Structural Tuning of

Article
pubs.acs.org/JPCC
Structural Tuning of Energetic Material Bis(1H‑tetrazol-5-yl)amine
Monohydrate under Pressures Probed by Vibrational Spectroscopy
and X‑ray Diffraction
Liang Zhou,† Nilesh Shinde,† Anguang Hu,‡ Cyril Cook,§ Muralee Murugesu,§ and Yang Song*,†
†
Department of Chemistry, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B7, Canada
Defense Research and Development Canada-Suffield, P.O. Box 4000 Stn Main, Medicine Hat, Alberta T1A 8K6, Canada
§
Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada
‡
S Supporting Information
*
ABSTRACT: As a high-energy density material, bis(1Htetrazol-5-yl)amine monohydrate (BTA·H2O) was investigated
at high pressures up to 25 GPa using in situ Raman
spectroscopy, infrared spectroscopy, X-ray diffraction, and ab
initio simulations. Upon compression, both the Raman and IR
vibrational bands were found to undergo continuous and
gradual broadening without significant change of the profile,
indicating pressure-induced structural disordering rather than
phase transition. X-ray diffraction patterns confirmed the
pressure effect on the structural evolutions of BTA·H2O. Upon
decompression, the back transformation was observed with almost identical Raman and IR spectra and X-ray pattern of the
recovered material, indicating the complete reversibility of the pressure-induced disordering of BTA·H2O and thus the high
chemical stability of the aromatic rings in BTA·H2O. Interestingly, in contrast with all of other Raman and IR modes of BTA·
H2O, which exhibit blue shifts, the N−H stretching mode shows a prominent red shift upon compression to ∼8 GPa, strongly
suggesting pressure-enhanced hydrogen bonding between BTA and H2O. The analysis of X-ray diffraction patterns of BTA·H2O
indicates that the unit-cell parameters undergo anisotropic compression rate. The pressure dependence of the unit-cell
parameters and volumes coincides with the behavior of the hydrogen-bonding enhancement. Aided with first-principles
simulations, these pressure-mediated structural modifications consistently suggest that hydrogen bonding played an important
role in the compression behavior and structural stability of BTA·H2O under high pressures.
1. INTRODUCTION
Compounds containing chains of multiple directly linked
nitrogen atoms are of great interest because they could release a
large amount of energy by generating molecular nitrogen as the
final product due to the substantial enthalpy difference between
the N−N or NN and NN bonds. Besides, the end-product
nitrogen is also highly favorable in terms of avoiding
environmental pollutions and health risks.1−4 Thus, these
compounds are considered desirable high-energy density
materials (HEDMs). Over the past few years, in particular,
great efforts have been directed toward the synthesis of
nitrogen-rich or polynitrogen compounds,5−9 among which
tetrazole-containing compounds and tetrazole salts are of
particular interest because of their high nitrogen content, high
heat of formation, as well as high thermal stability.10 Bis(1Htetrazol-5-yl)amine (BTA) is such a compound composed of
two tetrazole rings linked by one nitrogen atom that contains
82.5% nitrogen by weight. However, according to the UN
Recommendations for the “Transport of Dangerous Goods”,
BTA is classified as a sensitive reagent due to its impact (30 J)
and friction sensitivity (300 N),7 which makes it limited for
practical industrial applications. As its monohydrate form, BTA·
© 2014 American Chemical Society
H2O is sensitive toward neither impact (<100 J) nor friction
(<360 N), which makes it an ideal replacement to study the
properties of BTA, although the nitrogen content is slightly
lower (73.7%).11 Under ambient conditions, BTA·H2O
crystallizes into a monoclinic cell with space group P21/c (Z
= 4) and cell parameters a = 9.367 Å, b = 10.531 Å, c = 6.808 Å,
β = 90.42°, and V = 671.6 Å3.11 Its molecular and crystal
structures are shown in Figure 1, where the coordination
between H2O and BTA is via the N−H···O hydrogen bonding.
