Competing isomeric product channels in the 193 nm

JOURNAL OF CHEMICAL PHYSICS VOLUME 114, NUMBER 10 8 MARCH 2001

Competing isomeric product channels in the 193 nm photodissociationof 2-chloropropene and in the unimolecular dissociation of the2-propenyl radical

Julie A. Mueller, Bradley F. Parsons, and Laurie J. Butlera)

The James Franck Institute and Department of Chemistry, University of Chicago, Chicago, Illinois 60637

Fei Qi, Osman Sorkhabi, and Arthur G. SuitsChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720

~Received 10 August 2000; accepted 13 December 2000!

This paper presents product translational energy spectroscopy measurements of the primaryphotofragmentation channels of 2-chloropropene excited at 193 nm and of the unimoleculardissociation of the 2-propenyl radical. Tunable vacuum ultraviolet~VUV ! photoionization of theproducts allows us to distinguish between the various product isomers formed in these processes.The data show evidence for three significant primary reaction channels in the dissociation of2-chloropropene: An excited-state C–Cl fission channel producing fast Cl atoms, a C–Cl fissionchannel producing slow Cl atoms, and HCl elimination. A minor C–CH3 fission channel contributesas well. The measured branching of the major primary product channels is:@ fast C–Cl#:@slow C–Cl#:@HCl elimination#562%:23%:15%. The experiments also allow us toresolve selectively the product branching between the unimolecular dissociation channels of the2-propenyl radical, a high energy C3H5 isomer; we measure how the branching ratio between thetwo competing C–H fission channels changes as a function of the radical’s internal energy. The dataresolve the competition between the unimolecular H1allene and H1propyne product channels fromthe radical with internal energies from 0 to 18 kcal/mol above the H1propyne barrier. We find thatthe barrier to H1allene formation from this high-energy C3H5 radical is higher than the barrier toH1propyne formation, in agreement with recent theoretical calculations but in sharp contrast to thatpredicted for the most stable C3H5 isomer, the allyl radical. The experiments demonstrate a generaltechnique for selectively forming a particular CnHm isomer dispersed by internal energy due to theprimary photolysis, thus allowing us to determine the branching between unimolecular dissociationchannels as a function of the selected radical isomer’s internal energy. ©2001 American Instituteof Physics. @DOI: 10.1063/1.1345877#

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I. INTRODUCTION

One of the most basic goals of chemistry is to determthe products of chemical reactions when several pathware possible. This goal is often difficult to achieve whproducts include more than one isomer of the same speor when short-lived radicals are involved as either reactaor products. The 193 nm photodissociation2-chloropropene (H2CCClCH3) first provides the opportunity to investigate the competition between bond fission amolecular elimination product channels occurring in comtition on both the ground electronic state and excited etronic potential-energy surfaces. More importantly, theperimental method presented here also allows us to studycompeting C–H bond fission processes of the 2-properadical, a C3H5 radical isomer 19.4 kcal/mol higher ienergy1 than the allyl radical, with the radicals dispersedinternal energy in the primary photolysis. The isomerizatand unimolecular dissociation of this radical has receivconsiderable attention in studies where it was embeddeC3H5 kinetics1–3 or allyl radical dynamics4–6 studies. The

a!Electronic mail: [email protected]

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method presented here opens up the study of the unimollar dissociation processes of a wide range of normally esive isomeric radical species.

Despite its common use in organic synthesis andwell-characterized infrared~IR! spectroscopy,7–9 we foundno UV-vis absorption spectra of 2-chloropropene in theerature. However, because of the molecule’s structural slarity to the better-characterized vinyl chloride, we expecstrong absorption band in the region of 193 nm to excitprimarily pp* transition. This paper reports the UV absortion spectrum from 300 to 190 nm,ab initio calculations ofthe low-lying excited electronic states of 2-chloropropenand the emission spectrum from the molecule excited199.7 nm; those data confirm our expectation that absorpat 193 nm excites app* transition analogous to that in vinychloride.

A rather large number of dissociation and eliminatireactions are open to 2-chloropropene excited by a 193photon. In addition to the obvious primary reactions, itpossible for many of the primary products to have sufficieinternal energy to undergo secondary dissociation reactiIn the following list of energetically allowed reactions, seondary dissociation reactions are shown as indented andsome are given:

5 © 2001 American Institute of Physics

IP license or copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp

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4506 J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Mueller et al.

H2CvCClCH3→H2CvC* Cl1CH3,

DH'101.6 kcal/mol,10,11 ~1!

H2CvC* Cl→C2H21Cl, DH'22.6 kcal/mol,1,12 ~2!

H2CvCClCH3→H2CvC* CH31Cl,

DH0'93.4 kcal/mol,13 ~3!

H2CvC*CH3→H2CvCvCH2~allene!1H,

DH0535.0 kcal/mol,1 ~4!

H2CvC* CH3→HCwCCH3~propyne!1H,

DH0533.9 kcal/mol,1 ~5!

H2CvCClCH3→H2CvCvCH2~allene!1HCl,

DH0526.1 kcal/mol,14,15 ~6!

H2CvCvCH2→HCCCH2~propargyl!1H,

DH0587.8 kcal/mol,16 ~7!

H2CvCClCH3→HCwCCH3~propyne!1HCl,

DH0525.0 kcal/mol,17 ~8!

HCwCCH3→HCCCH2~propargyl!1H,

DH0588.7 kcal/mol,15 ~9!

H2CvCClCH3→HCCClCH31H,

DH'109 kcal/mol,10,18 ~10!

H2CvCClCH3→H2CCClCH21H, ~11!

H2CvCClCH3→CCClCH31H2,

DH'91 kcal/mol,10,16 ~12!

H2CvCClCH3→H2CCClCH1H2. ~13!

Elimination of HCl from 2-chloropropene can result in thproduction of either allene (H2CvCvCH2) or propyne(HCwCCH3), as indicated in reactions~6! and ~8!. If theelimination takes place across the double bond, forming ppyne, it may be useful to compare the result of t2-chloropropene elimination reaction with HCl eliminatioin vinyl chloride. In the latter molecule, both 1, 1 and 1,~i.e., 3-center and 4-center! eliminations are possible. Prevous work on vinyl chloride10,19 has led to the proposal23,25

that the HCl elimination takes place via a 1, 1 mechanisalthough the evidence is not conclusive. Since allenepropyne have different ionization potentials,15 it is in prin-ciple possible to distinguish which products are formedthe 2-chloropropene HCl elimination by measuring the ioization onset of the mass 40 photoproduct. A difficulty eists, however, because the barrier to isomerization from ppyne to allene

HCwCCH3→H2CvCvCH2, ~14!


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is only about 65 kcal/mol.1,27The maximum energy availablto the propyne products formed in HCl elimination is ov120 kcal/mol, thus the mass 40 products are susceptiblsecondary C–H bond fission as well as isomerization.

There are two particularly interesting pairs of competiproduct channels, the HCl elimination reactions~6! and ~8!in the dissociation of the 2-chloropropene parent andC–H bond fission reactions~4! and~5! in the secondary dis-sociation of the 2-propenyl radical. The calculated energeof each transition state are shown in Fig. 1. The barrier toHCl elimination forming allene~6! is nearly identical to theone for HCl elimination forming propyne~8!.30 Ab initiocalculations by Daviset al.1 to determine the barriers to hydrogen atom loss from the 2-propenyl radical to give tisomers allene and propyne@~4! and ~5!, respectively# showthese barriers to be very similar as well. Their results,tained at the G2~B3LYP! level of theory,1 give values of37.1 kcal/mol for the barrier~zero-point corrected! to disso-ciation yielding propyne, and 38.1 kcal/mol for the reactiyielding allene. Thus, assuming ground-state reactions,branching between the two HCl elimination reactions abetween the two C–H fission reactions is largely controlby the tightness of the transition state,31 providing a particu-larly clear example of the importance of freezing internrotors in controlling the rate of a chemical reaction.

In this work, after reporting the UV absorption spectruof 2-chloropropene and the emission from the moleculecited in thepp* absorption band, we focus attention on dtermining the competition between the primary dissociatreactions of 2-chloropropene and the secondary dissociachannels of the nascent 2-propenyl radical products. Wethe experimental method of photofragment translationalergy spectroscopy, measuring the photoproduct velocitytributions in a crossed laser-molecular beam apparatus,employing the scattering apparatus at the Advanced LSource in Berkeley32 to allow us to use tunable photoionization detection of the products. In this way we can not ondistinguish between allene and propyne products~whichhave different ionization energies!,15 but we can use theproducts’ neutral time-of-flight to determine which camfrom primary HCl elimination and which came from seconary C–H fission from the primary 2-propenyl product of raction ~3!. The experiments also provide one more excitipiece of information not previously revealed in any expement, but reported in a preliminary communication of thwork.33 Because the primary C–Cl bond fission channel pduces the high-energy 2-propenyl radical~an isomer 19.4kcal/mole higher in energy1 than the most stable C3H5 radi-cal isomer, the allyl radical! with neutral product recoil ve-locities which identify the internal energy of the nasceradical, the radicals are effectively dispersed by their interenergy. Then because C–H fission from the radicals formwith enough internal energy to dissociate produces masproducts travelling with essentially the same velocity as th2-propenyl parents, the arrival time of the mass 40 prodidentifies the internal energy of the radical from whichcame! Thus, we can measure how the branching betweenH1allene~4! and the H1propyne~5! product channels fromthe unimolecular dissociation of the 2-propenyl radic


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4507J. Chem. Phys., Vol. 114, No. 10, 8 March 2001 Photodissociation of 2-chloropropene

FIG. 1. ~Color! Energetics of the 2-chloropropene HCl elimination channels with calculated~Ref. 30! barriers assuming dissociation on the ground-stpotential-energy surface, and the C–H fission channels of the 2-propenyl radicals formed from C–Cl fission in 2-chloropropene. The inset shows tductkinetic energies imparted in C–Cl fission~Fig. 5! and producing 2-propenyl radicals in three internal energy groups: a group shown in orange with inenergies below the dissociation limit to H1propyne, a group shown in red with internal energies near but above the dissociation threshold, and theinternal energy group of radicals~blue!, with median internal energy about 15 kcal/mol above the barrier to H1propyne. The barrier energetics shown for thradical dissociation are taken from those calculated in Ref. 1.

