UV Resonance Raman Investigation of Pentaerythritol Tetranitrate Solution Photochemistry and Photoproduct Hydrolysis

Article abstract of DOI:10.1021/acs.jpca.7b07588

Ultraviolet resonance Raman spectroscopy (UVRR) is being developed for standoff trace explosives detection. To accomplish this, it is important to develop a deep understanding of the accompanying UV excited photochemistry of explosives, as well as the impact of reactions on the resulting photoproducts. In the work here we used 229 nm excited UVRR spectroscopy to monitor the photochemistry of pentaerythritol tetranitrate (PETN) in acetonitrile. We find that solutions of PETN in CD₃CN photodegrade with a quantum yield of 0.08 ± 0.02, as measured by high performance liquid chromatography (HPLC). The initial step in the 229 nm UV photolysis of PETN in CD₃CN is cleavage of an O-NO₂ bond to form NO₂. The accompanying photoproduct is pentaerythritol trinitrate (PETriN), (CH₂ONO₂)₃CCH₂OH formed by photolysis of a single O-NO₂. The resulting UVRR spectra show a dominant photoproduct band at ～1308 cm^-1, which derives from the symmetric stretch of dissolved NO₂. This photoproduct NO₂ is hydrolyzed by trace amounts of water, which downshifts this 1308 cm^-1 NO₂ Raman band due to the formation of molecular HNO₃. The dissociation of HNO₃ to NO₃^- in the presence of additional water results in an intense NO₃- symmetric stretching UVRR band at 1044 cm^-1.

Full text of DOI:10.1021/acs.jpca.7b07588

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Article
UV Resonance Raman Investigation of Pentaerythritol Tetranitrate
Solution Photochemistry and Photoproduct Hydrolysis
Katie L. Gares, Sergei V Bykov, and Sanford A. Asher
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07588 • Publication Date (Web): 25 Sep 2017
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UV Resonance Raman Investigation of Pentaerythritol Tetranitrate
Solution Photochemistry and Photoproduct Hydrolysis
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Katie L. Gares, Sergei V. Bykov, and Sanford A. Asher*
Department of Chemistry
University of Pittsburgh
Pittsburgh, PA 15260
(
412) 624-8570
asher@pitt.edu
ABSTRACT: Ultraviolet resonance Raman spectroscopy (UVRR) is being developed for standoff trace explosives detec-
tion. To accomplish this it is important to develop a deep understanding of the accompanying UV excited photochemistry
of explosives, as well as the impact of reactions on the resulting photoproducts. In the work here we used 229 nm excited
UVRR spectroscopy to monitor the photochemistry of pentaerythritol tetranitrate (PETN) in acetonitrile. We find that
solutions of PETN in CD CN photodegrade with a quantum yield of 0.08 ± 0.02, as measured by high performance liquid
3
chromatography (HPLC). The initial step in the 229 nm UV photolysis of PETN in CD CN is cleavage of an O-NO bond to
3
2
form NO . The accompanying photoproduct is pentaerythritol trinitrate (PETriN), (CH ONO ) CCH OH formed by pho-
2
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2
-1
tolysis of a single O-NO . The resulting UVRR spectra show a dominant photoproduct band at ~1308 cm , which derives
2
from the symmetric stretch of dissolved NO . This photoproduct NO is hydrolyzed by trace amounts of water, which
2
2
-1
-
downshifts this 1308 cm NO Raman band due to the formation of molecular HNO . The dissociation of HNO to NO in
the presence of additional water results in an intense NO symmetric stretching UVRR band at 1044 cm .
2
3
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3
-
-1
3
onance Raman spectroscopy (UVRR) can be used to dra-
matically increase these Raman cross sections, which can
dramatically increase the spectral sensitivity for detection
of trace explosives.
INTRODUCTION
Interest in explosives detection has dramatically in-
creased due to the increasing number of terrorist attacks
1
-9
utilizing improvised explosive devices (IEDs). It is es-
sential to develop methods to detect explosives and to
distinguish them from background interferents. Promis-
ing standoff detection methods are likely to involve laser
UVRR spectroscopy shows great promise as a sensitive
1, 5, 8, 11-15, 20
technique for trace explosive detection.