The development of new HEDM using high-pressure
approach has demonstrated promising scientific and technological potentials recently. Because the high-pressure technique
has proven to be a powerful tool in reducing the available
volume and increasing the electronic density in materials;
therefore, it could be used as a driving force to trigger chemical
reactions, especially on systems with unsaturated chemical
bonds.12 For instance, Eremets et al. synthesized the diamondlike polymeric nitrogen, which was considered to be a novel
Received: July 21, 2014
Revised: October 22, 2014
Published: October 24, 2014
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Figure 1. Molecular (A) and crystal (B) structures of BTA·H2O. The numbers in the parentheses in (A) label the carbon and nitrogen atomic
sequence. The hydrogen bonding between BTA and H2O is labeled with a dashed line in both (A) and (B) and with a bond length in angstrom
under ambient conditions labeled in (B). The orientation of the monoclinic unit cell is labeled with O (origin) and a, b, and c axes.
fluorescence line shift with an accuracy of ±0.05 GPa under
quasi-hydrostatic conditions. For IR measurements, spectralquality KBr powders were also loaded into the DAC used both
as pressure transmitting medium and to dilute the sample.
A customized Raman microspectroscopy system was used to
collect the Raman spectra. A single longitudinal mode, diode
pumped solid-state (DPSS) green laser with wavelength 532.10
nm was used as the excitation source. The scattered light was
then dispersed using an imaging spectrograph equipped with a
1200 lines/mm grating achieving a 0.1 cm−1 resolution. The
Raman signal was recorded using an ultrasensitive, liquidnitrogen-cooled, back-illuminated, charge-coupled device
(CCD) detector from Acton. The Rayleigh scattering was
removed by an edge filter. The system was calibrated by neon
lines with an uncertainty of ±1 cm−1.
A customized IR microspectroscopy system was used for all
room-temperature IR absorption measurements. A commercial
Fourier transform infrared (FTIR) spectrometer from Bruker
Optics (Model Vertex 80v) equipped with Globar mid-IR light
source constituted the main component of the micro-IR
system, which was operated under a vacuum of <5 mbar, such
that the absorption by H2O and CO2 was efficiently removed.
The transmitted IR beam was directed to a midband mercury
cadmium telluride (MCT) detector equipped with a ZnSe
window that allows measurements in the spectral range of 600
to 12 000 cm−1. This accessible mid-IR range is determined by
the available combination of the IR source and the detector
equipped in our system, while in situ high-pressure far-IR
measurements typically require synchrotron light source with
far-IR detector. The reference spectrum, that is, the absorption
of diamond anvils loaded with KBr but without any sample, was
later divided as background from each sample spectrum to
obtain the absorbance.
In situ angle-dispersive X-ray diffraction measurements were
carried out at room temperature using the 16BM-D beamline of
the HPCAT at the Advanced Photon Source. The incident
wavelength of the monochromatic beam was 0.4246 Å with a
beam size of 5 × 12 μm2 focused at the sample. The diffraction
data were recorded on a MAR345 imaging plate with an
exposure time of 240 s. Then, the 2D Debye−Scherrer
diffraction patterns were integrated by using the Fit2D
energetic material, directly from molecular nitrogen at pressures
above 110 GPa and temperatures above 2000 K.13 However, an
issue that limits the practical application of the polynitrogen
produced by this method as a HEDM is that the pressure and
temperature of synthesizing this material is too extreme. In
addition, the polynitrogen reversibly transforms back to
nitrogen upon pressure releasing and could not be recovered
under ambient conditions. Therefore, searching for proper
nitrogen-containing precursors that could lead to synthesis of
the polymeric nitrogens at lower pressures or temperatures and
that could be recovered is particularly desirable.14−16 In
addition, the detonation reactions of many of the HEDM
involve extreme pressure conditions. Understanding thermodynamics and mechanisms of phase transitions and chemical
reactions under high pressures are thus of fundamental
importance for further applications of these materials. For
this reason, many conventional explosives, such as RDX,17−19
HMX,20,21 and CL-20,22 and so on, have been extensively
investigated under different pressure−temperature conditions.
