

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Article
Formamidinium Nitroformate: An Insensitive RDX Alternative
Amanda F. Baxter, Igor Martin, Karl O Christe, and Ralf Haiges
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10200 • Publication Date (Web): 15 Oct 2018
Downloaded from http://pubs.acs.org on October 16, 2018
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Journal of the American Chemical Society
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Formamidinium Nitroformate: An Insensitive RDX Alternative
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Amanda F. Baxter, Igor Martin, Karl O. Christe and Ralf Haiges*
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Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los
Angeles, California 90089-1661, United States.
Energetic materials, high energy density materials, insensitive high explosive, nitroformate, trinitromethanide
ABSTRACT: Five nitroformate (trinitromethanide) salts featuring nitrogen-containing cations were prepared. The salts
were characterized by multinuclear NMR, IR and Raman spectroscopy, single-crystal X-ray analysis, as well as
differential thermal analysis, and friction and impact sensitivity testing. These experimental data are supplemented
with thermochemical calculations using the Gaussian-4 composite method, and the performance of these energetic
materials was calculated based on the Chapman-Jouguet thermodynamic detonation theory. Out of the five
+
-
compounds studied by us, the formamidinium salt, [CH(NH ) ] [C(NO ) ] , is most interesting. Its performance matches
2
2
2 3
that of RDX, while it is much less sensitive to impact and friction and, therefore, might be an excellent, less sensitive
replacement for RDX.
well-known for its insensitivity and used for the initiation
of nuclear weapons. However, its performance is inferior
to that of RDX.
INTRODUCTION
The design of novel high energy density materials
HEDMs) is not trivial because the potential candidates
(
Nitroform, (HC(NO ) ), is a very acidic (pK = 0.17) and
2 3
a
must combine excellent performance with low
sensitivity, two properties which are usually mutually
exclusive.
over-oxidized methane derivative. Its bright yellow salts
have long been of interest for HEDM applications owing
to their potential for high nitrogen and oxygen contents.
Although nitroform was first synthesized over a century
Single-crystal structure determinations are crucial for
a thorough analysis of HEDMs including density
determinations and rationalizing sensitivity trends. For
3
ago, only the structures of a relatively small number of
1
+ 4
its salts containing the counterions [Na(18-crown-6)] ,
example, it is known that crystal packing strongly affects
the sensitivity of HEDMs. Ordered planar layer-stacking
can absorb the kinetic energy of mechanical stimuli by
converting it into layer sliding. This results in reduced
+ 5-8
+ 9
+ 9
2+ 10
2+ 10
K ,
[
Rb ,
6
Cs , ,
[Cu(en) ] ,
[Cd(en) ] ,
3
3
2+ 11-12
+
+13
Ni(NH ) ] ,
, [Ph PMe] and [Ph PCH I] has been
3
3 3 2
determined. The crystal structure of the energetic
hydrazinium nitroformate (HNF) was first obtained
2
sensitivity.
5,
almost half a century ago and has since been improved.
1
4-15
RDX,
(research
department
explosive,
Some other potentially energetic trinitromethanide
cyclotrimethylene trinitramine), is currently the most
important explosive for military applications. The
outstanding properties of RDX as an explosive are based
on its high chemical stability and high explosive power
that considerably surpasses that of aromatic nitro
compounds, such as TNT, (trinitrotoluene). The use of
common high-performance explosives, such as RDX or
HMX (cyclotetramethylene tetranitramine), has become
less attractive due to numerous accidents involving
accidental initiation by impact or shock aboard ships,
aircrafts and ammunition trains. As a result, one of the
biggest challenges in worldwide energetic materials
research is to find insensitive high-performing explosives
salts containing various ammonium, pyridinium,
pyrazolium, imidazolium, triazolium, guanidinium
cations have also been structurally investigated.
addition, four energetic metal organic framework
compounds containing nitroformate anions have been
structurally characterized.
5
, 16-23
In
2
4-26
(IHE). TATB, (2,4,6-triamine-1,3,5-trinitrobenzene), is
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Chart 1. The nitroformate salts synthesized and
characterized in this work.
Bruker AMX500 or a Varian NMRS-500 spectrometer.
Chemical shifts are given relative to neat
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1
13
14
tetramethylsilane ( H, C) or neat CH NO ( N). Raman
3
2
In this report, we present the syntheses and
characterization of five nitroformate salts with nitrogen-
rich cations (Chart 1) of potential interest for energetic
materials applications. Salts 1-5 were fully characterized
by multinuclear NMR, IR and Raman spectroscopy along
with single-crystal structure determinations. We have
carried out thermochemical calculations using the
Gaussian-4 composite method along with the prediction
of energetic properties, including detonation pressures
and velocities, to evaluate the performance of these
compounds for HEDM applications. We have also
determined the decomposition temperatures and
sensitivities to impact and friction. Combining these
data, we discuss the potential efficacy of these materials
for HEDM applications.
spectra were recorded at ambient temperature in Pyrex
−1
glass tubes in the range 4000−80 cm on a Bruker Optics
Vertex 70/RAM II FT-RA spectrometer using a Nd:YAG
laser at 1064 nm. Infrared spectra were recorded in the
−1
range 4000−400 cm on a Bruker Optics Alpha FT-IR
spectrometer. Differential thermal analysis (DTA) curves
were recorded with a purge of dry nitrogen gas on an
OZM Research DTA552-Ex instrument with a heating rate
of 5 °C/min using the Meavy 2.2.0 software. The impact
and friction sensitivity data were determined on an OZM
Research BAM fall hammer BFH-10 and an OZM
Research BAM friction apparatus FSKM-10, respectively,
by averaging five individual measurements. Both
instruments were calibrated with RDX.
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9, 30
Synthesis of nitroform HC(NO ) :
4.010 g (35.8
2
3
The synthesis of methylhydrazinium nitroformate (1)
mmol) 4,6-dihydroxypyrimidine was added in portions to
concentrated sulfuric acid (20 mL) with stirring. When
the solid dissolved, the solution was cooled with an ice
bath and concentrated nitric acid (6.0 mL) was added
very slowly over the course of 15 minutes with stirring.