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changes as a function of internal energy in the radical wan energy resolution of about 3 kcal/mol~even though theradicals are formed with a wide range of internal energiethe primary photolysis step!. Comparing the measurechange in branching as a function of internal energy to RicRamsperger–Kassel–Marcus~RRKM! predictions allows usto extract information about the relative A-factors of the twdecomposition pathways and the relative heights of theriers to C–H fission from the high-energy 2-propenyl radiisomer. The data thus give an unprecedented windowthe unimolecular dissociation reactions of selectively pduced hydrocarbon radical isomers.

II. EXPERIMENT

A. UV-vis absorption spectrum and emissionspectroscopy

The gas-phase UV-vis absorption spectrum2-chloropropene was collected with a HP 8453 Diode-ArUV-Visible Spectrophotometer. 2-chloropropene boils at 2K, so covering the bottom of a room temperature 1 cm plength cuvette with sample provides enough vapor to acqthe spectrum. The 2-chloropropene in this and all other wdescribed in this paper was purchased from Aldrich~98%!and used without further purification.

The apparatus used to obtain the emission spectrum2-chloropropene excited at 199.7 nm has been descrpreviously.34–36 The laser crosses a free-jet expansion41% 2-chloropropene in He through a pulsed valve withmm diameter orifice. The gas mix is formed by bubbling Hthrough 2-chloropropene maintained at 0 °C, where its va


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pressure is;255 Torr; the total pressure of the gas mixtuis 615 Torr. We generate the 199.7 nm excitation lightRaman shifting the fourth harmonic of an injection-seedNd:YAG laser~Continuum Powerlite 9020! in 50 psi of H2.An input of about 100 mJ/pulse at 266 nm gives;100 mJ/pulse at 199.7 nm. The emission from dissociating molecuis imaged into a 0.275 m spectrometer~Acton SpectraPro275, slit width 3 mm! and dispersed with a 2400 gr/mmholographic grating~Milton Roy! onto an optical multichan-nel analyzer~EG&G PARC 1456B-990-HQ!. With this ex-perimental configuration, our resolution is about 25 cm21

full width at half maximum~FWHM!.

B. Photofragment translational spectroscopy

These experiments measure the recoil velocity distritions of fragments from the photodissociation2-chloropropene excited at 193.3 nm and the arrival timesthe secondary products from the dissociation of 2-properadicals formed in the primary photolysis. We carried out texperiments using the rotating-source, fixed-detector mlecular beam apparatus on the Chemical Dynamics Beamat Lawrence Berkeley National Laboratory’s AdvancLight Source~ALS!. The key feature of this apparatus, whichas been described in detail elsewhere,32 is that photofrag-ments are ionized in the detector using tunable VUV photfrom the ALS rather than by the more traditional electrbombardment method. This ‘‘soft’’ ionization largely prevents photofragments from cracking during the ionizati


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process, making it much easier to identify secondary disciation products than if electron bombardment ionizationused.

We generate the 11.6% 2-chloropropene–He pulsedlecular beam by bubbling helium through 2-chloropropecooled to226 °C and expanding the resulting gas mixtuthrough a General Valve with a 1 mmorifice. With a backingpressure near 800 Torr and a nozzle temperature of a150 °C there is no evidence of clusters in the molecubeam. Proper analysis of the data requires knowledge omolecular beam velocity and velocity spread, which we msure by directing the molecular beam straight into the detor and analyzing the shape of the ‘‘hole’’ the laser burinto the signal at the parent molecule mass. This procedgives a peak molecular beam velocity in the number dendistribution, N(v), of 1137 m/s and a velocity sprea(Dv/vpk) of 0.29 with Dv calculated from the full width athalf maximum ofN(v).

The laser used in this work is a Lambda Physik LPX-2excimer laser operating on the 193.3 nm ArF transitiVarying the laser power from 1.5 to 36 mJ/pulse showeddissociation to be the result of single-photon absorption,saturation effects become significant at the higher powerswe collect data at laser powers of 5–10 mJ/pulse. The labeam, focused to a 2 mm34 mm spot, intersects the molecular beam at an angle of 90°, perpendicular to the planefined by the molecular beam and the detector axis. Molecthat absorb light dissociate, and a small fraction of the pructs scatter into the detector, traveling 15.2 cm to the pwhere they are ionized by tunable VUV radiation from tALS. The characteristics of the light source have beenscribed in detail elsewhere.37 The photoionization efficiencycurves presented in this paper were taken with a light soubandwidth of 2.2% and most of the photofragment time-flight spectra~except the mass 40 10 eV spectrum, as noin its figure caption! were taken with a light source bandwidth of about 4.5%. A gas filter filled with about 25 Torr Aeliminates unwanted higher harmonics of the undulatordiation. A MgF2 window filters out any remaining highenergy photons when the probe energy is below ab10.5 eV.

Ions of the desired mass are selected using a quadrumass filter and counted as a function of time using a Ddetector. The previously calibrated ion flight time (aAmion)constant ofa55.81ms/amu1/2 was used to correct for thflight time of the ion through the mass spectrometer; figushow total (ion1neutral) flight times. Rough photofragmeangular distribution measurements of the Cl and HCl pructs showed both to be nearly isotropic, so an anisotrparameter of 0 is assumed for all product branching detenations.

III. RESULTS AND ANALYSIS

A. UV-vis absorption spectrum of 2-chloropropene

In Fig. 2 we present the UV-vis absorption spectrum2-chloropropene from 190 to 300 nm. As expected, itqualitatively similar to that of vinyl chloride.38 The mainfeature to note is the first strong absorption band growing


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near 220 nm. We believe the structure at the top of the pmay be the vibrational structure of the predissociative eltronic state accessed in the absorption. The spectrumtween 300 and 1100 nm, also acquired but not shown infigure, displays no absorption features detectable withspectrometer.

B. Emission spectroscopy of 2-chloropropene excitedat 199.7 nm

Figure 3 shows the emission spectrum of dissociat2-chloropropene excited at 199.7 nm. The spectrum is slar to the emission spectrum of vinyl chloride.35 We assignthe strongest feature, appearing near 1625 cm21, to n5 ~theCvC stretch! with a literature value of 1640 cm21.7 Thepeaks at 3256 and 4880 cm21 thus correspond to 2n5 and3n5 , respectively. Two quanta ofn19, the CvC twist, ~lit-erature value ofn19'692 cm21, 2n19'1384 cm21)7 giverise to the peak at 1378 cm21. The feature near 2990 cm21 isassigned asn3 , the CH2 symmetric stretch, which has a literature value of 2992 cm21.7 Finally, we assign the peak a646 cm21 as n12, the C–Cl stretch~literature value of 641cm21!.7 To understand the observed absorption and emisspectra, we performed configuration interaction with sin

FIG. 2. Absorption spectrum of gaseous 2-chloropropene. The spectromis likely saturated just below 220 nm, as we expect thepp* absorption to beas strong as in vinyl chloride.

FIG. 3. Emission spectrum of gaseous 2-chloropropene excited at 199.7plotted as a function of the wave number of the absorbed photon minuswave number of the emitted photon~the Raman shift!.


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electron excitations~CIS! ab initio calculations usingGAUSS-

IAN 9839 to characterize the nature of the electronic transitnear 200 nm. We used the MP2/6-311G(d,p) @~MP2!second-order Moller–Plesset# optimized geometry fromGAMESS-US40 for these calculations.30 The CIS calculation,restricted to singlet spin states, gave a vertical excitationergy to the bright state of 7.8411 eV, thus overestimatinby the 1 to 2 eV typical for CIS calculations, and identifiethe electronic character of the transition as primarilypp* ,similar to that in vinyl chloride. Thus, the forces in thFranck–Condon region act to stretch the CvC bond, result-ing in the strong emission to then5 feature as also observein vinyl chloride.35

C. Photofragment translational spectroscopyof 2-chloropropene

The data show evidence for several primary reactchannels: C–Cl fission~3!, HCl elimination @~6! and ~8!#,and a minor C–CH3 fission channel~1!, as well as severasubsequent secondary dissociation processes, as detailelow. Contributions from Reactions~10!–~13!, which involveloss of H and H2, may only be detected in the data presenhere if the dissociation releases considerable energy to puct translation. Time-of-flight~TOF! spectra collected for30 000 laser shots atm/e575~C3H4Cl1! and 74~C3H3Cl1!and a source angle of 5° show no significant signal, so ifproducts from loss of H or H2 are formed, those dissociationmust not be releasing more than 6.69 and 3.3 kcal/mol,spectively, to product translation.

