Decomposition of pentaerythritol tetranitrate [C(CH2ONO2)4] following electronicexcitationZijun Yu and Elliot R. Bernstein

Citation: The Journal of Chemical Physics 135, 154305 (2011); doi: 10.1063/1.3652893View online: http://dx.doi.org/10.1063/1.3652893View Table of Contents: http://aip.scitation.org/toc/jcp/135/15Published by the American Institute of Physics

THE JOURNAL OF CHEMICAL PHYSICS 135, 154305 (2011)

Decomposition of pentaerythritol tetranitrate [C(CH2ONO2)4]following electronic excitation

Zijun Yu and Elliot R. Bernsteina)

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA

(Received 15 July 2011; accepted 28 September 2011; published online 20 October 2011)

We report the experimental and theoretical study of the decomposition of gas phase pentaerythritoltetranitrate (PETN) [C(CH2ONO2)4] following electronic state excitation. PETN has received majorattention as an insensitive, high energy explosive; however, the mechanism and dynamics of thedecomposition of this material are not clear yet. The initial decomposition mechanism of PETN isexplored with nanosecond energy resolved spectroscopy and quantum chemical theory employingthe ONIOM algorithm at the complete active space self-consistent field (CASSCF) level. Thenitric oxide (NO) molecule is observed as an initial decomposition product from PETN at threeUV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of thethree excitation wavelengths coincide with the (0–0), (0–1), and (0–2) vibronic bands of the NOA 2�+ ← X 2� electronic transition, respectively. A unique excitation wavelength independentdissociation channel is observed for PETN, which generates the NO product with a rotationally cold(∼20 K) and a vibrationally hot (∼1300 K) distribution. Potential energy surface calculations at theONIOM(CASSCF:UFF) level of theory illustrate that conical intersections play an important role inthe decomposition mechanism. Electronically excited S1 PETN returns to the ground state throughthe (S1/S0)CI conical intersection, and undergoes a nitro-nitrite isomerization to generate the NOproduct. © 2011 American Institute of Physics. [doi:10.1063/1.3652893]

I. INTRODUCTION

Energetic material pentaerythritol tetranitrate (PETN)[C(CH2ONO2)4] has been the focus of attention for decadesdue to its numerous applications in the explosive core of in-dustrial detonating fuses, in the base charge of commercialblasting caps, in industrial boosters, in linear shaped charges,and in commercial and military detonators.1, 2 It is also usedas a vasodilator drug to treat certain heart conditions, such asfor management of angina.3 Thus, PETN has intensively beeninvestigated both experimentally and theoretically.4–11

Swayambunathan et al.4 have performed laser photofrag-mentation fragment detection (PF-FD) experiments near227 nm to detect trace concentrations of PETN. In their ex-periments, detection is accomplished by resonance enhancedmultiphoton ionization with miniature electrodes and bylaser-induced fluorescence with a photodetector: a nitric ox-ide (NO) fragment is found to be the major product. Using ananosecond time-resolved spectropyrometric system operatedat six discrete wavelengths between 350 and 700 nm, shocktemperatures of PETN single crystals have been measuredby Yoo et al.5 The results show that the shock sensitivity ofPETN is strongly dependent on crystal orientation: sensitivealong shock propagation normal to the (110) plane, but highlyinsensitive to it normal to the (100) plane. The detonationtemperature of PETN is, however, independent of crystal ori-entation. Wu and co-workers6 have performed semi-empiricalquantum mechanical molecular dynamics (MD) simulations

a)Author to whom correspondence should be addressed. Electronic mail:erb@lamar.Colostate.edu.

involving collisions of two PETN molecules at differentmolecular orientations and at several intermolecular sepa-rations. They point out that the dissociation mechanism ofPETN remains unimolecular, and the role of binary collisionsis to transfer the kinetic energy to excite internal degrees offreedom in PETN molecules; the dominant initial reactionpath during binary collision involves the bond breaking ofan O–NO2 group. This dissociation mechanism is insensitiveto mutual molecular orientations and is identical to thelowest energy pathway for unimolecular reactions involvingan isolated PETN molecule. The probability of collision-induced decomposition of PETN depends strongly on initialconditions, however, in agreement with the experimentallyobserved sensitivity of shock initiated detonation in bulkPETN along different crystalline orientations. However,the detailed mechanism of energy transfer from the binarycollision to vibrational modes that leads to unimolecular dis-sociation has not been studied. Landerville et al.7 performedBorn-Oppenheimer direct dynamics classical trajectorysimulations of bimolecular collisions of PETN moleculesto investigate the fundamental mechanisms of hypervelocitychemistry relevant to initiating reactions immediately behindthe shock wavefront in energetic molecular crystals. Thereactive case simulations show the formation of NO2 as thedominant reaction pathway in all cases, with H2CO forma-tion for cases in which a large amount of collision energyis localized in a particular arm of the molecule. Bond disso-ciation energies play a significant role in the decompositionmechanism.

