environmental impact of light
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Environmental Impact of Light. Fundamental processes in soil, atmospheric and aquatic systems Chemical and biochemical changes 2.iii.c (2 hrs). ‘Dark’ Reactions. Most organic compounds react very slowly, even with oxygen, at normal temperature. - PowerPoint PPT PresentationTRANSCRIPT
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Environmental Impact of Light
Fundamental processes in soil, atmospheric and aquatic systems
Chemical and biochemical changes2.iii.c (2 hrs)
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2Environmental processes / Chemical and biochemical changes / Impact of light
‘Dark’ Reactions
• Most organic compounds react very slowly, even with oxygen, at normal temperature.
• As a rule, they can be considered generally nonreactive.
• BUT !...
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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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4Environmental processes / Chemical and biochemical changes / Impact of light
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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6Environmental processes / Chemical and biochemical changes / Impact of light
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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10Environmental processes / Chemical and biochemical changes / Impact of light
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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11Environmental processes / Chemical and biochemical changes / Impact of light
Ground-statesinglet
Excited-statesinglet
Excited-statetriplet
S1 T10S
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12Environmental processes / Chemical and biochemical changes / Impact of light
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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13Environmental processes / Chemical and biochemical changes / Impact of light
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14Environmental processes / Chemical and biochemical changes / Impact of light
• 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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18Environmental processes / Chemical and biochemical changes / Impact of light
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