IV. ULTRAVIOLET SPECTROSCOPY
 Spectroscopy review:
 IR
 Molecular vibrations
 Information about bonding (functional groups)
 NMR
 Nuclear spin transitions
 Electronic/molecular environment of nuclei (C-H framework)
 MS
 Bombard with electrons to fragment
 Masses of molecular ion and fragments
UV SPECTROSCOPY
 UV Spectroscopy
 Electronic transitions of conjugated systems
 Length and structure of conjugated part of molecule
 Less common than other spectroscopy
 More specialized information
 UV region = most commonly 200-400 nm
 Visible = 400-800 nm
 UVA = 400-315 nm
 UVB = 315-280 nm
 UVC = below 280 nm
 Wavelength absorbed by molecule determined by
energy differences in p orbitals
UV SPECTROSCOPY
 p electron excited
from HOMO into
higher energy
LUMO
 p → p* transition
 Longer
wavelengths from
lower E
absorption
UV SPECTRUM
 Dissolve sample in solvent
 Put in quartz cell
 Reference cell (solvent only)
 Spectrometer compares intensity of light transmitted
through sample cell to light through reference cell

𝐼𝑟
A = log
= 𝜖bc
𝐼𝑠
(Beer ’s Law)
 e = molar absorptivity (measure of strength of absorption)
 b = pathlength of cell (usually ~1 cm)
 c = concentration in mol/L
UV SPECTRUM
 1,3-Butadiene:
UV SPECTROSCOPY
Molecule
Absorption Wavelength (l)
171 nm
217 nm
258 nm
454 nm
b-CAROTENE
 Absorbs at 454 nm
 Blue visible light
 Appears orange (carrots)
 White light – blue = orange
UV SPECTROSCOPY
 Correlation between structure and wavelength:
UV SPECTROSCOPY
 Predicting maximum wavelength:
 Woodward-Fieser rules
 Very specific and lots of them!
 Some generalizations:
 Add ~40 nm for each C=C
 Add 5 nm for each extra R group on C=C
 Examples:
PREDICT l max FOR THESE MOLECULES
VITAMIN D
 Two related compounds
 Cholecalciferol (D 3 ) and ergocalciferol (D 2 )
 Steroid derivatives; differ in side chain structure
 Function = increase absorption of Ca
 Deficiency
 Poor bone growth
 Rickets in children
 Osteoporosis in adults
 Vitamins not found in food; p recursors are
 D 3 precursor: found in dairy and fish
 D 2 precursor: found in some vegetables
 Conversion to D 3 and D 2 initiated by UVB sunlight (l = 295-300 nm)
VITAMIN D
*
 First step: electrocyclic ring opening
 Conrotatory or disrotatory?
 How should H be drawn on C marked with *?
 Second step: spontaneous thermal sigmatropic shift
 Where did H go?
 What type of shift?
 Products further oxidized to active forms in kidney and liver