1. 13.20 Ultraviolet-Visible (UV-VIS) Spectroscopy
Gives information about conjugated p electron systems 1

2. Transitions between electron energy states
gaps between electron energy levels are greater than those between vibrational levels
gap corresponds to wavelengths between 200 and 800 nm
DE = hn 2

3. X-axis is wavelength in nm (high energy at left, low energy at right)
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) 3

4. UV Spectrum of cis,trans-1,3-cyclooctadiene
2000
Molar absorptivity (e)
lmax 230 nm
emax 2630
1000
Wavelength, nm 4

5. pÆp* Transition in cis,trans-1,3-cyclooctadiene
LUMO
y3*
DE = hn y2
HOMO y1
Most stable p-electron configuration
p-Electron configuration of excited state 5

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 5

7. Methyl groups on double bond cause lmax to shift to longer wavelengths
Table (p 525)
Methyl groups on double bond cause lmax to shift to longer wavelengths H H H
CH3 C C H
CH3 H H
lmax 170 nm
lmax 188 nm 5

8. Table (p 525)
Extending conjugation has a larger effect on lmax; shift is again to longer wavelengths H H H H C C H C H H H H H
lmax 170 nm
lmax 217 nm 5

9. lmax 217 nm (conjugated diene)
Table (p 525) H C
lmax 217 nm (conjugated diene) C H
CH3
H3C
lmax 263 nm conjugated triene plus two methyl groups 5

10. orange-red pigment in tomatoes
Lycopene
orange-red pigment in tomatoes
lmax 505 nm 5