Spectroscopy with Milder Photons: Molecular Vibrations

Solar Radiation

• What wavelengths are present?

This depends on the wavelengths given off by the sun (black body radiation) AND the wavelengths absorbed by the atmosphere

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:

O2 + h --> 2 O

< 241 nm (496 kJ/mol)

O3 + h --> O2 + 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)

Chlorophyll

beta Carotene

• anthocyanin

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

CO2 + H2O --> (CH2O) + O2

2H2O --> 2H2 + O2

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”

• DNA• 2’ Deoxyribo Nucleic Acid

Photochemistry of Alkenes: DNA Damage

• Several akenes can be combined to form larger carbon “skeletons”

• Lets review the orbitals that might be involved

Ethene

1,3 Butadiene: Conjugation

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.

DNA: UV photochemistry

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?

Sunblock

• PABA para aminobenzoic acid: absorbs UV (bluish tinge)

• Titanium dioxide: scatters UV (bright white)

• Melanin, the skin pigment, is a similarly conjugated system, formed by joining several double bonds

• Eumelanin:black/brown• Pheomelanin: red/orange

Photodynamic Therapy

(Abs 390 - 450 nm)

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

Rising CO2

Global Warming: Expected Consequences

drought, new desertsocean ice melting: many major cities will be deluged

• Models based on historical data, considering “natural” cycles, have been open to debate

• Data from NASA Global

Hydrology and Climate Center

Global Energy Consumption, 1998based on talk by N. Lewis, Cal Tech

4.52

2.72.96

0.286

1.21

0.2860.828

00.5

11.5

22.5

33.5

44.5

5

TW

Oil Coal Biomass NuclearGas Hydro Renew

Total: 12.8 TW (15% Electricity)U.S.: 3.3 TW (10% Electricity)