Most
explosives show deep UV absorption bands below 260
1, 5
nm. Excitation of resonance Raman spectra also result
1
, 4-5, 10-15
spectroscopy methods
that irradiate target surfaces
in the absorption of numerous UV photons since the ab-
sorption cross sections exceed resonance Raman cross
and analyze the scattered or emitted light for evidence of
explosive species.
8
sections by ~10 -fold. Analytes cycle through their elec-
Raman spectroscopy is a promising laser spectroscopy
method for detection of explosives because it can detect
their unique vibrational signatures. Normal Raman spec-
tronic excited states myriad times prior to Raman scatter-
ing. During this absorption and relaxation cycling, the
analyte can undergo photolysis, which can decrease its
concentration. This photochemistry can also produce new
photochemical species. The resulting UVRR will show
characteristic spectral changes due to this analyte deple-
tion and photoproduct formation. We previously demon-
1
-4,
troscopy has already been used for explosive detection.
6, 10, 14, 16-19
Unfortunately, trace detection of explosives,
utilizing normal Raman, is impeded by the typically,
small normal Raman cross sections of explosives. UV res-
1
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strated the impact of photochemistry in the resonance capped 1 cm path length fused silica cuvette by using a
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Raman studies of nitrates, TNT, and RDX.
We also
Varian Cary 5000 UV-Vis/near-infrared (NIR) spectrome-
ter.
showed that these photochemically produced UVRR spec-
tral changes provide additional information that can aid
in identifying analytes.
A 3 mL, 1 mg/mL PETN in CD CN sample was used for
3
the photolysis absorption measurements. The sample was
obtained by evaporating a 3 mL sample of 1 mg/mL PETN
In the work here, we investigated the 229 nm UVRR so-
lution state photochemistry of the explosive pentaerythri-
tol tetranitrate (PETN). We determined the PETN photo-
chemical quantum yield and identified the initial photo-
products formed.
in CH OH (Accustandard). The PETN was then redis-
3
solved in 2 mL of CD CN (Acros Organics). The sample
3
was irradiated in a capped 1 cm path length fused silica
cuvette. The PETN was irradiated by an unfocused ~10
mW 229 nm CW laser beam for the times necessary to
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We base some of the observed photochemistry on the
extensive previous studies of photolysis of nitrate esters
11
achieve absorption of 0.5, 1, 3, 5, 11, 16 phot/molc. The
22
absorption spectra were then measured.
in the gas phase. This previous work was previously mo-
tivated by the need to characterize the nitrogen oxides
PETN Photolysis Quantum Yield. HPLC-HRMS was
used to determine the solution state quantum yield of the
PETN photolysis. A reversed phase 100 x 2 mm Phenom-
enex Luna HPLC column with 3 μm C-18 silica particles
was utilized for HPLC measurements. The loss of PETN
during photolysis was determined from the area of the
PETN + formate adduct peak in photolyzed solutions.
This peak elutes at 8.3 min with a 361 m/z as measured
with UV detection. A calibration curve was determined by
measuring HPLC of calibration standards of 1, 0.5, 0.25,
2
2
formed during atmospheric photochemistry. Most of
these studies used excitation wavelengths >290 nm, a
22
region of very weak absorption. For nitrate esters excit-
ed at λ>290 nm, photolysis involves cleavage of the O-
2
2-26
NO bond to form NO and an alkoxy radical.
2
2
We examined PETN photochemistry with 229 nm exci-
tation, a region of strong absorption. In some of the dis-
cussion here, it appears that the tail of the absorption
band excited with λ>290 nm light involves the same elec-
tronic transition that shows strong absorption with a
maximum at ~200 nm. Our results seem consistent with
the expectation that 229 nm excitation photochemistry
and the λ>290 nm atmospheric photochemistry have
identical mechanisms and photoproducts.
0
.13, 0.063 mg/mL solutions of PETN in CD CN. This cali-
3
bration curve was used to determine PETN concentra-
tions within the irradiated PETN samples.
UVRR and Absorption Studies of NO Hydrolysis in
2
Acetonitrile. The presence of water significantly alters
the PETN photoproduct distributions. We examined this
dependence by using UVRR and absorption spectra of
EXPERIMENTAL
solutions of NO with varying concentrations of water.
PETN Solution State UVRR
2
To obtain NO2(g), ~10 g of Pb(NO ) was placed into a
Raman Instrumentation. The UVRR instrumentation
3 2
27-28
round bottom flask and heated with a Meker-Fisher
was described previously.