Studies of PETN chemistry have appeared for crystalthermal decomposition and stability,4–11 but little information

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154305-2 Z. Yu and E. R. Bernstein J. Chem. Phys. 135, 154305 (2011)

is available for unimolecular, isolated molecule decompo-sition of PETN. Additionally, ignition processes involvingsparks, shock, slow/rapid heating, lasers, and arcs can all initi-ate the decomposition reaction of PETN by generating excitedelectronic states. Thus, investigations of the gas phase, iso-lated molecule decomposition of PETN following electronicexcitation will yield an improved understanding of the initialdecomposition mechanisms and dynamics for NO/NO2/NO3

containing energetic materials, at a fundamental level.In this work, we will focus on understanding the decom-

position mechanism and dynamics of PETN under unimolec-ular conditions. Nanosecond energy resolved spectroscopyis employed to investigate the excited electronic state de-composition mechanisms and dynamics of isolated gas phasePETN. The PETN parent molecule is excited to its (n,π*)excited electronic state by absorption of a single UV pho-ton and a dissociation product, nitric oxide, is observed bya one color (1+1) resonance enhanced two photon ioniza-tion (R2PI) scheme using time of flight mass spectrome-try (TOFMS) detection. Three vibronic transitions A 2�+(υ ′

= 0) ← X 2�(υ ′′ = 0,1,2) of the NO product molecule fromPETN are observed: the NO product from PETN is gener-ated through an excitation wavelength independent, nonadi-abatic, dissociation channel and is characterized by a rota-tionally cold (∼20 K) and a vibrationally hot (∼1300 K)internal energy distribution. Quantum mechanical calcula-tions [ONOIM (CASSCF:UFF)] are performed on groundand electronic excited states of PETN in order to gener-ate details about the decomposition mechanisms pertainingto PETN. Based on these calculations and the experimen-tal observations, we propose that PETN from the S1 stategoes back to the S0 state through a nonadiabatic mecha-nism involving a (S1/S0)CI conical intersection (CI) and, then,undergoes a nitro-nitrate isomerization on the S0 potentialenergy surface (PES), finally giving birth to the NO prod-uct. Therefore, present results provide several detailed in-sights into the unimolecular decomposition behavior of PETNfrom its excited electronic states. We note that a numberof other NO/NO2/NO3 containing energetic materials (suchas, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL20) and3,3′-diamino-4,4′-azoxyfurazan (DAAF)) (Refs. 12–14) fol-low a very similar decomposition mechanism from theirelectronic excited states.

II. EXPERIMENTAL PROCEDURE

A detailed experimental procedure for nanosecond massresolved excitation spectroscopy has been described in ourprevious publications.15, 16 Briefly, the experimental setupconsists of laser systems with nanosecond time durationpulses (10 Hz), a supersonic jet expansion nozzle with a laserdesorption attachment, and a time of flight mass spectrome-ter vacuum chamber. For the nanosecond laser experiments, asingle pump-probe laser beam at three separate wavelengths(226, 236, and 248 nm) is used both to initiate dissociationof PETN molecules and to detect NO following a one-color(1+1) R2PI scheme (A 2�+(υ ′ = 0) ← X 2�(υ ′′ = 0,1,2) and

I ← A transitions) through TOFMS. The three UV laser wave-lengths used in the nanosecond laser experiments are gener-ated by a pulsed dye laser, pumped by the second harmonic(532 nm) of a neodymium doped yttrium aluminum garnetlaser’s fundamental output (1.064 μm), in conjunction with anonlinear wavelength extension system. The typical pulse en-ergy of the UV laser is 50–400 μJ/pulse depending on the ex-act wavelength of interest for a one-color experiment, whichgives a laser beam intensity (I) ∼ (0.3–2.5) × 107 W/cm2 fora 8 ns pulse duration at a focused beam diameter of 0.5 mm.

The isolated gas phase PETN molecules are producedthrough a combination of matrix-assisted laser desorption andsupersonic jet expansion. The nozzle employed for the sam-ple beam generation is constructed from a Jordan Co. pulsedvalve and a laser desorption attachment.15 Sample drums formatrix desorption are prepared by wrapping a piece of porousfilter paper around a clean aluminum drum. A solution ofequimolar amounts of sample and matrix (R6G dye) in ace-tone is uniformly sprayed on the sample drum. An air atomiz-ing spray nozzle (Spraying System Co.) with siphon pressureof 10 psi is used to deposit the PETN and R6G on the drumsurface. During the spraying, the drum is rotated and heatedwith a halogen lamp to make sure that the coating is homo-geneous and dry. The dried sample drum is then placed inthe laser ablation head/nozzle assembly and put into a vac-uum chamber. In order to maintain a fresh sample area foreach laser ablation shot, a single motor is used to rotate andtranslate the sample drum simultaneously. PETN moleculesare desorbed from the drum by laser ablation at 532 nm, en-trained in the flow of helium carrier gas through a 2 × 60 mmchannel in the ablation head, and expanded into the vacuumchamber.17, 18

The experiment is run at a repetition rate of 10 Hz. Thetiming sequence for the pulsed nozzle, ablation laser, and ion-ization laser is controlled by a time delay generator (SRSDG535). The molecular beam is perpendicularly crossed by aUV laser beam that is focused to a spot size of about 0.5 mmat the ionization region of a time of flight mass spectrometer.A background pressure of 2 × 10−6 Torr is maintained in thevacuum chamber during the experiment. Ion signals are de-tected by a microchannel plate detector. Signals are recordedand processed on a personal computer using a boxcar aver-ager (SRS SR 250) and an analog-to-digital conversion card(Analog Devices RTI-800).

III. COMPUTATIONAL METHOD

The ONIOM method, which divides a large molecularsystem into two or three layers and treats them with differentlevels of theory and, then, combines those results into the fi-nal predictions, is used to execute the calculation for the largemolecular system PETN. In our previous work,19 the ONIOM(CASSCF:UFF) method has been employed to explore thenonadiabatically coupled ground and excited electronic statePESs of the isolated RDX molecule. In that calculation, one ofthe N–NO2 moieties of the RDX molecule is considered to bean active site. Electronic excitation of RDX is supposed to belocalized in the active site, which is treated with the CASSCFmethod. The remainder of the molecule was calculated by

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a universal force field (UFF)/molecular mechanics (MM) orHartree-Fock (HF)/quantum mechanics (QM) method to ob-tain the steric and electrostatic influences on the active site.Computational results predict that a nitro-nitrite isomeriza-tion on the S0 PES is the major decomposition channel for theelectronic excited RDX molecule. Nitro-nitrite isomerizationof RDX is found to occur through a series of conical intersec-tions and is finally predicated to produce rotationally cold butvibrationally hot distributions of NO products: this decom-position pathway is in good agreement with the experimentalobservation of rovibrational distributions of the NO product.Thus, this previous work19 indicates that the ONIOM methodcombined with the CASSCF method is a feasible means toexplore the excited electronic state PESs for large energeticmolecules.

