photo - Photochemical transformation reactions Direct...

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Unformatted text preview: Photochemical transformation reactions Direct photolysis = transformation of a compound due to its absorption of UV light Indirect photolysis = transformation of a compound due to its interaction with a reactant generated by the influence of UV light (photosensitizer or reactive oxygen species) Direct photolysis and light absorption Types of orbitals: bonding: (single) or (double) non-bonding: n (often lone pairs on hetero atoms such as N, O) anti-bonding: * (single) or * (double) Absorption of light causes electronic transitions: important transitions are usually n to * or to * HOMO and LUMO Ethylene Light Energy Energy E = hv = h (c/ ) where h = Planks constant = wavelength c = speed of light Longer wavelengths = less energy Bond E (kJ/mol) (nm) O-H 465 257 C-H 415 288 N-H 390 307 C-O 360 332 C-C 348 344 C-Cl 339 353 Br-Br 193 620 O-O 146 820 Light hitting earths surface = 290- 600nm (why?) Light absorption A I I C l o = + log ( ) ( ) [ ( ) ( ) ] Beer-Lambert Law where: A = absorbance I = light intensity (emerging vs. incident) = wavelength = absorption coefficient of the medium = absorption coefficient of the compound l = path length Fate of excited species Quantum yield: r ( ) depends on chemical structure, solvent, pH, ionic strength, etc. low activation energies: 10-30 kJ/mol in solution Compound h excitation C* Physical processes: vibrational loss of energy (heat transfer) energy loss by light emission (luminescence) energy transfer promoting an electron in another species (photosensitization) C Chemical reactions : fragmentation intramolecular rearrangement isomerization H atom abstraction dimerization electron transfer from or to the chemical Not C product(s) Chemical processes Reaction rates for direct photolysis in water:...
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This document was uploaded on 10/25/2011.

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photo - Photochemical transformation reactions Direct...

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