A Coherent Industries Inno-
29
+
burner until decomposition occurred, forming NO2(g)
The NO2(g) was drawn up into a syringe and bubbled into
3 mL of CD CN (Acros). Varying concentrations of water
.
va 300 FreD frequency doubled Ar laser generates 229
nm continuous wave (CW) light. A Spex Triplemate spec-
trograph was used to disperse the Raman scattered light,
which was detected by a Princeton Instruments CCD
camera (Spec-10).
3
were added to the NO /CD CN sample (0, 0.1, 0.5, 1, 3, and
2
3
5 % H O by volume). These samples were excited with ~5
2
mW of CW 229 nm light for the UVRR measurements.
The absorption was measured for each sample in a 1 cm
path length fused silica cuvette.
UVRR Measurements. The 2 mL, 1 mg/mL PETN in
CD CN sample used for UVRR measurements was ob-
3
tained by evaporating a 2 mL sample of 1 mg/mL PETN in
CH CN (Cerilliant). The PETN was then redissolved in 2
3
RESULTS AND DISCUSSION
mL of CD CN (Acros Organics). The samples were meas-
3
PETN Photolysis
ured in a 1 cm path length fused silica capped cuvette
with a magnetic stir bar. The 229 nm UVRR were excited
with an ~80 µm spot size laser beam.
The PETN sample was photolyzed by an unfocused ~10
mW 229 nm CW laser beam for the times necessary to
achieve absorption of 0.5, 1, 3, 5, 11, 16 photons per mole-
1
1
cule (phot/molc). The UVRR of the samples were meas-
ured after each irradiation period by exciting the UVRR
with ~4 mW of a ~80 µm spot size focused beam. The
calculated total absorbed phot/molc includes the irradia-
tion during the UVRR measurements. Quartz and CD CN
3
UVRR spectral contributions were numerically subtract-
ed. A 25 µL aliquot of these photolyzed samples was used
for the quantum yield determination using HPLC-HRMS.
Absorption Measurements. The absorption spectra of
the irradiated PETN/CD CN samples were measured in a
3
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sion degradation product of PETN.
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Other minor pho-
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toproduct peaks are also observed with HPLC-HRMS.
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Figure 1. 229 nm UVRR of PETN in CD CN with increasing
3
irradiation times (absorbed photons per molc). The NO in
2
-
1
Figure 2: PETN photolysis quantum yield measurements.
Dependence of PETN 361 m/z peak intensity measured by
HPLC-UV on the number of absorbed phot/molc during
initial stages of photolysis in the absence of water. The quan-
tum yield is determined from the slope.
CD CN spectrum shows the NO Raman band at 1308 cm .
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CD CN and quartz UVRR bands were subtracted.
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Figure 1 shows the UVRR spectra of PETN in CD CN as
3
the irradiation time increases. The initial PETN UVRR
-
1
spectra show PETN UVRR bands at 869 cm (O-N
We determined the photochemical quantum yield of
-
1
stretching and C-C stretching), at 1279 cm (C skeletal
5
PETN in CD CN by using HPLC to monitor the depend-
-
1
3
and CH bending), at 1295 cm (-NO symmetric stretch
2
ence of the 361 m/z PETN+ formate adduct peak intensity
on the number of absorbed photons/molc (Figure 2). The
measurements used a HPLC-HRMS calibration curve for
-
1
with CH wagging), at 1507 cm (CH scissoring), and at
2
2
-
1
1, 5, 30-31
1650 cm (-NO asymmetric stretching).
The initial
2
UVRR PETN spectrum with minimal irradiation shows
negligible contributions of photoproducts.
PETN in CD CN. We calculate a quantum yield of φ =
3
0
.08 ± 0.02.
The PETN Raman band intensities monotonically de-
crease as the irradiation time increases. The 5 phot/molc
PETN UVRR spectrum clearly shows a new strong over-
-
1
lapping photoproduct Raman band at 1308 cm . At longer
-
1
irradiation times, the PETN1295 cm Raman band intensi-
-
1
ty continues to decrease, while the 1308 cm photoprod-
uct Raman band intensity increases. There is clearly an
extensive photolysis of PETN at 16 phot/molc, where the
-
1
photoproduct 1308 cm Raman band dominates.
