Improving the Stability of Trinitramide by Chemical Substitution: N(NF2)(3) has Higher Stability and Excellent Propulsion Performance

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Yaempongsa, D., Brinck, A., Brinck, T. (2021)

Improving the Stability of Trinitramide by Chemical Substitution: N(NF2)(3) has Higher Stability and Excellent Propulsion Performance

Propellants, explosives, pyrotechnics, 46(2): 245-252 https://doi.org/10.1002/prep.202000305

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DOI: 10.1002/prep.202000305

Improving the Stability of Trinitramide by Chemical Substitution: N(NF ₂ ) ₃ has Higher Stability and Excellent Propulsion Performance

Dhebbajaj Yaempongsa,

^{[a, b]}

Ann Brinck,

^[a]

and Tore Brinck*

^[a]

Dedicated to Professor Thomas Klapötke on the Occasion of his 60

^th

Birthday.

Abstract: The potential for improving the stability of trini- tramide (N(NO

₂

)

₃

) by chemical substitution of the NO

₂

group has been investigated using Kohn-Sham density functional theory [M06-2X/6-31 + G(d,p)] and ab initio quantum chemistry [CBS-QB3]. Monosubstituted analogs are generally found to have a decreased N-NO

₂

bond dis- sociation enthalpy (BDE) because of increased stabilization of the N(NO

₂

)X radical intermediate resulting from the bond cleavage. This is particularly apparent for N(NO

₂

)

₂

NH

₂

, which has a BDE of only 54 kJ/mol. Instead it is shown that the stability of TNA can be significantly improved by sub- stituting all three NO

₂

for the NF

₂

group. The resulting mol- ecule, N(NF

₂

)

₃

, has a N N BDE of 138 kJ/mol, which is 17 kJ/

mol higher than the N N BDE of N(NO

₂

)

₃

. In contrast to N

(NO

₂

)

₃

, there are no indications that the stability of N(NF

₂

)

₃

is significantly reduced in polar solvents. Condensed phase properties of N(NF

₂

)

₃

have been estimated based on surface electrostatic potential calculations, and N(NF

₂

)

₃

is estimated to be a liquid in the approximate temperature range of 170–290 K because of its nonpolar character. The perform- ance of N(NF

₂

)

₃

in propellant formulations with fuels rich in hydrogen and/or aluminum has been investigated. N(NF

₂

)

₃

propellants are shown to outperform propellants based on standard oxidizers by up to 20 % in specific impulse and up to 100 % in density impulse. Compositions of N(NF

₂

)

₃

and HMX have significantly higher detonation performance than CL-20.

Keywords: Oxidizer · Fluorine · Difluoroamino · Propellant · High Energy Density Material

1 Introduction

Trinitramide (TNA, N(NO

₂

)

_3,

1) was first observed in 2010, and it is the largest nitrogen oxide that has been detected experimentally [1]. It has a high predicted heat of formation and density, and the computed propulsion performance of propellant formulations of TNA with hydrogen or aluminum rich fuels is excellent [1, 2].

TNA was first prepared and detected in our laboratory after a comprehensive computational study, which pro- vided guidance not only for the preparation but also for the detection of TNA [1]. The original synthesis featured a direct nitration of KN(NO

₂

)

₂

or NH

₄

N(NO

₂

)

₂

in acetonitrile at 30 ° C using NO

₂

BF

₄

as the nitrating agent. As predicted, TNA was found to decompose rapidly in solution and thus isolation of TNA was not possible. The formation of TNA was instead confirmed by direct spectroscopic observation (IR and NMR) of TNA and its decomposition products in the reaction ves- sel.

The gas phase stability of TNA is relatively high with a barrier for N N bond cleavage close to 120 kJ/mol accord- ing to the latest quantum chemical calculations [2]. In addi- tion to the homolytic dissociation of the N N bond, there is a second decomposition channel that involves a transfer of

a NO

₂

-group to form a high lying intermediate with a NO

₂

N

₂

O NO

₃

structure [1]. This decomposition channel has a very similar barrier as the direct N N cleavage and the decomposition half-life of TNA in the gas phase can be estimated to 15 years at 20 ° C [2]. However, the barrier for the NO

₂

-group transfer pathway is very sensitive to the en- vironment; in the gas phase the reaction can be charac- terized as a NO

2

-radical transfer, but in solution, it is gradu-

[a] D. Yaempongsa, A. Brinck, T. Brinck Applied Physical Chemistry Department of Chemistry KTH Royal Institute of Technology Stockholm SE-100 44, Sweden