In current work, we use the GAUSSIAN 09 package20 in-stalled on the Ember (NCSA, TeraGrid)) supercomputer toperform the ONIOM calculation. In this calculation, we con-sider two layer QM:MM combinations, for which the totalenergy of a molecule is expressed through an extrapolationscheme as

EONIOM = EHighactive site + ELow

real − ELowactive site.

Here, active site stands for a small part of the real molecule,at which a chemical or physical process being studied is lo-calized. An adequate high level is employed to describe accu-rately the behavior of the active site. A low level and cheaperMM method is employed for the full molecule (denoted asreal). As a consequence, the active site is treated with bothhigh and low levels of theory, and the full molecule is treatedwith only a low level of theory. The energy gradient is thederivative of the above expression with respect to the reactioncoordinate, and the optimization is performed on the total PESgradient. The (ELow

real − ELowactive site) term in the ONIOM energy

expression describes both the contribution from the low levelregion and the interaction between the high and low level re-gions. In other words, coupling between the two regions isevaluated at a low computational level of theory. In general,the ONIOM method cannot calculate the absolute energy ofthe real system, as the various subcalculations (MM) eitherhave different energy scales or are less accurate on the sameenergy scale (pure QM); however, the energy difference be-tween two points on a PES will be reproduced with consider-able accuracy.21–23

The active site and the remainder of the PETN moleculeconsidered for the present work are illustrated in Fig. 1. Thehigh level region is depicted by balls, and the low level regionis depicted by a wire frame. Here, electronic excitation is as-sumed to be localized entirely in the active site (more specif-ically in one O–NO2 unit) of the molecule. The CASSCF al-gorithm (to calculate E

Highactive site) is selected to treat the active

site because this approach can successfully describe excitedelectronic states with adequate accuracy. Choice of a low levelmethod (to calculate ELow

real − ELowactive site) is not straightforward,

however. Based on the assumption that electronic excitation iscompletely localized in the high level region (active site), onecan choose a ground state method (e.g., HF, UFF), becausethe electronic excitation is the same in ELow

real and ELowactive site

and, therefore, cancels in the difference (ELowreal − ELow

active site).

FIG. 1. Optimized structure of PETN at ONIOM(CASSCF(6,6)/6-31G(d):UFF) level of theory on the ground state. The high-level region ofPETN is depicted by balls, and the low-level region is depicted by a wireframe.

For the present study, low level ground state methods, such asUFF MM, are employed for calculation of the properties ofthe remainder of the PETN molecule. The difference of inter-action energies between the high and low level regions relatedto the ground and excited electronic-state structures of PETNis assumed to be negligible.24

To explore the excited state potential energy surfaces, theactive space comprises 6 electrons distributed in 6 orbitals,denoted as CASSCF(6,6). Orbitals used in the active spaceare one nonbonding nO orbital, one NO bonding orbital σ NO,one π -bonding orbital πO, one ONO π -antibonding orbitalπONO*, one NO antibonding orbitals σ NO*, and one NO2 an-tibonding orbitals σ ONO*, as illustrated in Fig. 2. Search forcritical points (minima, transition states (TSs), and CIs) onexcited electronic and ground state PESs for PETN is per-formed at the ONIOM[CASSCF(6,6)/6-31G(d):UFF] level oftheory with the GAUSSIAN 09 program. No symmetry restric-tions are applied during the calculations. Vertical excitationenergies are computed by state averaging over the groundstate and two singlet excited states with equal weights. Thegeometries of the critical points are optimized with state aver-aging over the S0 and S1 states with equal weights. Transitionstate structures are characterized by analytical frequency cal-culations. Equilibrium geometry calculations are conductedtaking the total charge as neutral and the spin multiplicity as1. The whole potential energy surfaces are calculated usingan intrinsic reaction coordinate (IRC) algorithm and a SCANalgorithm implemented in GAUSSIAN 09.

IV. EXPERIMENTAL RESULTS

The PETN parent molecule is excited to an uppervibronic state by absorption of a single UV photon: threedifferent excitation wavelengths (226, 236, and 248 nm) areemployed. The parent molecule decomposes into products

154305-4 Z. Yu and E. R. Bernstein J. Chem. Phys. 135, 154305 (2011)

FIG. 2. Orbitals used in active space for CASSCF calculation for PETN.

through specific decomposition pathways. Different possibledecomposition pathways of the PETN parent molecule areillustrated in Fig. 3. The PETN molecule potentially cangenerate a number of initial products, such as, NO afternitro-nitrite isomerization, NO2 and ONO2 through directN–O or C–O bond rupture, and HONO elimination. In theexperiments, however, NO is observed to be the major initialdissociation product in the decomposition of the electronicexcited PETN molecule. The NO product is probed using aone color (1+1) R2PI detection scheme through TOFMS.The three excitation wavelengths used in this work alsocorrespond to the resonance (0–0), (0–1), and (0–2) vibronicbands of the A 2�+ ← X 2� electronic transitions of theNO product, respectively. By scanning the nanosecond laserexcitation wavelength, a (1+1) R2PI rotationally resolvedspectrum of the NO product from different parent moleculesis obtained. The parent molecule has continuous absorptionin these wavelength regions.

