LECTURE 22 OPTICAL SPECTROSCOPY II UVVIS CHIROPTICAL

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LECTURE 22OPTICAL SPECTROSCOPY II UV/VIS -
CHIROPTICAL
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UV-VIS SPECTROMETERS ARE UBIQUITOUS
BEGIN HERE
P 8
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(No Transcript)
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10-5
10-5
? UV 300 kcal/mol
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What functional groups are observed in UV/vis
spectrum?
Excitation
LUMO s
C-C
UV/vis (200-600 nm) Functional groups Ketone Ester
Amide Conjugated D.B Not observed C-C C-H CC
(isolated)
l 125nm
HOMO s
LUMO p
CC
l 165nm
HOMO p
p4
LUMO
p3
CC-CC
l 217 nm
p2
HOMO
p1
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UV Absorbance
A log (I0/I) ecl
I intensity of the transmitted light I0
intensity of the incident light e the molar
excitation coefficient or molecular
absorption (lmax) c concentration of the sample
(mol/L) l path length (cm)
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P354
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Isoprene
A 0.8 Conc 4x10-5 M Path 1 cm
e 0.8/4x10-5x1 2x104 20000
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P354
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P352
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P356 (see also 359)
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P360
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• ???
• 3100cm-1 C-H
• 1700cm-1 CC
• m/z (APT) 136 C10H16
• U 3
• 5. 2 db 1 ring

pg 390
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• ???
• 3100cm-1 C-H
• 1700cm-1 CC
• m/z (APT) 136 C10H16
• U 3
• 5. 2 db 1 ring

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P356
217 20 237
217 20 237
253 15 268
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-2H (134)
-Me
119
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Optical Rotation
Chiral molecules can rotate the plane of the
polarized light.
?D 100a/lc
? Specific rotation D 589 nm (Na D line) l
path length (dm) c g/100mL
Moler Rotation ? ?(MW)/100 Plot of ? (or
? vs. ? ORD)
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(UV Chromophore)
(no chromophore)
P365
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First Positive Cotton effect
Second Negative Cotton effect
P366
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ORD CD
?? Diff in molar extinction between L R
components of polarized light.
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P366
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Octant Rule (Steroid type compounds)
Predict the sign of the cotton effect for
saturated cyclo-ketone compounds Carbonyl group
Absorption 300 nm
P367
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11a
11b
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How?
Using ORD-CD absolute configuration
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Using ORD-CD absolute configuration
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10 neg C.E. -0.91 (ORD) ? m
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CD exciton method (semi-empirical)
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-Relative stereochem from NOE -Absolute
stereochem from CD (dibenzoate derivative)
Neg max at 236, pos max at 252, indicating
11R,12R config
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SUMMARY

• UV/VIS involves transitions among electronic
  states of 300 Kcal in strength.
• For ?max gt 200 nm requires conjugated
  chromophore.
• Extinction coefficient (?) large for ???
  transitions and small for n?? transitions.
• Calculated ??? or n?? ?max values useful in
  comparing working structures.
• Moleclar rotation MD ? ?D(MW)/100. ORD
  plot of MD vs. ? CD plot of ?? vs ?
• Two useful empirical approahes (a) Octant Rule,
  (b) Exciton Chirality approach.

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Homework
Chapter 8 3-6 8 9 17 Chapter 9 1 4 6 9