A comparison of the thermal decomposition of nitramines and difluoramines

Article abstract of DOI:10.1021/jp002516a

The decomposition rates and product distributions of a number of nitro- and difluoramino-substituted six-membered rings were compared: nitrocyclohexane (I); 1,1-dinitro-cyclohexane (II); 1,1,4,4-tetranitrocyclohexane (III), 1,1,4,4-tetrakis(difluoramino)cyclohexane (IV); 1,4-dinitropiperazine (V); 1,4,4-trinitropiperidine (VI), and 4,4-bis(difluoramino)-1-nitropiperidine (VII). The study suggested the following order for susceptibility to decomposition: N-NO₂ > C-(NO₂)₂ > C-(NF₂)₂ The difference in bond energies among the compounds is small. Geminal bis(difluoramino) compounds appeared to be somewhat more stable than the corresponding gem-dinitro compounds though they released more heat during decomposition. Where a nitramine functionality was present, the nitroso analogue was observed as a major decomposition product. The decomposition of gem-bis(difluoramino) and gem-dinitro compounds exhibited similarities. Both experienced loss of one geminal NX₂ group followed by the rearrangement of the remaining NX₂. Where X was oxygen, loss of the initial nitro by homolysis was favored; rearrangement of the remaining nitro followed by homolysis of NO resulted in a C=O bond. Where X was fluorine, the initial difluoramino may have been lost as HNF₂. The remaining difluoramino reacted by losing fluorine, leaving C=NF or by losing HNF, resulting in =C-F; the latter was mainly observed.

Full text of DOI:10.1021/jp002516a

J. Phys. Chem. A 2001, 105, 579-590
579
A Comparison of the Thermal Decomposition of Nitramines and Difluoramines
J. C. Oxley,* J. L. Smith, and J. Zhang
Chemistry Department, UniVersity of Rhode Island, Kingston, Rhode Island 02881-0809
C. Bedford
NaVal Air Warfare Center, China Lake
ReceiVed: July 13, 2000; In Final Form: October 6, 2000
The decomposition rates and product distributions of a number of nitro- and difluoramino-substituted six-
membered rings were compared: nitrocyclohexane (I); 1,1-dinitro-cyclohexane (II); 1,1,4,4-tetranitrocyclo-
hexane (III), 1,1,4,4-tetrakis(difluoramino)cyclohexane (IV); 1,4-dinitropiperazine (V); 1,4,4-trinitropiperidine
(VI), and 4,4-bis(difluoramino)-1-nitropiperidine (VII). The study suggested the following order for
susceptibility to decomposition: N-NO2 > C-(NO2)2 > C-(NF2)2 The difference in bond energies among
the compounds is small. Geminal bis(difluoramino) compounds appeared to be somewhat more stable than
the corresponding gem-dinitro compounds though they released more heat during decomposition. Where a
nitramine functionality was present, the nitroso analogue was observed as a major decomposition product.
The decomposition of gem-bis(difluoramino) and gem-dinitro compounds exhibited similarities. Both
experienced loss of one geminal NX2 group followed by the rearrangement of the remaining NX2. Where X
was oxygen, loss of the initial nitro by homolysis was favored; rearrangement of the remaining nitro followed
by homolysis of NO resulted in a CdO bond. Where X was fluorine, the initial difluoramino may have been
lost as HNF2. The remaining difluoramino reacted by losing fluorine, leaving CdNF or by losing HNF,
resulting in dC-F; the latter was mainly observed.
Introduction
isomerization.8-11 The relative dominance of these pathways
depends on temperature; and since homolysis requires the most
energy, it is favored at high temperatures (Table 1).12,13
However, the isokinetic point between homolysis and elimina-
tion has been estimated to be much lower for gem-dinitro species
(370 K) as compared to mononitro- and vicinal dinitro-alkanes
(770 K).14 Cleavage of the gem-dinitro species is several orders
of magnitude faster than that of mononitroalkanes15 because the
bond is weaker due to inductive effects.16 Thus, homolysis is
considered to be the principal decomposition pathway in the
thermolysis of 2,2-dinitropropane and hexanitroethane.10
The presence of a nitro group signals a compound that
decomposes exothermically and which may require special
handling precautions. Indeed, most explosives contain NO2
groups. Another moiety with the same oxidizing and energetic
potential is the NF2 group. Since new routes for synthesizing
these NF2 compounds have become available,1,2 we have
examined the thermal stability of several new difluoramines and
their nitro analogues.
The N-NO2 linkage has frequently been studied in dimeth-
ylnitramine and the cyclic nitramines3-7 RDX and HMX. In
the latter two, alternating CH2N-NO2 groups make depolym-
erization with formation of N2O and CH2O an important
decomposition pathway.3-7 Proposed decomposition pathways
for RDX and HMX are shown in Figures1 and 2, respectively.6,7
Common to the thermolysis of all nitramines is the formation
of nitrosamine formed via N-NO2 homolysis, subsequent
reduction to NO, and reaction of NO with the amine:
f RCH2• + NO2
(2a)
(2b)
(2c)
RCH2-NO2 f •RdCH2 + HONO
f RCH2-ONO f RCH2O• + NO
Few gem-dinitroalkyl nitramines are known. One that has
found practical use is 1,3,3-trinitroazetidine (TNAZ).16,17 Unlike
many energetic materials, it is stable well above its melting point
(101 °C). In a theoretical study of its stability, only a 2 kcal/
mol energy difference was found between the products of
N-NO2 (44.6 kcal/mol) and C-NO2 scission (46.6 kcal/mol).18
We observed products from both decomposition pathways.19
Under the experimental conditions (neat thermolyses, 160 to
280 °C), N-NO2 was favored although its dominance is not
huge, 66% of the initial 10% decomposition. NO formed was
observed (by 15N-labeling experiments) to come from the
N-nitro group. A small DKIE (1.4) was observed as well as a
-NO2
R2N-NO2
8 R2N +NO 8 R2N-NO
(1)
Indeed, the observed activation energies for decomposition of
secondary nitramines falls in a range comparable to the N-NO2
bond energy (Table 1).
The C-NO2 linkage is found in a variety of nitroarenes and
in a more limited number of nitroalkanes. Three modes of
decomposition are generally postulated: (a) homolysis of the
C-NO2 bond; (b) inter- or intramolecular hydrogen transfer to
the nitro group, resulting in HONO loss; and (c) nitro/nitrite
10.1021/jp002516a CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/03/2001

