CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
VIBRATIONAL SPECTRA OF NITRO COMPOUNDS UNDER SHOCK
COMPRESSION
Takamichi Kobayashi, Toshimori Sekine, and Hongliang He
Advance Materials Laboratory, National Institute for Materials Science,
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Abstract Real-time vibrational spectra of shock-compressed nitro compounds have been measured
using a single-pulse laser Raman spectrometer in conjunction with a propellant gun and vibrational
mode-dependent behavior has been examined. The NO2 stretching mode shows smaller frequency shift
compared to other stretching modes, which may be attributed to increased intermolecular interaction
under pressure. Pressure-induced shift of nitromethane-d3 shows monotonic increase up to -5.0 GPa
However, above this pressure, the monotonic increase no longer exists and a more complicated behavior
is observed. Above ~ 8.5 GPa, a strong background emerges over the whole spectral range (500 ~ 2600
cm"1) and Raman bands are not detectable. A chemical reaction induced by a single shock may be
initiated at ~ 8.5 GPa.
INTRODUCTION
In situ vibrational spectroscopy provides essential
information on shock-induced phenomena such as
chemical reactions, intermolecular interactions, and
phase transitions. Especially, observation of vibrational mode-dependent behavior is important
because it can give a clear picture on how molecular
structure or crystal structure changes under shock
compression.
In the experiments described here, we focused on
the vibrational mode-dependent behavior of shockcompressed nitro compounds (nitrobenzene and
nitromethane-d3) to obtain information on intermolecular interaction and shock-induced reaction in
molecular liquids.
Increased intermolecular
interaction under pressure and shock-induced
initiation of a chemical reaction in nitromethane-d3
are discussed.
stage propellant gun (30 mm in bore diameter).
Aluminum impactors and aluminum driver plates
(base plates) were used with the impact velocity up to
—2.0 km/s to obtain single shock pressure up to ~8
GPa. For higher pressure experiments, a stainless
steel flyer was impacted on an aluminum driver plate.
Nitrobenzene and nitromethane-d3 were chosen as
our initial samples. They are relatively strong
Raman scatterers and shock experiments on
nitromethane have been performed by several
researchers. [1- 4]
In this experiment, deuterated nitromethane
(CD3NO2) was used because, in normal nitromethane
(CH3NO2), the NO2 stretching mode overlaps with
the CH3 bending mode. Liquid samples were
confined between a driver plate and a glass window.
Typical sample thickness was ~5 mm. Figure 1
shows a schematic diagram of the experimental setup.
The second harmonic of a Nd:YAG laser (532 nm, 8
ns) was used as an excitation light The excitation
laser pulse was introduced into the sample just
before the shock wave reached the rear surface of the
EXPERIMENT
Shock experiments were performed using a single
1255
However, in the case where relatively strong
intermolecular interaction such as hydrogen bonding
exists, the situation can become quite different.
sample. Raman frequency shifts of stretching
modes were measured against single shock pressure.
The uncertainties in the measured peak shifts were
about ± 3 cm"1. Since the Hugoniot for
nitromethane-d3 is not known, that for normal
nitromethane was used to estimate shock pressures
of nitromethane-ds by the shock impedance matching
method.
Mode-dependent behavior of pressure-induced
vibrational frequency shift
Observed Raman frequency shifts vs. single
shock pressure are summarized in Fig. 3. It is seen
that the NO2 stretching mode shows significantly
smaller blue shifts in both molecules. In the case of
nitromethane-ds, the C-N and the CD3 stretching
modes show similar blue shifts up to ~5 GPa and they
are much larger than that of the NO2 stretching mode.
