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Spectroscopy
 Atomic emission spectra
 UV/Vis spectra
 Infrared (IR)
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Spectroscopy
 Atomic emission spectrum. Recall.
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Spectroscopy
 An atomic light source may be
obtained by burning an element over
a flame (flame test). When this light
hits a prism, the different
wavelengths of light are refracted at
different angles.
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Spectroscopy
 Atomic emission spectrum. Recall.
 c =   and E = h 
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Spectroscopy
Absorption and Emission spectra
Evidences to support the atomic model.
1. All atoms have a unique emission/absorption spectrum. This
allows the identification of elements.
2. The spectra of the pure element is ALWAYS the same
regardless of where is the spectrum obtained. This suggest that
the separation of the energy levels for each atom is different but it
is constant for that element.
3. Not all lines appear equally separated. It suggests that not all
energy levels are equally separated.
4. The transition n=7 to n=1 releases more energy than the
transition n=5 to n=1. Therefore the lines associated with these
transitions will have different frequencies and wavelengths.
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Energy
Spectroscopy
n=7
n=6
n=5
n=4
n=3
IR
n=2
Vis
n=1
UV
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Ultraviolet/Visible or UV/Vis Spectroscopy
It examines transitions in electronic energy levels.
It is used in Lambert Beer’s law.
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a 1.8
b 1.6
s 1.4
o 1.2
r 1
b 0.8
a 0.6
n 0.4
c
0.2
e
0
200
300
400
500
600
700
wavelength (nm)
800
The peaks
correspond with
energy transitions
between MOs.
Peaks at about 480
nm and 600 nm
allows to calculate
the energy
transitions using
c =   and E = h 
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Ultraviolet/Visible or UV/Vis Spectroscopy
1.8
a
b 1.6
s 1.4
o
r 1.2
b 1
a
n 0.8
c 0.6
e
0.4
0.2
0
100
150
200
250
300
wavelength (nm)
This compound
absorbs in the UV
section and it is
colorless.
Any colored
molecule can be
used in UV/Vis
spectroscopy
experiments, as
they will absorb light
from the visual
spectrum.
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Ultraviolet/Visible or UV/Vis Spectroscopy
Recall Lambert Beer’s law
A=abc
where:
A is absorbance (amount of light absorbed by the
sample)
a: molar absorptivity (M-1 cm-1). Constant and specific
to the atom, ion or molecule and to the wavelength.
b: path length of sample (cm). Length from front to
back of the cuvette that is used. Larger cuvettes
absorb more as the light will encounter more particles.
c: concentration (M). When concentration increases,
absorbance increases as the light will encounter more
particles.
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IR Spectroscopy
 All covalent bonds in molecules experience
vibrations within the bond. See animation
 This vibrational frequency falls in the IR section of
the spectrum.
 Vibrational frequencies depend on the mass of the
atoms and the strength of the bonds.
 Recall frequency is related to wavelength. c =  
 Wavenumbers are the reciprocal of the wavelengths
measured in cm-1.
 IR spectra is often plotted with wavenumbers along
the top and wavelengths along the bottom, both in
the x axis.
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IR Spectroscopy
Bond type
Range of wavelengths
(m)
Range of
wavenumbers (cm-1)
-C-H
3.38 – 3.51
2960 - 2850
=C-H
3.23 – 3.33
3100 - 3000
C=C
5.95 – 6.17
1680 - 1620
O-H
2.74 – 4.00
3650 - 2500
N-H
2.94 – 3.13
3400 - 3200
C–O
7.69 – 10.00
1300 - 1000
C=O
5.56 – 6.13
1800 - 1630
Molecules that have the same functional groups
such as alcohols (O-H) or carboxylic acids (-COOH)
can be identified through IR since they will have
peaks within the same range.
Every compound have a unique IR spectrum that
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allows its identification.
IR Spectroscopy
O
║
Acetone CH3 – C – CH3
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IR Spectroscopy
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Visit the webcast below for a
better understanding of this
important technique.
 http://media.collegeboard.com/dig
italServices/swf/apwebcasts/chemistry/ap_chem_pe
s.html

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