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Photochemistry Lecture 8 Photodissociation Photodissociation ABCD + h? ? AB + CD Importance Atmospheric and astrophysical environment Primary step in photochemical processes – free radical production Fundamental studies of dynamics of chemical reactions Atmospheric Chemistry – the ozone hole In the stratosphere, ozone protects the earth from damaging UV radiation via the Chapman cycle O2 + h? → O + O (? 242 nm) O3 + h? → O2 + O (? 1180 nm) O + O2 + M ? O3 + M O + O3? O2 + O2 Solar energy converted into thermal energy…heating…temperature inversion. Catalytic destruction of ozone e.g., CF2Cl2 + h? ? CF2Cl + Cl Cl + O3 ? ClO + O2 ClO + O ? Cl + O2 Formation of reservoir species e.g., Cl + CH4 ? CH3 + HCl ClO + NO2 + M ? ClONO2 + M Antarctic ozone hole ClONO2 + HCl ? Cl2 + HNO3 Hetergeneous catalysis on polar stratospheric clouds Cl2 + h? ? Cl + Cl Regeneration of ozone destruction mechanism Smog formation Production of OH radical in troposphere via sequence… NO2 + h? ? NO + O O(1D) + H2O ? OH + OH Oxidation of hydrocarbons (with regeneration of OH and NO2 OH + RCH3 ? RCH2 + H2O ……+ O2 ? RCH2O2 …….. Wavefunctions in the continuum Simple photodissociation within a single electronic state is essentially forbidden Lifetime broadening of predissociating levels Upper state predissociation evident in linewidths of P and R branch transitions of Se2 Photodissociation of polyatomic molecules Potentially more than one product channel for sufficiently high photolysis energy e.g., formaldehyde CH2=O + h? ? H + HCO ? H2 + CO Latter requires rearrangement via 3-membered ring transition state Should generally consider dissociation in polyatomics as occurring via a form of predissociation…..energy transfer from initially excited state to a dissociative state. Energy requirements State in which excited molecule resides must be higher than dissociation energy For the halonaphthalenes X-Np Localization of excitation The weakest bond is most likely to