In this study, we report the first in situ high-pressure study of
BTA·H2O up to 25 GPa at room temperature using Raman and
IR spectroscopy, X-ray diffraction, as well as ab initio
simulations. In particular, we examine the possibility of
converting this high nitrogen content precursor to other
polymorphs with higher energy density using high pressure.
Our observations established important relationship between
the structures and stabilities of BTA at high pressure.
2. EXPERIMENTAL SECTION
BTA·H2O was synthesized following the previous literature.11
In brief, the reaction of sodium dicyanamide with two
equivalents of sodium azide and hydrochloric acid was
performed in an aqueous ethanol solution at 80 °C for 4 h
followed by refluxing of the reaction mixture for another 48 h
to yield BTA·H2O. A symmetrical DAC with two type-I
diamonds with 250 μm culets was used for the high-pressure
Raman measurements, while a pair of type-II diamonds with a
culet size of 350 μm was used for the IR measurements. Ruby
(Cr3+-doped α-Al2O3) chips as the pressure calibrant were
carefully placed inside the sample chamber before the sample
was loaded. The pressure was determined by using the R1 ruby
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program. The structural refinement was performed using GSAS
software package.
The first-principles simulations based on enthalpy minimization with target pressures were carried out by density
functional theory using the Perdew−Burke−Ernzerhof exchange-correlation functional, 23 as implemented in the
SIESTA.24 For the SIESTA package, the Troullier−Martins
norm-conserving pseudopotentials25 were utilized with carbon
and oxygen reference configurations of [He]2s222 and [He]2s22p4, along with customized SIESTA basis sets of numerical
double-ζ and polarization auxiliary basis functions. The cutoff
radius of pseudopotentials was 1.25 Å for carbon and 1.14 Å for
oxygen. A 20.0 Å cutoff was employed for the k-point sampling
in the hydrostatic pressure enthalpy minimization calculations,
in which a variable-cell-shape conjugate gradient method under
constant pressure was used without any constraint on
symmetry. In all simulations, cell shape, volume, and atomic
positions were optimized at the target pressure with the
magnitude of forces on each atom converged to smaller than
0.005 eV/Å and stress to smaller than 0.005 GPa.
Table 1. Selected Raman and IR Vibrational Frequencies
(cm−1) of BTA·H2O Observed at Ambient Pressure
refa
this work
peak no.
description
1
2
3
4
5
6
7
8
9
10
11
12
N−H stretch
C−N−C asym
C−N−C sym
NN stretch
CN stretch
CN stretch
N−N stretch
N−N stretch
ring vibrationb
ring vibrationb
ring vibrationb
ring vibrationb
Raman
1658
1570
1451
1347
1276
1259
1108
1065
1055
1033
IR
3459
1655
1560
1456
1352
1338
1282
1263
1108
1072
1053
1036
Raman
1649
1552
1455
1346
1267
IR
3456
1656
1556
1454
1352
1337
1282
1263
a
Ref 11. bAssignments of those modes were based on the vibrational
study on 5-aminotetrazole.26
GPa). The diffraction pattern can be indexed with the
monoclinic space group P21/c very well. The Rietveld
refinement (shown in Figure 3) indicates the high crystallinity
3. RESULTS AND DISCUSSION
3.1. Ambient-Pressure Structural Characterization.
The synthesized BTA·H2O was characterized under ambient
condition as the starting material. Raman and IR spectra of
BTA·H2O are depicted in Figure 2, where over 25 Raman and
Figure 3. Rietveld refinement of X-ray diffraction pattern of BTA·H2O
at near ambient pressure (i.e., 0.1 GPa) with cell parameters reported.
Figure 2. Raman spectrum of BTA·H2O (top) in comparison with IR
spectrum (bottom) in the spectral region 0−3600 cm−1, both collected
at near ambient pressure and room temperature. The omitted spectral
regions are due to the lack of spectroscopic features. Selected peaks
with vibrational assignments are labeled with peak number (see the
text).
of BTA·H2O with unit-cell parameters of a = 9.3806 Å, b =
10.5302 Å, c = 6.8244 Å, β = 89.11°, and V = 674.03 Å3,
consistent with the reference data.11 In addition, the distance
between hydrogen on NH group and oxygen in H2O was found
to be 1.967 Å with an N−H−O angle of 172.7°, suggesting a
favorable hydrogen bonding even under ambient condition.
