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1、CHAPTER2AComputationalStrategyforOrganicPhotochemistryMichaelA.Robb,*MarcoGaravelli,‘kMassimoOlivucci,tandFernandoBernard8“DepartmentofChemistry,King’sCollegeLondon,Strand,LondonWC2R2LS,UnitedKingdom,tIstitutodiChimicaOrganica,UniversitadegliStudidiSiena,V
2、iaAldoMoro,I-53100Siena,Italy,andSDipartimentodiChimica“G.Ciarnician”dell’llniversitadiBologna,ViaSelmi2,40126Bologna,ItalyINTRODUCTIONModelingPhotochemicalReactionsTheaimofthischapteristoprovideanintroductiontothepracticalcomputationalinvestigationofphoto
3、chemicalreactionmechanisms.Duringthelastdecadeorso,thespeedofcomputershasgrownconsiderably,andnowthecomputationalinvestigationofrealisticmodelsoforganiccompoundsisbecomingastandardpractice.Currentapplicationsrangefromtheinvestiga-tionofthemechanismofsynthe
4、ticallyusefulreactionstothestudyofshort-livedorganicintermediatesdetectedintheinterstellarmedium.Forthermalreactions,standardstate-of-the-artabinitioquantumchemicalmethodsarealreadycapableofprovidingacompletedescriptionofwhathappensattheReviewsinComputatio
5、nalChemistry,Volume15KennyB.LipkowitzandDonaldB.Boyd,EditorsWiley-VCH,JohnWileyandSons,Inc.,NewYork,020008788AComputationalStrategyforOrganicPhotochemistrymolecularlevelduringbond-breakingandbond-formingprocesses.Inparticu-lar,itispossibletocomputethetrans
6、itionstructurethatconnectsareactanttoaproductandtheassociatedenergybarrierwithalmostchemicalaccuracy(ca.1kcal/molerror).Furthermorethereactionpath(i.e.,theprogressionofthemolecularstructurefromthereactantstowardthetransitionstateandtheproduct)canbedetermin
7、ed,inacompletelyunbiasedway,bycomputingtheminimumenergypath(MEP)1connectingthereactanttotheproductonthe(3N-6)-dimensionalpotentialenergysurfaceofthesystem.Adetailedunderstandingofthereactionpathwayintheexcitedstatemanifoldwillincreaseourabilitytodesignnewa
8、ndtocontrolknownpho-tochemicalreactions.Asanexample,theconversionoflightintochemicalenergyinplantsandanimalsinvolvesextendedconjugatedmolecules-carotenoidsandretinals-boundinproteincomplexes.Theuseofsuchex-te