The (1+1) R2PI spectra of the three vibronic transitions,A 2�+(υ ′ = 0) ← X 2�(υ ′′ = 0,1,2) of the NO productobserved from the decomposition of the electronic excitedPETN molecule are shown in Fig. 4. The most intense fea-ture in each spectrum of NO can be assigned as the (Q11+P12)band head of each vibronic band,25, 26 and the less intense fea-tures within each spectrum are due to other rovibronic tran-sitions. Spectral simulations based on Boltzmann populationdistributions for the three vibronic transitions produce similar

FIG. 4. One color (1+1) R2PI spectra of the vibronic transitions A 2�+(υ ′= 0) ← X 2�(υ ′′ = 0,1,2) of the NO product from the decomposition of theelectronically excited PETN molecule. Rotational temperature simulationswith a Boltzmann population distribution show that the three observed vibra-tional levels of the ground electronic state have a cold rotational temperatureof ∼20 K.

rotational temperatures (∼20 K). The vibrational temperatureof the NO product from PETN can also be obtained by sim-ulating the relative intensities among the observed vibronicbands using a Boltzmann population distribution analysis andFranck-Condon factors. By comparing the experimental datawith simulations at different vibrational temperatures, the vi-brational temperature of the NO product from PETN is esti-mated to be ∼1300 K.

The NO product from decomposition of PETN is gen-erated following single photon absorption. This is confirmedby careful analysis of the linewidth of the NO mass peak.Extra kinetic energy from photodissociation following mul-tiphoton absorption would generate additional linewidth forproduct mass channels. Moreover, if the PETN molecule ab-sorbs multiple photons at 226 nm sequentially, it can pho-todissociate (fragment) into NO following ionization. Thiswill also lead to a broadening of the linewidth of the NO

FIG. 3. Different possible decomposition channels for PETN.

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FIG. 5. One color (1+1) R2PI spectra of the vibronic transitions A 2�+(υ ′= 0) ← X 2�(υ ′′ = 0) of the NO product from the decomposition ofthe electronically excited NO2. Rotational temperature is estimated to be∼130 K.

mass signal. The linewidth of the NO mass channel signal isobserved to be about 10 ns, which is the instrumentallinewidth of our laser/TOFMS setup. Therefore, we believethat the NO product is associated with a single photon ab-sorption by these molecules. Furthermore, the laser energyis reduced to ∼100 μJ/pulse, so that the multiphoton dis-sociation of PETN is avoided. Hot PETN parent moleculesthat can be generated in the matrix-assisted laser desorption(MALD) process would be effectively relaxed and cooled inthe highly collisional expansion environment through the su-personic nozzle.

Although the MALD technique is a good method to placeeasily fragmented, fragile molecules in the gas phase with-out fragmentation, a great effort is taken in our previous andpresent work to ensure that the sample molecules are not frag-mented in the ablation process.15, 16 Three different methodshave been employed to explore this issue: comparison of NOvelocity distributions from the nozzle, determination of arrivaltime for the NO signal intensity as a function of nozzle/pumplaser timing, and NO rotational and vibrational temperaturedeterminations as a function of NO source. These methodsdemonstrate that NO from PETN is generated at the ioniza-tion region of the TOFMS, not in the laser desorption regionof the nozzle.

The (1+1) R2PI spectrum of NO from photolysis of NO2

in the NO A 2�+(υ ′ = 0) ← X 2�(υ ′′ = 0) transition regionis presented in Fig. 5. The (0–0) vibronic band of the NOproduct is observed from photolysis of NO2 gas at 226 nmphotoexcitation: the spectrum is characterized by a rotationaltemperature of about 130 K. Comparing the rotational distri-bution of NO from NO2 gas and PETN, NO2 is excluded fromthe primary source for NO and the major reaction channelfor the decomposition of electronically excited PETN, sinceif NO2 loss were the major reaction channel for decomposi-tion of electronically excited PETN, NO2 would subsequentlydissociate to NO in the wavelength range used in our exper-iments, and thus NO from NO2 produced by PETN shouldpresent similar or hotter rotational distributions compared tothat from expansion cooled NO2 gas. NO from photolysis ofthe PETN molecule is, however, rotationally colder than thatfrom expansion cooled NO2 gas, which indicates that NO2

cannot be the precursor for the NO product and the majorreaction channel in the decomposition of the electronicallyexcited PETN molecule. Instead, a nitro-nitrite isomerizationchannel is proposed to be the mechanism of NO elimination

from the PETN molecule. With regard to the HONO reac-tion channel, we have determined that the HONO elimina-tion in all X–NO2 system is less than 5% of the NO elim-ination following the nitro-nitrite isomerization in previouswork.16, 27, 28 So, we have not presented this channel here as amajor competitor for the nitro-nitrite isomerization/NO elim-ination channel. The detailed description of different reactionchannels for X–NO2 systems including models and energeticsystems will be discussed in a future paper.