580 J. Phys. Chem. A, Vol. 105, No. 3, 2001
Oxley et al.
Figure 1. Decomposition pathway of RDX (ref 6).
other observations a mechanism was postulated:
-NF2
C(NF2)4
8 [‚C(NF2)3 f (NF2)2C(F)NF] -F8
(NF2)2CdNF
-NF2
8 [F_C(NF2)2 V-NF8 (NF2)CF2(NF)] -F8
Figure 2. Decomposition pathway of HMX (ref 7).
FC(NF2)3
F(NF2)CdNF f f N2 + CF4 + NF3 (4)
sensitivity to ammonia; it was speculated that this resulted from
the C-NO2 cleavage pathway in which hydrogen transfer to
the nitro group assisted in bond breakage.19 Brill examined the
decomposition of TNAZ, DNNC, and HNDZ (Figure 3),
anticipating the weaker C-(NO2)2 linkage would be the site of
initial homolysis.20,21 This was the case for DNNC and HNDZ,
but results were unclear for TNAZ.
Difluoramines have several decomposition routes possible;
the principal two are analogous to those of the nitro analogues.
Homolysis of C-NF2 (3a), like homolysis of C-NO2 (2a),
requires considerable energy. It is not usually favored in
monodifluoramines unless temperatures are above 400 °C.
Where hydrogen is available, transfer to the X (X ) O or F) of
R-NX2 provides a lower energy decomposition pathway. In
nitro compounds, the next step is usually loss of HONO (2b);
in difluoramines, HF is eliminated (3b). The compound resulting
from HF loss might be considered analogous to the nitroso
species formed by nitramine compounds (1). Grebennikov22,23
and Ross24 have shown that loss of HF is favored at low
temperatures for monodifluoramines with alpha hydrogen.
-NF2
F2C(NF2)2
8 [F2_C(NF2) V-NF8 (NF)CF3] -F8 F2CdNF
In the series XC(NO2)2(NF2), where X ) NO2, CH3, or F,
C-NO2 homolysis was found to be the first step in thermal
decomposition.22 The thermolysis of these compounds, in both
gas and liquid phase, was first-order and independent of phase,
pressure, surface-to-volume ratio, or inhibitors. Because the
C-NF2 bond energy is generally higher than that of C-NO2
in gem(dinitro) alkanes,22,26 it is not surprising that C-NO2
homolysis occurred first (Table 1). After NO2 loss, several
possible decomposition routes were postulated:22
-NO2
(NO2)XC(NO2)(NF2)
8
XC˙ (NO2)(NF2) f XC(dO)(NF2) + NO oxidation
XC(dNF)(NO2) +
F ejection of F (5)
XC(dNF)F +
RCH2 - NF2 f RCH2• + NF2
NO2 rearrangement
Ea ) 214 kJ/mol (51 kcal/mol)23 (3a)
Examining a series of compounds where difluoramino groups
were attached to primary, secondary, or tertiary carbons,
Grebennikov et al. observed that solution decomposition was 6
orders of magnitude faster than in gas phase; and the activation
energy of 110-120 kJ/mol (26-28 kcal/mol) was significantly
lower than observed for HF loss in the gas phase (176 kJ/mol
or 42 kcal/mol).23 On the basis of these observations, they
proposed that the liquid-phase decomposition involved ionic
dissociation of the N-F bond:
RCH2 - NF2 f R′CH2 d NF + HF
Ea ) 176 kJ/mol (42 kcal/mol, gas phase)23 (3b)
In compounds containing no hydrogen, FxC(NF2)4-x (x ) 0
to 2), Sullivan25 postulated a mechanism involving first-order
dissociation of NF2 followed by loss of one fluorine from a
second NF2 ligand. Added N2F4 did not affect the reaction rate;
therefore, researchers concluded that NF2 did not abstract F from
FC(NF2)3. Also of note is the fact that C(NF2)4 did not form
FC(NF2)3, nor did FC(NF2)3 form F2C(NF2)2. From these and
RCH2NF2 S [RCH2NF+][F-] f RCHdNF + HF (6)

