Transcript Document

Electronic Excitation by UV/Vis Spectroscopy :
X-ray:
core electron
excitation
UV:
valance
electronic
excitation
IR:
molecular
vibrations
Radio waves:
Nuclear spin states
(in a magnetic field)
Spectroscopic Techniques
UV-vis
UV-vis region
bonding electrons
Atomic Absorption
UV-vis region
atomic transitions (val. e-)
FT-IR
IR/Microwave
vibrations, rotations
Raman
IR/UV
vibrations
FT-NMR
Radio waves
nuclear spin states
X-Ray Spectroscopy
X-rays
inner electrons, elemental
X-ray Crystallography
X-rays
3-D structure
The wavelength and amount of light that a compound absorbs depends on
its molecular structure and the concentration of the compound used.
The concentration dependence follows Beer’s Law.
A=ebc
Where A is absorbance (no units, since A = log(P0 / P )
e is the molar absorbtivity with units of L mol-1 cm-1
b is the path length of the sample - that is, the path length of the cuvette in
which the sample is contained (typically in cm).
c is the concentration of the compound in solution, expressed in mol L-1
Characteristics of UV-Vis spectra of
Organic Molecules
• Absorb mostly in UV unless highly
conjugated
• Spectra are broad, usually to broad for
qualitative identification purposes
• Excellent for quantitative Beer’s Lawtype analyses
• The most common detector for an
HPLC
Molecules have quantized energy levels:
ex. electronic energy levels.
energy
energy
hv
}
= hv
Q: Where do these quantized energy levels come from?
A: The electronic configurations associated with bonding.
Each electronic energy level
(configuration) has
associated with it the many
vibrational energy levels we
examined with IR.
Broad spectra
• Overlapping vibrational and rotational
peaks
• Solvent effects
Ethane
C C


hv



H
C C
H
H
HH
H

max = 135 nm (a high energy transition)
Absorptions having max < 200 nm are difficult to observe because
everything (including quartz glass and air) absorbs in this spectral
region.
C C




= hv
=hc/
hv






Example: ethylene absorbs at longer wavelengths:
max = 165 nm e= 10,000
C O




n
hv
n




n

The n to pi* transition is at even lower wavelengths but is not
as strong as pi to pi* transitions. It is said to be “forbidden.”
Example:
Acetone:
n max = 188 nm ; e= 1860
n max = 279 nm ; e= 15
C C

135 nm
C C

165 nm
n
183 nm
weak

n
n
150 nm
188 nm
279 nm
weak
H
C O
C O
180 nm
C O
A
279 nm

Conjugated systems:
C
C
LUMO
HOMO
Preferred transition is between Highest Occupied Molecular Orbital
(HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).
Note: Additional conjugation (double bonds) lowers the HOMOLUMO energy gap:
Example:
1,3 butadiene:
max = 217 nm ; e= 21,000
1,3,5-hexatriene
max = 258 nm ; e= 35,000
Similar structures have similar UV spectra:
O
O
O
max = 238, 305 nm
max = 240, 311 nm
max = 173, 192 nm
Woodward-Fieser Rules for Dienes
Parent
Homoannular
=253 nm
Heteroannular
=214 nm
=217 (acyclic)
Increments for:
Double bond extending conjugation
Alkyl substituent or ring residue
Exocyclic double bond
Polar groupings:
-OC(O)CH3
-OR
-Cl, -Br
-NR2
-SR
Homoannular
heteroannular
C
+30
+5
+5
C C
C
+0
+6
+5
+60
+30
acyclic
exocyclic
For more than 4 conjugated double bonds:
max = 114 + 5(# of alkyl groups) + n(48.0-1.7n)
Lycopene:
max = 114 + 5(8) + 11*(48.0-1.7*11) = 476 nm
max(Actual) = 474.