*e-mail: tore@kth.se [b] D. Yaempongsa

Present address: Directorate of Armament Royal Thai Air Force

Bangkok 10120, Thailand

© 2021 The Authors. Propellants, Explosives, Pyrotechnics pub-

lished by Wiley-VCH GmbH. This is an open access article under

the terms of the Creative Commons Attribution Non-Commercial

License, which permits use, distribution and reproduction in any

medium, provided the original work is properly cited and is not

used for commercial purposes.

ally transformed into a NO

₂

-cation transfer with increasing solvent polarity [2]. Already in a nonpolar solvent, such as THF, the decomposition barrier is reduced by 20–40 kJ/mol.

In acetonitrile, which was used in the original synthesis, the decomposition barrier is less than 84 kJ/mol and the life- time is reduced to around 30 minutes at 30 ° C.

TNA is a molecule of low polarity and it has been pre- dicted to be a liquid at ambient temperature [2]. In the liq- uid phase, each TNA molecule will experience an environ- ment that is similar to a nonpolar solvent and thus the liquid stability is expected to be much lower than the sta- bility in the gas phase. Thus, any practical application of TNA would require it be to be used under cryogenic con- ditions where TNA would be in solid rather than liquid form.

Considering the apparent stability issues, which not only prevents the large scale synthesis and isolation of TNA, but also its practical use, it is imperative to increase the under- standing of the bonding in TNA to facilitate the design of analogs with improved stability. In this work, we have first analyzed a series of monosubstituted TNA analogs, where one NO

₂

group has been substituted for substituents of varying electron donating and electron accepting proper- ties. On the basis of this analysis we suggest a trisubstituted analog (N(NF

₂

)

₂

that not only has much improved stability, but also shows outstanding performance for use as an oxi- dizer in propellant and explosive formulations.

2 Computational Methods and Procedure

Geometrical structures and energies of TNA and the sub- stituted analogs together with their dissociation products have been optimized using Kohn-Sham density functional theory at the M06-2X/6-31 + G(d,p) level. The M06-2X [3] ex- change correlation functional has been shown to provide accurate geometries and energies for highly nitrated com- pounds, including TNA [2, 4].

Structures of TNA and N(NF

₂

)

₂

have also been optimized using coupled-cluster theory at the highly accurate CCSD/6- 311G(d) level. The structures are very similar to those ob- tained at the M06-2X/6-31 + G(d,p) level, which further sup- ports the use of the M06-2X functional.

Energies of N(NF

₂

)

₂

and its decomposition products were also calculated at the CBS-QB3 level, which involves computing structures and vibrational frequencies at the B3LYP/6-311G(d) level followed by CCSD(T) energies that are extrapolated to complete basis set limit [5, 6]. Earlier work has shown that B3LYP slightly overestimates the N N bond length of TNA, which led CBS-QB3 to overestimate the energy of TNA [2]. Similar problems were not observed for N(NF

₂

)

₂

and we expect the CBS-QB3 energy to be highly accurate. In order to calculate the gas phase enthalpy of formation for N(NF

₂

)

₂

, we used an isodesmic reaction (vide infra) and combined the computed energies with accurate experimental enthalpies of formation. All quantum chem-

ical calculations were performed using the Gaussian suite of computer programs [7].

In order to obtain condensed phase properties, we have computed the surface electrostatic potential at the B3PW91/6-31G(d,p) level using the hs95(ver.20190522) pro- gram of Brinck [8]. Enthalpies of vaporization and sub- limation have been computed from a statistical analysis of the surface electrostatic potential using a parameterized re- lationship developed by Politzer and coworkers [9]. The methodology originates from work by Brinck, Murray and Politzer [10, 11]. The solid density has been obtained by a similar type relationship, which use the scaled molecular volume of the 0.001 a.u. electron density contour together with an electrostatic potential correction that accounts for the effect of intermolecular interactions [12]. This relation- ship has only been parametrized for CHNO compounds, but tests by Yaempongsa show that it gives improved density predictions also for F-containing explosives [13]; the agree- ment with experimental densities is significantly improved compared to the method of determining the density from the unscaled volume of the 0.001 a.u. density contour [14].

