UV photodissociation dynamics of CHI2Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

摘要

Photolysisofgeminaldiiodoalkanesinthepresenceofmolecular oxygenhasbecomeanestablishedudroutetothelaboratoryproductionofseveralCriegeeintermediates,andsuchcompoundsalsohaveudmarinesources. Here,weexploretherolethat thetrihaloalkane,chlorodiiodomethane(CHI2Cl),mayudplayas aphotolyticprecursor for thechlorinatedCriegeeintermediateClCHOO.CHI2Clhasbeenudsynthesized and its UV absorption spectrum measured; relative to that of CH2I2 the spectrum isudshiftedtolongerwavelengthand thephotolysislifetimeiscalculatedtobelessthantwominutes.udThephotodissociationdynamicshavebeeninvestigatedusingDCsliceimaging,probinggroundstateudI and spin-orbit excited I* atoms with 2+1 REMPI and single-photon VUV ionization. TotaludtranslationalenergydistributionsarebimodalforIatomsand unimodalforI*,witharound72%ofudtheavailableenergypartitionedintotheinternaldegreesoffreedomoftheCHIClradicalproduct,udindependentofphotolysiswavelength.AbonddissociationenergyofD0 =1.73±0.11eVisinferredudfromthewavelengthdependenceofthetranslationalenergyrelease,whichisslightlyweakerthanudtypical C–I bonds. Analysis of the photofragment angular distributions indicate dissociation isudpromptandoccursprimarilyvia transitions tostatesofA″symmetry.Complementaryhigh-leveludMRCI calculations, including spin-orbitcoupling, havebeenperformed tocharacterize theexcitedudstates andconfirm thatstatesofA″symmetrywithhighly mixedsingletand tripletcharacterareudpredominantly responsible for the absorption spectrum. Transient absorption spectroscopy hasudbeenusedtomeasure theabsorptionspectrumofClCHOO producedfromthereactionofCHIClwithudO2 overtherange 345–440nm.Theabsorptionspectrum,tentativelyassignedtothesyn conformer,udisatshorterwavelengthsrelativetothatofCH2OOandshowsfarweakervibrationalstructure.