1. Atomic chlorine loss and subsequent secondarydissociation of the 2-propenyl radical

In Fig. 4 we show the TOF spectra ofm/e535~Cl1!collected for 200 000 shots at source-detector angles ofand 35° and a photoionization energy of 13.5 eV. Two mpeaks are evident: A sharp, fast one centered at;100ms anda broad, slow one near 150ms. Figure 5 shows theP(ET)’sobtained by forward convolution fitting of them/e535 spec-tra. Note that the two fastP(ET)’s shown in orange and recombine to give a single peak. We treat the two componeseparately rather than combining them into one distributbecause the C–Cl fission reactions releasing the largesergies to product translation~the Cl atoms from which areshown in the orange line! give momentum-matched mass 4partner radicals with internal energies low enough that tare stable to secondary dissociation. The TOF spectram/e541, C3H5

1 at angles of 15° and 35° and photoionizatienergies of 10.5 eV~with the MgF2 window! shown in Fig. 6thus contain only a single, sharp peak which is fit by torangeP(ET) in Fig. 5. This P(ET) corresponds to C–Cfission that releases enough energy to product translatioproduce a stable 2-propenyl radical. The fits to the massspectrum show that about 50% of the C–Cl bond fissevents yield stable 2-propenyl products.~If the 2-propenylradicals formed in the chlorine loss channel were all stablsecondary dissociation, no peaks should appear in the1

spectra that do not also appear in the 2-propenyl spectracept a small contribution from secondary dissociationC2H2Cl, R2!. 2-propenyl radicals momentum-matched to t


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slowest Cl atoms~27% of C–Cl fission events, shown iblue! and the slower of the two groups of Cl atoms contruting to the fast peak in Fig. 4~23% of C–Cl fission eventsshown in red! are formed with enough internal energydissociate, and do so. As a result, they are not detectemass 41, but rather at the secondary product masses. Tsecondary dissociation channels are energetically alloand potentially viable from a C3H5 isomer: H atom loss tomake allene, H atom loss to make propyne, and C–C bfission to CH31C2H2. ~The barriers to H2 elimination arelikely to be considerably higher.! Figure 7 shows them/e540~C3H4

1! TOF spectra obtained at a photoionization eergy of 11 eV. Two main peaks appear in these spectrasharp, fast one near 100ms and a broad, slow one centered150 ms. The two slowerP(ET) distributions shown in redand blue in Fig. 5 fit these peaks. This is to be expectedthe recoil velocity imparted to the mass 40 product from

FIG. 4. ~Color! Time-of-flight spectra collected with a photoionization eergy of 13.5 eV of them/e535 signal with source angle shown. The signis assigned to VUV ionization of the35Cl atom photofragments from C–Cbond fission in 2-chloropropene. Experimental data is shown with ocircles. Fits shown are calculated from the C–Cl bond fission product trlational energy distributions shown in Fig. 5~and in the inset in Fig. 1!.Since lower internal energy momentum-matched 2-propenyl radicalsproduced when more energy is imparted to translation in the C–Cl fissthe orange fit shows the Cl atoms momentum-matched to the stable ma2-propenyl radicals~see Fig. 6!, the red fit shows the Cl atoms momentummatched to near-threshold but dissociative 2-propenyl radicals that undC–H bond fission and give mass 40 allene/propyne products~see red con-tribution in Fig. 7!, and the blue fit shows the Cl atoms momentum-matchto higher internal energy dissociative 2-propenyl radicals that also undC–H bond fission and give mass 40 allene/propyne products~see blue con-tribution in Fig. 7!. Figure for 15° angle is adapted from Ref. 33 witpermission of the authors.


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C–H fission in 2-propenyl radical dissociation is so sm~mass 1 recoiling from mass 40! that the mass 40 propyneallene product must travel at essentially the same velocitits particular parent 2-propenyl radical. Thus, the time-arrival of each mass 40 product gives essentially no informtion on the energy partitioned to product translation insecondary dissociation, but it gives us critical informationthe internal energy of the parent 2-propenyl radical frowhich the mass 40 product came. The arrival time distrition of the propyne versus allene products from t2-propenyl radicals dispersed by internal energy in themary C–Cl photolysis step thus gives us a direct measurthe change in branching between the H1propyne and theH1allene radical dissociation channels as a function ofinternal energy in the radical. We exploit this, as discusbelow. Note that very few, if any, 2-propenyl radicals udergo C–C bond fission, since retaining the same relaprobabilities of the slowest~27%! and near-threshold~23%!2-propenyl radical products~as predicted from the relativintensities of the Cl atom momentum matched partne!gives the good fit shown in Fig. 7 to the mass 40 secondproducts from each group of radicals. The shoulder onfast side of the 100ms peak in Fig. 7 results from propyneallene produced in primary HCl elimination. This will bdiscussed further in Sec. III B 2.

We tried two different approaches to determining hothe branching between the H1allene and the H1propynechannel from the dissociation of the 2-propenyl radi

FIG. 5. ~Color! Product kinetic energy distributions,P(ET), for C–Cl bondfission in the 193 nm photodissociation of 2-chloropropene derived fforward convolution fitting of the signal in Fig. 4. These C–Cl bond fissdistributions produce three groups of momentum matched 2-propenylcal products in a ratio of 49.7%~orange!: 23.0% ~red!: 27.3% ~blue!. TheorangeP(ET) derived from fitting the fastest signal in the Cl atom TOF fithe remaining stable mass 41 products. The red and the blueP(ET)’s pro-duce dissociative 2-propenyl radicals with, respectively, near thresholdternal energies and internal energies centered on an energy 15 kcalhigher than the barrier to C–H fission in the radical, so the momentmatched partner dissociates and does not contribute to the mass 41 spein Fig. 6. However, since C–H bond fission in the dissociative radicals dnot alter the mass 40 product’s velocity from the velocity the parent radhad from the precursor’s C–Cl bond fission, we can predict the arrival tiin Fig. 7 of the mass 40 products from the radical dissociation from theand blueP(ET)’s and in this way identify the internal energy of the radicthat resulted in propyne or allene product at each arrival time in the masTOF spectrum in Fig. 7.


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changes with internal energy in the radical. First, we obtphotoionization efficiency~PIE! curves by measuring, asfunction of photoionization energy, the amount ofm/e540signal arriving in a particular time gate. Proper choice of ttime gate allows us to examine the allene/propyne dissotion products from the near-threshold internal ener2-propenyl radicals separately from the allene/propyne pructs from higher internal energy 2-propenyl radicals.~Theproducts with arrival times between 120 and 300ms are fromthe higher internal energy group of 2-propenyl radicals, wmedian internal energy 15 kcal/mol above the C–H fissbarrier.! Figure 8 shows the photoionization efficienccurves obtained for various time gates of them/e540 signal.For comparison, Fig. 9 shows the photoionization efficiencurves of molecular beams of pure allene (I.E.59.69 eV)41

and pure propyne (I.E.510.36 eV)41 with the same band-width photoionization source. Certainly, the partial Pcurves of the mass 40 signal from the dissociation of bothnear-threshold 2-propenyl radicals and the higher internalergy radicals are due primarily to propyne, but some allenlikely formed as well. At first glance, one might expect thtaking the right linear combinations of the pure allene apropyne curves should generate composites looking likem/e540 curves. Unfortunately, no combinations of the pucold allene and cold propyne photoionization curves yield

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40FIG. 6. ~Color! Time-of-flight spectra collected with a photoionization eergy of 10.5 eV of them/e541 signal with source angles shown. The signis assigned to VUV ionization of the stable C3H5 radicals from C–Cl bondfission events in 2-chloropropene releasing more than about 19 kcal/menergy to product translation. Experimental data is shown with open circFit shown is calculated from the orange C–Cl bond fission product trantional energy distribution shown in Fig. 5.


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FIG. 7. ~Color! Time-of-flight spectra collected with a photoionization eergy of 11.0 eV of them/e540 signal with source angles shown. The signis assigned to VUV ionization of the allene/propyne products from the umolecular dissociation of 2-propenyl radical formed from C–Cl bond fissin the 2-chloropropene precursor and from primary HCl elimination fr2-chloropropene. Experimental data is shown with open circles. The redthe blue fits shown predict the arrival times of the allene/propyne seconproduct from the 2-propenyl radical dissociation by noting that the veloof the allene/propyne secondary product should be nearly identical to ththe 2-propenyl radical from which it came. Thus the fit shown in red linethe products from near-threshold radical dissociation and the fit in bluefor products from the dissociation of higher internal energy 2-propenyl rcals is derived from the corresponding color C–Cl bond fissionP(ET) forthe two groups of dissociative radicals shown in Fig. 5. The fits uspredetermined relative amount of the mass 40 propyne/allene productthe higher internal energy radicals~blue line! to near threshold allenepropyne products~red line! of 1.185 as obtained, with correction for kinematic factors in the three-dimensional~3D! scattering, from the ratio of theCl atoms momentum-matched to the two groups of dissociative radicThe appropriateness of using this fixed ratio relies on the assumptionthe radicals dissociate only via C–H fission~negligible C–C fission! andthat the detection efficiency for allene at 11 eV photoionization energsimilar to that of propyne. The fit shown in long-dashed green line formass 40 products from HCl elimination that survive secondary dissociais calculated from the long-dashed green line portion of theP(ET) in Fig.13. Figure for 20° angle is adapted from Ref. 33 with permission ofauthors.


results that looked even remotely like the experimencurves, probably because the allene and propyne photofments are formed with significant vibrational excitation. Thcauses their ionization onsets, particularly that of allenebe red-shifted from those of the pure compounds, sophotoionization efficiency curve of vibrationally excited alene products is expected to have a more gradual onsetbe significantly red-shifted from that of colder allene showin Fig. 9. Thus, instead of simulating the photoionizaticurves to determine the fraction of allene in the massproducts from the lower and higher-internal-energy radicawe sought to extract how the branching ratio changes afunction of internal energy in the radical. To do this, wretook the 20° data atm/e540, but tuned the photoionization energy to 10 eV in order to detect allene, but n

i-n

ndryyofrei-

am

ls.at

isen

e

FIG. 8. Photoionization efficiency curves for mass 40 products detectedsource angle of 20° and integrated over flight times shown in the leg~For corresponding flight time ranges, see 20° mass 40 spectrum showthe middle frame in Fig. 7.! Data was taken with the narrower energy linwidth and corrected for VUV energy. Data points shown with error barswith straight-line extrapolations between each point to guide the eye. Indata shown, points below 10.5 eV were taken with the MgF2 filter in thephoton beam~see the Experiment section!; points at 10.5 eV and above wertaken without the filter. The figure is reproduced from Ref. 33 with permsion of the authors.

