UV Resonance Raman Investigation of Pentaerythritol TetranitrateSolution Photochemistry and Photoproduct HydrolysisKatie L. Gares, Sergei V. Bykov, and Sanford A. Asher*

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States

*S Supporting Information

ABSTRACT: Ultraviolet resonance Raman spectroscopy (UVRR) is beingdeveloped for standoff trace explosives detection. To accomplish this, it isimportant to develop a deep understanding of the accompanying UV excitedphotochemistry of explosives, as well as the impact of reactions on the resultingphotoproducts. In the work here we used 229 nm excited UVRR spectroscopyto monitor the photochemistry of pentaerythritol tetranitrate (PETN) inacetonitrile. We find that solutions of PETN in CD3CN photodegrade with aquantum yield of 0.08 ± 0.02, as measured by high performance liquidchromatography (HPLC). The initial step in the 229 nm UV photolysis ofPETN in CD3CN is cleavage of an O-NO2 bond to form NO2. Theaccompanying photoproduct is pentaerythritol trinitrate (PETriN),(CH2ONO2)3CCH2OH formed by photolysis of a single O-NO2. Theresulting UVRR spectra show a dominant photoproduct band at ∼1308cm−1, which derives from the symmetric stretch of dissolved NO2. This photoproduct NO2 is hydrolyzed by trace amounts ofwater, which downshifts this 1308 cm−1 NO2 Raman band due to the formation of molecular HNO3. The dissociation of HNO3to NO3

− in the presence of additional water results in an intense NO3- symmetric stretching UVRR band at 1044 cm−1.

■ INTRODUCTION

Interest in explosives detection has dramatically increased dueto the increasing number of terrorist attacks utilizingimprovised explosive devices (IEDs).1−9 It is essential todevelop methods to detect explosives and to distinguish themfrom background interferents. Promising standoff detectionmethods are likely to involve laser spectroscopy meth-ods1,4,5,10−15 that irradiate target surfaces and analyze thescattered or emitted light for evidence of explosive species.Raman spectroscopy is a promising laser spectroscopy

method for detection of explosives because it can detect theirunique vibrational signatures. Normal Raman spectroscopy hasalready been used for explosive detection.1−4,6,10,14,16−19

Unfortunately, trace detection of explosives, utilizing normalRaman, is impeded by the typically, small normal Raman crosssections of explosives. UV resonance Raman spectroscopy(UVRR) can be used to dramatically increase these Ramancross sections, which can dramatically increase the spectralsensitivity for detection of trace explosives.UVRR spectroscopy shows great promise as a sensitive

technique for trace explosive detection.1,5,8,11−15,20 Mostexplosives show deep UV absorption bands below 260 nm.1,5

Excitation of resonance Raman spectra also result in theabsorption of numerous UV photons since the absorption crosssections exceed resonance Raman cross sections by ∼108-fold.Analytes cycle through their electronic excited states myriadtimes prior to Raman scattering. During this absorption andrelaxation cycling, the analyte can undergo photolysis, whichcan decrease its concentration. This photochemistry can also

produce new photochemical species. The resulting UVRR willshow characteristic spectral changes due to this analytedepletion and photoproduct formation. We previouslydemonstrated the impact of photochemistry in the resonanceRaman studies of nitrates, TNT, and RDX.11,12,15,21 We alsoshowed that these photochemically produced UVRR spectralchanges provide additional information that can aid inidentifying analytes.In the work here, we investigated the 229 nm UVRR solution

state photochemistry of the explosive pentaerythritol tetrani-trate (PETN). We determined the PETN photochemicalquantum yield and identified the initial photoproducts formed.We base some of the observed photochemistry on the

extensive previous studies of photolysis of nitrate esters in thegas phase.22 This previous work was motivated by the need tocharacterize the nitrogen oxides formed during atmosphericphotochemistry.22 Most of these studies used excitationwavelengths >290 nm, a region of very weak absorption.22

For nitrate esters excited at λ>290 nm, photolysis involvescleavage of the O−NO2 bond to form NO2 and an alkoxyradical.22−26

We examined PETN photochemistry with 229 nm excitation,a region of strong absorption. In some of the discussion here, itappears that the tail of the absorption band excited with λ>290nm light involves the same electronic transition that shows

Received: July 31, 2017Revised: September 21, 2017Published: September 25, 2017

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strong absorption with a maximum at ∼200 nm. Our resultsseem consistent with the expectation that 229 nm excitationphotochemistry and the λ>290 nm atmospheric photo-chemistry have identical mechanisms and photoproducts.