3.2. Raman Spectra on Compression. Starting from
ambient pressure, Raman spectra of BTA·H2O were collected
upon compression to 25.4 GPa with selected spectra depicted
in Figure 4. Upon compression, all of the peaks became
broadened and weakened, especially for the modes below 350
cm−1 and the ring vibration modes in the range of 1000−1150
cm−1 (Figure 4a). Moreover, the N−N stretching modes at
1259 and 1276 cm−1 (peak nos. 7 and 8) began to merge into a
broadened singlet at 4.6 GPa. With increasing pressure, the
NN stretching mode at 1451 cm−1 is significantly weakened,
while the C−N−C symmetric stretching mode at 1658 cm−1
IR modes were observed in the displayed spectral region.
Compared with the Raman and IR studies by Klapotke et al.11
on crystalline BTA·H2O, our measurements are in close
agreement with theirs. We followed the assignment proposed
by Klapotke et al.11 and that for 5-aminotetrazole.26 Because of
the complexity of full assignment, we labeled only some
important modes in Figure 1 and Table 1. We note that both
the Raman and IR modes associated with H2O are much
weaker than BTA in BTA·H2O, and the known vibrational
frequencies of H2O were not observed, so therefore all major
bands are identified as from BTA. We also collected X-ray
diffraction pattern under near-ambient conditions (i.e., at ∼0.1
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Figure 4. Selected Raman spectra of BTA·H2O collected at room temperature on compression in the spectral region of 0−1250 (A) and 1250−1800
cm−1 (B) in the pressure region of 0−25.4 GPa. The relative intensities are normalized and thus directly comparable. The pressures in gigapascals are
labeled for each spectrum. Selected Raman modes with vibrational assignments are labeled with peak number. (See the text.)
(peak no. 3) remains the most prominent above 5.7 GPa. Upon
subsequent compression to 10.2 GPa, the NN and N−N
stretching modes are completely depleted. Instead, a new peak
located at 1553 cm−1 was observed (labeled as no. 13).
Concurrently, the C−N−C symmetric stretching mode merged
with an adjacent peak located at 1605 cm−1. When compressed
to above 18.4 GPa, the lattice modes vanished, indicating the
material has entered a disordered phase. At the highest pressure
of 25.4 GPa, the Raman profile is almost featureless, suggesting
that the material has become totally amorphous.
3.3. IR Spectra on Compression. Mid-IR spectra of BTA·
H2O were collected on compression to 24.5 GPa. Selected IR
absorption spectra as a function of pressure in the spectral
region of 600−3700 cm−1 are depicted in Figure 5. Similar to
the Raman spectra, most of the IR bands become broadened
with band merging upon compression. In particular, the NN
stretching mode at 1456 cm−1 (peak no. 4) is significantly
weakened and completely depleted when compressed to ∼7.7
GPa. Other modes including the CN stretching modes at
1352 and 1338 cm−1 (band nos. 5 and 6) as well as N−N
stretching modes at 1282 and 1263 cm−1 (band nos. 7 and 8)
significantly merged into broad bands at 11.0 GPa. Similarly,
the N−H stretching mode (peak no. 1), which could be not
observed in the Raman profile, shows pressure-induced band
broadening and weakening but persists to the highest pressure.
At the highest pressure of 24.5 GPa, only the C−N−C
stretching modes (peak nos. 2 and 3) remain prominent.
3.4. Pressure Effects on Raman and IR Modes. The
pressure dependence of the observed Raman and IR modes of
BTA·H 2 O were examined by plotting the vibrational
frequencies as a function of pressure, as shown in Figure 6.
