Synthesis and thermochemical properties of NF2-containing energetic salts

Article abstract of DOI:10.1016/j.jfluchem.2007.07.006

Ammonium, 1,5-diamino-4-methyl-tetrazolium and 4-amino-1-methyl-triazolium salts of 5-difluoroaminodifluoromethyl-tetrazolate (TA-CF₂NF₂) were prepared by metathesis reactions of silver 5-difluoroaminodifluoromethyl-tetrazolate and the corresponding iodides. All are thermally stable to ～150 °C. The ammonium salt has a density of 1.88 g cm^-3. The combination of the CBS-4 method and isodesmic bond separation reactions was found to be an economical and reliable method to estimate heats of formation for polyfluorinated molecules. The standard heats of formation (Δ_f H₂₉₈ °) of ammonium 5-difluoroaminodifluoromethyl-tetrazolate was calculated to be -53.13 kcal mol^-1 using the CBS-4 method. While its detonation pressures (P) and velocities (D) were estimated using Cheetah 4.0: P = 28.78 GPa; D = 8490 m s^-1; detonation properties for 1,5-diamino-4-methyl-tetrazolium salts of 5-difluoroaminomethyltetrazolate (TA-CH₂NF₂), 5-difluoroaminotetrazolate (TA-NF₂) and 5-difluoroaminodinitromethyl-tetrazolate (TA-C(NO₂)₂NF₂) are also compared based on predicted densities and computed heats of formation.

Full text of DOI:10.1016/j.jfluchem.2007.07.006

Journal of Fluorine Chemistry 128 (2007) 1410–1415
www.elsevier.com/locate/fluor
Synthesis and thermochemical properties of
NF2-containing energetic salts
*
Chengfeng Ye, Haixiang Gao, Jean’ne M. Shreeve
Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, United States
Received 9 June 2007; received in revised form 4 July 2007; accepted 5 July 2007
Available online 18 July 2007
Abstract
Ammonium, 1,5-diamino-4-methyl-tetrazolium and 4-amino-1-methyl-triazolium salts of 5-difluoroaminodifluoromethyl-tetrazolate (TA-
CF2NF2) were prepared by metathesis reactions of silver 5-difluoroaminodifluoromethyl-tetrazolate and the corresponding iodides. All
are thermally stable to ꢀ150 8C. The ammonium salt has a density of 1.88 g cmꢁ3. The combination of the CBS-4 method and isodesmic
bond separation reactions was found to be an economical and reliable method to estimate heats of formation for polyfluorinated molecules. The
standard heats of formation (DfHꢂ298) of ammonium 5-difluoroaminodifluoromethyl-tetrazolate was calculated to be ꢁ53.13 kcal molꢁ1 using the
CBS-4 method. While its detonation pressures (P) and velocities (D) were estimated using Cheetah 4.0: P = 28.78 GPa; D = 8490 m sꢁ1
;
detonation properties for 1,5-diamino-4-methyl-tetrazolium salts of 5-difluoroaminomethyltetrazolate (TA-CH2NF2), 5-difluoroaminotetrazolate
(TA-NF2) and 5-difluoroaminodinitromethyl-tetrazolate (TA-C(NO2)2NF2) are also compared based on predicted densities and computed heats of
formation.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Energetic materials; Fluorine; NF2 compounds; Quantum chemical calculations
1. Introduction
structures, NF2 compounds have higher impulse values than
NO2 compounds [6]. Currently, there are three main approaches
to the synthesis of energetic difluoroamines: addition of NF2 to
alkene using N2F4 [7], difluoroamination of heterocyclic and
dinitromethyl anions with NF2OSO2F [8] the formation of
geminal difluoramino groups by reacting ketones with HNF2 in
fuming sulfuric acid [4a]. The three most promising of the
known difluoramine candidates for use as energetic materials
are 3,3,7,7-tetrakis(difluoramino)-octahydro-1,5-dinitro-1,5-
diazocine (1, HNFX) [9], 1,1,3,5,5-pentanitro-1,5-bis(difluor-
amino)-3-azapentane (2) [10] and 1,10-difluoroamino-3,30,4,40-
tetranitro-5,50-bipyrazole (3) [11]. Although so far only
one low-density polymorph of 1.807 g cmꢁ3 was found for
HNFX [4a], volume additivity calculations [5,12] predict a
higher density of 1.914 or 1.919 g cmꢁ3 for this compound.