Propulsion performance in terms of specific impulse and combustion temperature of bipropellants has been com- puted using the rocket propulsion analysis program, RPA Lite edition 1.2.8 [15]. Computation of detonation proper- ties has been made based on Chapman-Jouguet theory us- ing the method of Keshavarz and Pouretedal [16], which is a revision and extension of the “simple method” of Kamlet [17, 18]. A computer program for computing the detonation properties of binary compositions was developed as part of this study.

As a measure of the oxidizing capacity of the oxidizers, we have computed their oxygen balance (OB) using the fol- lowing equation:

OBð%Þ ¼ 1600

M

_W

ð 2n

_C

þ n

_H

=2 n

_O

n

_F

=2 Þ (1)

where n

_X

refers to the number of atoms of the type X in the molecular formula. As indicated by the equation, a mole- cule can have a positive oxygen balance even if it lacks oxy- gen, pending that it has a surplus of fluorine.

3 Results and Discussion

3.1 Substituent Effects on Stability

To analyze the character and stabilization of the N N bond in TNA, we have studied a range of monosubstituted TNA analogs with different substituents. It has generally been observed that in highly nitrated systems the C-NO

₂

or N- NO

₂

bonds can be stabilized by electron donating sub- stituents, and particularly by resonance donating sub- stituents, such as NH

₂

[19]. A classic example is TATB, which has significantly improved stability compared to TNB and

Full Paper D. Yaempongsa, A. Brinck, T. Brinck

246

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246

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whose high stability and insensitivity at least partly can be attributed to the resonance interaction between the amino and nitro groups. A similar but much stronger effect is ob- served in H

₂

N NO

₂

, which has a N N BDE of 267 kJ/mol compared to 50 kJ/mol in O

₂

N NO

₂

[21]. Rather surpris- ingly, we found a very low N-NO

₂

BDE of 54 kJ/mol in N (NO

₂

)

₂

NH

₂

. This is 68 kJ/mol lower than the BDE of TNA. It is also noteworthy that the N NO

2

bond length is significantly reduced after the bond cleavage; it goes from 1.508 Å in N (NO

2

)

2

NH

2

to 1.393 Å in the formed N(NO

2

)NH

2

radical. A similar effect is seen in the N NH

₂

bond length, which de- creases from 1.364 to 1.308 Å. Thus, the BDE as well as the changes in the N N bond lengths support the inter- pretation of a much stronger resonance stabilization of the N(NO

₂

)NH

₂

radical in comparison with the parent molecule, N(NO

₂

)

₂

NH

₂

.

The result for N(NO

₂

)

₂

NH

₂

indicates that the relatively high stability of N(NO

₂

)

₃

is largely due to the symmetric na- ture of the molecule. Perturbations that reduce the symme- try, either by an external field as in a solvent or by an elec- tron donating substituent are found to reduce the stability.

On the basis of this observation, we decided to investigate a number of electron withdrawing substituents of varying nature with respect to their inductive and resonance with- drawing properties. All substituents except for CF

₃

were found to reduce the N-NO

₂

BDE relative to TNA. Interest- ingly there is no correlation between the BDE and the N- NO

₂

bond length of the molecule. As an example, N(NO

₂

)

₂

C (CN)=C(CN)

₂

which has a very low BDE of 64 kJ/mol, has the shortest N-NO

2

bond length of all the closed shell mole- cules. Again we believe that the low BDE is an effect of an increased resonance stabilization of the radical inter- mediate formed by the N NO

₂

bond cleavage, as C(CN)=C (CN)

₂

is a strong resonance acceptor [22]. The CF

₃

sub- stituent, which is considered an inductive acceptor, is found to increase the N NO

₂

BDE to 138/mol. At first this result may seem puzzling, but a similar interpretation as for the low BDEs of N(NO

₂

)

₂

NH

₂

and N(NO

₂

)

₂

C(CN)=C(CN)

₂

can be applied. The N(NO

2

)CF

3

radical is destabilized relative the N (NO

₂

)

₂

radical due to a higher degree of resonance stabiliza- tion in the latter, and this radical destabilization leads to a higher BDE of N(NO

₂

)

₂

CF

₃

compared to TNA. Unfortunately, N(NO

₂

)

₂

CF

₃

has a highly negative gas phase enthalpy of for- mation ( 506 kJ/mol) and we expected it to be of limited interest for energetic material applications. However as pointed out by a reviewer, a substance with negative en- thalpy of formation can still be a powerful explosive. The most prominent example is PETN, which has a enthalpy of formation similar to that of N(NO

₂

)

₂

CF

₃

. The potential for us- ing N(NO

2

)

2

CF

3

as performace enhancing additive to HMX has been investigated in a latter part of this article.

