1. Copyright © Houghton Mifflin Company.All rights reserved. 12a–1

2. From here…To here! Qualitative: Conjugated Pi System Quantitative: Concentration c

3. UV Protection - Sunscreen UV-A: 320 to 400 nm UV-B: 280 to 320 nm, cause tanning and sunburn. UV-C: 100 to 280 nm, most dangerous, but absorbed by ozone. Sunscreen active ingredients:

4. Valence Bond Theory (VB) vs. Molecular Orbital Theory (MO) Two H atoms VB MO Ethylene Benzene

5. Fig. 1-17, p. 21 σ-bond molecular orbitals

6. Fig. 14-1, p. 485 Π-bond molecular orbitals

7. 7 UV-Vis. Spectra (200 - 700 nm) × σ  σ * < 150 nm × n  σ * 150 - 250 nm √ n  π * 200 - 700 nm √ π  π * 200 - 700 nm UV-Vis. Spectra (200 - 700 nm)

8. Copyright © Houghton Mifflin Company.All rights reserved. 12a–8 UV-Vis spectrum of tetraphenylcyclopentadienone

9. HOMO Highest Occupied Molecular Orbital LUMO Lowest Unoccupied Molecular Orbital LUMO HOMO Π-molecular orbitals of 1,3-butadiene

10. UV-Vis Spectrophotometer

11. Fig. 14-12, p. 501 The electronic transition

12. Fig. 14-13, p. 501

13. Copyright © Houghton Mifflin Company.All rights reserved. 12a–13 six 2p atomic orbitals ground state excited state Antionding (4 nodes) Bonding (1 node) Bonding (2 nodes) Antionding (5 nodes) Antionding (6 nodes) Bonding (3 nodes) Conjugated p-orbitals alternating  - and  -bonds # AO’s in = # MO’s out UV absorption takes place b/t the HOMO and LUMO As # MO’s increases  E(HOMO-LUMO) decreases  => longer max

15. Copyright © Houghton Mifflin Company.All rights reserved. 12a–15 What compound would absorb the longest wavelength ultraviolet light? A B CD

16. Quinine Fluorescence in Tonic Water

17. Quinine Fluorescence in Tonic Water