When the addition was complete, the reaction mixture
was warmed slowly to room temperature and was
stirred for an additional 20 h. After this time, the reaction
mixture became pale yellow in color and was poured
onto 150 mL of ice water. Nitroform was extracted with
dichloromethane (3 x 25 mL) and the solution was dried
2
7
was briefly described in a patent. The compound was
later characterized by IR spectroscopy, elemental
analysis, thermal gravimetric analysis and impact
2
0
sensitivity testing. Methylammonium nitroformate (4)
has also been synthesized before but was characterized
only by elemental analysis.2 To the best of our
knowledge, the syntheses of 2, 3 and 5, as well as the
structure determinations of 1-5, have not yet been
reported. Ethylenediammonium dinitroformate (3) is a
rare example of an energetic, oxygen-rich CHNO salt
featuring two nitroformate anions. Only two examples of
such nitroformate salts have previously been deposited
8
with MgSO . The resulting crude nitroform solution was
4
stored at 0°C, and its processing performed in smaller
batches to allow a safer handling of solid potassium
nitroformate (KNF). To this end, 25 mL of the crude
nitroform/dichloromethane solution was cooled in an
ice bath and stirred while a saturated solution of KOH in
ethanol was added dropwise. A yellow precipitate
formed immediately. The addition was continued in this
way until no more precipitate was formed. The yellow
solid was carefully filtered off and washed with ether.
Mass of KNF: 2.050 g (10.8 mmol). KNF was suspended
in hexane (50 mL) and cooled in an ice bath.
Concentrated H SO (10 mL) was added dropwise with
29
in the Cambridge Structural Database, one of these
salts features the 4,4’,5,5’-tetraamino-3,3’-bi-1,2,4-
2
1
triazolium dication and the other the 3,6-diguanidino-
1
9
1
,2,4,5-tetrazinium dication.
EXPERIMENTAL SECTION
Caution! The compounds described herein are
energetic materials that can explode under certain
conditions (e.g., elevated temperatures, impact, friction,
or electric discharge). Furthermore, the combinations of
these compounds with organic solvents can be
potentially hazardous. Although the authors did not
experience any explosions during this work, nitroform
and its metal salts are known explosives. Therefore,
appropriate safety precautions, such as the use of blast
shields in a well-ventilated fume hood and personal
protective equipment (safety glasses, face shields, ear
plugs, as well as gloves and suits made from leather
and/or Kevlar), should always be taken when handling
these materials.
2
4
stirring. When the addition was complete, the solution
was stirred further for 1 h at 0 °C. The hexane layer was
separated, and nitroform extracted with hexane (3 x 15
mL). The combined hexane layers were cooled to -78 °C,
and a colorless precipitate formed. The hexane was
carefully decanted off, and the remaining hexane
removed in a dynamic vacuum at -78 °C overnight,
leaving behind colorless nitroform crystals; mass of the
14
colorless crystalline solid: 0.938 g (6.21 mmol). NMR: N
-1
Materials and apparatus: All chemicals and solvents
were obtained from commercial sources and were used
as received. NMR spectra were recorded at 298 K on a
(CDCl ) δ = -28.2 (s, Δυ = 11 Hz). IR (ATR, cm ) 3356 (s,
3 1/2
br), 1594 (m), 1308 (w), 1165 (m), 1042 (m), 877 (w), 574
(w).
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Synthesis of methylhydrazinium nitroformate
Synthesis of ethylenediammonium dinitroformate
[NH CH CH NH ][C(NO ) ] (3): solution of
1
2
20
[
CH NH NH ][C(NO ) ]
(1):
A
solution
of
A
3
2
2
2 3
3
2
2
3
2 3 2
methylhydrazine (140 µL, 2.66 mmol) in 2 mL of
dichloromethane was added dropwise to a solution of
nitroform (2.62 mmol) in dichloromethane (5 mL),
ethylenediamine (16.0 mg, 0.266 mmol) in methanol (1
mL) was added dropwise to a solution of nitroform (91.5
mg, 0.606 mmol) in methanol (5 mL) with stirring at 0 °C.
After the addition, the solution was stirred for an
additional 30 min, and then at room temperature for 1
h. The volatile compounds were pumped off at -45 °C
until a constant mass was achieved, affording crystals
suitable for X-ray diffraction. When the volatile materials
were removed at warmer temperatures, only the
monocation was obtained rather than the dication, even
when a large excess of nitroform was used. NMR
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
cooled with an ice bath under a flow of N . When the
2
addition was complete, the yellow solution was stirred
at 0 °C for 1.5 h and then at ambient temperature for 1
h. The volatiles were pumped off at -20 °C; mass of the
yellow solid residue: 507 mg; mass expected for 2.62
mmol [CH NH NH ][C(NO ) ] 524 mg. The yellow solid
0
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5
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7
8
9
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2
2
2 3
was deliquescent, but the water could subsequently be
pumped off, recovering the original yellow solid. This
was also evident by the unchanged IR spectrum in air
except for the increasing intensity of the water bands
over time. 1 was dissolved in methanol which was
removed in vacuo at -20 °C affording yellow crystals
1
2+ 13
(CD OD) H: δ = 2.68 (s, [NH (CH ) NH ] ), C: δ = 36.5
3
3
2 2
3
2+ 14
(s, [NH (CH ) NH ] ), N: δ = -32.9 (s, Δυ1/2 = 15 Hz,
3
2 2
3
-
2+
[C(NO ) ] ), -355.8 (s, Δυ = 156 Hz, [NH (CH ) NH ] ).
2
3 2
1/2
3
2 2
3
Raman (25 mW) 휈 = 3025 (0.1), 2995 (0.6), 2420 (0.3),
2319 (0.2), 2138 (0.1), 2083 (0.9), 1557 (0.7), 1488 (0.3),
1462 (0.7), 1388 (5.5), 1357 (1.5), 1340 (0.4), 1262 (1.5),
1157 (0.8), 1146 (3.3), 1132 (1.5), 1030 (0.1), 1009 (0.2),
971 (0.2), 870 (10.0), 789 (1.5), 750 (0.5), 735 (0.4), 677
(0.1), 482 (2.4), 465 (0.1), 428 (1.6), 421 (0.2). IR (ATR) 휈
= 3450 (w), 3249 (s), 3203 (w), 3068 (vs), 3033 (vw), 2942
(w), 2817 (w), 2661 (w), 2631 (vw), 2508 (m), 2411 (w),
1950 (m), 1719 (w), 1608 (s), 1542 (s), 1478 (m), 1452 (s),
1376 (s), 1271 (vs), 1145 (vs), 1059 (s), 1024 (m), 1008
(s), 941 (vw), 868 (s), 837 (m), 778 (vs), 732 (s), 672 (m),
625 (m), 565 (w), 539 (m), 482 (m), 416 (s).