Decomposition of Nitramines and Difluoramines
J. Phys. Chem. A, Vol. 105, No. 3, 2001 581
TABLE 1: Summary of Arrhenius Parametersa
Ea kJ/mol
Ea (kcal/m ol)
bond
reaction
homolysis
range
averageb
log A s-1
ref
C-ONO
N-NO2
40
44
44
45
46
47
53
50
47
66
70
56
48
50
62
60
45
44
41
51
49
45
43
60
27
42
47
45
168
184
184
188
192
196
222
210
197
276
294
234
201
209
259
252
188
184
171
213
205
188
180
251
113
176
196
188
13
5
5
44
45
18
3
8
13
8
8
12
8
10
14
8
14
10
22
3
14
18
14
24
23
24
25
29
43
38
37
49
53
47
12-18
13-20
13-17
16
20
C-NO2 arene
C-NO2 alkane
homolysis
61
39
72
61
isomerization
-HONO from CH or NH
homolysis
13
17.5
11.5
-HONO
41
39
39
46
50
43
C-NO2 gem,di
homolysis
18.0
17.5
-HONO
homolysis
-HF
39
24
40
47
29
54
11.5
17.5
C-NF2
13.5
16.0
C-NF2 gemc,di
homolysis
a Range of activation energies (Ea) are given for papers reporting multiple compounds. Average is also given. b A single entry in the “average”
compound means only one compound was reported. c Ross says gem NF2 should be 10 kcals less.
Experimental Section
Mono- and dinitrocyclohexanes (I and II) were purchased
from Aldrich. 1,1,4,4-Tetranitrocyclohexane (III),30 1,1,4,4-tetra-
kis(difluoramino)cyclohexane (IV),31 and 1,4-dinitropiperazine
(V)32,33 were synthesized following literature procedures. The
syntheses of 1,3,3-trinitropiperidine (VI) and 3,3-bis(difluor-
amino)-1-nitropiperidine (VII) have been described elsewhere.34
Figure 3. Structures of nitramines (ref 21).
3,3,7,7-Tetrakis(difluoramino)octahydro-1,5-dinitro-1,5-diazo-
cine (VIII, HNFX) was synthesized and provided by Chapman.1
Such a mechanism is supported by the observations that
Compounds are shown at the top of Table 2. Differential
difluoramine decomposition is accelerated by bases and the
scanning calorimetric (DSC) analyses were performed to
decomposition of bis(2,2-dinitropropyl)-N-fluoramine is ac-
celerated by acids, bases, and polar solvents.27 However, as
pointed out, a purely heterolytic E1 mechanism fails to explain
HF loss in nonpolar solvents and gas-phase decompositions.27
Gem-bis(difluoramines) without alpha hydrogen do not undergo
HF elimination. Probably the electron-withdrawing difluoramino
group makes the carbon less likely to support a positive
charge:25
determine the endothermic and exothermic activity of the
compounds; a TA 2910 DSC was used to record the change in
heat flux from the sample while its temperature was raised at a
constant rate (usually 20°/min.). Samples were run under
nitrogen flow and calibrated against indium. A typical experi-
ment involved heating from 50 °C to 500 °C samples (0.2-0.5
mg) in sealed glass capillary tubes (1.5 mm o.d., 0.28 mm wall
thickness, and 8 mm length) held in the DSC head by an
aluminum support. In other analyses, sample size was held about
the same, but glass tube size varied: for kinetics and condensed-
phase products analysis, tubes with inner diameters of 2.0 mm
and total volumes of 150 to 200 uL were used; for gas
chromatography (GC) evacuated tubes of inner diameters 0.9-
1.1 mm and 40-50 uL total volumes were employed. Solution
studies used 40 to 60 uL samples of 1% (by weight) solutions
(in benzene, d6 benzene, or acetone). When tetra-substituted
cyclohexanes were heated at 320 °C in the presence of acid or
base, 0.5 uL nitric acid or pyridine was added to 0.3-0.5 mg
samples.
(CH3)2CdN+(CH3)F S (CH3)2C+-N(CH3)F (ref 28) (7)
Ross et al. compared the thermal decomposition of bis-
(difluoramino)propanes where the difluoramino moieties were
vicinal and geminal.29 Pyrolyses were conducted at low pressure
in the range of 650 to 750 °C for the vicinal compound and in
the range of 450 to 550 °C for the geminal one. For 1,2-bis-
(difluoramino)propane the principal decomposition product was
propylene, suggesting the decomposition pathway reversed the
stepwise addition of N2F4 to propylene. For 2,2-bis(difluor-
amino)propane, the major products were (a) CH3C(NF)F (62%)
and (b) (CH2)2CdNF (21%). The initial step was thought to be
homolysis of C-NF2 followed by (a) loss of methyl and
rearrangement of the NF2 group or (b) loss of fluorine from the
NF2 group.
Gas products were identified by gas chromatography with
mass selective detection (GC/MS): a Hewlett-Packard (HP)
model 5890 GC, equipped with electronic pressure control and
a model 5971 electron impact quadrupole MS; a PoraPLOT Q