In general, in the spectral region of 0−1800 cm−1, all of the
Raman and IR modes exhibited pressure-induced blue shifts,
indicating that bonds became stiffened upon compression. The
linear pressure dependence of these modes with increasing
pressure suggested no phase transition during compression,
Figure 5. Selected IR spectra of BTA·H2O collected at room
temperature on compression in the spectral region of 600−3700 cm−1
in the pressure region of 0.5−24.5 GPa. The pressures in gigapascals
are labeled for each spectrum. Selected IR bands with vibrational
assignments are labeled with peak number. (See the text.)
although the material was found to transform to an amorphous
phase at higher pressures. Above 18 GPa, fewer Raman modes
observed (Figure 6a) may suggest the onset of order-todisorder transition. The lack of sharp change in the lattice
region as well as the significant profile broadening starting at
even lower pressures for both IR and Raman spectra
consistently suggest that structural modification of BTA·H2O
involves a broad pressure range rather than a clear boundary.
The most interesting observation, however, is the N−H
stretching mode in the spectral region of 3300−3500 cm−1,
which exhibited a substantial red shift at a rate of −17.5 cm−1/
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Figure 6. Pressure dependence of Raman modes (A) and IR modes (B) of BTA·H2O on compression. Different symbols represent modes with
different origins.
Figure 7. Selected Raman spectra of BTA·H2O collected at room temperature on decompression in the spectral region of 0−1250 (A) and 1250−
1800 cm−1 (B) in the pressure region of 0−25.4 GPa. The pressures in gigapascals are labeled for each spectrum.
GPa upon compression from ambient to ∼10 GPa. With
further compression to higher pressures beyond 10 GPa, the
pressure dependence of this mode is significantly reduced with
almost constant Raman shift in the pressure region between 10
and 25 GPa. The abnormal pressure dependence (i.e., the red
shift) is typically associated with the unique hydrogen bonding
involving this particular N−H bond, which will be discussed in
detail later.
3.5. Raman and IR Spectra on Decompression. Raman
and IR spectra were also collected on decompression all the
way down to near-ambient pressure to examine the highpressure stability of BTA·H2O. Starting from 25.4 GPa, selected
Raman spectra of BTA·H2O upon decompression to nearambient pressure were depicted in Figure 7. During
decompression, back transformation from broadened to wellresolved profiles was observed but with prominent hysteresis.
In particular, the profile remains broad with merged bands
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above 5.2 GPa, in strong contrast with that upon compression
at similar pressure (e.g., 5.7 GPa). This hysteresis is most likely
attributed to the strong intermolecular interaction with
contribution from hydrogen bondings. Nonetheless, the
recovered Raman spectrum of BTA·H2O highly resembles the
one taken under ambient conditions, indicating that the
pressure-induced broadening and structural modification is
completely reversible. Mid-IR spectra of BTA·H2O collected
from 24.5 GPa to near ambient pressure (Figure 8) show a
Figure 9. X-ray diffraction patterns of BTA·H2O on compression at
selected pressures up to 16.8 GPa collected upon compression and
decompression to ambient pressure. The pressures in gigapascals are
labeled for each spectrum. The radiation wavelength used is 0.4246 Å.
Because of the low symmetry of the unit cell and insufficient
number of reflections observed, quantitative Rietveld refinement to yield the full structural information on BTA·H2O at
different pressures is difficult. Instead, we performed Le Bail
refinement for each diffraction pattern to yield the unit cell
parameters. The normalized cell parameters and calculated unit
cell volume with respect to ambient pressure values are shown
in Figure 10 and its inset. The refinement indicates that the β
angle does not follow a monotonic trend but remain relatively
Figure 8. Selected IR spectra of BTA·H2O collected at room
temperature on decompression in the spectral region of 600−3700
cm−1 in the pressure region of 0−24.5 GPa. The pressures in
gigapascals are labeled for each spectrum.
similar trend as that observed in the Raman spectra. All IR
bands remain broad during decompression until ambient
pressure. The IR spectrum of the recovered material with
almost identical profile as the ambient-pressure one before
compression is consistent with the Raman results, again,
suggesting the full reversibility of the pressure modification of
BTA·H2O.