1
suitable for X-ray diffraction. NMR: (CD NO ) H: δ = 3.14
3
2
+
13
-
(
[
s, [CH NH NH ] ); C: δ = 149.5 (s, [C(NO ) ] ), 38.6 (s,
3 2 2 2 3
+
14
CH NH NH ] ); (MeOH-d4) N: δ = -30.6 (s, Δυ = 11
3
2
2
1/2
Hz, [C(NO ) ]). Raman (25 mW) 휈 = 3040 (0.6), 2972
2
3
(
(
(
1.5), 1621 (1.1), 1613 (1.1), 1555 (0.8), 1548 (1.0), 1469
2.0), 1407 (3.9), 1353 (2.8), 1338 (3.1), 1273 (3.2), 1163
2.3), 1152 (5.1), 998 (0.6), 941 (1.2), 892 (1.2), 870
(10.0), 839 (1.2), 790 (2.2), 471 (2.4), 430 (2.1), 420 (2.0),
386 (1.8), 374 (1.6), 290 (1.6), 249 (2.1), 236 (2.3). IR
(ATR) 휈 = 3335 (m), 3287 (w), 3118 (s), 2892 (w), 2785
vw), 2522 (vw), 1679 (vw), 1617 (vw), 1594 (m), 1535
s), 1475 (s), 1455 (vw), 1373 (s), 1246 (vs), 1134 (vs),
084 (m), 1010 (m), 864 (s), 784 (vs), 729 (vs), 531 (w),
40 (w), 417 (m).
(
(
1
4
Synthesis of methylammonium nitroformate
28
[
CH NH ][C(NO ) ] (4): A solution of methylamine (0.50
3 3 2 3
mmol) in methanol (2 mL) was added to a solution of
nitroform (0.20 mmol) in dichloromethane (1 mL) at 0 °C.
The solution was warmed to ambient temperature and
stirred overnight. The volatile compounds were pumped
off starting at -40 °C and warming slowly to ambient
temperature, affording yellow crystals suitable for X-ray
diffraction. Mass of the yellow solid: 32 mg; mass
expected for 0.20 mmol [CH NH ][C(NO ) ]: 36 mg.
Synthesis
of
formamidinium
nitroformate
[
NH CHNH ][C(NO ) ] (2): Formamidine acetate (55.0
2
2
2 3
mg, 0.528 mmol) was dissolved in methanol (2 mL). A
solution of nitroform (93.0 mg, 0.616 mmol) in methanol
(1 mL) was added dropwise at 0 °C. The solution was
warmed to ambient temperature and stirred for 30 min.
The volatile compounds were removed in vacuo first at -
0 °C and then at ambient temperature, affording yellow
crystals suitable for X-ray diffraction. Mass of the yellow
3
3
2 3
13
1
+
2
NMR: (CD OD), H: δ = 2.33 (s, [CH NH ] ), C: δ = 24.7
(s, [CH NH ] ), N: δ = -30.7 (s, Δυ = 11 Hz, [C(NO ) ] );
-364.6 (s, Δυ1/2 = 38 Hz, [CH NH ] ). Raman (25 mW) =
3
3
3
+
14
-
3
3
1/2
+
2 3
crystalline solid: 96 mg; mass expected for 0.528 mmol
3
3
1
[
NH CHNH ][C(NO ) ]: 103 mg. NMR: (CD OD) H: δ =
3154 (0.1), 3053 (0.3), 2986 (1.1), 2912 (0.1), 2839 (0.2),
2538 (0.1), 1613 (0.2), 1529 (0.3), 1497 (0.1), 1468 (1.2),
1391 (6.2), 1323 (0.4), 1292 (1.7), 1260 (0.6), 1166 (0.9),
1146 (4.1), 990 (2.3), 941 (1.1), 873 (10.0), 789 (1.8), 734
(1.6), 469 (2.3), 431 (0.8), 416 (0.4). IR (ATR) 휈 = 3223 (w),
3191 (vs), 2860 (w), 2756 (vw), 2756 (vw), 2544 (m), 2435
(w), 1612 (m), 1531 (m), 1448 (s), 1424 (m), 1390 (s),
1238 (vs), 1139 (vs), 989 (s), 938 (s), 871 (s), 785 (vs), 731
(vs), 513 (m), 430 (w), 416 (vw).
2
2
2 3
+
3
13
+
7
.81 (s, [NH CHNH ] ); C: δ = 180.9 (s, [NH CHNH ] );
2 2 2 2
1
4
N: δ = -32.6 (s, Δυ1/2 = 12 Hz, [C(NO ) ]). Raman (25
2 3
mW) 휈 = 3012 (0.3), 2936 (1.3), 1650 (0.3), 1548 (0.8),
1
1
9
466 (0.4), 1394 (0.3), 1382 (5.0), 1353 (2.6), 1334 (0.5),
280 (1.3), 1160 (1.1), 1150 (7.4), 1123 (1.2), 1047 (0.5),
17 (3.3), 871 (10.0), 787 (1.2), 715 (0.2), 653 (1.2), 612
(0.3), 571 (1.5), 546 (1.2), 477 (2.6), 431 (1.4), 420 (0.7).
IR (ATR) 휈 = 3359 (vw), 3189 (vs), 3019 (vw), 2927 (vw),
2899 (vw), 2837 (vw), 2763 (w), 1716 (s), 1686 (w), 1540
(s), 1480 (m), 1385 (s), 1344 (s), 1258 (vs), 1141 (vs), 1068
Synthesis of dimethylammonium nitroformate
[
(CH ) NH ][C(NO ) ] (5): Dimethylamine (0.305 mmol)
3 2 2 2 3
(
(
m), 1044 (w), 1011 (m), 918 (m), 824 (m), 783 (s), 725
w), 649 (vs), 569 (s), 543 (m), 417 (w).
was condensed onto a frozen solution of nitroform
0.420 mmol) in dichloromethane (2 mL) at -196 °C. The
(
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Journal of the American Chemical Society
Page 4 of 13
mixture was warmed to -40 °C and then for 1 h to -20 °C.
significantly shorter than that for most other methods
which rely on a seven day reaction time to prepare
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
A yellow precipitate was formed, and the volatile
materials were pumped off from -20 °C to 0 °C, leaving
behind a yellow solid (55 mg, mass expected for 0.305
mmol of [(CH ) NH ][C(NO ) ] 60 mg). A small amount of
5
-6, 19, 38
tetranitromethane.