582 J. Phys. Chem. A, Vol. 105, No. 3, 2001
Oxley et al.
TABLE 2: Rate Constants and Arrhenius Parameters
(Chrompack Co.) capillary column (25 mm × 0.25 mm i.d.);
and helium carrier gas. Column temperature was ramped at 15
°C/min from -80 °C to 180 °C and held for 5 min for gas
identification.35 Permanent gases (N2, CO, CO2, and N2O) were
quantified using GC with a thermal conductivity detector (GC/
TCD). The HP 5890 GC was equipped with a Heyesep DB 100/
120 (30′ × 1/8′′) column. The oven temperature was held at 35
°C for 5 min, then ramped to 190 °C at 10 °C/min. Authentic
gases were used for calibration.
After thermolysis, the residue in the sample tubes was
dissolved in either acetonitrile for high-performance liquid
chromatography (HPLC) or acetone for GC analyses. To
quantify the amount of sample remaining, an HP 1100 HPLC
with autosampler (10 uL injection), photodiode array detector,
and Hypersil BDS C18 (4.0 mm i.d. × 100 mm) column were
used. The column temperature was 38 °C. For compounds I
and II, the eluent flow rate was 0.8 mL/min. The initial mobile
phase was 15% acetonitrile and 85% water; the acetonitrile was
ramped to 40% in 5 min and to 100% by 10 min. For
compounds III, V, and VI, the flow rate was 1.0 mL/min, and
an isocratic mobile phase was used; acetonitrile was fixed at
30%, 48%, and 55% respectively. For compounds IV and VII,
GC (HP 5890) with flame ionization detection (FID) and a J&W
DB5 column (0.32 mm × 50 mm) was used to quantify
remaining sample. The injector temperature was 220 °C; the
detector temperature was 300 °C; and the column temperature
was held at 35 °C for 1 min, ramped at 10 °C/min to 75 °C,
then to 280 °C at 25 °C/min. Fraction remaining versus time at
temperature was assessed on average at eight different time
intervals. Data were plotted as zero-, first-, and second-order;
first-order plotting of the data had the best fit. Typically, linear
regression yielded 0.99 R2 values.
To identify the condensed phase decomposition products, a
Varian 3400 GC with J&W DB-5MS, (30 m × 0.25 mm i.d.)
coupled to a Finnigan-MAT TSQ-700 tandem MS was used.
The injector temperature was 220 °C; the transfer line was 260
°C; and the column temperature was ramped from 50 °C to
260 °C at 15 °C/min. The electron energy was 70 eV, with
emission current 200 uA; and the ion source was operated at
150 °C. In the chemical ionization (CI) mode, methane was
used as reagent gas. The thermolyzed samples were examined
by electron impact (EI), CI positive, and CI negative ionization
modes. Product assignments were based on the M+1 or M-1
ions in CI and/or fragmentation patterns. MS/MS was used to
study the fragmentation pathways of the compounds. Detailed
experimental conditions are published elsewhere.34
Results
Thermal Stability. Most of the substituted cyclohexanes and
nitropiperidines exhibited melting endotherms well below their
decomposition exotherms. This is surprising because many
energetic materials, such as RDX and HMX, do not show a
melt distinct from decomposition. The decomposition exotherm