FIG. 9. Photoionization efficiency curves of pulsed beams of allenepropyne. Data was taken with the narrower ALS linewidth used also in F8 and corrected for VUV energy. Points below 10.6 eV were taken withMgF2 filter in the ALS beam~see the Experiment section!; the points at 10.6eV are averages of the counts measured with and without the MgF2 filter;and the points above 10.6 eV were taken without the MgF2 filter. The neatallene and propyne beams were formed under modest expansion cond~stagnation pressure5340 Torr and nozzle at room temperature!, so vibra-tional cooling of these molecules is not dramatic.


hia

ithoyb-ow

o

e

ri-KM

dth1

onin

on

oo

tht

taal

, an

ot

otiaith30ie

of

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ionergy.


propyne, products from 2-propenyl radical dissociation. Tspectrum is shown in Fig. 10. It is clear by inspection ththe quantum yield of allene from the group of radicals whigher internal energy is higher than the quantum yieldallene from the group of radicals with lower internal energUsing the two kinematically corrected probability ratios otained from fitting the relative intensities of the fast and slpeaks in each spectrum~Figs. 7 and 10!

S fallene

fallene1fpropyneD

high Eint radicals

S fallene

fallene1fpropyneD

low Eint radicals

5

S ~fallene!high Eint radicals

~fallene! low EintradicalsD

S ~fallene1fpropyne!high Eint radicals

~fallene1fpropyne! low Eint radicalsD

5~2.39!

~1.185!52.0~10.05/20.15!,

gives the following change in branching to the H1allenechannel with internal energy of the dissociating radical:

Thus, we experimentally determine that the fraction2-propenyl radicals that dissociate to give H1allene productsis 2.0 ~10.05/20.15! times larger for dissociation of thgroup of radicals with higher internal energy~median energy15 kcal/mole above the barrier to H1propyne) than from thegroup of radicals with lower internal energy. This expemental result is in reasonable agreement with the RRprediction of 2.2 for the change in yield of H1allene prod-ucts as the internal energy of the dissociating group of racals is changed from the near-threshold distribution tohigher-energy distribution having median internal energykcal/mol above the barrier. We give further informationthe RRKM calculations below. Although we cannot obtaan absolute branching ratio between the 2-propenyl secary dissociations forming allene,~4!, and propyne,~5!, wecan use RRKM theory to predict the rates for formationallene and propyne in the products from the near-threshinternal energy radicals. We can then derive, from howrelative intensities of the peaks in Fig. 7 change in goingthe 10 eV spectrum in Fig. 10, what the experimenbranching to the H1allene channel is for the higher internenergy group of radicals. Assumingfallene is 10.3 ~62!%from the near-threshold internal energy group of radicalsdetermined from the RRKM calculations, the relative intesities of the signal in the mass 40 spectrum at 10 eV phionization energy show the branching to the H1allene chan-nel increases to @10.3(62)%(2.015)521(64)%# fordissociation of the higher internal energy group2-propenyl radicals. This result is consistent with the parPIE curve of the products from the group of radicals whigher internal energy and arrival times between 120 andms, which has an ionization onset at slightly lower energthan the faster, near-threshold group.


st

f.

f

i-e5

d-

fldeol

s-o-

fl

0s

More information on the internal energy dependencethe branching between the H1allene and H1propyne chan-nels from the dissociation of 2-propenyl radicals is apparin fitting the velocity distribution of the allene products frothe near-threshold internal energy dissociative 2-properadicals. While fits of the mass 41 and mass 35 sigshowed that 2-propenyl radicals could be formed wenough internal energy to dissociate from primary C–bond fission events releasing less than 18.4 kcal/mol~or a bithigher! in translation, acceptable fits of the allene data in F10 could not be achieved until we assumed the internalergy required to access the H1allene channel was higherThe short-dashed lineP(ET) in Fig. 11 that fit the arrivaltimes of the allene products from low internal energy disciative C3H5 shows this clearly; H1propyne products appeared as products from radicals with higher kinetic energnear 19 kcal/mol, corresponding to a lower internal enerthan did H1allene products. Thus, the barrier to H1alleneformation is higher than the barrier to H1propyne formation.

In addition, we could not fit the data in Fig. 10 accepably by assuming the branching to the H1allene channel wasconstant for the products from the low internal energy groof radicals shown in red~although this gave a decent fit fothe higher internal energy group of radicals shown in bl

FIG. 10. ~Color! Time-of-flight spectrum of the allene products from thunimolecular dissociation of 2-propenyl radicals formed from C–Cl bofission in the 2-chloropropene precursor and from the allene/propyne pucts from primary HCl elimination from 2-chloropropene that are stablesecondary dissociation. The spectrum was collected by integrating the sfor 33106 laser shots at a source angle of 20° with a photoionization eneof 10.0 eV and with small light source apertures (4 mm35 mm) to achievea narrow~2.2%! bandwidth. Experimental data is shown with open circleThe red-dashed fit for the arrival times of the allene secondary product fthe dissociation of near-threshold 2-propenyl radicals is calculated frompartial C–Cl primary bond fissionP(ET) in red-dashed line in Fig. 11corresponding to C–Cl bond fission events that produced dissocia2-propenyl radicals near threshold that went on to dissociate to H1allene.The blue–dot–dashed fit for the arrival times of the allene secondary puct from the dissociation of higher internal energy 2-propenyl radicalscalculated from the partial C–Cl bond fissionP(ET) in blue-dot–dashedline in Fig. 11 corresponding to primary C–Cl bond fission events2-chloropropene that produced higher internal energy dissocia2-propenyl radicals that went on to dissociate to H1allene. The greendashed-line fit shown for the allene/propyne products from HCl eliminatthat survive secondary dissociation is calculated from the higher enportion of the HCl eliminationP(ET) in Fig. 13 shown in green dashed-lineFigure is adapted from Ref. 33 with permission of the authors.


en

thrgd

diulw

-a

thteh

rgms

thtost

ri-molomeher

ofnly

d inm-ry

e

110aiza-hedtha-al

Cld.Cl

real.to

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allyle

-dis-

nalthisfast

eseiesion.t to

ne/nent

to

lissheucen

r-roradthe

-

roldatithet

the


where the change in branching as a function of internalergy is expected to be slower!. A much better fit to the datawas obtained when we assumed the branching toH1allene channel increased with increasing internal eneof the radical even within the group of allene/propyne proucts from the low internal energy group of 2-propenyl racals. In other words, when fitting the allene products resing from the radicals with near-threshold internal energy,did not use a constant 10.3% fraction of theP(ET) ~shown inred in Fig. 5! that fit the total allene1propyne spectrumshown in Fig. 7. Instead, theP(ET) that fits the allene products of the near-threshold group of 2-propenyl radicals wobtained by weighting the red distribution in Fig. 5 wicalculated RRKM rate constants to determine, at each innal energy, the fraction that gives allene products. Tweighting factor is justkallene/(kallene1kpropyne). Ignoring theparent 2-chloropropene molecules’ spread in internal enewe crudely identified the 2-propenyl radicals formed froprimary C–Cl bond fission releasing 17.4 kcal/mol in tranlation as having an internal energy 0 kcal/mol abovebarrier to H1allene and 1 kcal/mol above the barrierH1propyne. These barriers were used to obtain the pairenergy-specific RRKM rate constants needed to calculateweightedP(ET) in Fig. 11 ~dashed red line! that fit the al-lene products’ arrival times in Fig. 10.~In reality, we observe

FIG. 11. ~Color! Partial product kinetic energy distributions for the C–Cbond fission events that produced 2-propenyl radical that went on to dciate to H1allene are shown in red dashed and blue dot-dashed line. TP(ET)’s gave a good fit to the arrival times of the secondary allene prodin the mass 40 time-of-flight spectrum taken at 10 eV photoionizationergy shown in Fig. 10. Comparison of the red-dashed C–Cl fissionP(ET)that fit the allene signal in Fig. 10 with the solid line redP(ET) taken fromFig. 5 that fit the total propyne1allene signal from the dissociation of neathreshold dissociative radicals from C–Cl fission shows that the barrieH1allene is higher than the barrier to H1propyne and that the branching tH1allene increases with increasing internal energy in the dissociativecal. Iteratively tested by forward convolution fitting the data in Fig. 10,shape of both partialP(ET)’s shown were calculated by weighting the number of dissociative radicals at each internal energy~shown in Fig. 5 andderived from the Cl atom TOF! with the fractional RRKM rate constant foallene formation, but only the fit of the time-of arrival of the near-threshallene products was sensitive to this weighting. Accounting for kinemfactors in the 3D scattering, the relative yield of allene product fromhigher internal energy radicals to near threshold allene products was dmined to be 2.39 by varying the ratio to obtain the best fit to the spectrumFig. 10. Figure is adapted in part from Ref. 33 with permission ofauthors.