Drawbacks of this method are
the need for the extra steps of converting the crude
nitroformate to the sensitive potassium nitroformate,
followed by conversion to free nitroform with conc
H SO , and the explosion-hazards associated with
3
2
2
2 3
the solid was re-dissolved in methanol, cooled to -35 °C
and upon solvent removal afforded crystals suitable for
2
4
1
X-ray diffraction. NMR: (CD CN) H: δ = 8.14 (br,
washing potassium nitroformate with ether and
extracting free nitroform with hexane. The safest
method for the preparation of nitroform is the reaction
of isopropyl alcohol with an excess of nitric acid and the
extraction of the nitroform with methylene chloride. This
method has been described as Procedure 21-01A in the
3
+
3
+
[
(CH ) NH ] ), 2.53 (t, J = 5.6 Hz, [(CH ) NH ] ); (DMSO-
3 2 2 HH 3 2 2
1
3
+
14
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
0
1
2
3
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5
6
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8
9
0
d6) C: δ = 35.3 (s, [(CH ) NH ] ); N: δ = -29.6 (s, Δυ
= 9 Hz, [C(NO ) ] ), -355.5 (t, J = 55.6 Hz, Δυ1/2 = 134
Hz, [(CH ) NH ] ). Raman (25 mW) 휈 = 3046 (1.0), 2977
3
2
2
1
1/2
-
2
3
+
NH
3
2
2
(2.5), 2921 (0.2), 2858 (0.3), 2821 (0.4), 1605 (0.2), 1546
40
(
(
(
0.9), 1463 (1.3), 1410 (0.9), 1390 (6.0), 1336 (1.5), 1271
1.4), 1157 (3.9), 1143 (3.2), 1083 (0.3), 1045 (0.4), 1017
0.3), 942 (0.3), 886 (3.4), 870 (10.0), 850 (1.1), 790 (1.1),
book of Ledgard and is based on the patented Rockwell
International commercial production process for
41
nitroform.
7
84 (0.6), 731 (0.2), 677 (0.2), 473 (2.4), 427 (1.4), 417
1.0). IR (ATR) 휈 = 3469 (vw), 3170 (s), 2974 (w), 2811 (w),
521 (vw), 2428 (m), 1730 (vw), 1590 (s), 1536 (s), 1487
m), 1467 (s), 1438 (m), 1416 (m), 1387 (w), 1363 (m),
334 (vw), 1267 (vs), 1247 (m), 1157 (s), 1143 (vs), 1080
m), 1016 (s), 984 (w), 939 (m), 883 (m), 868 (s), 836 (s),
82 (vs), 727 (s), 625 (m), 570 (m), 524 (m), 471 (m), 417
(s).
X-ray Crystal Structure Determination: The single-
The syntheses of salts 1-5 involve the reaction of the
corresponding nitrogenous base with acidic nitroform
via the slow addition of one reagent to the other at low
temperature using methanol or dichloromethane as
solvent. Removal of the volatile materials resulted in the
isolation of the desired salts as yellow solids in excellent
yields. Salts 1-5 were characterized by multinuclear
(
2
(
1
(
7
-
NMR, IR and Raman spectroscopies. The C(NO ) anion
2
3
resonates as a narrow (Δυ1/2 = 9-15 Hz) singlet at about
14
crystal X-ray diffraction data were collected on a Bruker
SMART APEX DUO 3-circle platform diffractometer using
Mo Kα radiation. The frames were integrated using the
SAINT algorithm to give the hkl files corrected for
 = -30 ppm in the N NMR spectra and as a singlet at
13
19
about  = 150 ppm in the C spectra.
3
1
Lp/decay. The absorption correction was performed
3
2
33
using the SADABS program.
Using Olex2
the
3
4
structures were solved by Intrinsic Phasing (ShelXT )
3
5
and refined with the ShelXL refinement package using
Least Squares minimization. All non-hydrogen atoms
were refined anisotropically. Figures were prepared
3
6
using the CSD Mercury software. The Hirshfeld analysis
3
7
was carried out using CrystalExplorer 17 and the
3
8
published definition of the Hirshfeld surface and the
mapped properties. Further crystallographic details for
compounds 1-5 can be obtained from the Cambridge
Crystallographic Data Centre [CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax (+44) 1223-336-033; e-mail
deposit@ccdc.cam.ac.uk] on quoting the deposition
numbers 1847171 - 1847175.
Figure 1. Asymmetric unit in the crystal structure of
. Selected bond distances (Å) and angles (°): C2 N3
1
1.3673(14), C2 N4 1.4609(14), C2 N5 1.3729(14), N3
O1 1.2436(12), N3 O2 1.2616(12), N4 O3 1.2172(14),
N4 O4 1.2268(14), N5 O5 1.2533(12), N5 O6
1
1
.2442(12), N3 C2 N4 116.25(9), N3 C2 N5
27.81(9), O1 N3 O2 122.35(9), O3 N4 O4
RESULTS
Synthesis
125.03(11), O5 N5 O6 122.70(9).
Although there are many methods published for the
synthesis of nitroform, we found the method by Langlet
et al. to be convenient at the laboratory scale as it
requires only commercial-grade concentrated nitric acid
X-ray Crystal Structure Determination
Slowly grown, quality crystals suitable for single crystal
X-ray analysis are typically obtained by the gradual
removal of the solvent from a concentrated solution.
However, concentrated solutions of nitroformate salts
3
0
rather than freshly distilled anhydrous nitric acid.
Furthermore, the reaction time of 12 hours is
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Journal of the American Chemical Society
unit of the structure consists of one [MeNH NH ] cation
+
tend to decompose at room temperature according to
Eq. 1.