■ EXPERIMENTAL SECTION

PETN Solution State UVRR. Raman Instrumentation.The UVRR instrumentation was described previously.27,28 ACoherent Industries Innova 300 FreD frequency doubled Ar+

laser generates 229 nm continuous wave (CW) light. A SpexTriplemate spectrograph was used to disperse the Ramanscattered light, which was detected by a Princeton InstrumentsCCD camera (Spec-10).UVRR Measurements. The 2 mL, 1 mg/mL PETN in

CD3CN sample used for UVRR measurements was obtained byevaporating a 2 mL sample of 1 mg/mL PETN in CH3CN(Cerilliant). The PETN was then redissolved in 2 mL ofCD3CN (Acros Organics). The samples were measured in a 1cm path length fused silica capped cuvette with a magnetic stirbar. The 229 nm UVRR were excited with an ∼80 μm spot sizelaser beam.The PETN sample was photolyzed by an unfocused ∼10

mW 229 nm CW laser beam for the times necessary to achieveabsorption of 0.5, 1, 3, 5, 11, 16 photons per molecule (phot/molc).11 The UVRR of the samples were measured after eachirradiation period by exciting the UVRR with ∼4 mW of a ∼ 80μm spot size focused beam. The calculated total absorbedphot/molc includes the irradiation during the UVRR measure-ments. Quartz and CD3CN UVR spectral contributions werenumerically subtracted. A 25 μL aliquot of these photolyzedsamples was used for the quantum yield determination usingHPLC-HRMS.Absorption Measurements. The absorption spectra of the

irradiated PETN/CD3CN samples were measured in a capped1 cm path length fused silica cuvette by using a Varian Cary5000 UV−vis/near-infrared (NIR) spectrometer.A 3 mL, 1 mg/mL PETN in CD3CN sample was used for the

photolysis absorption measurements. The sample was obtainedby evaporating a 3 mL sample of 1 mg/mL PETN in CH3OH(Accustandard). The PETN was then redissolved in 2 mL ofCD3CN (Acros Organics). The sample was irradiated in acapped 1 cm path length fused silica cuvette. The PETN wasirradiated by an unfocused ∼10 mW 229 nm CW laser beamfor the times necessary to achieve absorption of 0.5, 1, 3, 5, 11,16 phot/molc.11 The absorption spectra were then measured.PETN Photolysis Quantum Yield. HPLC-HRMS was used

to determine the solution state quantum yield of the PETNphotolysis. A reversed phase 100 × 2 mm Phenomenex LunaHPLC column with 3 μm C-18 silica particles was utilized forHPLC measurements. The loss of PETN during photolysis wasdetermined from the area of the PETN + formate adduct peakin photolyzed solutions. This peak elutes at 8.3 min with a 361m/z as measured with UV detection. A calibration curve wasdetermined by measuring HPLC of calibration standards of 1,0.5, 0.25, 0.13, 0.063 mg/mL solutions of PETN in CD3CN.This calibration curve was used to determine PETNconcentrations within the irradiated PETN samples.UVRR and Absorption Studies of NO2 Hydrolysis in

Acetonitrile. The presence of water significantly alters thePETN photoproduct distributions. We examined this depend-ence by using UVRR and absorption spectra of solutions ofNO2 with varying concentrations of water.

To obtain NO2(g), ∼ 10 g of Pb(NO3)2 was placed into around-bottom flask and heated with a Meker-Fisher burneruntil decomposition occurred, forming NO2(g).

29 The NO2(g)was drawn up into a syringe and bubbled into 3 mL of CD3CN(Acros). Varying concentrations of water were added to theNO2/CD3CN sample (0, 0.1, 0.5, 1, 3, and 5% H2O byvolume). These samples were excited with ∼5 mW of CW 229nm light for the UVRR measurements. The absorption wasmeasured for each sample in a 1 cm path length fused silicacuvette.

■ RESULTS AND DISCUSSIONPETN Photolysis. Figure 1 shows the UVRR spectra of

PETN in CD3CN as the irradiation time increases. The initial

PETN UVRR spectra show PETN UVRR bands at 869 cm−1

(O−N stretching and C−C stretching), at 1279 cm−1 (C5skeletal and CH bending), at 1295 cm−1 (−NO2 symmetricstretch with CH2 wagging), at 1507 cm

−1 (CH2 scissoring), andat 1650 cm−1 (−NO2 asymmetric stretching).

1,5,30,31 The initialUVRR PETN spectrum with minimal irradiation showsnegligible contributions of photoproducts.The PETN Raman band intensities monotonically decrease

as the irradiation time increases. The 5 phot/molc PETNUVRR spectrum clearly shows a new strong overlappingphotoproduct Raman band at 1308 cm−1. At longer irradiationtimes, the PETN 1295 cm−1 Raman band intensity continues todecrease, while the 1308 cm−1 photoproduct Raman bandintensity increases. There is clearly an extensive photolysis ofPETN at 16 phot/molc, where the photoproduct 1308cm−1Raman band dominates.Figure 1 also shows the UVRR spectrum of pure NO2

dissolved in CD3CN. The UVRR shows that the dominatingNO2 symmetric stretch Raman band occurs at 1308 cm−1 inCD3CN. Thus, Figure 1 allows us to conclude that NO2 is aPETN photoproduct of 229 nm excitation.