-

ey--t-e

s

r-e

y,

-e

ofhe

some propyne products from 2-propenyl radicals from pmary photolysis events that released more than 18.4 kcal/to translation because some of the 2-propenyl radicals cfrom 2-chloropropene precursors with internal energy higthan the most-probable value.!

In summary, we obtain good fits to the arrival timesallene derived from near-threshold 2-propenyl radicals oif we assume:~i! The barrier to H1allene is higher than thebarrier to H1propyne and~ii ! the branching to the H1allenechannel increases with internal energy above the H1allenebarrier. In the discussion we compare the barriers obtainefitting the data in this way with those predicted from a cobination of thermodynamics and density-functional theocalculations.

2. HCl elimination and subsequent secondarydecomposition of the m Ä40 product

The m/e536(HCl1!TOF spectra acquired at sourcangles of 20° and 40° (photoionization energy513.5 eV,200 000 shots) exhibit single, broad peaks centered nearms, as shown in Fig. 12; theP(ET) used to fit these spectrappears as the solid green line in Fig. 13. The photoiontion efficiency curve of the HCl product is shown as tdashed line in Fig. 14. Taking into account the energy wiof the ALS, the HCl reaction product shows a photoioniztion onset of 10.5 eV, over 2.2 eV lower than the usuionization potential of 12.74 eV.41 Given the large H–Clseparation in the transition state, one would expect the Hformed in the elimination process to be vibrationally exciteThe redshift of the ionization onset, also apparent in the Hproduct from vinyl chloride photolysis,23 indicates that theHCl product is indeed internally excited; unfortunately, theis currently no method for obtaining the degree of vibrationexcitation of a molecule from its photoionization spectrum

To identify the mass 40 momentum-matched partnerthe HCl from primary HCl elimination, we can use thP(ET) in Fig. 13 to predict its time-of-flight spectrum. However, note that if propyne/allene is formed with aboutkcal/mol of internal energy it will dissociate topropargyl1H,15 and that the energy partitioned to internenergy of the HCl elimination products is rough124 kcal/mol-ET ~using the parent precursor’s most probabinternal energy of 1.2 kcal/mol!. Thus we expect the laterarriving allene/propyne products to be lost to secondarysociation to propargyl radical (mass 39!1H if the HCl part-ner does not carry away more than 35 kcal/mol of interenergy. We can characterize the mass 40 products lost inway by measuring the spectrum at mass 39. Thus, theshoulder to the sharp peak centered at 100ms in Fig. 7, them/e540 spectrum, momentum-matches to the HCl, as thallene/propyne products are formed with internal energlow enough that they are stable to secondary dissociatHowever, as expected, in order to obtain a satisfactory fithe m/e540 data, we need to truncate theP(ET) used to fitthe m/e536 data. This truncatedP(ET), shown in dashedgreen line in Fig. 13, represents about 40% of the allepropyne products. About 60% of the allene and propyformed in the HCl elimination reaction thus had sufficieenergy to undergo secondary loss of a hydrogen atom

o-sets-

to

i-

ceer-in


isseC

le

isi

Cl-y15

itioroCsl iypgezaa

in-

ltsnyl

ughen-s of

.c-V,of

yhenth

r-m-yl

10Thel-line

di-ichmay

assthe

ntsach


yield propargyl radicals, detected atm/e539, C3H31, the

spectrum of which is shown in Fig. 15. Fortunately, thsecondary dissociation gives mass 39 products with estially the same time-of flight distribution as the mass 40 Helimination products would have had~as with the 2-propenyradical’s secondary dissociation, loss of an H atom barchanges the heavier product’s velocity!. Therefore, one canidentify the signal from propargyl radicals formed in thsecondary dissociation by using the portion of the HCl elimnation P(ET) not used in fitting the stable mass 40 Helimination products in them/e540 TOF spectrum; the predicted time-of-flight distribution of the secondary propargradicals is shown in the green dot-dashed line in Fig.Although the truncation point in the HCl eliminationP(ET)is shown as rather sharp in Fig. 13, a more gradual transbetween stable and unstable mass 40 HCl elimination pucts can also fit the data, as one would expect if the Hproduct is formed with a distribution of internal energieUsing the very uncertain sharp truncation near 17 kcal/motranslation suggests the HCl co-fragment carriers away tcally about 18 kcal/mol of internal energy. The very larredshift in the ionization onset observed in the photoionition spectrum of the HCl product implies, however, thsome HCl product is probably formed with much higherternal energy. The remaining signal in them/e539 TOF

FIG. 12. ~Color! Time-of-flight spectra of the HCl products from primarHCl elimination from 2-chloropropene. The spectrum was collected witphotoionization energy of 13.5 eV at the source angles shown. Experimdata is shown with open circles. The green line fit for the arrival times ofHCl product is calculated from the total HCl eliminationP(ET) in solidgreen line in Fig. 13.


n-l

ly

-

l.

nd-l

.ni-

-t

spectrum in Fig. 15 is adequately fit by assuming it resufrom a daughter ion of the stable mass 41 neutral 2-properadicals also detected at parent mass in Fig. 6. Althothere are no prior experimental results on the ionizationergy of the 2-propenyl radical or the appearance energieits ion daughters, observing a C3H3

1 daughter ion from highlyvibrationally excited ~within 0–20 kcal/mole of theH1propyne barrier! 2-propenyl radicals is not too surprisingThe cyclic C3H3

1 ion is resonance stabilized and the spetrum in Fig. 15 is taken at a photoionization energy of 11 econsiderably higher than the expected ionization energythe 2-propenyl radical of about 8 eV,42 so is likely above theappearance energy of the stable43 cyclic C3H3

1 daughter ion.A preliminary photoionization efficiency

atale

FIG. 13. ~Color! Product kinetic-energy distribution,P(ET), for primaryHCl elimination from 2-chloropropene in green solid line derived from foward convolution fitting of the signal in Fig. 12. Some of the momentumatched allene/propyne undergoes secondary dissociation to proparg1Hso the flight time of the remaining stable allene/propyne in Figs. 7 andare fit with the higher energy part of this shown in green-dashed line.remaining lower energy part of theP(ET) adequately predicts the arrivatimes of the propargyl secondary product as shown in green dot–dashedin Fig. 15 ~since losing an H from allene/propyne results in propargyl racals traveling with essentially the same velocity as the propargyl from whit came!, but the mass 40 products and the mass 39 secondary productsalso be fit with partialP(ET)’s that overlap more significantly.

FIG. 14. Photoionization efficiency curves for mass 41 products and m36 products detected at a source angle of 20°. Data was taken withnarrower energy linewidth and corrected for VUV power. Data poishown with error bars and with straight-line extrapolations between epoint to guide the eye. The MgF2 filter ~see Experiment! was used whenacquiring them/e541 data, but not for them/e536 data.


d,is

als

ne

io

toal

naencaW

bynthuche

ethth

a-n, tltssoonssaa

hef

ldg.mo-

a-eoldat-

re-the

ld,tionre-

o-

con-i-10cts

-lto

ch

e

ve

ioninge ao-

val

woble

41sedasttlyinlythen-o-

at

roena-n

fro


curve of the stable, but highly vibrationally excite2-propenyl radicals is shown as the solid line in Fig. 14; itlikely redshifted from what the spectrum of the cold radicwould be.