Figure 1 also shows the UVRR spectrum of pure NO2
dissolved in CD CN. The UVRR shows that the dominat-
3
ing NO symmetric stretch Raman band occurs at 1308
2
-1
cm in CD CN. Thus, Figure 1 allows us to conclude that
3
NO is a PETN photoproduct of 229 nm excitation.
2
We used HPLC-HRMS to study the photoproducts
formed during PETN photolysis. In the early stages of
photolysis (1 phot/molc), we observe a photoproduct that
has the m/z value of 316, which is pentaerythritol trini-
trate (PETriN), (CH ONO ) CCH OH + formate adduct.
2
2
3
2
Studies measuring PETN using electrospray ionization in
the negative ion mode in liquid chromatography mass
spectrometry (LCMS) have shown that nitrate esters form
3
2
Figure 3: Absorption spectra of PETN in CD CN (1 cm path
adduct ions rather than molecular ions. PETriN results
from cleavage of a single O-NO2 bond of PETN,
C(CH ONO ) . PETriN has been observed as a post explo-
3
length) upon 229 nm irradiation of 0.5, 1, 3, 5, 11, and 16 ab-
sorbed phot/molc.
2
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Figure 3 shows the dependence of the PETN absorption
spectrum on increasing 229 nm light absorption. PETN
has an absorption maximum at ~ 200 nm that derives
from two π ꢀ π* electronic transitions at ~ 208 nm and ~
Figure 4 shows the UVRR spectra of NO dissolved in
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CD CN at increasing volume fractions of H O. The NO
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spectrum without H O added shows the NO symmetric
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stretching Raman band at 1308 cm , and a N O Raman
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1
39
1
.
87 nm localized on each of the four PETN O–NO groups
band at ~1383 cm (coupled NO symmetric stretching).
2
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5
During photolysis an increase in absorption occurs
At 0.1 % volume fraction of H O, we observe that the
2
-1
-1
between 300-350 nm which derives from formation of
N O , a dimer of NO . The dimer is in equilibrium with
1
308 cm NO Raman band is downshifted to 1306 cm .
2
We assign this 1306 cm band to the symmetric stretching
vibration of undissociated HNO3, as clearly shown by
-1
2
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2
40
the monomer photoproduct, NO . The NO absorption
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maximum occurs at ~400 nm. In addition, a series of
sharp absorption features are observed between 350-400
the Figure 5 absorption spectra and discussion below. We
also observe the HNO3 asymmetric stretching Raman
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nm that derive from nitrous acid (HNO ). We hypothe-
2
band at ~1668 cm in the Figure 4 0.1 % volume fraction
40
size that the nitrous acid results from the reaction of NO
with trace amounts of water.
2
H O UVRR spectrum. HNO is formed by the reaction
2
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2
9, 37-38
29, 37-38, 41-42
between NO and H O at room temperature,
2
2
The presence of water significantly alters the PETN
photoproduct UVRR spectra. We find that under humid
2
NO + H O� HNO +HNO
2
2
3
2
-1
The HNO photoproduct is observed in the Figure 5 ab-
2
lab conditions the 1308 cm photoproduct UVRR band of
sorption measurements. We do not observe UVRR HNO2
bands likely because they have small Raman cross sec-
tions.
Figure 1 downshifts due to NO hydrolysis.
2
Hydrolysis of NO2 Photoproducts dissolved in
CD CN. We investigated the hydrolysis of NO by meas-
3
2
The HNO vibrational frequencies depend on interac-
3
uring the absorption and UVRR of NO in the presence of
2
tions with the solvent. The frequency dependence in wet
solvents is probably dominated by water hydrogen bond-
different volume fractions of water (Figures 4 and 5).
4
0, 43-46
ing.
We observe that, with increasing water con-
-
1
centration, the HNO 1306 cm Raman band (0.1 % H O)
downshifts to 1305 cm (0.5 % H O). This Raman band
then further downshifts to 1303 cm (1 % H O) likely due
3
2
-
1
2
-
1
2
to increased water hydrogen bonding to HNO . At ~1%
3
-
H O, HNO begins to dissociate to nitrate (NO ) as indi-
2
3
3
-
cated by the appearance of the NO symmetric stretching
Raman band at 1044 cm .
3
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1 15, 21
-
-1
The NO3 1044 cm Raman
band is first observed at 1 % volume fraction water con-
centration with an intensity close to that of the HNO
Raman band at 1303 cm .
