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
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
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