In determining whether the primary HCl eliminatiofrom 2-chloropropene excited at 193 nm producallene1HCl~6! or propyne1HCl~8!, two factors must beconsidered. First, not only are about 60% of the eliminatproducts formed with sufficient internal energy~89kcal/mol!16 to undergo secondary dissociationpropargyl1H, but also the isomerization barrier betweenlene and propyne is around 65 kcal/mol.1,27–29Thus, we canonly measure the branching between the two HCl elimition channels for the events that produce propyne/allproducts stable to secondary dissociation, and a signififraction of these may isomerize before we detect them.expect the equilibrium between the two C3H4 isomers to fa-vor propyne, both because propyne is lower in energyabout 1 kcal/mole, and more importantly, because prophas an internal rotor that increases its statistical weight inloose transition state. In addition, the propyne/allene prodnot lost to secondary dissociation typically still has muhigher internal energy than the propyne and allene from sondary dissociation of the 2-propenyl radicals~where the netreaction is H1Cl1allene/propyne). Thus, if the propynphotoionization efficiency curve redshifts significantly wiincreasing internal energy, we may not be able to assurethe vibrationally excited propyne product from HCl elimintion would not be ionized at the 10 eV photoionization eergy used to detect the products in Fig. 10. Neverthelesspartial photoionization efficiency~PIE! curve showing signafrom 80–100ms, Fig. 8, for the allene/propyne producfrom HCl elimination that do not undergo secondary disciation clearly shows a shoulder from allene photoionizatiFor the allene/propyne products not lost to secondary diciation we can also attempt to determine the fraction oflene relative to the allene fraction determined for the ne

FIG. 15. ~Color! Time-of-flight spectrum of photofragments giving signalm/e539, C3H3

1 showing a contribution in green from the C3H3 neutralpropargyl products from the secondary dissociation of propyne/allene fHCl elimination events releasing smaller amounts of energy to kineticergy ~see Fig. 13, caption!, and a contribution in orange from the dissocitive ionization of the stable 2-propenyl radical, from primary C–Cl fissioExperimental data are shown with open circles. The figure is adaptedRef. 33 with permission of the authors.


s

n

-

-ente

yeect

c-

at

-he

-.o-l-r-

threshold group of dissociating 2-propenyl radicals. Tkinematically corrected ratio of relative contribution oallene1propyne from HCl elimination to the near threshoallene1propyne from 2-propenyl radical dissociation in Fi7 is 0.296, while the relative amount of allene/propyne froHCl elimination to allene from near-threshold radical dissciation at 10 eV photoionization~Fig. 10! is 2.25. Thus, ifthe 10 eV spectrum only detects allene from HCl elimintion, the branching to allene from HCl elimination would b7.6 times the branching to allene from the near-threshdissociation of 2-propenyl radicals. Using 10.3% for the lter ~as predicted from RRKM calculations! we might con-clude that if the surviving mass 40 HCl products are repsentative of the nascent product branching, that 79% ofHCl eliminations form allene1HCl ~6! and only 21% formpropyne1HCl. However, the propyne formed in the HCelimination channel is probably highly vibrationally exciteso it is dangerous to assume that the 10 eV photoionizawould selectively detect the allene product. Indeed, our pliminary electronic structure calculations30 on the two HClelimination barriers suggest that if the HCl elimination prceeds by internal conversion, the propyne1HCl channelshould be the dominant one. Thus the increased relativetribution from the shoulder resulting from the fast HCl elimnation apparent in Fig. 10 probably reflects the fact thateV photons can ionize both allene and propyne produfrom HCl elimination ~since the propyne is highly vibrationally excited! but only allene products from 2-propenyradical dissociation. Propyne product can also isomerizeallene if it has over 65 kcal/mol of internal energy, as muof the HCl elimination C3H4 products do. The small signal inthe PIE spectrum in Fig. 9 below 9 eV is likely from thphotoionization of highly vibrationally excited allene fromHCl elimination, as it is most apparent in the PIE curintegrated over the arrival times of stable C3H4 product fromHCl elimination.

3. Primary photodissociation product branching

The next section~Sec. III C 4! discusses a minor C–CH3

fission channel identified as a primary photodissociatchannel for 2-chloropropene excited at 193 nm. Assumthat the branching to that channel is small, we can derivbranching fraction for the direct C–Cl fission channel prducing fast Cl atoms~hereafter ‘‘fast C–Cl’’!, the C–Cl fis-sion channel producing slow Cl atoms appearing with arritimes peaking near 150ms in Fig. 4 ~hereafter ‘‘slowC–Cl’’ !, and HCl elimination from 2-chloropropene~to pro-duce allene or propyne!. The fractional contribution from thefast C–Cl channel is obtained by adding together the tcontributions to the fast peak in Fig. 4, one producing stamass 41 product and one producing dissociative massproduct with near-threshold internal energy, as discusabove. The kinematically corrected ratio between the fC–Cl channel and the slow C–Cl channel is derived direcfrom fitting the relative signals from each in Fig. 4. To obtathe relative contribution from HCl elimination, we need onassume that the vibrationally excited propyne/allene fromHCl elimination is detected as efficiently at 11 eV photoioization energy as the colder propyne/allene from the unim

m-

.m


usheuco-,

nt

thr a1

ha

thuery

ar

ome

stladia

,

ui

e-oaciareth

atg

ue to

ing


lecular dissociation of the 2-propenyl radicals, and thenthe kinematically corrected contributions to the fits of tmass 40 TOF spectrum in Fig. 7. Then the primary prodbranching, accounting for only the HCl elimination that prduces propyne/allene stable to secondary dissociation@ fast C–Cl# :@slow C–Cl# :@HCl elimination producing stableproducts#568%:26%:6%. If weaccount for all the HClprimary elimination events~accepting our estimate that 60%of the allene/propyne products from HCl eliminatiounderwent secondary dissociation! then the primary producbranching ratio is:@fast C–Cl#:@slow C–Cl#:@HCl elimi-nation#562%:23%:15%. These ratios of course assumeour detection sensitivity to propyne and allene are simila11 eV photoionization energy and that the sensitivity ateV to vibrationally excited propyne/allene~from HCl elimi-nation! stable to secondary dissociation is also similar to tfor the colder C3H4 products from C–H fission in the2-propenyl radical.

4. Methyl loss and subsequent secondarydissociation of the chlorovinyl radical

Fig. 16~A! shows the TOF spectrum collected atm/e561(C2H2Cl1) at a source angle of 15°~photoioni-zation energy510.5 eV, 400 000 laser shots!. The signal ismomentum-matched to the signal atm/e515 (CH3

1, col-lected for 750 000 shots at 20°! shown in Fig. 16~B! so weassign these spectra to primary C–CH3 bond fission in2-chloropropene to form methyl1chlorovinyl radical. Figure17 shows the primary C–CH3 bond fissionP(ET) used to fitthe methyl and C2H2Cl data. Although them/e515 signalwas collected for almost twice as many laser shots assignal in Fig. 16, the signal-to-noise ratio remains poor dto the low-relative absorption cross section for the 10.5photons used.44 As discussed in the Introduction, secondadissociation of C2H2Cl to C2H21Cl ~2! is energetically al-lowed. It is difficult to assess the amount of this seconddissociation because the signal atm/e526, C2H2

1, @Fig.16~C!# clearly shows some signal that probably results frthe C2H2

1 daughter ion from dissociative ionization of thchlorovinyl radical, C2H2Cl, from primary C–CH3 fission.The fastest signal in the spectrum also appears at too faarrival time to be due to acetylene from the unimolecudecomposition of C2H2Cl from primary one-photon induceC–CH3 bond fission in 2-chloropropene. Secondary dissoction of C2H2Cl into C2HCl1H is also energetically allowedbut no significant signal was evident atm/e560 after440 000 laser shots.

IV. DISCUSSION

This discussion is organized in two parts. First, we focon the competition between C–Cl bond fission, HCl elimnation, and C–CH3 fission of the 2-chloropropene and rlated molecules, and how that competition might evolvethe excited-state and ground-state potential energy surfof the molecule. Then we turn to the unimolecular dissoction channels of the 2-propenyl radicals, comparing oursults with predictions of recent theoretical calculations ofC–H fission transition states for reactions~4! and ~5! andwith isomerization transition states to other C3H5 isomers.


e

t

is:

att

1

t

eeV

y

anr

-

s-

nes--

e

FIG. 16. ~A! Time-of-flight spectrum of photofragments giving signalm/e561, C2H2Cl1. ~B! Time-of-flight spectrum of photofragments givinsignal atm/e515, CH3

1 . Fits shown in smooth solid line in~A! and ~B!were calculated from theP(ET) for the C–CH3 bond fission channel in2-chloropropene shown in Fig. 17.~C! Time-of-flight spectrum of photo-fragments giving signal atm/e526, C2H2

1 showing suggested contributionfrom dissociative ionization of the chlorovinyl radical, C2H2Cl, from pri-mary C–CH3 fission. The peak centered near 90ms may be due, in part, tosecondary unimolecular dissociation of C2H2Cl to Cl1acetylene, but thefastest signal in the spectrum appears at too fast an arrival time to be dsecondary acetylene products from the primary 1-photon C–CH3 dissocia-tion.

FIG. 17. Product kinetic-energy distribution,P(ET), for the C–CH3 bondfission channel in 2-chloropropene derived from forward convolution fittof Figs. 16~A! and 16~B!.


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leealg

fislath

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allhatulde--

d tonthehenk

or

uta-nal

for

Clelsgest-atenylo-ies

ac-Ite,

ted

d


We end by suggesting how the unimolecular dissociatpathways of the other straight-chain C3H5 radical isomers,allyl and 1-propenyl, might differ from those observed hefor 2-propenyl radical and indicate the future directionsour ongoing experiments on these systems.