The analysis of the substituent effects on the N-NO

₂

BDE of the TNA analogs indicates that the development of com- pounds with improved stability can follow two different strategies. One approach is to increase the BDE by design- ing compounds where the initial bond cleavage generates a radical intermediate that is highly destabilized. N(NO

₂

)

₂

CF

₃

is an example of a compound whose relatively high stability to some extent is afforded by this effect. However, this type of strategy is difficult to control as the target molecule may end up following an alternative decomposition mechanism with a lower barrier, and it is very difficult to anticipate ev- ery potential decomposition mechanism. Instead, it is pref- erable to enhance the stability by increasing the intrinsic bond strength of the weakest bonds in the target molecule.

Thus, to design a molecule with improved stability com- pared to TNA, we should not decrease the symmetry of the molecule but rather find an alternative substituent to re- place all NO

2

groups of the molecule

.

In this pursuit we were inspired by Politzer, who has shown that replacing NO

₂

for NF

₂

in nitroamines generally increases the N N bond strength by more than 15 kJ/mol [23]. Indeed, the N (NF

₂

)

₃

molecule has a computed N N BDE of 140 kJ/mol, which is 18 kJ/mol higher than the N-N BDE of TNA. The short N N bond length of 1.395 Å, which is shorter than in hydrazine (1.421 Å) and tetrafluorohydrazine (1.453 Å), is in- dicative of a high intrinsic bond strength. The geometries of TNA and N(NF

₂

)

₃

are depicted in Figure 1. It should be not- ed that whereas the N N bond length and BDE of TNA are very sensitive to the computational method, we do not find a similar variation for N(NF

₂

)

₃

. As an example, the N N bond length in N(NF

₂

)

₃

is very close to 1.40 Å, independent of method. This supports the notion that N(NF

₂

)

₃

has a higher intrinsic bond stabilization than TNA.

The relatively high barrier for N N bond cleavage

should render N(NF

₂

)

₃

sufficiently stable for practical appli-

cations. However, it is necessary to also consider alternative

decomposition pathways. Politzer has suggested that 1,1-

difluorohydrazines, which also incorporates the N NF

2

func-

tionality, may decompose via the facile loss of a F due to a

resonance induced charge delocalization of the positive

charge on the formed NF

⁺

to the adjacent N [24]. How-

ever, it was argued that this destabilizing effect can be

counteracted by the introduction of electron withdrawing

groups on the second nitrogen, such as in (NC)

₂

NNF

₂

. Con-

sidering that NF

₂

is strongly electron withdrawing and that

N(NF

₂

)

₃

can be written (F

₂

N)

₂

NNF

₂

[25], this so called

anomeric effect is not likely to significantly reduce the sta-

bility of N(NF

₂

)

₃

. We have also considered a F atom transfer

to another NF

2

group to form NF

3

. However, our calcu-

lations indicate that the barrier for this pathway is too high

in energy for the reaction to be relevant for the decom-

position of N(NF

₂

)

₃

. Thus in contrast to TNA, we do not ex-

pect that the stability of N(NF

₂

)

₃

will be significantly re-

duced in polar solvents.

3.3 Properties of N(NF

₂

)

₃

We have estimated the gas phase enthalpy of formation of N(NF

₂

)

₃

based on quantum calculations at the CBS-QB3 lev- el. A direct calculation of DH

^�_f

ðgÞ following the approach of [6] renders a value of 109 kJ/mol. However, using an iso- desmic reaction (N(NF

₂

)

₃

+ 4 NH

₃

!NF

₃

+ 3 N

₂

H

₄

) and ex-

perimental DH

^�_f

ðgÞ for NH

₃

, NF

₃

and N

₂

H

₄

gives a DH

^�_f

ðgÞ val- ue of 120 kJ/mol for N(NF

₂

)

₃

as shown in Table 1. We believe this value to be more accurate as the direct calcu- lation on NF

₃

indicates that the CBS-QB3 method under- estimates the energy of a N F bond by circa 2 kJ/mol [6].

We have also performed surface electrostatic potential calculations of N(NF

₂

)

₃

at the 0.001 au isodensity contour to Figure 1. The structures of TNA and N(NF

2

)

3

computed at the CCSD/6-311G(d) level are shown at the top. Bond lengths are given in Ang- strom. Surface electrostatic potentials [V

_S

(r)] computed at the B3PW91/6-31G(d,p) level are shown at the bottom. The surface is defined by the 0.001 au isodensity contour. The V

S

(r) maps clearly show that N(NF

2

)

3

is less polar in character than TNA and thus will form weaker intermolecular interactions.

