13.20 UltravioletVisible UVVIS Spectroscopy

1
13.20Ultraviolet-Visible (UV-VIS) Spectroscopy

• Gives information about conjugated p electron
  systems

2
Transitions between electron energy states

• gaps between electron energy levels are greater
  than thosebetween vibrational levels
• gap corresponds to wavelengthsbetween 200 and
  800 nm

DE hn
3
Conventions in UV-VIS

• X-axis is wavelength in nm (high energy at left,
  low energy at right)
• lmax is the wavelength of maximum absorption and
  is related to electronic makeup of molecule
  especially p electron system
• Y axis is a measure of absorption of
  electromagnetic radiation expressed as molar
  absorptivity (e)

4
UV Spectrum of cis,trans-1,3-cyclooctadiene
2000
Molarabsorptivity (e)
lmax 230 nm
emax 2630
1000
200 220 240 260 280
Wavelength, nm
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pÆp Transition in cis,trans-1,3-cyclooctadiene
y4
y3
LUMO
DE hn
y2
HOMO
y1
Most stable p-electron configuration
p-Electron configuration of excited state
6
pÆp Transition in Alkenes

• HOMO-LUMO energy gap is affected by substituents
  on double bond
• as HOMO-LUMO energy difference decreases
  (smaller DE), lmax shifts to longer wavelengths

7
Table 13.5 (p 525)

• Methyl groups on double bond cause lmax to shift
  to longer wavelengths

H
H
H
CH3
H
CH3
H
H
lmax 170 nm
lmax 188 nm
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Table 13.5 (p 525)

• Extending conjugation has a larger effect on
  lmax shift is again to longer wavelengths

H
H
H
H
H
H
H
H
H
H
lmax 170 nm
lmax 217 nm
9
Table 13.5 (p 525)
lmax 217 nm(conjugated diene)
lmax 263 nmconjugated triene plus two methyl
groups
10
Lycopene

• orange-red pigment in tomatoes

lmax 505 nm