MOLPAK [13] also indicates that additional polymorphs with
densities up to 2.03 g cmꢁ3 may exist in a manner similar to
HMX whose low-density polymorph (a-HMX, density =
1.84 g cmꢁ3) was first isolated. 2 has a density of 2.045 g cmꢁ3
3 which is extremely high for an acyclic compound, and 3
at 1.923 g cmꢁ3 is also significant for an energetic material
[10].
During the past two decades, considerable effort has been
focused on the development of high energy density materials
(HEDM) with higher performance and/or enhanced insensi-
tivity with respect to thermolysis, shock, friction, and
electrostatic discharge [1–3]. These modern HEDMs derive
most of their energy from: (1) oxidation of the carbon
backbone, and (2) their very high positive heats of formation.
Recently, using difluoramines as energetic materials, especially
for weapon systems containing aluminum and boron appeared
to offer intriguing possibilities [4]. Because on decomposition
in the presence of a hydrogen source, the NF2 compounds
produce a larger number of moles of gas per gram, by releasing
HF, than found for NO2 compounds. In addition, the
introduction of the dense NF2 group (2.30 g cmꢁ3 for NF2
vs. 2.17 g cmꢁ3 for NO2)[5] would increase the density of an
energetic material. Earlier calculations show that with similar
* Corresponding author. Tel.: +1 208 885 6215; fax: +1 208 885 9146.
E-mail address: jshreeve@uidaho.edu (J.M. Shreeve).
0022-1139/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2007.07.006

C. Ye et al. / Journal of Fluorine Chemistry 128 (2007) 1410–1415
1411
Ionic liquids/salts with high nitrogen content and high heats
of formation were regarded as a new family of explosives or
propellants, mainly because of their low vapor pressures, and
adjustable properties brought about by changing the cation or
anion counterparts, and because of their rather high densities.
Here we report three typical energetic salts/liquid synthesized
from 5-difluoroaminodifluoromethyl-tetrazole for which
F2NCF2CN is the precursor [14].
Scheme 2. Isodesmic reactions for fluorinated compounds.
The heats of formation of the energetic salts were calculated
using Gaussian 03 [16]. For fluorinated species, small basis
sets may cause large deviations in these calculations. Initially,
a comparison calculation was performed for 13 C1 and C2
fluorinated species by isodesmic reactions (Eqs. (1–5),
Scheme 2) correlated to CH3F (Scheme 1) (DHꢂ298 (CH3F,
gas) = ꢁ56.5 kcal molꢁ1) [17], using G2 [18], G3 [19], CBS-Q
[20], CBS-4 [21], and B3LYP/6-311+G(3df,2p)// B3LYP/6-
31g(d). Generally, due to a poor UHF/3-21G* geometry, the
errors for the CBS-4M method are much larger than those found
for G2, G3, CBS-Q, especially for a large system.
2. Result and discussion
2.1. Preparation and characterization data for NF2-
containing energetic salts
Based on our earlier work [14], the ammonium 5-
difluoroaminodifluoromethyl-tetrazolate (5) was readily
obtained by a neutralization reaction. After being transformed
into its silver salt, and followed by metathesis reaction with
corresponding iodides, two ionic solid/liquid (6, 7) were
obtained in good yield (Scheme 1). Salts 5 and 6 are solids at
room temperature, but 7 is a very non-viscous liquid with a
density of 1.40 g cmꢁ3 which may be attributed to the
asymmetric character and lack of hydrogen bonding in the
cation. The three compounds are thermally stable to ꢀ150 8C.
The ammonium salt (5) has a rather high density of
1.88 g cmꢁ3. The densities of the energetic salts were also
predicted using our newly established volume parameters
method for salts/liquids [15]. For 5 and 6, the predicted
densities are reasonably close to the experimental values.