2
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5
-
and one [C(NO ) ] anion associated through hydrogen
2 3
bonding (Figure 1). As expected, the methylhydrazinium
cation is protonated on the internal nitrogen rather than
(
1)
39-40
the terminal nitrogen.
. Two nitro groups of the
nitroformate anion lie in one plane while the third nitro
group is approximately perpendicular to this plane with
a N5-C2-N4-O3 torsion angle of 85.0(1)°. The in-plane
nitro group features C-N bond distances of 1.3673(14) Å
and 1.3729(14) Å whereas the C-N distance of the third
nitro group is longer by about 0.1 Å. Only the in-plane
nitro groups participate in significant hydrogen bonding
with the cation. Four strong NHO hydrogen bonds with
N···O distances between 2.821(1) Å and 3.295(1) Å, and
N-H···O angles between 133(1)° and 161(1)° between the
cations and anions of 1 (Figure 1) result in a complex
hydrogen bonding network.
We therefore relied on the use of low temperatures
for the slow removal of the solvent in vacuo to facilitate
the crystal growth while minimizing the potential for
decomposition. In this way, we succeeded in growing
crystals suitable for single crystal X-ray analysis for all
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
five
nitroformate
salts.
Selected
relevant
crystallographic data and parameters used in the
structure determination are summarized in Table 1.
The methylhydrazinium salt 1 crystallizes in the
monoclinic space group P2 /n (Z = 4). The asymmetric
1
Table 1. Crystallographic and structure determination details for salts 1-5.
1
2
3
4
5
EMPIRICAL FORMULA
FORMULA WEIGHT
TEMPERATURE/K
C
2
H
7
N
5
O
6
C
2
H
5
N
5
O
6
C
4
H
10
N
8
O
12
C
2
H
6
N
4
O
6
C
3
H
8
N
4
O
6
197.13
100(2)
195.11
100(2)
362.20
100(2)
182.11
100(2)
196.13
100(2)
CRYSTAL SYSTEM
monoclini
c
monoclinic
triclinic
monoclinic orthorhombic
SPACE GROUP
P2 /n
1
P1
C2/m
Pnma
8.226(2)
8.403(2)
10.367(3)
90
P2 /c
1
A/Å
8.5003(11)
9.2107(12)
6.764(4)
7.374(5)
10.023(4)
9.240(3)
8.250(3)
90
7.409(3)
9.031(4)
12.671(5)
90
B/Å
C/Å
10.3713(14) 8.058(5)
Α/°
90
98.976(11)
Β/°
108.348(2)
90
100.324(11) 116.017(5)
109.945(10) 90
90
92.666(7)
90
Γ/°
90
VOLUME/Å3
770.73(18)
4
361.3(4)
2
686.6(4)
2
716.6(3)
4
846.8(6)
4
Z
Ρ
CALC/G/CM3
1.699
0.166
408.0
18851
2358
1.794
0.177
200.0
9189
2204
0.0861
1.752
0.174
372.0
8579
1116
0.0533
1.688
0.168
376.0
16233
1158
1.538
0.148
408.0
19784
2578
Μ/MM-1
F(000)
REFLECTIONS COLLECTED
INDEPENDENT REFLECTIONS
R
INT
0.0498
0.0403
0.0450
2
8
578/0/12
DATA/RESTRAINTS/PARAMETERS
GOODNESS-OF-FIT ON F2
2358/0/131 2204/0/138 1116/0/71
1158/0/106
1.037
0.967
1.052
1.048
0.920
R
1
[I>=2Σ (I)]
0.0338
0.0525
0.0327
0.0350
0.0385
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0.0769 0.0989 0.0843
Page 6 of 13
WR
2
[I>=2Σ (I)]
0.0907
0.1120
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Salt 2 crystallizes in the triclinic space group P1 with
one cation-anion pair in the asymmetric unit and two
asymmetric units in the unit cell. The crystal density at
interlayer hydrogen bonds is 3.119(3) Å with a C-H···O
angle of 122(2)°. The interlayer spacing is 3.097(3) Å.
3
100(2) K determined by X-ray diffraction is 1.794 g/cm
3
and the density at 298 K is estimated to 1.74 g/cm using
41
empirical data. The formamidinium cation is nearly
planar. The H-N-C-H torsion angles are about 5(3)° and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
75(3)° (Figure 2). Both C-N distances in the cation
(1.293(3) Å and 1.304(4) Å) are identical within the
margins of error and are significantly shorter than a
typical C-N single bond of about 1.47 Å. Similar to the
anion of 1, the nitroformate anion of 2 has two nitro
groups coplanar with the plane formed by the C N N N
Figure 3. Two layers formed through hydrogen
bonding in the solid-state structure of 2.
Because the sensitivities of compounds are strongly
affected by crystal packing, the solid-state structure of
salt 2 was further investigated by a Hirshfeld surface
analysis, a convenient tool for the investigation of
1
1 2 3
fragment and one which is approximately perpendicular
N3-C1-N2-O3 86.2(2)°). The C-N distances in the anion
(
show a similar pattern as in the anion of 1, i.e., the C-N
distances of the two co-planar NO2 groups are
approximately 0.1 A shorter than the one of the third
group.
4
2
intermolecular interactions. Hirshfeld surface based
tools are particularly suitable for visualizing variations in
the intermolecular interactions of compounds. A
quantitative analysis of the intermolecular interaction of
a compound can be provided through fingerprint plots.
The Hirshfeld surfaces (isovalue: 0.5) of the
trinitromethanide and formamidinium ions in the solid-
state structure of salt 2 were generated using a very high
-
surface resolution. For the [C(NO ) ] anion, the 3D d
2
3
norm
surfaces were mapped over a fixed color scale of -0.5163
+
(
red) to +0.9129 (blue) and for the [HC(NH ) ] cation
2
2
over -0.5161 (red) to +1.0518 (blue) . In both cases, the
Shape Index was mapped in the color range of -1.0 to
Figure 2. Asymmetric unit of 2. Selected bond
distances (Å) and angles (°): C1 N1 1.370(3), C1 N2
1.462(3), C1 N3 1.367(3), N1 O1 1.250(2), N1 O2
1.253(2), N2 O3 1.215(2), N2 O4 1.224(2), N3 O5
+
1.0, and the Curvedness was in the range of -4.0 to +0.4.
