Copyright Houghton Mifflin Company All rights reserved 12

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

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

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

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

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

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

UV-Vis Spectrophotometer

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

Fig. 14 -13, p. 501

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

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

Fluorescence in Tonic Water Quinine

Fluorescence in Tonic Water Quinine