V. THEORETICAL RESULTS

A. Equilibrium geometry of PETN

The optimization of PETN on the ground state usingthe two layer ONIOM methodology described in Sec. III isconducted by using the CASSCF method with a 6-31G(d)basis set for the high level region and UFF for the lowlevel region. The optimized Frank-Condon (FC) geometryof PETN on the ground state is shown in Fig. 1. PETNhas a number of conformers. Here, only the conformer be-longing to point group S4 is chosen to execute the ONIOMcombined with CASSCF methodology to explore the po-tential energy surfaces of PETN, since the S4 point groupstructure of PETN is the most stable conformer.29 In gen-eral, with a good combination of theory levels for the ac-tive site and the low level region of the molecule for anONIOM methodology, the predicted geometrical parametersshould be similar to those predicted by a pure quantum me-chanical calculation. Therefore, a comparison of geometricalparameters of PETN predicted at the ONIOM(CASSCF/6-31G(d):UFF) and MP2/6-31G(d) levels of theory is also per-formed to judge appropriateness of the CASSCF:UFF combi-nation used in the present ONIOM methodology. Note thatcomparison of geometrical parameters for PETN predictedat the ONIOM(CASSCF:UFF) and pure quantum mechani-cal CASSCF levels of theory is impossible to perform be-cause the PETN molecule is too large to be treated with apure CASSCF multiconfiguration SCF level of theory. Thegeometrical parameters for PETN computed at the MP2/6-31G(d) and ONIOM(CASSCF(6,6)/6-31G(d):UFF) levels oftheory are given in Table I (for atom labels, see Fig. 1). Theagreement between the ONIOM(CASSCF:UFF) and MP2 re-sults for the active site of PETN is excellent, in general. Thissuggests that the CASSCF:UFF combination in the ONIOMalgorithm is an adequate method to predict the structure ofPETN, and even more to explore the potential energy surfacesof PETN.

B. Electronic transition of PETN

The vertical excitation energies for PETN, com-puted at the ONIOM(CASSCF(6,6)/6-31G(d):UFF) opti-mized Franck-Condon geometry (i.e., the ground state min-imum) are listed in Table II (the calculated energies have notbeen corrected for zero point energy). The calculated verti-cal excitation energies of 4.9 ev for S1 and 6.0 ev for S2 arein agreement with the experimental UV-Vis absorption max-ima of 4.8 and 6.5 ev, which is shown in Fig. 6. CASSCFcalculations show that the two lowest lying excited states for

154305-6 Z. Yu and E. R. Bernstein J. Chem. Phys. 135, 154305 (2011)

TABLE I. Geometrical parameters of PETN at MP2/6-31G(d) andONIOM(CASSCF(6,6)/6-31G(d):UFF) (for atom labels see Fig. 1).

MP2 CASSCF

Bond lengths (Å)C2O5 1.435 1.426O5N6 1.422 1.345N6O7 1.200 1.193N6O8 1.191 1.186

Bond angles (◦)C2O5N6 114.1 115.1O5N6O7 116.9 118.6O7N6O8 130.8 127.9

Dihedral angle (◦)C2O5N6O7 −0.46 − 0.24

PETN are n → π* transitions, which is consistent with theresults for nitromethane, dimethylnitramine, and RDX, as wehave published previously,19, 27, 28 even though the HOMO andLUMO orbitals are π and π* orbitals (see Fig. 2). Both exper-iments and calculations30–32 indicate that the electrons excitedfor the S0 → S1 transitions of these NO2-containing systemsalmost exclusively come from the nonbonding orbital nO ofNO2. These NO2 localized n → π* transitions are similar forPETN, nitromehtane, dimethylnitamine, RDX, HMX, etc. Inaddition, by comparing the three excitation energies (5.49 eVat 226 nm, 5.25 eV at 236 nm, and 5.00 eV at 248 nm) used inthe experiment with the calculated vertical excitation energies(4.9 and 6.0 eV) of the S1 (n,π*) and S2 (n,π*) exited states ofthe PETN molecule, one can find that the PETN molecule isexcited to its S1 (n,π*) excited state by these three excitationwavelengths.

C. Electronic PESs of PETN

A schematic one-dimensional plot of the multidimen-sional singlet PESs (S0 and S1) of PETN is presented inFig. 7. The plot is along the minimum-energy path coordi-nates available to PETN from the Franck-Condon point on S1,“S1/(n,π*),” with locations and structures of different criticalpoints and CIs. All the structures for the important pointslabeled in Fig. 7 are presented in Fig. 8. “FC geometry” isthe optimized minimum energy structure of PETN on theground state S0. “S0,TS” is the tight transition state of thenitro-nitrite isomerization on S0. “(S1/S0)CI” is the conicalintersection between S0 and S1: the smallest adiabatic energygap between the S0 and S1 surfaces near the “(S1/S0)CI” ofPETN is computed to be ∼2 cm−1, which suggests that theS1 and S0 surfaces are strongly nonadiabatically coupled atthis point (CI). “S0,inter” is the intermediate structure between

TABLE II. Vertical transition energy (without zero point energy correction)at FC geometry.

Compound Excited state Ecal (eV) Eexp (eV)

PETN S1/(n,π*) 4.9 4.8S2/(n,π*) 6.0 6.5

FIG. 6. The UV-Vis absorption spectrum of PETN in acetonitrile.

the tight transition state “S0,TS” and the conical intersection“(S1/S0)CI,” which is obtained by IRC from the conical inter-section “(S1/S0)CI.” “S0,nitrite” is the nitrite structure of PETNafter nitro-nitrite isomerization on S0 and “S0,TS,NO-elimin”is the transition state for NO elimination from the nitritestructure of PETN on S0. “S1,min” is the optimized minimumenergy structure of PETN on the first excited state S1. “S1,TS”is the transition state on S1 between “S1,min” and “(S1/S0)CI”on S1. “S1,nitrite” is the nitrite structure of PETN afternitro-nitrite isomerization on S1 and “S1,TS,NO-elimin” is thetransition state for NO elimination from the nitrite structureof PETN on S1. “S1,TS,nitro-nitrite” is the transition state betweenthe conical intersection “(S1/S0)CI” and the nitrite structure

FIG. 7. A schematic one-dimensional projection of the multidimensionalpotential energy surfaces of PETN computed at ONIOM(CASSCF(6,6)/6-31G(d):UFF) level of theory. The labeled relative CASSCF energies of thecritical points (minimum, conical intersection, transition state) on the PESswith respect to the FC geometry (S0) are not corrected for zero point energy.The potential energy surfaces are calculated by the IRC and SCAN algo-rithms: the FC geometry and S0,inter are the two minima connected by thetransition state S0,TS; S0,inter and S0,nitrite are the two minima connected bythe conical intersection (S1/S0)CI; (S1/S0)CI and S1,min are the two minimaconnected by the transition state S1,TS; (S1/S0)CI and S1,nitrite are the twominima connected by the transition state S1,TS,nitro-nitrite.

