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

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

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:

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

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

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

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

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

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

UV-Vis Spectrophotometer

Fig , p. 501 The electronic transition

Fig , p. 501

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

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

Quinine Fluorescence in Tonic Water

Quinine Fluorescence in Tonic Water