Table 1. N-NO

2

bond lengths and bond dissociation enthalpy (BDE) computed at the M06-2X/6-31 + G(d,p) level for the dissociation of TNA (N(NO

2

)

3

) and substituted TNA analogs.

Molecule N-NO

2

Bond length N-NO

2

Bond length

in dissoc. prod.

^[a]

BDE

N(NO

2

)

3

1.502 1.459 122, 121

^[d]

N(NO

2

)

2

NH

2

1.508/1.497

^[b]

1.393 54

N(NO

₂

)

₂

CN 1.487 1.459 98

N(NO

2

)

2

NF

2

1.520 1.456 103

N(NO

2

)

2

CF

3

1.494/1.474

^[b]

1.424 138

N(NO

2

)

2

C(CN) = C(CN)

2

1.480/1.469

^[b]

1.434 64

N(NF

2

)

3

1.395

^[c]

1.350

^[c]

140, 138

^[e]

[a]

Bond length in the radical formed after N-NO

2

cleavage.

[b]

The two N-NO

2

bonds are of different length.

[c]

Refers to the N-NF

2

bond.

[d]

Best theoretical estimate from Ref. [1, 2].

[e]

Best theoretical estimate based on CBS-QB3 calculations.

Full Paper D. Yaempongsa, A. Brinck, T. Brinck

248

© 2021 The Authors. Propellants, Explosives, Pyrotechnics published by Wiley-VCH GmbH Propellants Explos. Pyrotech. 2021, 46, 245–252

248

www.pep.wiley-vch.de

first be noted that the surface electrostatic potential [V

_S

(r)]

of N(NF

₂

)

₃

shows the molecule to be of low polarity. Al- though the central nitrogen has a slightly pyramidal coordi- nation with a N N N angle of 115 degrees, the minimum in V

_S

(r) (V

_S,min

) associated with the lone pair region has a pos- itive value of 10 kJ/mol. This shows that the electron den- sity of the lone pair is strongly diminished by the electron withdrawing NF

2

groups. The situation is similar to TNA, where the V

_S,min

value at the corresponding position is 0 kJ/

mol. The most negative position is instead found at the lone pair region of each NF

₂

group with a value of -25 kJ/

mol. The most positive V

_S,max

has value of 59 kJ/mol and is found outside the NF

₂

nitrogen at the extension of the N F bond, and it is indicative of a σ-hole. Overall N(NF

₂

)

₃

shows a smaller variation in V

_S

(r) than TNA, indicating that TNA is more polar than N(NF

₂

)

₃

. In TNA the most positive V

_S,max

has a value of 155 kJ/mol, and it is found at the central nitrogen but on the opposite side of the lone pair region. There are negative regions over the oxygens with the lowest V

S,min

amounting to 34 kJ/mol. The larger variation in V

_S

(r) for TNA is also reflected in higher predicted enthalpies of va- porization and sublimation compared to N(NF

₂

)

₃

. The en- thalpy of vaporization and enthalpy of sublimation for N (NF

₂

)

₃

are 25 kJ/mol and 31 kJ/mol, respectively. The corre- sponding values for TNA are 31 and 43 kJ/mol. Using Trou- ton's rule (ΔH

_vap

=0.085T

_b

), we estimate the boiling point of N(NF

₂

)

₃

to 290 K (16 ° C). The melting point prediction has higher uncertainty, but empirical relationships of ref. [27]

indicate a value of around 170 K (ca 100 ° C). Thus, for practical applications N(NF

2

)

3

is likely to be used in the liq- uid form, which probably will require a temperature below room temperature or pressurized conditions. The solid den- sity is estimated to 2.32 kg/dm

³

based on Politzer's relation- ship (2.47 g/cm

³

from the 0.001 au electron density con- tour) [12, 14]. We estimate the liquid density to 2.0 g/cm

³

based on empirical relationships between solid and liquid densities for organic compounds [11, 26]. The condensed phase properties of TNA and N(NF

₂

)

₃

are summarized in Ta- ble 2.