A. Primary photodissociation channelsof 2-chloropropene at 193 nm

There are several interesting comparisons betweenprimary photofragmentation channels of 2-chloropropenecited at 193 nm and those of vinyl chloride20–26 and allylchloride45 at the same wavelength. First, all three molecuevidence the ‘‘fast C–Cl’’ bond fission channel discusshere, while only vinyl chloride and 2-chloropropene, notlyl chloride, evidence significant branching to a photoframentation channel producing slow Cl atoms. The C–Clsion producing fast Cl atoms likely proceeds via a simimechanism in all three molecules, predissociation ofnominalpp* excited state via ansC–Cl* ~or psC–Cl* at non-planar twisted geometries! repulsive state, which partitionconsiderable energy to product recoil translational eneThe distribution of energies partitioned to product translatfor this fast C–Cl channel is very similar in vinyl chloridand 2-chloropropene, but is shifted to almost 20 kcal/mhigher recoil translational energies in allyl chloride. Thmay well be due to the significant35 admixture ofnsC–Cl*character in thepp* excited state in allyl chloride; that molecule feels a repulsive force acting to stretch the C–Cl beven in the Franck–Condon region, thus partitioning menergy to product translation. The slow C–Cl fission chanis also much more minor in allyl chloride, contributing lethan 3% of the total C–Cl fission.45 It may be that repulsiveforces in the Franck–Condon region of the excited stateallyl chloride act to stretch the C–Cl bond before the mecules have a chance to sample regions of the excitedfrom which they may undergo internal conversion to tground-state potential-energy surface. The slow C–Cl fisschannel in vinyl chloride was attributed to molecules thhad undergone internal conversion to the ground electrostate before dissociation. This is also the likely mechanfor the slow C–Cl fission channel in 2-chloropropene. Hoever, one consistency check is to compare the measP(ET) for the slow C–Cl fission channel with a statisticdistribution of energies to product translation predictedRRKM theory. To calculate an RRKMP(ET), we arbitrarilychose a C–Cl bond distance of 2.5 Å on the ground-sreaction coordinate to locate a reasonable point to calcufrequencies at the ‘‘transition state,’’ optimizing the othbond lengths and angles at the MP2 level of theory. Fig18 shows a comparison between this RRKMP(ET) and themeasuredP(ET) from Fig. 5 for the C–Cl bond fission weare attributing to dissociation of 2-chloropropene onground-state potential-energy surface. The compariclearly supports the assignment of the slow C–Cl bondsion to a ground state mechanism.~We also used RRKMtheory to estimate the probable branching to C–H fiss~10! upon internal conversion because the work of Blaet al. on vinyl chloride detected primary C–H fission anattributed it to internal conversion. Because the barrier


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C–H fission is much higher than the barrier to C–Cl fissioour RRKM rate constants for C–H fission30 were a factor of6.5 smaller than for C–Cl fission; detecting this smbranching to C–H fission is made harder by the fact tmost of such presumed statistical C–H fission events wopartition less than 6.7 kcal/mol into product recoil. Threcenter H2 elimination is more likely to appear as a significant, though minor, dissociation channel, but it is expectehave a small exit barrier so is also unlikely to partitioenough energy to product translation to be observable atheavier photofragment in these experiments. Telimination/concerted isomerization mechanism that Blaet al.23 invoke to explain the high-recoil kinetic energies fH2 elimination in vinyl chloride is not possible in2-chloropropene as there is no alpha H atom to migrate.!

HCl elimination also occurs in all three molecules, bwith marked differences. One might attribute HCl elimintion in 2-chloropropene to reactions occurring after interconversion to the ground electronic state, as proposedvinyl chloride.23,25 In contrast, in allyl chloride two distinctproduct kinetic-energy distributions were observed for Helimination, and the branching between those two channshowed a marked nozzle temperature dependence, suging at least one of them might occur via an excited-stelimination process.45 Returning to the comparison betweevinyl chloride and 2-chloropropene, the elimination in vinchloride was attributed to a 1,1-elimination with synchrnous isomerization to acetylene to explain the high energpartitioned to product translation.23,25 2-chloropropene doesnot have a 1,1-elimination process available~there is a me-thyl rather than an H on the C–Cl carbon! so, of the two HClelimination channels allowed for 2-chloropropene, only retion ~8!, forming propyne, has an analog in vinyl chloride.is analogous to the 1,2-elimination channel in vinyl chloridwhich forms acetylene directly; this pathway was discounin assigning the HCl elimination becauseab initio calcula-

FIG. 18. ~Color! Comparison between the experimental C–Cl fissionP(ET)~from Fig. 5! measured for the slow Cl atoms with an RRKM predicteP(ET).


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tions showed it had a higher barrier than 1,1-eliminatiHowever, we note here that the measuredP(ET) and the HClPIE curves for HCl elimination in 2-chloropropene are rmarkably similar to those in vinyl chloride, indicating a 1,elimination mechanism may dominate in both molecules.though there is some evidence in the PIE spectrum focontribution from the HCl elimination channel i2-chloropropene which does not have an analog in vichloride, it is likely that 1,2-elimination in 2-chloropropenis dominated by the HCl1propyne channel. We base thexpectation on first assuming that HCl elimination procevia internal conversion and then predicting the rate constfor the two 1,2-elimination channels with RRKM theory46

using calculatedab initio barriers and vibrational frequencieat each transition state. The zero-point-correctedab initiobarrier heights calculated by Parsonset al.30 are nearly iden-tical @74.2 kcal/mol for~6! and 73.3 kcal/mol for~8! at theMP2/6-311G(d,p) level of theory# and the molecule haover 147.9 kcal mol21 of internal energy~see Fig. 1!. Thusthe ratio of our calculated30 RRKM rate constants for the twochannels,k8 /k651.7 favoring propyne, results primarilfrom the fact that the HCl1propyne channel does not freezthe internal methyl rotor while the HCl1allene channel doesinducing a tighter transition state in reaction~6! than in re-action ~8!.

B. Competing C–H bond fission channels in the2-propenyl radical unimolecular dissociation

The experiments presented here selectively resolvedproduct branching between the unimolecular dissociachannels of a high-energy C3H5 isomer, the 2-propenyl radical, measuring how the branching ratio between thecompeting C–H fission channels changes as a functioninternal energy. This is a unique capability that is easilytended to probing the internal energy dependence ofcompetition between unimolecular dissociation channelsbroad spectrum of hydrocarbon radicals that have eludedrect experimental probes in the past. Although the branchbetween the dissociation channels of the 2-propenyl radas a function of internal energy has never been measbefore, the system is small enough that high qualityab initiocalculations1,4 have been reported, adding to earlier2,3,6 com-putational and empirical estimates of isomerization andsociation barriers in the C3H5 systems, which allow us topredict the branching between product channels and hochanges with internal energy in the radical. We make tcomparison below.

The experimental data showed the following. First, tfraction of 2-propenyl radicals that dissociate to giH1allene products is 2.0~10.05/20.15! times larger for dis-sociation of the group of radicals with higher internal ener~median energy 15 kcal/mole above the barrierH1propyne) than from the group of radicals with lower iternal energy. Second, fitting the arrival times of allenerived from near-threshold 2-propenyl radicals showed tha~i!the barrier to H1allene is higher than the barrier tH1propyne and~ii ! the branching to the H1allene channelincreases with internal energy above the H1allene barrier. Infact, we used the fitting of the near-threshold H1allene prod-


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uct channel in Fig. 10 to determine the internal energyquired to surmount the barrier to H1propyne and the fittingof products from both internal energy groups to test whetthe relative barrier heights predicted for the H1propyne andH1allene channels in recent G2~B3LYP! calculations1 areconsistent with our experimental results. We now descrthe RRKM calculation used for this purpose and the resu

We used RRKM theory30,46,47to obtain both a quantitative prediction of how the branching between the H1alleneand H1propyne channels changes as a function of interenergy in the 2-propenyl radical and to predict the shapethe P(ET) in red-dashed line in Fig. 11 that fit the neathreshold allene products and gave us a barrier forH1allene product channel. We use the frequencies ofradical and transition state complexes and relevant momof inertia48 calculated by Davis, Law and Wang1 to calculateRRKM rate constants for reactions~4! and ~5!. Their zero-point-corrected barrier to H1allene ~4!, 38.1 kcal mol21, isone kcal mol21 higher than the barrier to H1propyne~5!; seeFig. 1. ~The barrier energy difference is in qualitative agrement with the 2 kcal/mol energy difference determined inab initio work of Deyerlet al.4!. Thus we expect the groupof dissociating 2-propenyl radicals with the lower internenergies to favor propyne products for barrier energy reasalone. This expectation is borne out by fitting the 10 eV TOspectrum, which reveals the internal energy dependencthe product branching even within the low-internal-energroup of dissociating radicals as shown in Figs. 10 andTo fit the time-of-arrival distribution of the near-threshoallene products, we already knew the number of dissociaradicals at each energy from momentum matching withmeasured Cl atom TOF that are momentum-matched tonear-threshold 2-propenyl radicals. If these radicals brancbetween H1allene and H1propyne with a ratio independenof the internal energy of the radical, we could have fit tTOF of the allene products with the near-threshold~red!P(ET) in Fig. 5, but that gave a very poor fit. The dashowed clearly that the onset of the H1allene channel was ahigher internal energies~lower kinetic energies in Cl1C3H5

recoil! than H1propyne and that the branching to thH1allene channel increased with internal energy in the dsociating radical. Indeed, we used the RRKM calculationspredict the shape of the red-dashed lineP(ET) in Fig. 11 thatwe used to fit the near-threshold allene products. We simweighted the distribution of dissociative radicals at eachergy with the RRKM prediction for what fraction of radicaat each internal energy would dissociate to H1allene insteadof H1propyne, kallene(E* )/(kallene(E* )1kpropyne(E8* )),where the available energy above the H1allene barrier (E* )was, because of the 1 kcal/mol difference in barrier heig1 kcal/mol higher thanE8* for H1propyne. We originallyassumed that the heats of formation of reactants and pucts, particularly 2-chloropropene, and the H1propyne/allene barrier heights with respect to the two product asymtotes were not accurate enough to derive anE* correspond-ing to each kinetic energy for the RRKM predictedP(ET) inFig. 11. Instead, we arbitrarily assigned a particularET inC–Cl fission as producing the radical with anE8* 50 for theH1propyne channel, and calculated the RRKM rate co


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stants to generate a predictedP(ET) to fit the allene productsin Fig. 10. Iterating this, we found that assigningET517.462 kcal/mol as producing 2-propenyl radical withE* 50gave the red-dashedP(ET) in Fig. 11 that fit the near-threshold H1allene signal well. ~with ET518.4622 kcal/mol giving E8* 50 for the H1propyne channel!.This experimental determination is in remarkably goagreement with the prediction derived from computed heof formation. Calculating the kinetic energy required to prduce 2-propenyl radicals with an internal energy at the brier to H1allene, we get

ET,threshold,allene5Ehn1Eint,mp2DH0 K~314!2Eexit barrier

5~147.911.22128.423.1! kcal/mol

517.663.5 kcal/mol.