Decomposition of Nitramines and Difluoramines
J. Phys. Chem. A, Vol. 105, No. 3, 2001 583
TABLE 3: Gaseous Decomposition Products
of most of the compounds was in the range 270-300 °C (Table
2). Comparing the nitrocyclohexanes, it is obvious that the fewer
functional groups attached to the ring, the more stable the
compound (i.e., the higher the temperature of the exothermic
maximum). As nitro groups are added to cyclohexane, the DSC
exothermic maximum drops from 350 °C to 270 °C. Comparison
of the nitro- and difluoramino- analogues suggests that the
difluoramino compounds may be slightly more stable, but upon
decomposition they release more heat. The isothermal rate
constants at 240 °C showed the same trends observed in the
DSC data. The decomposition of the tetra-substituted cyclo-
hexanes and the nitropiperidines appeared first-order out to 50%
decomposition; 1,4,4-trinitropiperidine followed first-order ki-
netics to over three half-lives. The decomposition of nitrocyclo-
hexane and dinitrocyclohexane deviated from first-order after
about 40% decomposition. Since these two compounds were
used only for purposes of comparison, their first-order rate
constants were calculated using only the first 40% data. The
four compounds of interest were thermolyzed neat or in benzene,
in deutero or proteo benzene, or neat in the presence of acidic
or basic species (Table 2). Little variation in rate was observed,
suggesting both first-order reaction and one where proton
transfer was not a critical factor in the rate-determining step.
At low fraction reacted, the reaction products were soluble in
acetone or acetonitrile; at high levels of decomposition, an
insoluble tar was formed.
3,3-bis(difluoramino)nitropiperidine (VII) were heated at 320
°C for 24 h to effect complete decomposition. Most of the
decomposition products were condensed; only 1,1,4,4-tetrani-
trocyclohexane (III) formed more than 2 mol of permanent gas
per mole compound. Decomposition gases were identified by
GC/MS; the major decomposition gases, (nitrogen, nitrous oxide,
carbon monoxide, and carbon dioxide) were quantified by GC/
TCD (Table 3). Nitrogen gas was the primary fate of the
nitrogen atoms. Nitrous oxide was observed mainly in the
decomposition of the nitramines, suggesting that it arises directly
from the nitramine functionality or from its decomposition
product nitrosamine. Water and acetonitrile were formed by all
compounds examined (Table 3). All three nitramines exhibited
the aromatized parent ring as a product; 1,1,4,4-tetrakis-
(difluoramino)cyclohexane formed fluorinated benzenes; and the
nitrocyclohexanes formed small quantities of benzene. The
surprising observation was the presence of oxygenated decom-
position gases (carbon monoxide, carbon dioxide and nitrous
oxide) in the thermolysis of tetrakis(difluoramino)cyclohexane.
After ruling out the possibility of a GC leak or an impurity in
the starting material, we surmised that the compound was
reacting with residual air in the “evacuated” reaction tube. This
supposition is supported by the fact that when the sample was
sealed under air the quantities of all oxygenated decomposition
gases increased. In some runs, silicon tetrafluoride rather than
HF was observed. The SiF4 would be directly formed from the
reaction of HF with the glass reaction vessel:
Neat samples of 1,1-dinitrocyclohexane (II); 1,1,4,4-tetranitro-
cyclohexane (III); 1,1,4,4-tetrakis(difluoramino)cyclohexane
(IV); 1,4-dinitropiperazine (V); 1,3,3-trinitropiperidine (VI); and
4HF + SiO2 ) 2H2O + SiF4
(8)

TABLE 4: GC/MS Analysis of the Decomposition Products

Decomposition of Nitramines and Difluoramines
J. Phys. Chem. A, Vol. 105, No. 3, 2001 585