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FIG. 8. Structures of PETN at different critical points on PESs calculated atONIOM[CASSCF(6,6)/6-31G(d):UFF] level of theory. “FC geometry” is theoptimized minimum energy structure of PETN on the ground state S0. “S0,TS”is the tight transition state of the nitro-nitrite isomerization on S0. “(S1/S0)CI”is the conical intersection between S0 and S1. “S0,inter” is the intermediatestructure between the tight transition state “S0,TS” and the conical intersec-tion “(S1/S0)CI,” which is obtained by the IRC from the conical intersection“(S1/S0)CI.” “S1,min” is the optimized minimum energy structure of PETN onthe first excited state S1. “S1,TS” is the transition state on S1 between “S1,min”and “(S1/S0)CI” on S1. “S1,TS,nitro-nitrite” is the transition state between theconical intersection “(S1/S0)CI” and the nitrite structure of PETN “S1,nitrite”on S1. “S0,nitrite” and “S1,nitrite” are the nitrite structures of PETN afternitro-nitrite isomerization on S0 and S1, respectively. “S0,TS,NO-elimin” and“S1,TS,NO-elimin” are the transition states of NO elimination from the nitritestructure of PETN on S0 and S1, respectively. The arrows in the structure ofthe transition states show the reaction coordinate of the imaginary frequency.

of PETN “S1,nitrite” on S1, which is obtained by searching fora TS between “(S1/S0)CI” and “S1,nitrite” using the CASSCFmultireference algorithm. The potential energy surfaces areexplored by the IRC and SCAN algorithms, and then they arecombined together to build the whole reaction path.

In Fig. 7, the relative CASSCF energies of the criti-cal points (minimum, conical intersection, transition state)on both excited S1 and ground S0 PESs of PETN with re-spect to the energy of ground state FC geometry are alsopresent (the calculated energies have not been corrected forzero point energy). And the blue dotted arrows (labeled 1, 2,3) in Fig. 7 indicate different possible decomposition chan-nels for the excited state decomposition of PETN along dif-ferent relevant nuclear coordinates. One decomposition path-way of PETN from the FC point of the S1 surface would bealong the S1 surface, which bypasses the (S1/S0)CI conical in-tersection, and remains on the S1 PES (arrow 1). The moleculemust then overcome the S1,TS,nitro-nitrite transition state to initi-ate the nitro-nitrite isomerization on the S1 surface. This pro-cess, however, requires an activation energy barrier of 2 eVto be surmounted on the S1 surface of PETN with respectto the (S1/S0)CI conical intersection (see Fig. 7). For subse-quent NO elimination on the S1 PES, PETN must surmountthe S1,TS,NO-elimin transition state subsequent to the nitro-nitriteisomerization. Then, the energy barrier to subsequent NO lossis about 0.7 eV with respect to the S1,nitrite local minimum.The unstable normal mode of vibration for the NO elimina-tion transition state on the S1 surface of PETN clearly showsa large component of force on only the N atom of the elimi-nated NO (see Fig. 8 “S1,TS,NO-elimin”). This force imbalance isexpected to produce considerable torque on the terminal NOmoiety. Therefore, decomposition of the molecule throughnitro-nitrite isomerization followed by NO elimination on theS1 surface is expected to produce a rotationally hot NO prod-uct. To proceed through this pathway, however, PETN mustfirst surmount a high activation barrier (2 eV) and then a bar-rier of 0.7 eV on the S1 surface. Even though the photon en-ergy is about 5.5 eV at 226 nm, which is a little higher than themaximum energy barrier S1,TS,NO-elimin (5.3 eV with respect tothe FC geometry of the ground state), all the photon energywould be required to overcome the energy barriers, withoutrotation and vibration excitation of the PETN molecule in thedissociation process. Therefore, such high activation barrierson the S1 PES render the pathway untenable.

Based on the conclusion above, one can find that the de-cay path, from the S1 state FC point to the minimum of theS1 state (S1,min), surmounting the energy barrier on the S1 sur-face, and finally reaching the (S1/S0)CI conical intersection, isthe most energetically favorable decomposition pathway forPETN: this pathway will then lead the molecule to its groundstate PES. After the molecule comes back to the ground statethrough (S1/S0)CI, a number of fragmentation channels are en-ergetically available for PETN. To understand these differentpossible channels on the ground state PES of PETN, differ-ent critical points and related minimum energy paths on theS0 state of PETN must be explored. Undoubtedly, the over-all most favorable path, which is determined by many fac-tors such as the steepest descent and the largest �E, from(S1/S0)CI would be the primary decomposition channel for

154305-8 Z. Yu and E. R. Bernstein J. Chem. Phys. 135, 154305 (2011)

this molecule.33 A key feature of crossing geometries is thatmore than one relaxation direction might exist. From calcula-tional results, two most possible energy decay pathways canbe identified on the S0 PES: one involves coming back to FCminimum geometry after surmounting an activation barrier(blue dotted line, arrow 2), and the other involves evolutionalong the deepest descent path, which results in a nitro-nitriteisomerization (blue dotted line, arrow 3). Before addressingthe most favorable path connecting to (S1/S0)CI on the groundstate, we first point out a number of important insights for theground state decomposition of PETN.

