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

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

From here… To here!

Qualitative:Conjugated Pi System

Quantitative: Concentration c

or

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:

N

OC8H17

O

O

O

O

Octyl 4-N,N-dimethylaminobenzoatePadimate O, max 310 nm

2-Ethylhexyl p-methoxycinnamatemax 310 nm

O OH

O

2-hydroxy-4-methoxybenzophenoneOxybenzone, max 288 and 325 nm

OH

O

O

Homomenthyl salicylatemosalate max 309 nm

Valence Bond Theory (VB) vs. Molecular Orbital Theory

(MO) Two H atoms VB MO

C CH H

H H

Ethylene

Benzene

Fig. 1-17, p. 21

σ-bond molecular orbitals

Fig. 14-1, p. 485

Π-bond molecular orbitals

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)

Copyright © Houghton Mifflin Company.All rights reserved. 12a–8

UV-Vis spectrum of tetraphenylcyclopentadienone

O

CC

CC

H

H

H

H

H

H

HOMOHighest Occupied Molecular Orbital

LUMOLowest Unoccupied Molecular Orbital

LUMO

HOMO

Π-molecular orbitals of 1,3-butadiene

UV-Vis Spectrophotometer

Fig. 14-12, p. 501

The electronic transition

Fig. 14-13, p. 501

Copyright © Houghton Mifflin Company.All rights reserved. 12a–13

CC

CC

H

H

C

H

H

H

C

H

H

H

hsix 2p atomic orbitals

groundstate

excitedstate

Antionding(4 nodes)

Bonding(1 node)

Bonding(2 nodes)

Antionding(5 nodes)

Antionding(6 nodes)

Bonding(3 nodes)

E

• 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?

AB

C D

Quinine

Fluorescence in Tonic Water

Quinine

Fluorescence in Tonic Water