Environmental Impact of Light
Fundamental processes in soil, atmospheric and aquatic systems
Chemical and biochemical changes2.iii.c (2 hrs)
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‘Dark’ Reactions
• Most organic compounds react very slowly, even with oxygen, at normal temperature.
• As a rule, they can be considered generally nonreactive.
• BUT !...
Environmental processes / Chemical and biochemical changes / Impact of light
3
Let Be Light !
• Visible and ultraviolet radiation can promote the reactivity of almost all compounds.
• Recall a well known fact that items exposed (northern hemisphere) on southern side of buildings decay more quickly than those on northern side.
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On the side facing south, the wooden (and metal) items decay faster
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Museum item
Side exposed to light Side not exposed to light
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Basics of Photochemistry
• Primary photophysical process.
• Subsequent chemical change(s)
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Photophysical process
• Actually, it means absorption of light quanta.• It promotes molecule from ground- to excited
state (energy rich one).
• But, to catch the light quanta, molecule has to have a CHROMOPHORE !
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Chromophores
• Chromophores are structural details in molecule(s) that enable high(er) probability of light absorption.
• Normally, it comprises double bonds in molecule, i.e., π-electrons:
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Chromophore
• Double bond
• Aromatic ring
• Far more efficient are polarized double bonds.
C=O:
C=N-
C=C
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Photophysical process• Once molecule acquires light quantum, it has
many ways to decay, involving the chemical change.
• Simplified approach tells that molecule can exist in ground state singlet or triplet– The same holds for excited state
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Ground-statesinglet
Excited-statesinglet
Excited-statetriplet
S1 T10S
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hν(d)(b)
(c)E
S1
S0
T1
hν hν(a)
ES1S0 T1
hν
hν
hν
(a)
(b)(c)
(d)
r
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• Singlets and triplets are known as multiplicities of state.
• Electronic transition among states of different multiplicities is not likely to happen (is ‘forbidden’),
• But, as many other things, something forbidden still could happen.
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• These ‘violations’ could be promoted by, so called, SENSITIZERS.
OHO OH
COO
+
S
N
NMe 2Me 2 N+
CN
CN
Fluoresceineee
Methylene blue Dicyanoanthracene
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• Sensitizers quickly, and in good yields populate excited triplet states.
• Triplets live long enough to encounter other molecules.
hν(d)(b)
(c)E
S1
S0
T1
hν hν(a)
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Sensitization
h ν
in te rs ys te m
C r o ss in g
S e n s (S 0) S e n s (S 1)
S e n s (S 1) S e n s (T 1)
S e n s (T 1) + V O C (S 0) S e n s (S 0) + V O C (T 1)))))
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Photophysics → Photochemistry
C=O:
CH3-CO-CH3 hν CH3CO + CH3 ))CH3COCOCH3 + CH3CH3
Norrish type I
+
R
O H
R
OH
hνR
R
R
OHR
Enol
Norrish type II
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• Primary chemical intermediates in photochemical metathesis are organic free radicals.
• Free radicals are highly reactive.• Radicals react with almost
everything!
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Additional sensitization(singlet oxygen)
h ν
in te rs y s te m
C r o ss in g
S e n s (S 0) S e n s (S 1)
S e n s (S 1) S e n s (T 1)
S e n s (T 1) + O 2 (T 0) S e n s (S 0) + O 2 (S 1)))))
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Coatings & Erosion
• VOC-s exposed to light can be promoted to more reactive compounds.
• Oxygen-containing derivatives can be aggressive to living organisms and to artifacts.
• Reactive intermediates may lead to polymers forming sticky coating.
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Everything matters
• Ozone is not emitted directly from industrial sources and vehicles.
• It is formed in troposphere as a result of reactions involving oxides of nitrogen and volatile organic compounds.
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What pollution we can control?
–Industry,–Traffic,–Agriculture– and (….. leisure)
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Internet Resources
• http://www.iaq.dk/index.html• http://www.airclim.org/airAndEnvironment/A
E_chp3.htm• http://www.epa.gov/region4/airqualitytoolkit/1
0_Glossaries/US NPS - Air Quality Glossary.pdf
• http://www.heritage.xtd.pl/pdf/full_czop.pdf• http://www.epa.gov/iaq/voc.html
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• http://www.environment.gov.au/soe/2006/publications/commentaries/atmosphere/glossary.html
• http://www.airimpacts.org/documents/local/aqbook.pdf