If the PETN molecule surmounts the activation bar-rier and goes back to FC minimum geometry after crossingthrough the (S1/S0)CI CI (this is energetically favorable), thedynamical behavior will act as the thermal decomposition ofPETN on the ground state PES. The ONIOM(CASSCF:UFF)level of theory predicts activation energies for the O–NO2

bond dissociation energy barrier from PETN and the NOelimination barrier following nitro-nitrite isomerization of0.97 and 2.6 eV (see Fig. 8), respectively. Therefore, theseresults indicate that, if the PETN molecule is thermally disso-ciated on the ground state surface only, the major decomposi-tion channel should be O–NO2 bond dissociation because thischannel is associated with the lowest activation barrier. In or-der to elucidate the first chemical events that trigger chemistrybehind a shock wave front, Landerville et al. have performeddensity functional theory direct dynamics simulations setupby choosing molecular pairs colliding head on along specificcrystallographic directions [100], [110], and [111] for PETN.The results show that each reactive collision case producesNO2 as its first and major product.34, 35

If the molecule evolves from the upper excited state (S1)to the ground state, however, the (S1/S0)CI conical intersec-tion becomes important, and the reaction path taken dependson a number of factors associated with the S0 PES connectingto (S1/S0)CI. Figure 7 (blue dotted line, arrow 3) directly in-dicates that, if the PETN molecule comes back to the groundstate following the (S1/S0)CI CI, the most favorable path is as-sociated with the nitro-nitrite isomerization, which is a barri-erless channel. These paths are explicitly calculated using theIRC algorithm. The PETN molecule isomerizes to a nitriteform on its S0 surface and then undergoes NO elimination,surmounting an energy barrier (∼0.9 eV), to the NO elim-ination transition state. The force components on N and Oatoms for the unstable normal mode of vibration for the NOelimination transition state (S0,TS,NO-elimin) on the S0 surfaceof PETN are almost parallel (see Fig. 8 “S0,TS,NO-elimin”). Thistransition state is thereby predicted to produce little torqueon the terminal NO moiety. Therefore, decomposition of thePETN molecule, following nonadiabatic relaxation throughthe (S1/S0)CI conical intersection followed by NO elimina-tion on the ground state surface, is expected to produce a ro-tationally cold NO product. Additionally, following verticalexcitation of PETN to its higher vibronic manifold of the S1

surface, the molecule undergoes rapid nonadiabatic internalconversion from S1 to S0 through the (S1/S0)CI conical inter-section and thereafter from the nitro-nitrite isomerization toNO elimination. Thus, on this pathway, the molecule storessignificant electronic excitation energy in its vibrational de-

grees of freedom on the ground state surface. Moreover, com-pared to the bond length 1.21 Å of the NO at TS, the equi-librium bond length of the NO is about 1.14 Å, so there is agreat chance to excite the vibration of NO. Based on the abovediscussion, a rotationally cold and vibrationally hot distribu-tion of NO product from the S0 surface is reasonable. Kuklja’sgroup finds that 1,1-Diamino-2,2-dinitroethylene (DADNE)(Ref. 36) also produces a NO moiety from excited state de-composition, and produces a NO2 moiety upon ground statedecomposition, even though DADNE has a C–NO2 bond, dif-ferent from the O–NO2 bond of PETN. X–NO2 systems ap-parently have similar reaction mechanisms due to the pres-ence of the NO2 moiety.

VI. DISCUSSION

In this work, the energetic material PETN is studied bothexperimentally and theoretically to elucidate its decomposi-tion pathways and initial fragmentation upon electronic stateexcitation. We have observed NO from the dissociation ofPETN, and we conclude that it is the major initial productfor its excited electronic state decomposition mechanism. Inour experiments, the PETN parent molecule is excited to itsfirst excited electronic state using different excitation wave-lengths (226, 236, and 248 nm) that also correspond to theresonance transitions A 2�+(υ ′ = 0) ← X 2�(υ ′′ = 0,1,2)of the NO molecule. The NO product from PETN displayssimilar cold rotational distributions (∼20 K) at all three ex-citation wavelengths (226, 236, and 248 nm). Similar rovi-bronic spectral structures for the NO product from PETN atdifferent excitation wavelengths indicate that the decomposi-tion of PETN from its S1 excited electronic state follows a sin-gle, excitation wavelength independent dissociation channel.Thus, we conclude that decomposition of the electronicallyexcited energetic material PETN takes place through the samepathway at the three excitation wavelengths used in this ex-periment. Compared with the results for RDX, HMX, CL20,and others,12–14 NO/NO2/NO3 containing energetic materi-als show similar NO product rovibronic distributions, that is,cold rotational and hot vibrational distributions. This behav-ior can be rationalized by the fact that PETN, and the otherrelated NO/NO2/NO3 containing energetic molecules, musthave similar and potentially characteristic dissociative excitedstate mechanisms, as described above, that all provide barrier-less, single deactivation channels (e.g., Sn → Sn−1 → . . . →S0) for excited state initiated decomposition.