The surfaces were chosen to be transparent. The
Hirshfeld surface (dnorm, Shape Index and Curvedness) of
the trinitromethanide anion in 2 is shown in Figure 4.
1.260(2), N3 O6 1.241(2), C2 N4 1.304(3), C2 N5
1.294(3), N1 C1 N2 115.84(18), N1 C1 N3
127.23(18), N2 C1 N3 116.35(18), N4 C2 N5
124.3(2).
The crystal packing of compound 2 is interesting and
consistent with the observed low sensitivities of the
material (see Performance Calculations and Sensitivities
2
section). The formamidinium and trinitromethanide
ions form nearly planar layers involving relatively strong
N-H···O hydrogen bonds (Figure 3). The NH groups of
2
the cation serve as hydrogen bond donors to the oxygen
atoms of the two co-planar NO groups of the anion. The
2
donor-acceptor distances are in the range of 2.933(3) Å
to 3.059(3) Å. The observed N-H···O angles are between
131(3)° and 176(3)°. Additional hydrogen bonds occur
between the cation-anion layers with a C-H group of a
cation from one layer serving as donor to one oxygen
atom of the perpendicular NO -group of an anion in the
2
next layer (Figure 3). The donor-acceptor distance of the
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It can be seen clearly from Figure 4 that three of the
1
2
3
4
5
6
7
8
9
four oxygen atoms of the co-planar nitro-groups in the
anion act as acceptor sites for hydrogen bonding from
NH -groups of the cations within the same layer. The
2
-
third, perpendicular NO group of the C(NO ) anion is
2
2 3
an acceptor site for the longer inter-planar hydrogen
bonding for which the C-H hydrogen atom of the cation
acts as the donor. The Shape Index is highly sensitive to
very subtle changes in the surface shape and reveals
where two molecular surfaces touch one another. The
Curvedness is the measurement of “how much shape”
there is in a crystal. The flat areas of the surface
correspond to low values of curvedness, whereas sharp
curvature areas correspond to high values of curvedness
and usually tend to divide the surface into patches,
indicating interactions between neighboring molecules.
Except for the area close to the perpendicular nitro
group, mapping of the Shape Index and Curvedness show
virtually flat surfaces above and below the anion,
indicative of planar stacking. A similar conclusion can be
drawn from the Hirshfeld surface plots of the
formamidinium cation. In the dnorm mapping, close
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
interactions can be spotted as red areas around the NH2
hydrogen atoms (Figure 5a). The mappings of the Shape
Index and Curvedness show again virtually flat surfaces
at the top and bottom of the cation (Figures 5b and 5c).
The combination of the distances from a point on the
Hirshfeld surface to the nearest nucleus outside the
surface (d ) and inside the surface (d ) in the form of a 2D
e
i
fingerprint plot provides information about these
quantities. The resulting plots are sensitive to the
immediate environment of the molecule or ion. The 2D
fingerprint plots can be deconstructed to highlight
particular atom pair contacts. This deconstruction
enables the separation of contributions from different
interaction types that overlap in the full fingerprint. The
2D fingerprint plots of the cation in salt 2 (Figure 6) show
O···H and O···O interactions with contributions to the
total Hirshfeld surface of 55.4 % and 25.1 %, respectively.
The 2D fingerprint plots of the formamidinium ion
Figure 4. Hirshfeld surfaces of the anion in 2: a) 3D
dnorm surface, b) Shape Index, c) Curvedness.
The 3D dnorm surface is used to identify very close
intermolecular interactions. The value of dnorm is
negative or positive when intermolecular contacts were
shorter or longer than the sum of van der Waals (vdW)
radii, respectively. The dnorm values are mapped onto the
Hirshfeld surface using a red-blue-white color scheme as
follows: red regions represent closer contacts and a
negative dnorm value; blue regions represent longer
contacts and a positive dnorm value; and white regions
represent the distance of contacts equal to exactly the
vdW separation with a dnorm value of zero.
(
Figure 7) show H···O and C···O interactions with
contributions to the total Hirshfeld surface of 72.7 % and
.2 %, respectively. This reveals that the solid-state
4
structure is dominated by cation-anion interactions
through O···H and to a much lesser extent through C···O
contacts. Additional anion-anion interactions with O···O
contacts are also present.
Figure 5. Hirshfeld surfaces of the cation in 2: a) 3D
dnorm surface, b) Shape Index, c) Curvedness.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7. The cation and anion in the solid-state
structure of 3 including additional O and H atoms
from neighboring anions and cations. Selected
bond distances (Å) and angles (°): C1 N1 1.4883(18),
C2 N2 1.3691(11), C2 N3 1.4633(17), N2 O1
1.2464(11), N2 O2 1.2581(11), N3 O3 1.2170(16), N3
O4 1.2265(16), N2 C2 N3 116.03(6), N2 C2 N2
1
1
26.47(12), O1 N2 O2 122.20(8), O3 N3 O4
24.71(12).
Salt 4 crystallizes in the orthorhombic space group
Pnma. The asymmetric unit contains a H C-NH unit of
2
2
the cation and a C(NO ) unit of the anion. The full cation
2
2
and anions are generated by symmetry. The structure
suffers from a positional disorder for the three NO -
2
-
groups of the [C(NO ) ] anion. Consequently, this
2
3
Figure 6. 2D fingerprint plots of the ions in 2: a)
anion full, b) anion O···H contacts, c) anion O···O
contacts, d) cation full, e) cation H···O contacts, and
f) cation C···O contacts showing the percentages of
contacts contributing to the total Hirshfeld surface
areas.
structure is not discussed in detail.
Salt 3 crystallizes in the monoclinic space group C2/m.