However, the CBS-4 model is very much faster computa-
tionally. The errors of CBS-4 are systematic and may be greatly
reduced by the use of isodesmic reactions [21]. These lend
credibility to the investigation of large systems for which G2, G3
and CBS-Q methods are not feasible. As shown in Table 1, CBS-
4 has a mean absolute deviation (MAD) of 0.95 kcal molꢁ1 for
the 13 test sets which is comparable with results obtained when
G2, G3, or CBS-Q is used and is much more accurate than a large
basis set of 6–311+G(3df,2p) at B3LYP level on B3LYP/6-
31g(d) geometry.
Scheme 1. Synthesis of NF2-containing energetic salts.

1412
C. Ye et al. / Journal of Fluorine Chemistry 128 (2007) 1410–1415
Table 1
Calculated and literature heats of formation for fluorinated compoundsa
Compound
DfHꢂ298
Deviation (literature–calculated)
Literaturea
G2
G3
CBS-Q
CBS-4M
DFTb
CH2F2
ꢁ108.10
ꢁ166.20
ꢁ223.14
ꢁ105.66
ꢁ65.78c
ꢁ106.97c
ꢁ118.79
ꢁ178.20
ꢁ211.11
ꢁ321.27
ꢁ118.40
ꢁ212.33
ꢁ246.51
ꢁ0.22
0.41
ꢁ0.12
0.52
0.03
0.75
0.00
0.74
ꢁ0.99
ꢁ2.12
ꢁ4.67
ꢁ1.11
ꢁ0.79
ꢁ1.00
0.38
CHF3
CF4
0.34
0.32
0.75
1.17
CH3COF
C2H5F
ꢁ0.35
ꢁ0.52
ꢁ0.17
1.95
0.05
0.07
ꢁ0.21
ꢁ0.80
ꢁ0.99
1.61
ꢁ0.45
ꢁ0.01
2.12
ꢁ0.40
0.10
CH2F-CH2F
CF2H-CH3
CF3CH3
CF2H-CF2H
C2F6
1.38
2.54
2.98
3.20
2.50
ꢁ1.05
ꢁ2.89
ꢁ6.24
ꢁ2.46
ꢁ4.80
ꢁ3.62
0.72
0.97
1.36
ꢁ0.55
1.07
2.37
2.33
2.61
CF3CN
1.63
1.45
1.80
0.79
CF3CH2OH
CF3COOH
ꢁ0.59
0.55
ꢁ0.50
0.51
ꢁ0.12
0.67
ꢁ1.29
ꢁ0.59
0.95
Mean absolute deviation (MAD)
0.95
0.95
1.02
2.47
a
From Ref. [22] unless otherwise stated.
b
B3LYP/6-311+G(3df,2p)// B3LYP/6-31g(d).
Ref. [23].
c
The heat of formation, DfHꢂ298, for CH3F, ꢁ56.5 kcal molꢁ1
Standard heats of formation of the species involved in the
isodesmic reactions are ꢁ17.83 kcal molꢁ1 for CH4 [22],
ꢁ20.08 kcal molꢁ1for C2H6 [22], 47.9 kcal molꢁ1 for tetra-
zolate [31], ꢁ17.76 kcal molꢁ1 for CH3NO2 [22], while
ꢁ23.44 kcal molꢁ1 was used as DHꢂ298 for CH3NF2 which
was calculated at G3 level of theory by isodesmic bond
separation reaction (Eq. (10)).
used here is a calculated value [17] computed using complete
basis set limit extrapolation where no empirical parameters are
involved, because of the great uncertainty in the experimental
value reported for its heat of formation. For example, in the
JANAF tables [24] the value for DfHꢂ298 (CH3F, g) is given as
ꢁ56 ꢃ 6.9; ꢁ60 ꢃ 3.5 kcal molꢁ1 (NIST benchmark data-
base)[25]. Different calculation methods also lead to quite
different results: ꢁ57.4 ꢃ 1.2 kcal molꢁ1 (based on isogyric
and hydrogenation reactions) [26]; ꢁ55.6 ꢃ 2 kcal molꢁ1 [27];
and ꢁ55.9 ꢃ 1 kcal molꢁ1 [28], ꢁ55.8 to ꢁ58.6 kcal molꢁ1
using atomization and isodesmic bond separation reactions
when correlated to CF4 using G2, G2(MP2), CBS-4, CBS-Q,
CBS-APNO, BAC-MP4, etc. [29].