Figure 1. 229 nm UVRR of PETN in CD3CN with increasingirradiation times (absorbed photons per molc). The NO2 in CD3CNspectrum shows the NO2 Raman band at 1308 cm−1. CD3CN andquartz UVR bands were subtracted.

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We used HPLC-HRMS to study the photoproducts formedduring PETN photolysis. In the early stages of photolysis (1phot/molc), we observe a photoproduct that has the m/z valueof 316, which is pentaerythritol trinitrate (PETriN),(CH2ONO2)3CCH2OH + formate adduct. Studies measuringPETN using electrospray ionization in the negative ion mode inliquid chromatography mass spectrometry (LCMS) haveshown that nitrate esters form adduct ions rather thanmolecular ions.32 PETriN results from cleavage of a single O-NO2 bond of PETN, C(CH2ONO2)4. PETriN has beenobserved as a post explosion degradation product of PETN.33,34

Other minor photoproduct peaks are also observed withHPLC-HRMS.We determined the photochemical quantum yield of PETN

in CD3CN by using HPLC to monitor the dependence of the361 m/z PETN+ formate adduct peak intensity on the numberof absorbed photons/molc (Figure 2). The measurements useda HPLC-HRMS calibration curve for PETN in CD3CN. Wecalculate a quantum yield of φ = 0.08 ± 0.02.

Figure 3 shows the dependence of the PETN absorptionspectrum on increasing 229 nm light absorption. PETN has anabsorption maximum at ∼200 nm that derives from two π →π* electronic transitions at ∼208 nm and ∼187 nm localized oneach of the four PETN O−NO2 groups.

35 During photolysis anincrease in absorption occurs between 300 and 350 nm whichderives from formation of N2O4, a dimer of NO2. The dimer isin equilibrium with the monomer photoproduct, NO2. TheNO2 absorption maximum occurs at ∼400 nm.36 In addition, aseries of sharp absorption features are observed between 350and 400 nm that derive from nitrous acid (HNO2). Wehypothesize that the nitrous acid results from the reaction ofNO2 with trace amounts of water.29,37,38

The presence of water significantly alters the PETNphotoproduct UVRR spectra. We find that under humid labconditions the 1308 cm−1 photoproduct UVRR band of Figure1 downshifts due to NO2 hydrolysis.Hydrolysis of NO2 Photoproducts Dissolved in CD3CN.

We investigated the hydrolysis of NO2 by measuring theabsorption and UVRR of NO2 in the presence of differentvolume fractions of water (Figures 4 and 5).Figure 4 shows the UVRR spectra of NO2 dissolved in

CD3CN at increasing volume fractions of H2O. The NO2spectrum without H2O added shows the NO2 symmetric

stretching Raman band at 1308 cm−1, and a N2O4 Raman bandat ∼1383 cm−1(coupled NO2 symmetric stretching).

39

Figure 2. PETN photolysis quantum yield measurements. Depend-ence of PETN 361 m/z peak intensity measured by HPLC-UV on thenumber of absorbed phot/molc during initial stages of photolysis inthe absence of water. The quantum yield is determined from the slope.

Figure 3. Absorption spectra of PETN in CD3CN (1 cm path length)upon 229 nm irradiation of 0.5, 1, 3, 5, 11, and 16 absorbed phot/molc.

Figure 4. UVRR excited at 229 nm of NO2 dissolved in CD3CN withaddition of 0.1, 0.5, 1, 3, 5% volume fractions of H2O. The spectra arenormalized to the 2061 cm−1 band of CD3CN. Quartz and CD3CNRaman bands are subtracted. This sample was also used for theabsorption measurements of Figure 5.

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At 0.1% volume fraction of H2O, we observe that the 1308cm−1 NO2 Raman band is downshifted to 1306 cm−1. Weassign this 1306 cm−1 band to the symmetric stretchingvibration of undissociated HNO3,

40 as clearly shown by theFigure 5 absorption spectra and discussion below. We alsoobserve the HNO3 asymmetric stretching Raman band at∼1668 cm−1 in the Figure 4 0.1% volume fraction H2O UVRRspectrum.40 HNO3 is formed by the reaction between NO2 andH2O at room temperature,29,37,38,41,42

+ → +2NO H O HNO HNO2 2 3 2

The HNO2 photoproduct is observed in the Figure 5absorption measurements. We do not observe UVRR HNO2bands likely because they have small Raman cross sections.The HNO3 vibrational frequencies depend on interactions

with the solvent. The frequency dependence in wet solvents isprobably dominated by water hydrogen bonding.40,43−46 Weobserve that, with increasing water concentration, the HNO31306 cm−1 Raman band (0.1% H2O) downshifts to 1305 cm−1

