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 + hn --> 2 O
< 241 nm (496 kJ/mol)
O3 + hn --> 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 + hn --> Chl+ + eEnergy 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
(Abs 390 - 450 nm)
Like many molecules,
porphyrins are selectively
taken up by cancer cells
The porphyrin excited states react
with water and oxygen to
generate reactive/toxic
chemical species (radicals, and
excited electronic states).
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 reradiated, 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 deserts
ocean 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, 1998
based on talk by N. Lewis, Cal Tech
5
4.5
4
3.5
3
TW 2.5
2
1.5
1
0.5
0
4.52
2.7
2.96
1.21
0.828
0.286
Oil
Gas
Coal
0.286
Hydro Biomass Renew Nuclear
Total: 12.8 TW (15% Electricity)
U.S.: 3.3 TW (10% Electricity)