3.6. X-ray Diffraction Patterns of BTA·H2O. Figure 9
shows the selected X-ray diffraction patterns of BTA·H2O
collected upon compression. In general, all of the reflections
shift to higher 2θ angle (i.e., lower d spacings) with increasing
pressure, corresponding to the contracted unit cell parameters
and cell volume. However, the diffraction patterns from near
ambient pressure, that is, 0.1 GPa to the highest pressure 16.8
GPa, remain similar, indicating no phase transitions in terms of
crystal structures in the entire investigated pressure range,
consistent with the Raman and IR measurements. Moreover,
the reflections become significantly weakened above 11.9 GPa,
and at the highest pressure of 16.8 GPa, only two reflections
corresponding to (120) and (102) of P21/c space group can be
observed, indicating the low crystallinity and high disorder, as
suggested by Raman and IR measurements. Then, the
diffraction patterns were also collected upon decompression.
The recovered pattern of BTA·H2O (bottom pattern in Figure
9) is almost identical to the pattern collected at near-ambient
pressure before compression, again, indicating that the
pressure-induced structural modification and disordering is
reversible.
Figure 10. Normalized unit cell volume versus pressure (black
squares) of BTA·H2O on compression and fitted equation of state (red
curve) using second-order Birch−Murnaghan equation of state. The
inset shows normalized monoclinic unit-cell parameters for a, b, and c
of BTA·H2O on compression. The vertical dashed line denotes the
pressure at which the monotonic contraction of a and c axes changed.
(See the text.)
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constant at ∼87° with some fluctuations. Apparently, a
monotonic contraction for b axis is observed in the entire
pressure region but with larger rate below 10 GPa. The
corresponding unit cell volume behaves similarly to b axis. In
contrast, a and c axes exhibit a monotonic contraction below 10
GPa, above which the contraction stopped and fluctuated. We
attempted to fit the pressure−volume data with second-order
Birch−Murnaghan equation of state (EOS) (i.e., B′ is fixed at
4) but found that a single function cannot fit the entire pressure
region satisfactorily. Instead, the pressure region below 10 GPa
can be fitted by a second-order Birch−Murnaghan EOS
reasonably well, which yields bulk modulus B0 = 45.1 GPa.
The implications of the abnormal behavior of a and c axes as
well as the P−V relations above 10 GPa will be discussed later.
3.7. Discussion. Our collective in situ high-pressure Raman
and IR spectroscopic data and X-ray diffraction measurements
of BTA·H2O convincingly suggested that the chemical structure
of BTA·H2O is highly stable up to 25 GPa without any phase
transitions. This observation is in strong contrast to many
aromatic systems investigated at high pressures previously.
Earlier extensive high-pressure studies on benzene revealed as
many as five phases when compressed to 23 GPa and
irreversible chemical transformations to a cross-linked polymer.27 Pyridine, a heteroaromtic system isoelectronic to
benzene, was found to undergo a similar phase transition
sequence and to irreversibly amorphize upon compression to
about 22 GPa.28 It is believed that high-pressure reactivity is
strongly associated with these aromatic rings stacked in a
parallel configuration along a crystalline axis of the unit cell, and
thus favoring an efficient overlapping of the π electron densities
of the nearest neighbor molecules. Compared to these sixmembered rings, five-membered rings with higher internal
strains are expected be less stable and thus have lower
transformation pressures than benzene. Indeed, thiophene was
found to exist in two phases below 10 GPa and undergo an
irreversible phase transition above 16 GPa.29 Furan also
exhibited two solid phases below 12 GPa, and the ring-opening
reactions were observed to start at ∼10 GPa.30 In addition to
these isolated aromatic systems, systems containing aromatic
moieties, such as azobenzene and hydrazobenzene, were also
found to undergo phase transitions at a similar pressure of 10
GPa, although the final amorphization reversibility differs
depending on the conjugation structures.31 Apparently, BTA·
H2O that contains two aromatic tetrazole rings does not follow
any of the previous studied aromatic systems.