Also we reported previously that the NO2 stretching
mode of nitrobenzene (1436 cm1) shows much
RESULTS AND DISCUSSION
Typical Raman spectra of nitrobenzene and
nitromethane-d3 under ambient and shock pressure
are shown in Fig. 2. Only totally symmetric
stretching modes were selected for investigation
because they are well separated from other bands and
also generally more intense. They are the NO2
stretching mode (1346 cm"1) and the C-H stretching
mode (3082 cm"1) of nitrobenzene and the C-N
stretching mode (895 cm"1), the NO2 stretching mode
(1390 cm"1), and the CD3 stretching mode (2283
cm 1 ) of nitromethane-d3.[5]
It is seen in Fig. 2 that all Raman bands mentioned
above are blue shifted under shock compression but
the magnitude of the shift depends on vibrational
mode. In general vibrational frequency of a
stretching mode increases with pressure because
bond length is reduced and effective force constant
at the new equilibrium position is usually larger than
that at the original equilibrium position. [6]
Target
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(a)
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• : under shock (5.6 GPa)
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NO, stretch
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1500
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: under shock (4.75 GPa)
*2
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CN stretch
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CD 3 stretch
§20
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i 10
OL-_
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1000
1500
2000
2500
Raman shift (cm -1)
FIGURE 1
arrangement.
FIGURE 2. Raman spectra of (a) nitrobenzene and (b)
nitromethane-ds under ambient and shock pressure.
Schematic diagram of single-pulse laser Raman
1256
smaller blue shift than the C-C stretching mode (992
cm"1) and the C-H stretching mode (3080 cm"1) of
benzene.[7,8] There seems to exist some kind of
softening mechanism to account for the small blue
shift of the NO2 stretching mode under compression.
It is noted here that, in general, vibrational bands
become broader and peak positions may shift with
temperature due to hot bands. [9] In our spectra, it
is difficult to see this hot band effect because of low
resolution. One of the other possible explanations
for the softening mechanism of the NO2 stretching
mode may be an increased intermolecular interaction
70
J
1
1
1
under compression. In static high-pressure
experiments of some hydrogen-bonded solids,
softening of vibrational modes with pressure has been
observed. [10-11] An example is the softening of
the O-H stretching mode in H2O ice. The
vibrational frequency of this band decreases with
pressure until the band disappears at -60 GPa.[11]
Above this pressure it is reported that nonmolecular,
symmetric hydrogen-bonded state is formed, where
the proton is delocalized along O-O directions. A
Raman study of shock-compressed liquid water by
Holmes et.al. indicated that hydrogen bonding
diminishes with increasing shock pressure,[12]
which may be due to high temperature effect.
There are some reports on weak hydrogen bonding in
ambient nitromethane [13] and hydrogen bond
formation at high pressures [4,14]. If the situation
of nitro compounds is similar to that of H2O ice,
softening of the NO2 stretching mode may take place
under shock compression while the O--H or O--D
bonds between molecules become stronger.
Small Raman frequency
shifts under
compression observed for the NO2 stretching mode
may be explained as a result of two factors, i. e., (1)
the pressure-induced softening mechanism in
hydrogen-bonded materials which decreases the
vibrational frequency and (2) the general pressureinduced hardening mechanism which increases
vibrational frequencies of stretching modes. The
cancellation of these two effects may be responsible
for the observed small frequency shifts of the NO2
stretching mode. This seems to explain above
mentioned fact that the CD3 stretching mode shows
similar blue shifts to those of the C-N stretching
mode. It is inferred from analogy with C-H
stretching modes of other molecules with little
intermolecular interaction[8] that the CD3 stretching
mode would show larger frequency shifts than
observed unless influenced by some kind of
softening mechanism such as pressure-induced
intermoleculer interaction.
1 _
(a) Nitrobenzene
^ 60—
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o
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(b) Nitromethane-d
20
§ 10
o«
PH
5
0
1
1
1
2
4
6
8
Shock pressure (GPa)
FIGURE 3. Raman frequency shift vs. shock pressure, (a)
D:C-H stretching mode (3082 cm"1), O:NO 2 stretching mode
(1436 cm"1), (b) D: CD3 stretching mode (2283 cm"1), O:
NO2 sketching mode (1390 cm"1), •: C-N stretching mode
(895 cm"1). Experimental uncertainties in frequency shift
measurements are ~db 5 cm"1 for nitrobenzene and ~t 3 cm" for
nitromethane.