The structure is rather symmetric, and the asymmetric
unit contains only one CH-NH moiety of the cation and
2
a C(NO ) moiety of the anion. The full cation and anions
2
2
are generated by symmetry. The geometry of the cation
and anion are both as expected (Figure 7). The two in-
plane nitro groups of the anion are related to one
another by symmetry, and the third is nearly
perpendicular to the plane with the O4-N3-C2-N2 torsion
angle equal to 96.4°. Again, two shorter C-N distances of
1.3691(11) Å and a longer C-N distance of 1.4633(17) Å
are found in the trinitromethanide anion. The cations
and anions are associated through a nitrogen bonding
network with donor-acceptor distances of 2.899(8)
Figure 8. The cation and anion in the solid-state
structure of 4. Only one orientation of the
-
[
C(NO ) ] anion is shown for clarity. Selected bond
2
3
lengths (Å) and angles (°): C1 N1 1.485(3), C2 N2
1.369(3), C2 N3 1.467(3), N2 O1 1.284(8), N2 O2
1.15(1), N3 O3 1.211(3), N3 O4 1.235(3), N2 C2 N2
125.3(7), N2 C2 N3 109.4(6), O1 N2 O2 128.9(6), O3
(
N1H···O1) and 2.956(8) (N1H···O2) Å with bond angles
N3 O4 124.1(2).
of 132(2)° and 161(2)°, respectively.
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1
Journal of the American Chemical Society
The methylammonium cation exhibits a staggered
linking the cations to the anions, but in all cases, only the
oxygen atoms of the in-plane nitro groups participate in
hydrogen bonding.
conformation with C3v symmetry while the
trinitromethanide anion shows again the familiar
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
geometry with two co-planar NO groups (Figure 8).
Unlike the three previously discussed structures, the
2
The crystal packing of salt 2 is special among the five
studied nitroformate salt as it is the only structure that
exhibits planar layer-stacking. This distinctive structural
feature explains its unique insensitivity towards friction
and impact due to absorbing mechanical stimuli-induced
kinetic energy by converting it into layer sliding.
third NO -group deviates more from a perpendicular
2
orientation with a N2-C2-N3-O4 torsion angle of 60(2)°.
The cations and anions are associated through hydrogen
bonding between the NH -groups of the cations and the
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
two co-planar NO -groups of the anions. The donor-
2
acceptor distances of the hydrogen bonds range from
2.97(1) to 3.17(1) Å.
Thermochemical Calculations
In addition to the structural analyses of 1-5, we were
interested in evaluating the efficacy of these materials
for HEDM applications. This required the determination
of accurate enthalpies of formation. According to a Born-
Haber cycle, the heat of formation of an ionic solid can
be computed by adding the heats of formation of the
gaseous cation and anion and subtracting the lattice
enthalpy (Eq. 2).
(2)
The computations were performed with the Gaussian
9 suite of programs. The structure optimization and
44
0
frequency
calculations
together
with
the
Figure 9. The cation and anion in the solid-state
structure of 5 including additional O atoms from a
neighboring anion. Selected bond distances (Å) and
angles (°): C1 N1 1.4842(19), C2 N1 1.4874(18), C3
N2 1.3669(17), C3 N3 1.3794(17), C3 N4 1.4585(17),
C1 N1 C2 113.87(11), N2 C3 N3 127.16(11), N2 C3
N4 116.56(11), O1 N2 O2 121.86(11), O3 N3 O4
thermochemical calculations were performed using the
43
Gaussian-4 composite method. A true global minimum
on the potential energy surface was confirmed by the
absence of imaginary frequencies.
Table 2. G4 atomization energies and enthalpies of
formation at 0 and 298 K for the gas-phase ions of interest
to this work.
122.69(11), O5 N4 O6 124.54(12).
Ion
D
º
(M)a
ΔH
f
° 0 K a ΔH
° 298
f
The dimethylammonium salt 5 crystallizes in the
a
K
monoclinic space group P2 /c with one cation-anion pair
1
-
C(NO
NH4+
2
)
3
912.7
164.1
441.1
497.6
294.9
575.7
-51.2
154.9
151.2
155.6
188.3
180.8
-54.7
152.2
146.6
151.2
183.9
174.7
in the asymmetric unit and four asymmetric units in the
-
unit cell (Figure 9). The conformation of the [C(NO ) ]
2
3
anion is like the one in salt 4 and deviates from the ones
+
CH
3
NH
3
exhibited by salts 1-3. Two of the NO -groups are co-
2
+
2
NH
2
2
CHNH
2
planar while the third one deviates significantly from a
perpendicular orientation with a N3-C3-N4-O5 torsion
angle of 71.5(2)°. The cations and anions are again
associated by a hydrogen bonding network between the
+
NH
NH
3
CH
3
NH
2
NH
+
NH -group of the cation and oxygen atoms of the nitro
2
+
groups in the anion as acceptors. The donor-acceptor
distances are between 2.840(2) Å and 3.228(2) Å with
hydrogen bond angles between 139(2)° and 157(2)°.
(CH ) NH
719.0
681.8
146.5
399.5
140.4
391.0
3
2
2
(NH
3
)
2
(CH
)
2 2
2+
To summarize the structural characterization of 1-5,
the nitroformate anion crystallizes with pseudo-C2h
symmetry with two co-planar nitro groups and the third
a) All quantities in kcal/mol.
The G4 thermochemical output was transformed to
the gaseous heats of formation for each ion following the
NO -group approximately perpendicular to that plane.
2
44
The distortion from this idealized geometry is the most
pronounced for the methyl- and dimethylammonium
salts 4 and 5. Salts 1-5 all exhibit NHO hydrogen bonds
procedure outlined by Ochterski. Experimental heats of
formation at 0 K for the atomic elements along with their
4
5
entropy corrections were extracted from Curtiss et al.
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Journal of the American Chemical Society
Page 10 of 13
The computed thermochemical data for the gas-phase
ions of interest to this work are summarized in Table 2.
calculate the explosive performances based on the tradi-
tional Chapman–Jouguet thermodynamic detonation
theory using CHEETAH 8.0. The results of these
calculations along with the decomposition temperatures
determined by differential thermal analysis and impact
and friction sensitivities for ANF, HNF, 1-5 and some
conventional explosives are summarized in Table 4.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5
0
The final step in the computation of the enthalpy of
formation of the solid is the estimation of the lattice
enthalpy for the ionic salt by the volume-based
thermodynamics approach of Jenkins using the molar
volume derived from the crystal structure along with the
appropriate coefficients based on the stoichiometry of
All the nitroformate salts reported here have higher
detonation pressures and velocities than TNT. 1 has a
detonation pressure and velocity exceeding those of
PETN; however, it deliquesces to the extent that yellow
crystals are immediately liquefied upon exposure to
moist air, hindering our ability to accurately measure its
sensitivity, and of course, its practical application. Salt 4
also surpasses PETN in terms of performance along with
being less sensitive to impact and friction; however, it
suffers from poor thermal stability. 3 has a high oxygen
balance and good performance, surpassing that of TATB.