Close examination of the molecular and crystal structure and
compression behavior of BTA·H2O is of fundamental
importance to understand its high thermodynamic stability
over the large pressure region. Under near-ambient conditions,
the BTA molecule has a trans conformation with N−H on the
aromatic ring pointing to opposite side of the C−N−C bridge.
Although N(1) is in sp3 hybridization, the crystal structure
suggests that the entire BTA molecule is almost flat (i.e., the
torsion angles of involving C1−N1−C2 and ring atoms are
within ±2° of either 0 or 180°; see Figure 1). The N1−H bond
is also within 3° with the molecular backbone plane. In the unit
cell, the molecular plane is mostly parallel with b axis and forms
sheet structures with neighboring molecules stacked along the
[102] direction roughly. The unit-cell parameters as a function
of pressure (Figure 10 inset) indicate strong anisotropic
compression with the b axis as the preferential compression
direction. At ∼8 GPa, the contraction ratios of a/a0 and c/c0 are
only 96 and 98%, while those for b/b0 are 93%, significantly
larger than a and c. The more prominent compression along
the b axis suggests that BTA molecules within the same sheet
are drawn closer to each other, while the intersheet distance
along the molecular stacking direction does not change
significantly. As a result, no effective pressure-induced overlapping of the π electron densities in between the nearest BTA
molecules located on neighbor sheet is expected. This different
pressure-dependent structural behavior of BTA·H2O than that
of benzene and other aromatic systems is likely a major
contributor to the phase stability of BTA·H2 O upon
compression.
Another significant contributing factor could be associated
with the hydrogen bonding between N−H and H2O. The
prominent red shift of the N−H stretching mode below 10 GPa
convincingly indicates the weakening of the N−H bond as a
result of the strengthening of the N−H···O bond upon
compression. The crystal structure indicates that although the
H2O molecular plane intercepts with BTA sheets with a
significant angle, the O atoms are mostly located on the BTA
sheets as an effective hydrogen bonding acceptor for N−H. The
preferential contraction along the b axis upon compression
conveniently favors the hydrogen bonding interaction by
driving the BTA and H2O together within the same sheet. At
9.4 GPa, the H···O distance is estimated to be 1.639 Å, 17%
shorter than under ambient conditions, strongly supporting
pressure-enhanced hydrogen-bonding interaction. Above 10
GPa, however, the red shift stopped and IR frequency of N−H
bond became almost constant with compression, indicating that
the hydrogen bonds have reached the maximum strength and
become stabilized above this pressure. The pressure-facilitated
hydrogen-bonding stabilization has also been observed in other
systems such as acrylic acid and ethylene glycol.32,33
Concurrently, although the b axis continues to contract, the a
and c axes show a significant amount of resistance to
compression. This can be interpreted as a more prominent
pressure effect on the intramolecular interactions when the
intermolecular distance reached a threshold, such that the
repulsive energy becomes dominant over the hydrogen bonding
stabilization. Above this pressure, the crystal lattice becomes
significantly distorted and the molecular structure is substantially modified from the original planar or sheet orientations by
external stress. Consequently, the unit-cell volume also exhibits
a much less contraction rate, deviating from the original trend
below 10 GPa, a pressure region dominated by strong
hydrogen-bonding interactions. Although there is no phase
transition leading to different crystal structures at 10 GPa, the
strongly hydrogen-bonding-mediated crystal lattice apparently
makes the description of compression behavior by a single EOS
insufficient, where only mechanical properties of the materials
are assessed but the significant chemical interactions are left
unconsidered. Upon decompression, the large hysteresis is also
highly likely to be associated with the strong hydrogen-bonding
effect, such that the intermolecular dissociation was observed
only when external pressure was completely removed.