The heat of formation of 5-difluoroaminodifluoromethyl-
tetrazolate (TA-CF2NF2) was found to be ꢁ75.71 kcal molꢁ1
(Eq. (6)) and its derivatives, e.g. 5-difluoroaminomethylte-
trazolate (TA-CH2NF2), 5-difluoroaminotetrazolate (TA-NF2)
and 5-difluoroaminodinitromethyl-tetrazolate [30] (TA-
C(NO2)2NF2) have heats of formation of 23.02, 31.54,
27.28 kcal molꢁ1, respectively (Table 2).
The mean absolute deviation (MAD) of the isodesmic
reactions found using the four methods (G2, G3, CBS-Q and
CBS-4) for the 13 fluorinated molecules listed in Table 1 is
highly dependent on the heat of formation of CH3F. As shown
in Fig. 1, when values for DfHꢂ298 (CH3F) fall into the range of
ꢁ56.4 ꢃ 0.2 kcal molꢁ1, the MAD for G2, G3, CBS-Q and
CBS-4 isodesmic bond separation reactions approach the
lowest point on the curves, and are less than 1.0 kcal molꢁ1
.
Accordingly, a value of ꢁ56.4 ꢃ 0.2 kcal molꢁ1 as the standard
heat of formation for CH3F at 298 K, and ꢁ54.5 ꢃ 0.2
kcal molꢁ1 at 0 K can be derived based on the current method.
This value agrees perfectly with the ꢁ56.5 ꢃ 1.0 kcal molꢁ1
proposed by Dixon and co-workers [17]. This number further
confirms that ꢁ56.5 kcal molꢁ1 used for heat of formation for
CH3F is very credible.
Considering that most of the errors in calculation of the heats
of formation for energetic salts may derive from the estimation
of lattice energy, CBS-4 is sufficient for the current purpose.
Isodesmic bond separation reactions given below (Scheme 3)
were used to compute the heats of formation of the NF2-
containing anions: TA-CF2NF2 (Eq. (6)), TA-CH2NF2 (Eq. (7)),
TA-NF2 (Eq. (8)), TA-C(NO2)2NF2 (Eq. (9)) [30].
Fig. 1. MAD of 13 testing sets vs. DHꢂ298 (CH3F).

C. Ye et al. / Journal of Fluorine Chemistry 128 (2007) 1410–1415
1413
Scheme 3. Isodesmic reactions for NF2-containing anions.
Table 2
Heats of formation and detonation properties of NF2-containing salts
Densitya (g cmꢁ3
)
DHL
DfHꢂ298
P (GPa)c
V (m sꢁ1 c
)
b
Salts
DfHꢂ298 (anion)
DfHꢂ298 (cation)
Anion
Cation
150.57d
239.96
239.96
239.96
239.96
1.90 (1.88)
1.77 (1.69)
1.636
127.99
111.69
114.65
117.64
108.48
ꢁ53.13
52.56
28.78
23.55
24.82
28.81
32.45
8490
7905
8272
8752
8582
+
TA-CF2NF2
TA-CF2NF2
TA-CH2NF2
TA-NF2
NH4
ꢁ75.71
ꢁ75.71
23.02
DAMTe
DAMTe
DAMTe
DAMTe
148.33
153.86
158.76
31.54
1.705
TA-C(NO2)2NF2
27.28
1.794
DfH2ꢂ98 and lattice enthalpy (DHL), in kcal molꢁ1
a
.
Predicted based in Ref. [15], experimental value listed in parenthesis.
Predicted based in Ref. [33]. DHL (kcal molꢁ1) = 473.5 ꢄ (W/r)1/3 + 26.0.
Calculated using CHEETAH 4.0
b
c
d
e
NIST web book, Ref. [31].
DAMT: 1,5-diamino-4-methyl-tetrazolium.