3.4 Performance of N(NF

₂

)

₃

Propellants

The computed propulsion performance of N(NF

₂

)

₃

in pro- pellant formulations with some common fuels are listed in Table 3. Furthermore, the performance of N(NF

₂

)

₃

is com- pared to commonly used oxidizers and TNA. Beginning with the hydrazine formulations optimized with respect to spe- cific impulse, TNF at 78 % by weight increases the specific impulse by as much as 15 % compared to the optimum N

₂

O

₄

composition (57 %). The increase in density impulse is much higher, 55 %, and it is mainly due to higher loading of oxidizer in the N(NF

₂

)

₃

propellant. In comparison with liquid oxygen (LOX), the performance advantage of N(NF

₂

)

₃

in spe- cific impulse and density impulse is 7 % and 65 %, re-

spectively. N(NF

₂

)

₃

also performs considerably better than TNA by 12 % and 29 %. The combustion temperature is very high (4543 K) for the optimized N(NF

2

)

3

propellant, but the temperature is reduced to 3375 K at 59 % loading with a penalty of a reduced specific impulse by only 6 %.

For the liquid hydrogen propellants, it can be noted that LOX has an advantage in specific impulse over N(NF

₂

)

₃

by 8 %. However, the density impulse of the N(NF

₂

)

₃

propel- lant is twice that of the LOX propellant. Again we note that the combustion temperature (3844 K) is high for the opti- mized (92 %) N(NF

₂

)

₃

propellant. A reduction to 88 % N(NF

₂

)

₃

reduces the temperature to 3172 K, with only a minor re- duction in specific impulse but a significant decrease in density impulse. Compared to TNA, the optimized N(NF

2

)

3

propellant is slightly better in specific impulse but superior in density impulse.

In the case of the Al-based propellant, N(NF

₂

)

₃

outper- forms the ammonium perchlorate and TNA propellants by almost 20 % in specific impulse and 18 % in density impulse.

However, the combustion temperature is exceedingly high, 5260 K. Aluminum hydride is potentially a better fuel alter- native as the combustion of the corresponding propellant will, in addition to AlF

3

, generate HF molecules. HF is accel- erated by the strongly exothermic combustion reaction and thereby increases the specific impulse. The optimum N (NF

₂

)

₃

:AlH

₃

propellant (82 : 18) has a 9 % higher specific im- pulse and a similar density impulse as the N(NF

₂

)

₃

:Al propel- lant. The combustion temperature of the former is slightly lower and can be decreased further by increasing the AlH

₃

content.

3.5 Performance of N(NF

₂

)

₃

in Explosive Compositions

TNA and N(NF

₂

)

₃

are not of direct interest as explosives as they are primarly oxidizers. However, they are high in energy and can be used as additives to improve the performance of explosives that have a negative oxygen balance and that are rich in hydrogen. In this work we have investigated composi-

TNA N(NF

2

)

3

Molecular weight (MW), g/mol 152.02 170.02 Heat of formation (ΔH

^of

(g) ), kJ/mol 229 120 Heat of sublimation (ΔH

sub

), kJ/mol 43 31 Heat of vaporization (ΔH

vap

), kJ/mol 31 25 Melting/boiling point (mp/bp)

,

K 280

^[b]

/360

^[c]

170/290

^[c]

Density of solid/liquid, g/cm

³

2.0

^[a]

/1.7

^[d]

2.32/2.0

^[d]

Oxygen balance, % vs CO

2e

+ 63.15 % + 28.23 %

[a]

Computed value from Ref. [1].

[b]

Estimated value from Ref. [2].

[c]

Estimated bp based on Trouton’s rule.

[d]

Estimated value based on empirical relationships from Ref.

[11, 26].

[e]

Computed using Eq. (1).

tions of HMX with TNA and N(NF

₂

)

₃

as additives (Table 4)

_.

HMX is a hydrogen rich explosive with excellent performance and low sensitivity. The detonation pressure of HMX is increased from 39.2 to 42.8 GPa, and the detonation velocity from 9.29 to 9.66 km/s by the addition of 23.5 % by weight of TNA. This is a significant improvement in detonation performance, but this ideal HMX/TNA composition is still not as proficient as CL- 20. N(NF

₂

)

₃

is a significantly more effective additive due to its lower formal oxygen balance (Eq. (1)), which results in a high- er loading of the oxidizer compared to the optimum HMX:

TNA composition. At the optimum loading of 43 % N(NF

₂

)

₃

, the HMX:N(NF

₂

)

₃

composition outperforms CL-20 with a deto- nation pressure of 47.3 GPa and a detonation velocity of 10.11 km/s.

