Principles of Spectroscopy
Wavelength
(nm)
10-4
10-1
10 – 400 (most
useful: 200-400)
400-700
Type
Visible
Gamma Rays
X-rays
Ultraviolet
Causes
changes to
Nucleus
Inner Electrons
Pi electrons
Visible
Valence e-
400
500
600
700
104
Infrared
109
1014
Microwaves
Radiowaves
Molecular
vibrations
Rotations
Nuclear Spin
Energy/Frequency
Violet
Blue
Green
Yellow
Orange
Red
En
er
g
y
D
e
cr
e
as
es
Fr
e
q
u
e
n
cy
D
e
cr
e
as
es
W
a
v
el
e
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gt
h
In
cr
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es
Only certain amounts of energy can be absorbed by molecules, atoms, electrons.
 Because of this we say energy is quantized. It is only transferred between species in certain sized packets.
 Because of this we can use absorption and emission of energy to identify species.
 The energy absorbed by a species is unique because of the unique energy levels for each
atom/element/molecule.
 Spectroscopy involves the absorption and emission of electromagnetic radiation. According to E=hv and c
= v, we can calculate the energy absorbed/emitted if we know the wavelength of the radiation.
We consider emission spectra or absorption spectra
o Absorption: gain in energy to higher energy state. We can determine wavelengths absorbed by looking at the
wavelengths that are missing (comparing continuous spectra to transmitted wavelengths)
o Emission: gained energy is given off as electron/molecule goes back to ground state. We can see which
energies are given off.
UV/VIS Spectroscopy
Ultraviolet Radiation
o Causes changes to higher energy states for the pi electrons in unsaturated molecules
o UV spectra are recorded by irradiating the substance with continuously varying UV light. When the
wavelength of light corresponds to the amount of energy required to promote a pi electron in an
unsaturated molecule to a higher level, energy is absorbed. This absorption is detected and plotted
(absorption vs. wavelength).
o UV is useful for detecting the presence of double bonds and conjugation
o Conjugation occurs when there are multiple double bonds in a molecule that are separated by only
one single bond.
o Conjugation occurs because of the location of pi-bonds. Recall that pi bonds
occur due to axially overplapping p-orbitals. When two pi bonds occur in the
same molecule, and close enough to each other, the pi-bonds can “bond” with
each other, making a network of pi-bonds which we call a system of
conjugation.
o The energy absorbed by a pi electron, decreases with increased conjugation.
CH2
H2C
H2C
conjugated
CH2
Not conjugated
CH2
H2C
o
o
o
Energy absorbed by pi-electron decreases with conjugation,
1,3-butadiene absorbs 217 nm, 1,3,5 hexatriene absorbes 258 nm (remember that as
wavelength increases, energy decreases)
This property can be used to distinguish between isomers because one the conjugated system
will absorb UV light at a higher wavelength (lower energy):
CH3
CH3
H2C
H2C
Conjugated
1,3-hexadiene
Un-conjugated
1,4-hexadiene
When the system of conjugation is extensive, as it is when many benzene rings are attached, the energy
absorbed is low enough to be in the visible region. Generally, a molecule with eight or more conjugated
double bonds will absorb light in the visible region (400 to 750). This explains the color of chlorophyll, retinol,
and indicators like phenolphthalein.
The greater the number of conjugated multiple bonds, the longer the wavelength at which the compound
absorbs light.
O
O
OPhenolpthalein: O
HIn, Clear, transmits all
colors
O
Phenolpthalein:
In-, Pink
**Notice that it turns pink when the system of
conjugation increases so it begins to absorb
some wavelengths of visible light and
reflect/transmit others, hence the pink color
FD & C Blue Food Coloring
Based on the demonstration in class, does conjugation increase or
decrease for this molecule in increasingly acidic solutions?
Molecules called anthocyanins are found in flower petals, leaves, blueberries,
blackberries, strawberries, cranberries, cherries, apples and grapes, and in red cabbage. Anthocyanins cause the
beautiful red/purple color that you see in roses and in the fall, in maple leaves. Anthocyanins are produced in leaves
once chlorophyll (which is green) degenerates in the fall. Nature produces over 300 structurally distinct anthocyanins.
By boiling red cabbage leaves, we can take out (extract) the anthocyanins. To get concentrated anthocyanins, cut a
head of red cabbage into small pieces (about bite-sized) and boil for about 10 minutes in as little water as necessary
to cover the cabbage. Slowly pour (decant) the anthocyanin solution into a jar and store it in the fridge. Anthocyanins
dissolved in water (in aqueous solution) change color depending upon the amount of H + and HO- in the water. (That is
the same thing as saying that the absorbance of light by anthocyanins depends on pH.) Flowers such as hydrangea
contain anthocyanins, and their color is sensitive to the acidity of the soil. The anthocyanins in red cabbage turn bright
pink in acid solutions, reddish-purple in neutral solutions and green in basic solutions. But anthocyanins from other
plants have other pH/color profiles. The effect of pH on blueberry anthocyanins is shown above.