3
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1
-
The NO concentration dramatically increases with wa-
3
ter concentration as indicated by the dominating 1044
-1
cm Raman band intensity in the 3 and 5 % water spectra.
-
-1
In the 5 % H O spectrum, the NO 1044 cm Raman band
2
3
-1
becomes ~5 times more intense than the NO 1308 cm
2
Raman band if no H O is present.
2
This is an important result for UVRR detection of PETN
-
because NO3 is the ultimate photoproduct in the pres-
-
ence of H O, and NO3 shows a dominating UVRR cross
2
-
2
section. We do not observe the nitrite (NO ) photoprod-
uct because the weak acid HNO will not dissociate under
2
-
29
these conditions to form NO .
2
Figure 4: UVRR excited at 229 nm of NO2 dissolved in
CD CN with addition of 0.1, 0.5, 1, 3, 5 % volume fractions of
3
-1
H O. The spectra are normalized to the 2061 cm band of
2
CD CN. Quartz and CD CN Raman bands are subtracted.
3
3
This sample was also used for the absorption measurements
of Figure 5.
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NO . Absorbance measurements show that NO and its
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dimer N O coexist in equilibrium in CD CN.
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The NO2 photoproduct undergoes hydrolysis in the
presence of trace amounts of water forming HNO and
3
-1
HNO . The 1308 cm NO2 photoproduct Raman band
downshifts to 1306 cm upon conversion to HNO . This
Raman band further downshifts to 1303 cm upon an in-
2
-1
3
-1
crease in water concentration likely due to increased wa-
ter hydrogen bonding to the HNO . At higher water con-
3
- .
centrations HNO dissociates to form NO The very large
3
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-
UVRR 229 nm cross section of the NO symmetric stretch
3
causes this band to dominate the UVRR of PETN in the
presence of trace water.
ASSOCIATED CONTENT
Figure 5: Absorption spectra of NO dissolved in CD CN at
Supporting Information
2
3
increasing volume fractions of H O (0.1, 0.5, 1, 3, and 5 %)
2
Absorption spectrum of NO , N O , and HNO
2
2
2
4
measured using a 1.0 cm path length cuvette. This is the
same sample used for the UVRR measurements of Figure 4.
(
PDF) This information is available free of charge via the
Internet at http://pubs.acs.org
Figure 5 shows that the absorption spectrum of NO2
AUTHOR INFORMATION
dissolved in CD CN is dominated by the N O dimer ab-
3
2
4
sorption, which has a strong gas phase absorption maxi-
Corresponding Author
* Email: asher@pitt.edu
4
7-49
mum at ~ 340 nm.
NO gives rise to a broad absorp-
2
3
6, 49
tion band with a maximum at ~400 nm.
N O coexist in equilibrium in acetonitrile.
NO and
2
3
7, 50
Figure S-1
2
4
of the supporting information shows the absorption spec-
ACKNOWLEDGEMENT
trum of gaseous monomer NO , while Figure S-2 shows
2
We gratefully acknowledge funding from ONR grants
N00014-12-1-0021 and N00014-16-1-2681.
the similar absorption spectrum of gaseous N O . A larger
2
4
absorption is observed for N O in Figure 5 compared to
2
4
that of NO because the dimer is favored in CD CN solu-
2
3
5
0
tion.
The N O and NO absorption bands essentially disap-
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2
4
2
1.
Ghosh, M.; Wang, L.; Asher, S. A., Deep-
pear upon addition of 0.1 % H O, indicating conversion of
2
Ultraviolet Resonance Raman Excitation Profiles of
N O and NO to HNO and HNO . Undissociated HNO
3
2
4
2
2
3
Nh no , Petn, Tnt, Hmx, and Rdx. Appl. Spectrosc. 2012,
4
3
shows an absorption maximum at ~260 nm, which derives
5
1-53
66, 1013-1021.
Moore, D. S., Instrumentation for Trace
from an nꢀπ* transition.
This absorption band de-
2.
creases in intensity as the water concentration increases,
indicating HNO3 dissociation, which agrees with the
Detection of High Explosives. Rev. Sci. Instrum. 2004, 75,
2499-2512.
-
UVRR spectra that show intense bands of NO . HNO
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Figure S-3 of the supporting information
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2
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3
8
PETN Raman band intensities upon photolysis and the
-
1
appearance of a photoproduct Raman band at 1308 cm ,
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