This result corresponds closely to our experimental valueET,threshold,allene517.462 kcal/mol. The key number in theequation that could not be experimentally determined,therefore, was only available from calculations, is the diffence in energy between the barrier to H1allene and theasymptotic product energy. The barrier energy determinethe G3~B3LYP! calculations by Daviset al.1 was 3.1 kcal/mole above the H1allene asymptote. Our threshold agrewith this to within 0.2 kcal/mol, well within the uncertaintin heats of formation used to make the comparison.~In con-trast, if it were the allyl radical dissociating to H1allene, thebarrier energy is predicted by Daviset al.1 to be 5.6 kcal/molhigher than the H1allene asymptote.! Thus the excellentcomparison with our experimental threshold supports the3.1 kcal/mol exit barrier~barrier to reverse reaction! for the2-propenyl radical dissociation to H1allene. There are noother experimental determinations of this exit barrier, asother experiment has been able to produce the 2-propradical selectively and detect its branching to H1allene as afunction of internal energy above the H1allene asymptote~The other numbers used in the equation above are fthermodynamic measurements as follows. We assumeusing in the equation above the most probable internalergy for the 2-chloropropene precursor,Eint,mp,1.2 kcal/molat a nozzle temperature of 150 °C, which assumes no vitional cooling in the expansion in He, gives us a reasonaway to estimate the kinetic energy at which the radicalsmost likely to be formed withE* 50. To calculate theDH0 K

for H2CvCClCH3→H2CvCvCH2~allene!1H1Cl @~3! 1~4!# we use literature15 values for the heats of formation atK for the Cl and the H and calculated values f2-chloropropene49 and allene.50 Experimental heats of formation for 2-chloropropene51 and allene15 are only available at298 K, so we use the G3 result at 0 K for 2-chloropropene of21.362.5 kcal/mol and the G3 result at 0 K for allene of46.862.5 kcal/mol. We should note that the literatuvalues52,53 for DH f of the 2-propenyl radical yield values oDHrxn for C–Cl bond fission in 2-chloropropene that gapredictions for the internal energy in the radical product tare inconsistent with the onset for secondary dissociatiothe radical observed here. Therefore, we recommend insusing the calculated heats of formation above and the re


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from Davis et al.1 that the zero-point energy of th2-propenyl radical is 35 kcal/mol lower than the H1alleneasymptote. This gives a derived heat of formation at 0 K of63.4 kcal/mol for the 2-propenyl radical and a C–Cl boenergy in 2-chloropropene of 93.4 kcal/mol.!

The RRKM calculations using the transition statesDavis et al.1 thus gave predictions in good agreement wour experimental results for both the exit barrier in t2-propenyl radical→H1allene unimolecular dissociation~4!and for the change in branching between the H1propyne andH1allene channels as a function of internal energy from 018 kcal/mol above the H1propyne barrier. Experimentallywe found that the quantum yield of allene from the highinternal energy group of radicals, with median internal eergy 15 kcal/mol above the H1propyne barrier, is 2.0~10.05/20.15! times higher than the quantum yield of allenfrom the lower internal energy group of radicals. The RRKcalculations, appropriately averaged over the internal enedistributions for the two groups of radicals using theE* andE8* corresponding to each kinetic-energy release,equately predict the observed change in branching, preding a ratio of 2.2. Calibrating the experimental yield for thhigher internal energy group of radicals with the RRKyield predicted for the lower internal energy radicals, we fiexperimentally that only 21% of the 2-propenyl radicals wmedian internal energy 15 kcal/mol above the H1allene bar-rier give H1allene products. Note that the C–H fission prouct branching we resolve here from 2-propenyl radicalsvery different than the dominance of H1allene productsfrom allyl radical decomposition.4,5 The rate of isomerizationof 2-propenyl radical to allyl radical at the energies in owork must be small compared to the dissociation rates2-propenyl radicals; indeed there is considerable disagment in the literature on the height of the isomerization brier, some estimates1,4 having the isomerization barriehigher than the 2-propenyl radical C–H bond fission barrieand some2,3,6,54having it lower.

Figure 1 depicts a very interesting comparison offerby the (H1propyne!/~H1allene) product branching for th2-propenyl radical dissociation and the HCl1propyne–HCl1allene product branching in HCl elimination from2-chloropropene. Although the C–H fission reactions pceed through loose transition states and the HCl eliminathrough tight transition states, in both types of reactionsbranching to the propyne products is predicted to be larthan the branching to allene products. The prediction perseven at very high excess energies above the barrier, scannot be attributed to the very small barrier energy diffences for producing allene versus propyne. Thus any atteto predict the preference for propyne formation basedrelative barrier heights or endothermicities alone wouldgive the RRKM preference for propyne1H products at thesehigher internal energies. The preference for propyne formtion over allene formation clearly results from the fact thpropyne formation does not freeze an internal rotor at


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transition state in either the HCl elimination or the radicC–H fission reactions, while the rotor is frozen for alleformation in either type of reaction. The preference for tpropyne products in these elimination and bond fission stems is thus a nice example of the general effect onconstants of the tightening of the transition state whemethyl rotor is frozen. These experiments were only ableconfirm the preference for propyne products in t2-propenyl radical C–H fission reactions~the high vibra-tional excitation of the propyne product from the HCl elimnation thwarted our attempt to determine the product isousing tunable VUV photoionization!. Work is underway inother labs regarding the HCl elimination channels in the thmal decomposition of 2-chloropropene.55

The experiments presented here open the door to sting the branching between unimolecular dissociation chnels for selectively-produced hydrocarbon radical isomera function of internal energy in the radical isomer. Given tkey role played by hydrocarbon radical isomers in combtion and atmospheric processes, a role embedded in kinmechanisms for such processes, the opportunities openehere are very exciting. We are currently using the samethod to compare the C–C and two C–H fission unimlecular decomposition channels of the allyl, 2-propenyl, a1-propenyl C3H5 radical isomers and the corresponding umolecular reactions of several key C4H7 radical isomers.

ACKNOWLEDGMENTS

This work was supported by the National Science Fodation under Grant Nos. CHE-9619376 and CHE-99749~L.J.B.! and by the Director, Office of Science, OfficeBasic Energy Sciences, Chemical Sciences Division ofU.S. DOE, contract No. DE-AC03-76SF00098~A.G.S.!. TheALS is supported by the Materials Sciences Division of tDOE under the latter contract. J.M. gratefully acknowledgsupport from the Camille and Henry Dreyfus FoundatioPostdoctoral Program in Environmental Chemistry. Thethors thank B. Ruscic for pursuing two key calculationsheats of formation that contributed to this work, JohanMiller for carrying out the RRKM calculations and Bill Hasfor helpful advice on them, and Congjun ‘‘Panda’’ Wang faid in taking the UV-vis absorption spectrum. We also thaD. Neumark for donating the allene used to obtain the callene photoionization efficiency curve in this paper.

1Calculated in S. G. Davis, C. K. Law, and H. Wang, J. Phys. Chem103, 5889~1999!.

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hoes, A. Greenberg, and J. F. Liebman~Blackie, London, 1996!, p. 22.12Assumes the C2H2 vinylidene radical product isomerizes on a time sca

rapid compared to the dissociation process, so energy released inisomerization is included inDHrxn .

13Estimated from a DH0 of 128.4 kcal/mol for H2CvCClCH3

→H2CvCvCH2~allene!1H1Cl calculated as described in the Discusion and subtracting the endothermicity at 0 K of~4! calculated at theG2~B3LYP! level of theory in Ref. 1.

14Calculated from heats of formation at 0 K in Ref. 15 and acalculated heatof formation at 0 K at the G3 level of theory for 2-chloropropene byRuscic referenced in the Discussion.

15H. M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, J. PhChem. Ref. Data Suppl.6, Suppl. 1, 774~1977!.

16A. M. Mebel, W. M. Jackson, A. H. H. Chang, and S. H. Lin, J. AmChem. Soc.120, 5751~1998! but note that the allene-propyne zero-poienergy level difference in this paper is 0.2 kcal/mol less than that giveRef. 1.

17Calculated as described in Ref. 14 and subtracting the 1.1 kcal/mol zpoint energy level difference between allene and propyne reporteRef. 1.

18Estimated by assuming they have similar endothermicities as the angous reactions in vinyl chloride, cleavage of thetrans-b C–H bond and H2elimination to form chlorovinylidene for the analogs to~8! and ~10!, re-spectively, calculated in Ref. 10.

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48The 59 cm21 mode is replaced by a free rotor with I51.9 amu Å2, H.

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Competing isomeric product channels in the 193 nm