However, it is somewhat sensitive to impact and begins
to thermally decompose at 105 °C. Furthermore, the
isolation of 3 was the most challenging as removal of the
volatile materials in vacuo at temperatures ≥-10 °C
resulted in the isolation of the 1:1 salt regardless of the
stoichiometry used. To obtain the desired di-cation, the
methanol solvent was removed under vacuum at -45 °C.
Considering performance, sensitivity and the ease of
synthesis and handling, 2 proves to be the best candidate
for HEDM applications. Although thermally slightly less
stable than RDX by 33 °C, its performance is comparable
to that of RDX while being far less sensitive to impact and
friction. Furthermore, 2 is conveniently prepared from
nitroform and commercially available formamidine
acetate and is hydrolytically stable upon storage in moist
air at ambient temperature for at least several weeks.
4
6-47
the salt.
Selected properties of 1-5 together with
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ammonium nitroformate (ANF) and hydrazinium
nitroformate (HNF) are summarized in Table 3. Our
values of -34.6 and -0.4 kcal/mol for ANF and HNF,
respectively, agree well with the values of -35.9 and +0.8
kcal/mol previously obtained at the MP2 level of theory
7
with an aug-cc-pVDZ basis set.
Table 3. Properties of nitroformate salts.
Ma
ρb
ΔH
c
ΔH
°d
L
f
ANF
168.10 1.910
183.09 1.938
197.11 1.699
195.09 1.794
362.17 1.752
182.10 1.706
132.1
129.6
122.7
124.8
297.2
125.1
119.7
-34.6 (-35.9)
-0.4 (+0.8)
-2.7
HNF
1
2
3
4
5
-28.4
-15.7
-33.2
46.09
1.583
-34.1
3
a) Molecular weight (g/mol); b) density (g/cm ); c) lattice enthalpy
kcal/mol); d) solid state enthalpy of formation at 298 K (kcal/mol).
(
Values in parentheses are from ref 5.
Performance Calculations and Sensitivities
The resultant heats of formation for the solids along
with the experimental crystal densities were used to
Table 4. Explosive performance and sensitivity of nitroformate salts compared to some common explosives.
Ωa
P [GPa]b
35.882
41.323
32.281
34.133
33.568
31.420
25.512
22.349
V [km s-1]c
8.887
T
ex [K]d
IS [J]e
FS [N]f
T
dec [°C]g
35
2-548
965
14-3648
1165
125-12948
11520
ANF
HNF
1
17.58
12.18
-12.17
-4.10
-4.42
-8.79
-32.63
-73.97
-21.61
-55.78
2545.5
2865.1
3648.3
3489.6
3967.1
3630.3
3406.1
3221.6
3432.351
2483.9
9.419
8.694
-
-
2
8.811
>100
35
>360
>360
>360
>360
35350
12051
35053
171
3
8.755
105
4
8.530
>100
>100
1550
7.551
>5053
95
5
7.896
110
TNT
RDX
TATB
6.90049
8.75049
8.41152
29550
20450
35053
34.149
32.652
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Page 11 of 13
Journal of the American Chemical Society
8.26054 32
3682.5
PETN
-10.12
30.654
602
1602
1
2
2
a) oxygen balance given as % conversion to CO ; b) detonation pressure; c) shock velocity; d) explosion temperature; e) impact sensitivity; f)
friction sensitivity; g) decomposition temperature as determined by the onset of the DTA exotherm.
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The insensitivity of formamidinium trinitromethanide,
, relative to the other nitroformate salts is not
surprising when considering its packing diagram (Figure
). Since 2 possesses a pseudo-planar cation, it is the
only structure which can form nearly planar cation-anion
layer-stacking.
*Ralf Haiges, E-mail: haiges@usc.edu
2
Funding Sources
Financial support was provided by the Office of Naval
Research and the Loker Hydrocarbon Research Foundation.
A.F.B. acknowledges NSF for a graduate research fellowship.
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACKNOWLEDGMENT
SUMMARY
The authors thank Drs. Ross Wagner, William W. Wilson,
Piyush Deokar as well as Thomas Saal and Jonas Schaab for
their help and stimulating discussions. We would also like to
thank Dr. Edward F. C. Byrd for his suggestions regarding the
thermochemical calculations.
In conclusion, five salts featuring the energetic
nitroformate anion have been prepared and
characterized by multinuclear NMR, IR and Raman
spectroscopies as well as single-crystal structure
determinations. The nitroformate anions exhibit
pseudo-C2 symmetry in which two of the nitro groups
h
ABBREVIATIONS
lie in a plane and the third is approximately
perpendicular to the plane. All the species exhibit NHO
hydrogen bonding between the cations and the in-plane
nitro groups of the anions. Atomization energies and
enthalpies of formation were computed for all the ions
HEDM, high energy density material; IS, impact sensitivity;
ANF,
ammonium
nitroformate;
HNF,
hydrazinium
nitroformate; IHE, insensitive high explosive; TNT,
trinitrotoluene; RDX, cyclotrimethylenetrinitramine; TATB,
triaminotrinitrobenzene; PETN, pentaerythritol tetranitrate.
4
3
in the gas-phase using the G4 composite method. The
lattice enthalpies were calculated using a volume-based
thermodynamic approach to obtain the relevant solid-
state enthalpies of formation of the salts. Finally, to
evaluate the efficacy of these materials for HEDM
applications, the detonation properties were calculated
and sensitivities to impact, friction and heat were
experimentally determined. Of the salts reported here,
formamidinium trinitromethanide, 2, was found to be a
promising HEDM candidate with an outstanding
performance that slightly exceeds that of RDX, and an
impact and friction sensitivity which is much lower than
that of RDX and comparable to that of TATB. The salt
should therefore be considered for an Insensitive High
Explosive, the "holy grail" in energetic materials
research. The observed insensitivity of 2 can be
explained by its nearly planar cation-anion layer-stacking
and an extensive network of hydrogen bonding.
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