Finally, we also investigated the potential for using N (NO

₂

)

₂

CF

₃

(s) as an additive to HMX. Despite the strongly negative enthalpy of formation of 558 kJ/mol, a 40 % loading of N(NO

2

)

2

CF

3

significantly improves the perform- ance compared with pure HMX; the detonation pressure is close to the optimum HMX:TNA composition, and the deto- nation pressure is in between pure HMX and the HMX:TNA composition. In this case the improvement in performance is mainly a result of the high density of solid N(NO

₂

)

₂

CF

₃

, and using N(NO

₂

)

₂

CF

₃

in liquid form would not significantly improve the performance compared to pure HMX. The melting point of N(NO

₂

)

₂

CF

₃

is estimated to around 280 K.

Table 3. Computed propulsion performance of TNA-based propellants compared to propellants based on conventional oxidizers and fuels.

^[a]

Oxidizer (O) Fuel (F) O : F Ratio at maximum I

sp

Specific Impulse, (I

sp

) (s)

Density Impulse (I

d

) (kg,s/dm

³

)

Combustion Temperature (T

c

) (K)

O

2

(l) N

2

H

4

(l) 48 : 52 313 333 3392

N

2

O

4

(l) N

2

H

4

(l) 57 : 43 293 356 3250

TNA(s) N

2

H

4

(l) 59 : 41 300 427 3375

N(NF

2

)

3

(l) N

2

H

4

(l) 64 : 36

^[b]

321 473 3795

N(NF

2

)

3

(l) N

2

H

4

(l) 78 : 22 336 552 4543

O

2

(l) H

2

(l) 80 : 20 390 111 2947

N

₂

O

₄

(l) H

₂

(l) 85 : 15 342 122 2762

TNA(s) H

2

(l) 86 : 14 350 141 2891

N(NF

2

)

3

(l) H

2

(l) 88 : 12

^[b]

360 163 3172

N(NF

₂

)

₃

(l) H

₂

(l) 92 : 8 363 221 3844

NH

4

ClO

4

(s) Al(s) 73 : 27 248 522 4171

TNA(s) Al(s) 75 : 25 247 528 4648

N(NF

2

)

3

(l) Al(s) 81 : 19 296 623 5260

NH

4

ClO

4

(s) AlH

3

(s) 58 : 42 293 504 3637

TNA(s) AlH

3

(s) 57 : 43 302 525 4156

N(NF

2

)

3

(l) AlH

3

(s) 73 : 27

^[b

320 585 4570

N(NF

₂

)

₃

(l) AlH

₃

(s) 82 : 18 330 620 5054

[a]

The RPA Lite edition 1.2.8 was used for all performance computations [15]. A chamber pressure of 7 MPa and a nozzle expansion to atmospheric pressure (0.1 MPa) was assumed. ΔH

^of

values were taken as provided in the RPA software. ΔH

^of

(TNA) = 229 kJ/mol, ΔH

^of

(N (NF

₂

)

₃

) = 120 kJ/mol. Density impulses were calculated using the following densities (in g/cm

³

): 1(TNA) = 2.0, 1(N(NF

₂

)

₃

) = 2.0 1(O

₂

) = 1.141, 1 (N

2

O

4

) = 1.443, 1(N

2

H

4

) = 1.005, 1(H

2

) = 0.0678, 1(AlH

3

) = 1.486, 1(NH

4

ClO

4

) = 1.95.

[b]

Composition is not optimized for maximum I

sp.

Table 4. Computed detonation properties of high explosives with and without added oxidizer (TNA or N(NF

2

)

3

).

^[a]

Explosive (E) Oxidizer (O) E : O Mass ratio Density g/cm

³

Detonation pressure, GPa Detonation velocity, km/s

HMX – 100 : 0 1.91 39.2 9.29

HMX TNA(s) 76 : 24

^[b]

1.93 42.8 9.66

HMX N(NF

2

)

3

(l) 57 : 43

^[b]

1.95 47.3 10.11

HMX N(NO

2

)

2

CF

3

(s) 60 : 40 1.99 42.3 9.52

CL-20 – 100:0 2.04 45.2 9.76

[a]

Detonation properties were computed from the following ΔH

^of

(in kJ/mol) and densities (in g/cm

³

). H

^of

(TNA) = 229, ΔH

^of

(N(NF

2

)

3

) = 120, ΔH

o

f

(N(NO

2

)

2

CF

3

) = 558 kJ/mol, 120 ΔH

^of

(HMX) = 75, ΔH

^of

(CL-20) = 377, 1(TNA) = 2.0, 1(N(NF

2

)

3

) = 2.0, 1(N(NO

2

)

2

CF

3

) = 2.1, 1(HMX) = 1.91, 1(CL-20) = 2.04.

