Preview for 1513

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Preview for 15-13
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UV/Visible Spectroscopy

• Anderson University

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Electromagnetic Spectrum
frequency (n)
high
low
energy
X-Ray Ultraviolet
infrared micro radio

wave wave
visible
ultraviolet
wavelength (l)
short
long
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Principles of UV-Visible Spectroscopy
Chromophores - groups of atoms responsible for
these transitions
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UV Visible Spectrum
lmax
UV/visible bands are broad due to overlap of
rotational and vibrational states of a molecule.
A
Wavelength in nm (l)
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Transmittance and Absorbance
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Solvents and Cells for UV Spectroscopy
solvent
Sample cell
200 nm
400 nm
800 nm
uv
visible
Glass or plastic
quartz
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Common Chromophores Alkenes
?
E
165 nm
? 15,000
?
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Common ChromophoresCarbonyl Compounds
?
E
n
?
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Possible transitions
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Conjugation
Alternate single and double bonds overlap of
p-orbitals
Conjugation lowers energy of ? ? ? transition
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1,3-butadiene
antibonding
bonding
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UV Absorption of Conjugated Alkenes

• Increasing conjugation gives
• longer wavelength absorption
• more intense absorption

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b-Carotene
11 double bonds
lmax 460 nm
Lycopene
11 conjugated double bonds, 13 total
lmax 474 nm
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Robert Burns Woodward 1965 Nobel Prize in
Chemistry
First synthesized quinine, cholesterol and
cortisone (1951), chlorophyll (1960),
tetracycline (1962), Vitamin B12 (1971- Included
over 100 reactions), deduced structures For
penicillin (1945), strychnine (1947),
etc. Widely known as the father of modern
organic synthesis.
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Woodward-Fieser Rules for Dienes

• Base Value
• heteroannular 214 nm
• homoannular 253 nm
• Additions for
• alkyl group or ring residue 5 nm
• double bond extending conjugation 30 nm
• exocyclic double bond 5 nm

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Calculate the ?max for the following compounds
253 (base) 15 (3 ring residues) 5 (exocyclic
CC) ------------ 273 nm calc. Observed 275 nm
214 (base) 15 (3 ring residues) 5 (1
exocyclic CC) ----------- 234 nm calc. Observed
235 nm.
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Aromatic Compounds
Substituents shift 255 nm band to higher
wavelength