To get theoretical support and understanding for these ex-perimental observations, the evolution pathways from excitedelectronic states with respect to the relevant reaction coor-dinates have to be known. We have, therefore, explored thePESs for PETN through the ONIOM(CASSCF:UFF) method(see Fig. 7). From calculation results, conical intersections arenow firmly established to be the key features in the excitedelectronic state chemistry of the polyatomic molecules.37–39

The concept of a conical intersection was first introduced byvon Neumann and Wigner40 and has now been recognized asessential for describing the dynamics and mechanisms for de-activation of a range of polyatomic molecules from their ex-cited electronic states to their ground electronic PES.41, 42 One

154305-9 Decomposition of PETN J. Chem. Phys. 135, 154305 (2011)

of their most important implications is the rapid and efficientinternal conversion from upper to lower electronic state sur-faces through radiationless transitions. A radiationless transi-tion through a conical intersection involves rapid conversionof electronic energy of the upper electronic state to vibra-tional energy (thermal energy) of the lower electronic state.As mentioned before, the excitation wavelengths used in ourexperiment correspond to the transition to the S1(n,π*) ex-cited state of the NO2 moiety. Thus, for PETN, the energyof the (S1/S0)CI conical intersection is lower than the threeexcitation energies employed, which indicates that due to ex-citation to the S1(n,π*) state, electronic energy can be rapidlythermalized through an almost barrierless nonadiabatic dis-sociation pathway from the FC point of the S1(n,π*) statethrough the (S1/S0)CI conical intersection to the S0 state PES.This dynamical process will generate NO product with coldrotational and hot vibrational distributions: the unstable nor-mal mode of vibration associated with the relevant transitionstate on the ground electronic state surface does not generatea resultant torque on the NO moiety during dissociation of theparent molecule. This conclusion is in a good agreement withour experimental results.

Our previous research on nitramine and other energeticmolecules12–14, 16, 19 indicates that upon electronic excitationto the S2 or S1 state, the molecule follows a barrierless, mostfavorable descent pathway connecting the FC point of the S2

or S1 state to nitro-nitrite isomerization through the (S2/S1)CI

and (S1/S0)CI CIs, and then gives rotationally cold and vibra-tionally hot NO product. Even though the N–NO2 moiety isdifferent from the O–NO2 moiety, they both display a simi-lar decomposition dynamics from the excited state, and ob-tain a similar experimental result (rotationally cold and vi-brationally hot NO product). One reason for this similarityof behavior may be that with our chosen experimental wave-lengths, excitations for O–NO2 or N–NO2 parent moleculesare (n,π*) states. Thus, they will have some similarities ofPESs that may result in the similar barrierless, most favorablepathways for decomposition. The model system dimethyl-nitramine (DMNA) also has the N–NO2 moiety, and thesame (n,π*) transitions for the first and second electronic ex-cited states. But, in this instance, however, the NO productfrom DMNA displays hot rotational and cold vibrational en-ergy distributions. Calculations reveal that DMNA, and othernonenergetic model systems,13, 27, 28 decompose on their S1

(or higher) electronically excited states rather than on theirground electronic states S0; thereby, not all the excitation en-ergy is available to break internal bonds to generate reactivefragments and radicals for further reactions, as may be re-quired for true energetic behavior.

Combing all our previous experiments and theo-retical calculations on energetic materials and modelsystems,12–14, 19, 27 one can find that conical intersections playan important role in the decomposition mechanisms for both.The nitramine and related (NO/NO2/NO3) energetic materi-als all experience a NO elimination decomposition channel,and only generate a rotationally cold and vibrationally hotdistribution of NO product. The decomposition dynamics arealso similar for these species: the electronically excited ener-getic molecules follow a nonadiabatic relaxation to the ground

state through a series of conical intersections, undergo a nitro-nitrite isomerization on the S0 surface, followed by NO elim-ination. The model systems also give a NO product from theirexcited state decomposition, but the energy distribution forNO is not unique: some give rotationally hot and vibrationallycold distributions of NO, while some give rotationally coldand vibrationally hot distributions of NO. This behavior isexcitation wavelength dependent. Therefore, these distinct re-sults for energetic materials and model systems may be usedin the future to predict, characterize, and distinguish energeticsystems from nonenergetic or model systems.

VII. CONCLUSIONS

The excited electronic state decomposition of PETNhas been investigated via nanosecond energy resolved spec-troscopy. PETN generates NO as an initial decompositionproduct at the nanosecond laser excitation wavelengths (248,236, 226 nm), and gives a hot vibrational (∼1300 K) and coldrotational (∼20 K) distributions of the NO product that areindependent of excitation wavelengths. Based on the exper-imental observations and ONIOM combined with CASSCFcalculations, we conclude that the decomposition of electron-ically excited PETN occurs through a barrierless overall mostfavorable descent, lowest energy pathway connecting the FCpoint of S1 state to nitro-nitrite isomerization on S0 throughthe (S1/S0)CI conical intersection, followed by NO eliminationon S0. Force components on N and O atoms for the unstablenormal mode of vibration for the NO elimination transitionstate on the S0 surface of PETN are predicted to produce littletorque on the terminal NO moiety: this results in cold rota-tional (20 K) distributions of the NO product. On the contrary,if PETN molecule is thermally dissociated on the groundstate surface only, the major decomposition channel should beO–NO2 bond dissociation to yield the NO2 product becausethis channel is associated with the lowest S0 activation barrier.

According to our previous results on energetic materialsand model systems, one may conclude: 1. decomposition ofelectronically excited NO2 containing energetic materials oc-curs to yield NO product from their highly vibrationally ex-cited S0 states after following a series of conical intersectionsto their ground electronic states and 2. these molecules giverotationally cold and vibrationally hot distributions of NO.In the decomposition process, conical intersections play acrucial role in the initial decomposition mechanics for elec-tronically excited energetic materials. In order to understandultrafast internal conversion dynamics through the conical in-tersection, further investigation will be performed to studyNO generation from PETN with ∼100 fs time resolution.

ACKNOWLEDGMENTS

These studies were supported by a grant from the U.S.Army Research Office (ARO, FA9550-10-1-0454) and inpart by the U.S. National Science Foundation (NSF) throughTeraGrid supercomputer resources provided by NCSA underGrant No. TG-CHE090094. We thank Dr. Yuanqing Guoand Dr. Atanu Bhattacharya for many helpful discussionsconcerning the experiments and calculations for this system.

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