Shock-induced Reaction in nitromethane-ds
Shock-induced initiation of chemical reaction in
nitromethane-h3 has been studied by several
researchers. In single shock experiments, Renlund
et.al. [2] suggested the initial stage of reaction near 6.0
1257
B., J. Phys. Chem. 95, 3037 (1991).
6. M. R. Zakin and D. R. Herschback, J. Chem. Phys. 85,
2376(1986).
7. Kobayashi. T., and Sekine, T., in Shock Compression of
Condensed Matter -1999, edited byM. D. Furnish, L.
C. Chhabildas, and R. S. Hixson, AIP Conference
Proceedings 505, New York, 2000, pp. 951-954.
8.Kobayashi. T.,and Sekine T.,Phys. Rev. B62, 5281
(2000).
9. D. S. Moore, J. Phys. Chem. A, 105, 4660 (2001).
10. Wolanin. Ph. Pruzan, E.,Gauthier. M., ChervinJ. C.,
Canny .B., Hausermann. D., and Hanfland. M., J.Phys.
Chem. B, 101, 6230(1997).
11.Goncharov. F., Stmzhkin.V. V., Mao. H, and
Hemley.R. l.,Phys. Rev. Lett. 83, 1998 (1999).
12. Holmes,. N. C., Nellis,. W. J., and Graham,. W. B.,
Phys. Rev. Lett. 55,2433 (1985).
13. E. Knoezinger, H Kollhoff, and R. Wittenbeck, Ber.
Bunsenges. Phys. Chem. 86, 929 (1982).
14. D. M. Adams and J. Haines, J. Phys.: Condens.Matter
3,9503(1991).
15. Pieimarini G. J., Block S., and Miller P. L, J. Phys.
Chem. 93,457 (1989).
GPa by in situ Raman measurements and Von
Holle[3] suggested a reaction at above 7.0 GPa by
time-resolved infrared radiometry. In this study,
deuterated nitromethane was used and the results are
somewhat different from those of normal
nitromethane. Under static pressure, different
reactivity between normal nitromethane and
deuterated nitromethane has been reported. [15]
In Fig. 3, it is seen that pressure-induced blue
shift of the C-N stretching mode suddenly drops
down at -5.0 GPa and starts increasing again at
higher pressures.
The CD3 stretching mode
displays similar behavior but the drop at -5.0 GPa is
not as large. A similar drop in Raman peak shift for
the C-N stretching mode at -6 GPa was observed in
normal nitromethane.2 Pressure-induced frequency
shift of the NO2 stretching mode appears to level off
at around this pressure. Up to 8.3 GPa, Raman
bands of nitro methane-cb are observable but above
8.5 GPa strong non-resonant emission suddenly
emerges throughout the observed spectral range (500
~ 2600 cm"1) and the background jumps up by nearly
two orders of magnitude and Raman bands are no
longer detectable. This may be due to emission
from reaction products and thus a chemical reaction
by single shock may be initiated at -8.5 GPa.
ACKNOWLEDGMENTS
The authors would like to thank David S. Moore,
Los Alamos National Laboratory, for his helpful
comments and discussions. We also thank Harumi
Otsuka for preparing the manuscript.
REFERENCES
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D. L., and HoltW. T., in Shock Waves in Condensed
Matter -1985, edited by Y. M. Gupta, Plenum Press,
1986, pp. 207-210.
2. Relund.A. M., and Trott.W. M., in Shock Compression
of Condensed Matter-1989, edited by S. C. Schmidt, I
N. Johnson, and L. W. Davidson, Hsevier Science
Publishers B. V., 1990, pp. 875-878.
3. Von Holle.W. G., in Shock Waves in Condensed
Matter - 1981, edited by W. J. Nellis, L. Seaman, and
R. A Graham, AIP Conference Proceedings 78, New
York, 1982, pp. 287-291.
4.Winey.J.M, and Gupta.Y. M., J.Phys. Chem. B101,
10733 (1997).
5.HilLJ. R., Moore.D. S., SchmidtS. C., and Storm.C.
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