(0.5% H2O). This Raman band then further downshifts to 1303cm−1 (1% H2O) likely due to increased water hydrogenbonding to HNO3. At ∼ 1% H2O, HNO3 begins to dissociateto nitrate (NO3

−) as indicated by the appearance of the NO3−

symmetric stretching Raman band at 1044 cm−1.15,21 TheNO3

− 1044 cm−1 Raman band is first observed at 1% volumefraction water concentration with an intensity close to that ofthe HNO3 Raman band at 1303 cm−1.The NO3

− concentration dramatically increases with waterconcentration as indicated by the dominating 1044 cm−1

Raman band intensity in the 3 and 5% water spectra. In the5% H2O spectrum, the NO3

− 1044 cm−1 Raman band becomes∼5 times more intense than the NO2 1308 cm

−1 Raman band ifno H2O is present.This is an important result for UVRR detection of PETN

because NO3− is the ultimate photoproduct in the presence of

H2O, and NO3− shows a dominating UVRR cross section. We

do not observe the nitrite (NO2−) photoproduct because the

weak acid HNO2 will not dissociate under these conditions toform NO2

−.29

Figure 5 shows that the absorption spectrum of NO2dissolved in CD3CN is dominated by the N2O4 dimerabsorption, which has a strong gas phase absorption maximumat ∼340 nm.47−49 NO2 gives rise to a broad absorption bandwith a maximum at ∼400 nm.36,49 NO2 and N2O4 coexist in

equilibrium in acetonitrile.37,50 Figure S-1 of the SupportingInformation shows the absorption spectrum of gaseousmonomer NO2, while Figure S-2 shows the similar absorptionspectrum of gaseous N2O4. A larger absorption is observed forN2O4 in Figure 5 compared to that of NO2 because the dimer isfavored in CD3CN solution.50

The N2O4 and NO2 absorption bands essentially disappearupon addition of 0.1% H2O, indicating conversion of N2O4 andNO2 to HNO2 and HNO3. Undissociated HNO3 shows anabsorption maximum at ∼260 nm, which derives from an n →π* transition.51−53 This absorption band decreases in intensityas the water concentration increases, indicating HNO3dissociation, which agrees with the UVRR spectra that showintense bands of NO3

−. HNO2 shows distinct absorption bandsbetween ∼300−400 nm (Figure 5).54−56 Figure S-3 of theSupporting Information shows the absorption spectrum ofgaseous HNO2. In contrast to HNO3, the HNO2 bands persistwith increasing concentration of water because HNO2 does notdissociate in the presence of the strong acid, HNO3.

■ CONCLUSIONWe examined the 229 nm photochemistry of PETN dissolvedin CD3CN. The initial photolysis of PETN involves loss of oneof the four nitrate ester, (−ONO2), groups forming thephotoproducts PETriN [(CH2ONO2)3CCH2OH] and NO2.We measure a quantum yield for PETN/CD3CN photolysis ofφ ∼ 0.08 ± 0.02. The UVRR spectra of PETN/CD3CN show adecrease in the PETN Raman band intensities upon photolysisand the appearance of a photoproduct Raman band at 1308cm−1, which results from the symmetric stretch of dissolvedNO2. Absorbance measurements show that NO2 and its dimerN2O4 coexist in equilibrium in CD3CN.The NO2 photoproduct undergoes hydrolysis in the presence

of trace amounts of water forming HNO3 and HNO2. The1308 cm−1 NO2 photoproduct Raman band downshifts to 1306cm−1 upon conversion to HNO3. This Raman band furtherdownshifts to 1303 cm−1 upon an increase in waterconcentration likely due to increased water hydrogen bondingto the HNO3. At higher water concentrations HNO3 dissociatesto form NO3

−. The very large UVRR 229 nm cross section ofthe NO3

− symmetric stretch causes this band to dominate theUVRR of photolyzed PETN in the presence of trace water.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpca.7b07588.

Absorption spectrum of NO2, N2O4, and HNO2 (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: asher@pitt.edu.

ORCIDKatie L. Gares: 0000-0002-2147-6169NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge funding from ONR Grants N00014-12-1-0021 and N00014-16-1-2681.

Figure 5. Absorption spectra of NO2 dissolved in CD3CN atincreasing volume fractions of H2O (0.1, 0.5, 1, 3, and 5%) measuredusing a 1.0 cm path length cuvette. This is the same sample used forthe UVRR measurements of Figure 4.

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The Journal of Physical Chemistry A Article

DOI: 10.1021/acs.jpca.7b07588J. Phys. Chem. A 2017, 121, 7889−7894

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