Unimolecular Rate Constant and Threshold Energy for the HF Eliminaton from Chemically Activated CF3C

The reactions of CF3CHFCF3 in the gas phase are of greatinterest because CF3CHFCF3 is used as a fire suppressant inplace of Halon 1301, CF3Br;1–6 as a working fluid in refrigerationand air conditioning applications;7–9 as a foam blowing agentwith fire retardant capabilities;10 and as a propellant in medical sprays.11–13 Modeling studies of low-pressure and atmospheric pressure combustio

6996J.Phys.Chem.A2010,114,6996–7002

UnimolecularRateConstantandThresholdEnergyfortheHFEliminationfromChemicallyActivatedCF3CHFCF3

JulianaR.Duncan,MichaelS.Roach,BrookeSibilaStiles,andBertE.Holmes*

DepartmentofChemistry,UniVersityofNorthCarolinaatAsheVille,OneUniVersityHeights,AsheVille,NorthCarolina28804-8511

ReceiVed:January8,2010;ReVisedManuscriptReceiVed:May18,2010

CombinationofCF3CHFandCF3radicalsatroomtemperaturegeneratedchemicallyactivatedCF3CHFCF3moleculeswith95(3kcal/molofinternalenergythatdecomposebylossofHF,initiallyattachedtoadjacentcarbons,withanexperimentalunimolecularrateconstantof(4.5(1.1)×102s-1.DensityfunctionaltheorywasusedtomodeltheunimolecularrateconstantforHFelimination,kHF,todetermineathresholdenergyof75(2kcal/mol.

1.Introduction

ThereactionsofCF3CHFCF3inthegasphaseareofgreatinterestbecauseCF3CHFCF3isusedasa resuppressantinplaceofHalon1301,CF3Br;1–6asaworking uidinrefrigerationandairconditioningapplications;7–9asafoamblowingagentwith reretardantcapabilities;10andasapropellantinmedicalsprays.11–13Modelingstudiesoflow-pressureandatmosphericpressurecombustionrelyuponaccuratekineticdatafortheunimoleculardecompositionofCF3CHFCF3anditsbimolecularreactionswithOHradicalsandotherreactivespecies.Experi-mentalandcomputationalstudies14–26oftheproductpro leforcombustionofCF3CHFCF3andofthereactionsbetweenCF3CHFCF3andOHradicalsoratomicspecies(F,Cl,H,O,etc)haveappeared.

Ashocktubepyrolysis26ofCF3CHFCF3overthetemperaturerange1200-1500KconcludedthatthermaldecompositiondominatedtheremovalprocessesofCF3CHFCF3andthatHFeliminationpredominatesoverC-Cbond ssion.Theymodeledthecombustionpro leusinganassumedArrheniusactivationenergy,Ea,for1,2-HFlossof69.6kcal/mol.TheEawasestimatedbyadding38.8kcal/moltotheenthalpychangefortheHFeliminationreactionandcorrespondstoathresholdenergy,E0(HF),of67.4kcal/mol;theA-factorwasassumedtobe7.9×1012s-1.Amodelingstudy1usinglaser-induced uorescencedetectionofradicalspeciesintheCF3CHFCF3inhibitedCH4+O2 ameagreedthatunimolecularHFdecompositionwasdominant,butconcludedthatC-Cbondrupturewasincreasinglyimportantasthe ametemperatureincreased,anditappearedthatthesameArrheniusparameterswereusedastheearlier26work.Averyrecentthermaldecompositioninvestigation19busingUVphotoelectrondetectionalsofoundthat1,2-HFeliminationwasfavoredbutsuggestedthatformationofHCF3+CF2dCF2,ratherthanC-Cbondrupture,becamemoreimportantathighertemperatures.Amodelingstudyoftheproductpro lefromthereactionbetweenCF3CHFCF3andeitheratomichydrogenorO(3P)usedEa)69.5kcal/mol23forHFelimination,butasimilarapproach27fortheinhibitionbyCF3CHFCF3onshockheatedethane-oxygen-argonmixturesusedEa)75.6kcal/mol.

*Towhomcorrespondenceshouldbeaddressed.E-mail:bholmes@unca.edu.

AtheoreticalinvestigationattheQCISD(T)levelrecom-mended28E0)79.2kcal/mol,but vedifferentlevelsoftheorypredictedanE0intherange82.3-89.1kcal/molandB3LYPwith6-31G(d)gaveabarrierof75.4kcal/mol.TheE0istypicallytwoorthreekcal/mollowerthanEaathightemper-ature;thus,thetheoreticalworksuggestedanactivationenergybetween75and90kcal/mol.The amepro lemodelingstudies1,26agreedthatunimoleculareliminationofHFfromCF3CHFCF3isanimportantstepinthecomplexreactionchemistryofinhibited amesand,becauseaprecisethresholdenergyisessentialtoaccuratelymodelthecombustionsystem,anexperimentalinvestigationofthe1,2-HFeliminationreactionfromCF3CHFCF3seemswarranted.

WewillmeasuretheunimolecularrateconstantforHFlossfromCF3CHFCF3chemicallyactivatedbythecombinationofCF3andCHFCF3radicals.Theaverageenergy, E ,ofthechemicallyactivatedCF3CHFCF3isestimatedas95kcal/molfromtheCF3CHF-CF3bonddissociationenergy,D0(CF3CHF-CF3),plustheaveragethermalenergyofthecombiningradicals.TheRRKMtheory29willbeusedtocalculatetherateconstantforHFeliminationfromCF3CHFCF3,withanenergyequalto E ,andthethresholdenergy,E0,willbevarieduntilthecomputedunimolecularrateconstantmatchestheexperimentalresult.InputparametersneededfortheRRKMcomputationoftherateconstant(vibrationalfrequenciesandmomentsofinertia)willbecalculatedwithdensityfunctionaltheory(DFT)usingtheGaussian03suite30ofprograms.Asystematicstudy31ofDFTandabinitiomethodsforthe1,2-HF(DF)eliminationfromCF3CH3(CF3CD3)determinedthatB3LYPorB3PW91togetherwitheithera6-31G(d′,p′)oracc-PVDZbasissetgavethebestagreementbetweencomputedandexperimentalrateconstantsandthekineticisotopeeffect.Inourrecentwork,32–34DFTusingtheB3PW91methodwitha6-31G(d′,p′)ora6-311+G(2d,p)basissetwasfoundtogivecloseagreementbetweenthecomputedandexperimentalassignedE0(HF)forninehalopro-panesandonehalobutane.

TherearethreemainsourcesofuncertaintyinmatchingthecomputedrateconstanttotheexperimentalmeasurementtodeterminetheE0forHFelimination:(1)TheenthalpyofformationforCF3CHFCF3,theCF3CHFradical,andtheCF3radicalareusedtodetermineD0(CF3CHF-CF3).Theexperi-mentaluncertaintyinthermochemistryforthese uorocarbonspeciesisoften(1-2kcal/molleadingtoanuncertaintyof

10.1021/jp100195e 2010AmericanChemicalSociety

PublishedonWeb06/10/2010

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