586 J. Phys. Chem. A, Vol. 105, No. 3, 2001
Oxley et al.
Figure 4. Decomposition products of nitramines VI and VII compared.
Condensed Phase Products. Mass Fragmentation. Mass
fragmentation studies were performed on the neat compounds
(III, IV, VI, VII, VIII). The MS/MS studies indicated that six-
membered rings (III, IV, VI, VII) shared the same initial
fragmentation step: loss of one of the geminal (C-NX2) groups,
where X ) F or O. In the case of the eight-membered ring,
HNFX (VIII), the initial step was N-NO2 fragmentation. The
second fragmentation step for the other two nitramines (VI, VII)
was cleavage of N-NO2. For the two cyclohexanes (III, IV),
the second major fragmentation was loss of another NX2 group,
probably para to the first. A common fate of the gem-dinitro
groups in compounds III and VI was loss of NO to form a
ketone. The gem-NF2 groups in the difluoramines (IV, VII, and
VIII) formed fluorinated compounds or fluoroimine (dNF).
Further details of these studies are published elsewhere.34
Thermolysis Products. GC/MS (EI+, CI+, and/or CI-
detection) was used to identify thermolysis products of neat (240
°C and 320 °C) or solution (benzene or acetone 320 °C) samples.
At 240 °C, products were examined as a function of percent
decomposition (10 to 90%). A wider variety of products were
formed at 320 °C though the principal products remained the
same. Product assignments were based on the apparent molec-
ular mass and fragmentation patterns, and in some cases,
comparison with authentic samples (Table 4). For the nitramines
at high or low temperature, neat or in solvent (benzene or
acetone), the corresponding nitrosoamine was the principle
product. Dinitropiperazine (V) when heated at 320 °C for 3 s
(95% decomposition) formed both the mono- and dinitroso
analogues (Table 4). In addition to nitrosamine (VI.1), trinitro-
piperidine (VI) formed products resulting from the loss of the
geminal nitro groups: a large amount of N-nitrosopiperidin-4-
one (VI.2); a moderate amount of N-nitropiperidin-4-one (VI.3);
and small amounts of nitropyridine (VI.4) and N-nitroso-4-nitro-
piperidine (VI.5). At 240 °C 4,4-dinitropiperidine (VI.6) was
also a major product; it was not observed at 320 °C. 4,4-Bis-
(difluoramino)-1-nitropiperidine (VII) formed a large amount
of the parent nitrosamine (VII.1), moderate amounts of (difluor-
amino)pyridine (VII.4), and small quantities of 4-fluoroimino-
N-nitrosopiperidine (VII.2) and VII.3, where reaction occurred
at the difluoramine site (Figure 4). A small amount of a species
(III.6) with m/z of 184 was observed in the decomposition of
both nitropiperidines (VI and VII) and tetranitrocyclohexane
(III). This species was only observed in the thermolysis of the
nitropiperidines when they were heated in benzene, but for
tetranitrocyclohexane it was observed in the neat thermolysis
as well as the one in benzene. The fragmentation pattern of the
compound, which was identical in the thermolysis of all three
species, was consistent with a dinitrophenol or a dinitrocyclo-
hexadiene-4-one. The former would appear more likely than
the latter, but the fragmentation pattern differed significantly
from that of the commercially available 2,5-dinitrophenol.
The MS fragmentation patterns for the thermal decomposition
products of dinitro-, tetranitro- and tetrakis(difluoramino)-
cyclohexanes indicated that the ring remained intact (Table 4).
The major products of the nitrocyclohexanes involved the
Figure 5. Thermal stability comparison of compounds studied.
replacement of the geminal nitro groups by a ketone functional-
ity; thus, 1,1-dinitrocyclohexane, heated for 15 s at 320 °C
formed cyclohexanone (II.1, confirmed with an authentic
sample); and 1,1,4,4-tetranitrocyclohexane compounds III.1,
III.2, and III.3 (Table 4). It is difficult to assign with certainty
the other products of dinitrocyclohexane. Two assignments fit
equally well the fragmentation patterns of II.2 and II.3:
nitrocyclohexene or the dimer, di(nitrocyclohexane) and di-
(nitrosocyclohexane), or nitrosocyclohexyl-dinitrocyclohexane.
1,1-Gem-dinitrocyclohexane is known to undergo electrochemi-
cal reduction to form 1,1′-dinitrobicyclohexyl15 and to replace
one of the gem-nitro groups with certain nucleophiles;36
therefore, such reactivity may occur during the thermolysis or
in the MS instrument.
Neat or in solvent, at high or low temperatures 1,1,4,4-
tetranitrocyclohexane formed dinitrocyclohexanone (III.3) as a
major product and dinitrobenzene (III.5) as a minor one. At
320 °C cyclohexanedione (III.1) and benzoquinone (III.2) were
also major decomposition products. (The chlorine containing
species (III.4) was an impurity found in some batches of the
tetranitrocyclohexane.) The two principal decomposition prod-
ucts of neat 1,1,4,4-tetrakis(difluoramino)cyclohexane had GC/
MS peaks with m/z of 202 and a fragmentation pattern consistent
with the formula C6H7(NF2)2F. In solvent several other peaks
were observed. Possible structural assignments are shown in
Table 4. Three peaks (IV.5, IV.9, IV.10) with m/z of 149 and
fragmentation pattern suggesting C6H6(NF2)F were observed.
Difluorobenzene and trifluorobenzene were identified in the
decomposition gases (Table 3).
Discussion
The decomposition rates of nitrocyclohexane, 1,1-dinitro-
cyclohexane, and 1,1,4,4-tetranitrocyclohexane increased as the

