Incident Light on Earth Water: 725, 820 and 975 Oxygen 760 - 780 Oxygen and Ozone <400 Water Carbon Dioxide and other Greenhouse gasses >1200
Troposphere: weather Commercial airplanes and jets: 11 km, 1/3 to 1/5 atmosphere Stratosphere: from 10% to 0.1% of our density, from 18 km to > 50 km horizontal air flow Oxygen and Ozone in upper stratosphere is v. imp for UV Oxygen rich atmosphere is due to plants and algae (not originally here)
Oxygen, Ozone and UV Stratosphere: O 2 + h --> 2 O < 241 nm (496 kJ/mol) O 3 + h --> O 2 + O 200 - 350 nm
What happens to the visible light once it reaches the earth? The pigments of life Chlorophyll Heme (e.g. Myoglobin) Carotenoids, retinal etc. These chromophores are conjugated molecules …
Wavelength and Molecule Size As the size of the conjugated system gets larger, the wavelength gets longer. Which is larger, the molecule or the wavelength? How much larger? Wavelength of 200 nm, molecule length is < 1 nm. Wavelength is much much larger.
Red lamp: emits at 600 Blue lamp: emit at 450 Red dye: absorbs at 400-500 (passing at 600) Indigo dye: absorb at 500-600 (passing at 450)
How Many Photons Are Needed? Chl + h --> Chl + + e - Energy in a 600-650 nm photon: ~200 kJ/mole Energy needed for reactions below is ~ 500 kJ/mole oxygen CO 2 + H 2 O --> (CH 2 O) + O 2 2H 2 O --> 2H 2 + O 2 4 Photons/ 4 electrons/ 4 protons/ one oxygen
Some UV Radiation Does Reach the Earth How is UV distinct from visible? Almost double the energy per photon! Makes radicals, causes significant chemical reactions Skin cancer, begins with “DNA damage”
Combining two alkenes Reactants: three double bonds (612 kJ/mole), one single bond (348 kJ/mole) 2184 Product: one double bond, five single bonds 2352 More stable If aligned by matching the orbitals of two reactants: highest occupied and lowest unoccupied, can react.
UV Photochemistry Reactants: two double bonds (612 kJ/mole), 1224 Product: four single bonds (348 kJ/mole), 1392 More stable However… cannot align orbitals properly in ground state to react. Use light: reaction begins from an excited state, e.g. LUMO and LUMO can match Many alkenes undergo bond forming reactions in response to light.
Summary: UV and Visible Visible light is sufficient for electronic transitions and sometimes for eject electrons from substances if thery are large pigments with loosely bound electrons. The latter is the basis for the generation of essentially all fuels. Rarely are these are destructive processes. Ultraviolet light is sufficient for larger electronic transitions, to eject electrons from many substances, and to facilitate otherwise impossible chemical reactions. Commonly these are destructive processes.
Sunscreen Para aminobenzoic acid (PABA) blocks some of the UV irradiation (right). Titanium oxide is also used for this purpose, but it has no chromophore. How can this work?
Photodynamic Therapy The porphyrin excited states react with water and oxygen to generate reactive/toxic chemical species (radicals, and excited electronic states). (Abs 390 - 450 nm) Like many molecules, porphyrins are selectively taken up by cancer cells
The Greenhouse: Visible Light Penetrates, but IR?
Greenhouse Visible Light Comes to Earth from Sun Infrared Leaves earth: does it fail to escape atmosphere? It can either pass through the atmosphere, be absorbed and re- radiated, or be absorbed and converted to heat.
Greenhouse Which atmospheric gasses “trap” the heat from the IR radiation