To better understand the chemical interactions of BTA·H2O
at high pressures, we performed first-principles enthalpy
minimization simulations with target pressures. We found
that above ∼3 GPa the strong coordination between BTA and
H2O already favors a proton transfer leading to the formation
of hydronium cation H3O+ and BTA anion. Upon proton
transfer, the tetrazole ring carrying the negative charge remains
aromatic with the charge delocalized across the five-membered
ring, and thus the entire BTA molecule maintains its original
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geometry. However, such proton transfer is reversible and thus
the proton is rapidly hopping between BTA and H2O. These
dynamic structural modifications of BTA·H2O may be
evidenced by the subtle changes in the spectroscopic profiles
at high pressure. For instance, the new peak at 1553 cm−1 close
to the C−N−C stretching (labeled no. 13) observed at 10.2
GPa could be due to the nonequivalent tetrazole rings as a
result of the deprotonation of one of the rings. In addition, the
simulation suggests that several weaker hydrogen bonds could
also be formed between adjacent molecules, contributing to the
stabilization of the BTA·H2O system. The first-principles
simulation to show the proton hopping and molecular
interactions in animation is provided in the Supporting
Information.
Finally, our original attempt was to produce polymeric chains
of nitrogen using BTA as a precursor via the ring opening or
cross-linking between BTA molecules instead of observing the
high chemical stability of the materials in such a broad pressure
range. Although previous work on tetrazole derivatives or
tetrazole-containing compounds34−37 suggests that tetrazole
rings, in general, have high stability upon compression, it would
be of fundamental interests to examine the origin of the
pressure stability of pure tetrazole as an isolated aromatic
system without the complication of other contributing factors
such as hydrogen bonding to understand the structures and
stabilities of this class of compounds at high pressures.
Moreover, the reversible amorphization might be dictated by
slow kinetics. Thus, investigating BTA and other tetrazole
derivative at elevated temperature may ultimately lead to the
formation of polymeric nitrogen as energetic materials.
Article
ASSOCIATED CONTENT
* Supporting Information
S
Snapshot and animation to show the proton hopping and
molecular interactions of the BTA·H2O system based on firstprinciples simulations. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by a Discovery Grant, a Research
Tools and Instruments Grant from the Natural Science and
Engineering Research Council of Canada, a Leaders Opportunity Fund from the Canadian Foundation for Innovation, an
Early Researcher Award from the Ontario Ministry of Research
and Innovation, a Petro-Canada Young Innovator Award, and
by Defense Research and Development Canada under contract
no. W7702-135601. We acknowledge Dr. D. Ikuta for his
technical assistance for the X-ray diffraction experiments. This
work was partially performed at HPCAT (Sector 16),
Advanced Photon Source (APS), Argonne National Laboratory.
HPCAT operations are supported by DOE-NNSA under award
no. DE-NA0001974 and DOE-BES under award no. DE-FG0299ER45775 with partial instrumentation funding by NSF.
■
4. CONCLUSIONS
In summary, we report the first in situ high-pressure
investigation on bis(1H-tetrazol-5-yl)amine monohydrate
(BTA·H2O) by Raman spectroscopy, FTIR spectroscopy, and
synchrotron X-ray diffraction aided with ab initio simulations.
The spectroscopic data suggest BTA·H2O is stable up to 25
GPa without phase transitions. The broadened Raman and IR
profiles at high pressures, however, indicate that material
undergoes pressure-induced disordering. The structural modification by pressure was found completely reversible upon
decompression. The red shift of the N−H stretching mode
strongly indicates the pressure-enhanced hydrogen bonding
between BTA and H2O. The X-ray diffraction patterns
collected on compression to 17 GPa not only confirmed the
structural modifications probed by vibrational spectroscopy but
also provided interesting and quantitative information in
understanding the high chemical stability of BTA under
compression. In particular, the anisotropic compression along
different unit cell axes suggests that external stress was mainly
rendered along the BTA·H2O molecular plane and sheets
containing the BTA molecules. As a result, the distance
between BTA and H2O is shortened, thus strengthening the
N−H···O hydrogen bonding. In contrast, the distance between
molecular sheets along the stacking direction remains relatively
less compressed such that no effective overlapping of the π
electron density allows irreversible chemical reaction via crosslinking. First-principles simulations show the possible formation of hydronium cation at high pressure via proton transfer
from BTA to H2O and additional hydrogen bonding,
contributing to the overall stability of the system.
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