UltravioletVisible UVVIS Spectroscopy 13'21

1
Ultraviolet-Visible (UV-VIS) Spectroscopy (13.21)

• If there are pi bonds in the molecule, are they
  conjugated?

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

?E h?
3
Conventions in UV-VIS

• X-axis -- wavelength in nm ( 200-800 nm)
• Y-axis - the molar absorptivity (?)
• from Beer-Lambert Law
• ?max --- wavelength of maximum absorption

4
UV Spectrum of cis,trans-1,3-cyclooctadiene
2000
Molarabsorptivity (?)
?max 230 nm
?max 2630
1000
200 220 240 260 280
Wavelength, nm
5
??? Transition in cis,trans-1,3-cyclooctadiene
???
???
LUMO
??
HOMO
??
Ground or normal state
Excited State
6
??? Transition in Alkenes

• HOMO-LUMO energy gap is affected by substituents
  on double bond
• Smaller HOMO-LUMO energy differences give
  longer wavelengths

7
Table 13.5 (p 566)

• Methyl groups on double bond cause ?max to shift
  to longer wavelengths

H
H
H
CH3
H
CH3
H
H
?max 170 nm
?max 188 nm
8
Table 13.5 (p 566)

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

H
H
H
H
H
H
H
H
H
H
?max 170 nm
?max 217 nm
9
Table 13.5 (p 566)
?max 217 nm(conjugated diene)
?max 263 nmconjugated triene plus two methyl
groups
10
Lycopene

• orange-red pigment in tomatoes

?max 505 nm