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hydroxyl groups, the latter become visible in negative ion
IL with the 1-ethyl-3-methylimidazolium (EMI) cation was
used.
detection mode by deprotonation of OH groups (Figure 1b).
For example, the peak at m/z 126 corresponds to deproton-
ated ammeline and the peak at m/z 218 to hydroxy-substi-
tuted melem. The latter dominates the spectrum in negative
ion mode probably because melem is most predisposed to
hydrolysis to its hydroxy-substituted version.
Thermal decomposition into melamine-like cyclic azines
has been reported for a number of energetic materials.[16,17]
For example, the formation of melamine, melem, melon, and
ammeline has been observed from dicyandiamide, diamino-
glyoxime, and diaminofurazan when heated at a rate of
1008CsÀ1 at a pressure of up to 1000 psi of Ar.[16] The DCA
ILs with nitrogen-containing cations have recently been
reported to condensate into triazine rings at ~ 5008C, and
upon further heating (up to ca. 10008C) gave rise to dense
nitrogen-doped carbon materials.[18]
To explore the factors responsible for the formation of
precipitate during the reaction of DCA ILs with nitric acid, a
set of experiments was performed in which various DCA ILs
were mixed in bulk with aqueous HNO3 (10 %vol). As a
result of the lower concentration of components, the reaction
was much slower and no ignition occurred. Still, we observed
vigorous bubbling, thus indicating release of volatile products,
and finally, after about 1 min, formation of a precipitate.
Under these conditions, SEM images (see the Supporting
Information) reveal that the precipitate has a larger particle
size distribution as compared to the precipitate formed during
hypergolic ignition. Solid- and liquid-phase products were
isolated by centrifuging and then analyzed separately. The
solution phase was diluted in water (ꢀ 10À3) and then analyzed
using direct-infusion ESI-MS. Figure 2a shows the resulting
mass spectrum in negative ion detection mode when the DCA
The spectrum is dominated by clusters with the molecular
composition of [EMI+]nÀ1[NO3 ]n (n ꢀ 1), thus pointing at the
À
À
formation of [EMI+][NO3 ] salt during the reaction. It can be
concluded from this observation that HNO3 and [EMI+]-
[DCAÀ] IL undergo ion exchange—EMI+ pairs with NO3À to
form water-soluble salt, while DCAÀ interacts with protons to
yield the precipitate (Scheme 2). The EMI+ cations remain
intact (Figure 2a), thus indicating that the temperature does
not reach decomposition threshold during the reaction.[19]
The precipitate was washed in water and then dissolved in
ammonium hydroxide (10%vol) for ESI-MS analysis. Anal-
ogous to the precipitate formed under the conditions of
hypergolic ignition (Figure 1), the precipitate from the model
reaction between DCA ILs and aqueous HNO3 also reveals
the presence of melamine and its oligomers (Figure 2b and c),
including the one at m/z 454 (453 Da, Scheme 2). However, as
follows from the mass spectrum, new polymerization channels
arise: the peak at m/z 194 in positive ion mode and m/z 200 in
negative. Based on tandem MS analysis (Scheme 1), the
compound at m/z 194 was found to consist of dicyanamide
attached to melamine (Scheme 2, 193 Da). This observation
points possibly to a lower-energy polymerization pathway of
melamine than that associated with the intermediate at m/z
169 observed under hypergolic conditions (Scheme 2,
168 Da). The peak at m/z 200 was identified as a dicyanamide
trimer, known as tricyanomelaminate (Scheme 1 and
Scheme 2). In our experiments, each tricyanomelaminate
molecule originates from three DCA anions and three
protons donated by nitric acid, and this is in full agreement
with the ion exchange reaction mechanism proposed above.
We suggest that the formation of tricyanomelaminate
becomes a dominant polymerization channel at lower con-
centration of reagents because less ammonia is eliminated
during the reaction, which decelerates the concurrent poly-
merization of DCA into melamine (Scheme 2).
The fact that the composition of precipitate does not
depend on the IL cation suggests that the latter does not take
part in the reaction. In agreement with this hypothesis, we
found that sodium dicyanamide (Na DCA) produces the
same azine species when reacted with aqueous HNO3 under
nitrogen atmosphere, including melamine, melam, melem,
ammeline, and tricyanomelaminate. Therefore, it can be
concluded that ammonia necessary for the synthesis of
melamine polymers is formed during the reaction between
DCA and HNO3; possibly, it originates from the dinitrobiuret
intermediate.[6] The latter easily decomposes into HNCO,[20]
which is then hydrolyzed to yield NH3 and CO2.[21] The
reaction does not occur when HCl, CH3COOH, or aqueous
NaNO3 are used instead of HNO3 to oxidize Na DCA. It is
also worth noting that while mere heating of Na DCA up to
about 3008C does generate tricyanomelaminate,[22] mixing
Na DCA with aqueous HNO3 (10 %vol) yields tricyanome-
laminate as well as melamine and its oligomers without
notable increase in temperature. These observations indicate
that the formation of precipitate in our experiments cannot be
attributed to heating or change in pH. The critical compo-
nents to induce polymerization are DCA and nitrate anions as
Figure 2. Products of the reaction between 1-ethyl-3-methylimidazo-
lium dicyanamide and aqueous nitric acid (10 %vol) analyzed by ESI-
MS: a) solution phase (diluted 103 times in pure water) analyzed in
negative ion detection mode; b) and c) are the MS of the precipitate
(dissolved in ammonium hydroxide) analyzed in positive and negative
ion detection modes, respectively.
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8634 –8637