It is likely that the stability of TA-C(NO2)2NF2 would be low
where DHL is the lattice enthalpy, for 1:1, non-linear, polya-
tomic salt, it was estimated by [33]
since –NO2 and NF2 on the same carbon tends to exert a
destabilizing influence weakening both the C–NO2 and C–NF2
bonds. Actually, the ammonium and hydrazium salts of TA-
C(NO2)2NF2 have proven to be highly sensitive [30]. For TA-
CH2NF2, there is a possibility of intramolecular elimination of
HF, in comparison, salt 5 is quite stable and insensitive.The heat
of formation of the cation 1,5-diamino-4-methyl-tetrazolium
(DAMT) was calculated to be 239.96 kcal molꢁ1 based on the
ꢀ
ꢁ
W
DHL ðkcal molꢁ1Þ ¼ 473:5 ꢄ
1=3 þ 26:0
r
Detonation properties were also estimated using Cheetah 4.0
[34]. From Table 2, it can be seen that although the ammonium
salt of TA-CF2NF2 has a low heat of formation, its detonation
properties, e.g. detonation pressure (P) and detonation velocity
(V) are comparable with those of TA-NF2 salts, and are superior
to those of TNT (P = 20.6 GPa, V = 6700 m sꢁ1). This is
mainly due to the much higher density of 5. The calculated
¨
G3 electronic energetic reported by Klapotke et al. [32]. Thus,
the heat of formation can be calculated by:
DfHꢂ298 ðsaltÞ ¼ DfH2ꢂ98 ðcationÞ þ DfH2ꢂ98 ðanionÞ ꢁ DHL;

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C. Ye et al. / Journal of Fluorine Chemistry 128 (2007) 1410–1415
detonation velocity of compound 5, and DAMT salt of TA-NF2
are comparable with that of RDX (P = 34.4 GPa, D =
8750 m sꢁ1).
3.4. 1-Methyl-4-amino-1,2,4-triazolium 5-
difluoroaminodifluoromethyl-tetrazolate (7)
In summary, three NF2-containing energetic salts/liquids
were synthesized.
Compound 7 was prepared using the procedure above with
1-methyl-4-amino-1,2,4-triazolium iodide [35], colorless
liquid, yield 99%. Glass transition temperature (Tg) ꢁ38 8C.
Decomposition temperature (Td) 151 8C. 1H NMR (CD3CN): d
4.09 (s, 3H, CH3), 6.80 (bs, 2H, N-NH2), 8.82 (s, H, C-H); 9.84
(s, H, C-H); 13C NMR (CD3CN): d 39.1 (–CH3), 120.4
(–CF2NF2, tt, J = 264.8, 13.9 Hz), 143.2, 145.1, 153.1 (t, C-
CF2NF2, J = 27.6 Hz). 19F NMR (CD3CN): d 18.6 (s, –
CF2NF2, 2F), ꢁ100.7 (s, CF2NF2, 2F). Anal. calcd for
C5H7F4N9 (MW 269.16) C, 22.31; H, 2.62; N, 46.83; Found C,
22.30; H, 2.62; N, 45.97.
Each of these salts show moderate thermal stability to about
150 8C. The ammonium 5-difluoroaminodifluoromethyl-tetra-
zolate (5) has a rather high density of 1.88 g cmꢁ3. Their heats
of formation were calculated using CBS-4 with isodesmic
reactions. The results show that detonation properties of salt 5
are superior to those of TNT.
3. Experimental
3.1. General methods
Acknowledgements
1H, 13C NMR, and 19F NMR spectra were recorded on a
300 MHz nuclear magnetic resonance spectrometer operating
at 300.13, 75.48, and 282 MHz, respectively. Chemical shifts
were reported relative to TMS or CFCl3. Elemental analyses
were measured with an Exeter Analytical CE-440 elemental
analyzer. Gaseous N2F4 was purchased from Elf Atochem
North America, Inc.
The authors gratefully acknowledge the support of
HDTRA1-07-1-0024), NSF (CHE-0315275), and ONR
(N00014-06-1-1032).
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1
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