[b]

Composition with oxygen balance of 0 as defined by Eq. (1).

Full Paper D. Yaempongsa, A. Brinck, T. Brinck

250

© 2021 The Authors. Propellants, Explosives, Pyrotechnics published by Wiley-VCH GmbH Propellants Explos. Pyrotech. 2021, 46, 245–252

250

www.pep.wiley-vch.de

Our analysis of the monosubstituted TNA analogs indicates that most substituents (X) decrease the BDE for N-NO

₂

cleavage by stabilizing the N(NO

₂

)X radical that is formed in the dissociation reaction. This is particularly apparent with the NH

₂

substituent, which lowers the BDE to 54 kJ/mol. A lowering we attribute to the high resonance stabilization of the N(NO

2

)NH

2

radical that is formed by the N-NO

2

cleav- age. We believe that a similar effect is the reason behind the low BDE of N(NO

2

)

2

C(CN)=C(CN)

2

, as C(CN)=C(CN)

2

is a strongly resonance accepting substituent. The only singly substituted TNA analog that has an increased BDE is N (NO

₂

)

₂

CF

₃

, and here the most plausible explanation is that the inductive electron accepting substituent (CF

₃

) destabi- lize the N(NO

₂

)CF

₃

radical.

Rather than improving stability by destabilizing the ini- tial decomposition intermediate, we advocate the strategy to increase the intrinsic bond strength of the weakest bonds in the parent molecule. Following this approach we have suggested N(NF

₂

)

₃

as a substitute for TNA, as N-NF

₂

bonds are known to be stronger than N NO

₂

bonds. N(NF

₂

)

₃

has an increased N N BDE of 17 kJ/mol compared to TNA, and we have not found any alternative decomposition mechanism that is likely to reduce the stability.

Because of the many N N single bonds, N(NF

₂

)

₃

has a high internal energy, and the gas phase heat of formation has been computed to 120 kJ/mol. Surface electrostatic po- tential calculations show it to be of low polarity, and the vaporization enthalpy has been predicted to only 25 kJ/mol.

N(NF

2

)

3

is likely to be used in liquid form as the melting point is estimated to 170 K.

We have computed the propulsion performance of N (NF

₂

)

₃

in propellant formulations with hydrazine, liquid hy- drogen, aluminum and aluminum hydride as fuels. The N (NF

₂

)

₃

formulations outperform propellants based on stan- dard oxidizers, such as N

₂

O

₄

, LOX and ammonium perchlo- rate, by up to 20 % in specific impulse and up to 100 % in density impulse. N(NF

₂

)

₃

also performs much better than TNA with all fuels. This is mainly an effect of the much high oxidizer loadings that can be afforded with N(NF

₂

)

₃

, and it is a result of the lower oxygen balance of N(NF

₂

)

₃

.

Although N(NF

₂

)

₃

propellants excel in performance, their practical use will not be without problems. In particular, a fluorine based propellant can never be categorized as green, and there are severe health and environmental is- sues to consider both in the production and use of fluorine based propellants. However for applications where extreme performance is needed, N(NF

2

)

3

could become a com- petitive alternative to traditional oxygen-based oxidizers.

We have also considered the use of N(NF

2

)

3

as a per- formance enhancing additive to hydrogen rich explosives.

The performance of HMX can be significantly improved by the addition of up to 43 % N(NF

₂

)

₃

; the resulting composi- tion outperforms CL-20 by a wide margin in terms of deto- nation pressure and detonation velocity. However, there are

alized applications, we do not expect that N(NF

₂

)

₃

will be- come of any greater importance as a component in ex- plosive formulations.

We have not investigated the prospects for synthesizing and isolating N(NF

₂

)

₃

, but the stability analysis indicates that there should be much better potential for large scale syn- thesis of N(NF

₂

)

₃

compared to TNA. With some anticipation we leave this topic to be explored by the synthesis experts of the field. However, independent of the outcome, we hope that this study have demonstrated the significant im- provements in terms of stability and performance that can be gained by substituting NO

₂

with NF

₂

in nitrogen rich en- ergetic materials.

Data Availability Statement

Data available on request from the corresponding author.

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Manuscript received: November 24, 2020 Revised manuscript received: December 10, 2020 Version of record online: January 26, 2021

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252

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