Decomposition of Nitramines and Difluoramines
J. Phys. Chem. A, Vol. 105, No. 3, 2001 587
SCHEME 1: Thermal Decomposition Mechanism of
1,1,4,4-Tetranitrocyclohexane
SCHEME 2: Thermal Decomposition Mechanism of
1,4,4-Trinitropiperidine
group in their place. At 320 °C, neat or in solution, 1,1,4,4,-
tetranitrocyclohexane (III) formed 1,4-cyclohexanedione (III.1),
1,4-benzoquinone (III.2), and dinitrocyclohexanone (III.3) as
the major decomposition products. Of these, only III.3 was
observed at 240 °C. It is likely that the ketone functionality is
formed via bond homolysis:
number of nitro groups increased. The increase in rate and
decrease in the DSC exothermic maximum are especially
dramatic between nitrocyclohexane and 1,1-dinitrocyclohexane
illustrating lower energy decomposition routes available to gem
dinitro species. Inductive effects should weaken C-N bond in
gem dinitro species relative to mononitro compounds,3 and steric
effects should also make loss of a nitro more energetically
favorable. Figure 5 is a schematic of the relative thermal
stabilities of the various species.
-NO2
>C-(NO2) 2
8 >C-NO2 f
>C-O-NO -NO8 >CdO (9)
At lower temperature (240 °C versus 320 °C) such homolysis
would be less favored, and HNO2 elimination would dominate.
Indeed, at 240 °C, 1,4-dinitrobenzene (III.5), a modest product
at 320 °C, became a major product (identified by comparison
with an authentic sample). This species (III.5) and dinitrocyclo-
hexadiene (III.7) apparently resulted from HNO2 loss (see
Scheme 1). While we speculate that these products are the result
of a five-centered monomolecular reaction, the multitude of
oxidized ring species attest to the availability of oxidizing
species. Nitrogen dioxide is a likely oxidizing agent; indeed,
nitrobenzene and nitrophenol were observed when the ther-
molysis was performed in benzene solution (1%). Interestingly,
neat tetranitrocyclohexane formed dinitrobenzene (III.5) but not
nitrobenzene. The reason that aromatization is achieved via loss
of two rather than three molecules of HNO2 is not obvious.
The same trend was observed in the nitramines (VI and VII);
they formed substituted pyridines (VI.4 and VII.4) resulting from
the loss of two NX2 groups (X ) O or F) and subsequent
oxidation; however, in these cases, pyridine was also observed.
The major decomposition products of the nitramines (V, VI,
and VII) are the nitrosamine analogues,5 supporting N-NO2
homolysis as a major decomposition route. However, for both
VI and VII, loss of one or both of the gem NX2 groups is also
The fact that compounds III through VII all decompose at
about the same rate (within an order of magnitude at 240 °C)
reflects the similarity in activation energies for gem-dinitro, gem-
bis(difluoramino), and nitramine functionalities (Table 1). In
the temperature range 200 to 280 °C the two nitramines (VI
and VII) decomposed at almost identical rates and with
essentially identical activation energies, and trinitropiperidine
(VI) decomposed more quickly than dinitrocyclohexane (II).37
These two observations are taken to indicate that in this
temperature range homolysis of the nitramine functionality is
the rate-determining step. This is in line with the lower bond
energy for N-NO2 compared to gem C-NO2 (e.g., 44.6 and
46.6 kcal/mol, respectively, as calculated for TNAZ.18 The initial
loss of N-nitro also agrees with the observations of Grebenni-
kov.22 Comparing compounds III and IV, it appears species with
gem-bis(difluoramino) groups may be slightly more stable than
those with gem-dinitro groups.
The gem-dinitro groups in piperidine (VI) or cyclohexane
(III or II) decomposed to leave a single nitro group or a dO

588 J. Phys. Chem. A, Vol. 105, No. 3, 2001
Oxley et al.
SCHEME 3: Thermal Decomposition Mechanism of 4,4-bis(Difluoramino)-1-nitropiperidine
an important decomposition route. The fates of the gem-dinitro
groups on the nitropiperidine (VI) ring are similar to the fates
observed for these groups on the cyclohexane ring (III). Like
compound III, at 320 °C the gem-dinitro functionality of
trinitropiperidine (VI) appeared to favor C-NO2 homolysis over
HNO2 elimination, producing as major products the nitrosamine
(VI.1), N-nitrosopiperidin-4-one (VI.2) and N-nitropiperidin-4-
one (VI.3) (Scheme 2).
imine CdNF, via loss of fluorine or another ligand:25,29
-NF2
FxC(NF2)4-x (where x ) 0-2)
(Fx)(NF2)2-xCdNF + F• (10)
2,2-(NF2)2C3H6 f (CH3)2CdNF (21%)+
CH3(F)CdNF (62%) (11)
8
Loss of one NF2 group appears to be a major pathway for
the difluoramines IV and VII. Abstraction of HF, common in
the decomposition of difluoramines containing alpha hydro-
gen,23,24 is not an option for geminal bis(difluoramino) groups;
and we observed no solvent or pH effect on the decomposition
rate of 1,1,4,4-tetrakis(difluoramino)cyclohexane (IV) which
might support an E1 type ionic decomposition process.23
Nevertheless, product distribution suggests loss of HNF2 is an
important decomposition pathway. Although the gem-dinitro
groups favor homolysis at the temperatures of this study, the
C-NF2 linkage may be sufficiently stronger that homolysis is
not preferred.
However, only minor products appeared to contain fluoroimine
(Table 4). As geminal dinitro compounds decompose to leave
“)O”, geminal difluoramino groups decompose to the “-F”
analogues.
While the principal fate of 4,4-bis(difluoramino)-1-nitropi-
peridine (VII) was formation of the nitrosamine (VII.1),
indicating the importance of N-NO2 homolysis, a considerable
amount of 4-difluoraminopyridine (VII.4) was formed. The most
straightforward way to form difluoraminopyridine (VII.4) would
be by the loss of two hydrogen during the elimination of NF2
and NO2 and a final oxidation by an external molecule such as
NO2 or NF2 (Scheme 3). The five-centered elimination of HNO2
is well-known though it is not favored in unsubstituted piperidine
Once one gem-difluoramino group is lost, literature suggests
the remaining difluoramine would rearrange to form a fluoro-

Decomposition of Nitramines and Difluoramines
J. Phys. Chem. A, Vol. 105, No. 3, 2001 589
SCHEME 4: Thermal Decomposition Mechanism of 1,1,4,4-tetrakis(Difluoramino)Cyclohexane
at these temperatures.5 Indeed, trinitropiperidine formed only
minor amounts of the analogous product VI.4 (Figure 4). Loss
of HNF2 has not previously been postulated, but HNF2 is an
isolatable species.38 Because HNF2 is known to add across
double bonds, the reverse reaction is probable; furthermore, NF2
is known to abstract hydrogen.39,40 Thus, it is possible that HNF2
is formed and undergoes subsequent reaction.41 (NF exists; it
is isoelectronic with molecular oxygen; and, like NF2, it is
known to dimerize.42)
remaining NF2, subsequent loss of HNF. One would think that
if route (a) were followed, the -ene and/or -diene would be
observed. They are not. This is the same objection we have to
the proposed formation of VII.3 and VII.5 in Scheme 3. The
loss of two radicals in route (b) would be energetically
disfavored. Routes (c) and (d) both begin with NF2 homolysis;
but by analogy to 4,4-bis(difluoramino)-1-nitropiperidine (VII),
loss of HNF2 should be favored.
Conclusions
HNF2 f HF + NF
Because the bond energies of N-NO2, C-(NO2)2, and
C-(NF2)2 are close in magnitude (Table 1), decomposition
kinetics alone do not allow us to determine the initial site of
decomposition. However, comparing analogues of C-(NX2)2,
where X ) F or O, it appears that C-(NF2)2 is the most stable.
This trend in stability has also been calculated for N-NX2
compounds.43 For the various nitramines, nitroso-nitramines
were always the principal products, strongly suggesting that
N-NO2 homolysis was the first step in their decomposition.
(In the case of the four-membered ring TNAZ with both N-NO2
and C-(NO2)2 functionalities, we had previously observed the
product distribution favored N-NO2 scission.)19 In the decom-
position of 1,1,4,4-tetranitrocyclohexane, where the formation
of thermodynamically stable nitrobenzene would indicate
consecutive lose of HNO2, that product was not observed. The
replacement of C-(NO2)2 with CdO again suggests C-NO2
homolysis is important for compounds containing gem-dinitro
groups. In contrast, the major decomposition products of 1,1,4,4-
tetrakis(difluoramino)cyclohexane and 4,4-bis(difluoramino)-
1-nitropiperidine appear to result from HNF2 loss.
NF + NF f FNdNF
(12)
Minor decomposition products of 4,4-bis(difluoramino)-1-
nitropiperidine (VII), such as VII.5 or VII.3, appeared to have
fluorine but not “dNF” attached directly to the ring; thermolysis
products of 1,1,4,4-tetrakis(difluoramino)cyclohexane (IV) also
appeared to have fluorine attached to the ring without an
accompanying dNF group. In Scheme 3 we speculate that VII.3
could have formed by one of two routes: addition of HF across
a point of unsaturation or by rearrangement of NF2 followed
by loss of HNF. The troubling aspect of the first route is that
the precursor unsaturated ring (m/z 126) was not observed, even
though among the decomposition gases was pyridine, which
we envision was formed via loss of three molecules of HNX2
(where X ) O or F, Scheme 3). The presence of F unpaired
with NF is also difficult to rationalize in the decomposition
products of 1,1,4,4-tetrakis(difluoramino)cyclohexane (IV),
where the two principal decomposition products have m/z of
202. Four possible reaction modes were considered (Scheme
4) though none is entirely satisfactory: (a) loss of two molecules
of HNF2; (b) loss of two NF2 radicals; (c) loss of one NF2
radical, rearrangement of the remaining NF2, subsequent addition
of HF; (d) loss of one NF2 radical, rearrangement of the
Acknowledgment. The authors thank Dr. Judah Goldwasser
and Dr. Richard Miller of the Office of Naval Research for

590 J. Phys. Chem. A, Vol. 105, No. 3, 2001
Oxley et al.
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