INTRODUCTION TO
SPECTROSCOPY
Spectroscopy
Spectroscopy is a general term referring to the
interactions of various types of electromagnetic
radiation with matter.
Exactly how the radiation interacts with matter is
directly dependent on the energy of the radiation.
Spectroscopy
The higher energy ultraviolet and visible
wavelengths affect the energy levels of the outer
electrons.
Infrared radiation is absorbed by matter resulting
in rotation and/or vibration of molecules.
Radio waves are used in nuclear magnetic
Resonance and affect the spin of nuclei in a
magnetic field.
THE ELECTROMAGNETIC SPECTRUM
Important: As the wavelength gets shorter,
the energy of the radiation increases.
PARTICLE NATURE
OF
RADIATION
Electromagnetic radiation is also described as
having the properties of particles.
Molecules exist in a certain number of possible
states corresponding to definite amounts of
energy.
Molecules can absorb energy and change to a
higher energy level called the excited state.
The amount of energy absorbed in this transition
is exactly equal to the energy difference
between the states.
Energy Level Diagram for an Atom of Sodium
E0
Ground State
330 nm
E1
590 nm
E2
Energy Level Diagram for a Simple Molecule
E2
Relaxation from the E2
energy state to E0 may
go to different vibrational
energy states, emitting
different wavelengths.
E1
Excitation to the next
electronic energy
level caused by
adsorption of
specific wavelengths
E0
Ground State
e4
e3
e2
e1
Vibrational Energy
Levels
UV/VIS SPECTROSCOPY
Visible
(380-780 nanometers)
Ultraviolet (UV) (10 – 380 nanometers).
Below about 200 nm, air absorbs the UV light
and instruments must be operated under a vacuum
How many µm is 780 nanometers?
What is the corresponding wave number?
Absorption of ultraviolet and visible light only
takes place in molecules with valence electrons
of low excitation energy.
bonding p antibonding p transitions
have high molar absorbtivities
Energy
Antibonding s
Antibonding p
e
Non-bonding
e
Bonding p
e
e
Bonding s
Absorbs below 200 nm
not seen in typical UV spectra
Wavelengths Absorbed by Functional
Groups
Again, demonstrates the moieties contributing
to absorbance from 200-800 nm, because pi
electron functions and atoms having no
bonding valence shell electron pairs.
Wavelengths Absorbed by Functional
Groups
What is the
absorbance
max?
Example of a Method to Determine the
Absorption Spectra of an Organic
Compound
Woodward’s Rules For Conjugated Carbonyl
Compounds
Aldehyde:
208 nm
Extended conjugation:
30 nm
Homodiene component:
39 nm
a-Alkyl groups or ring residues: 10 nm
d-Alkyl groups or ring residues: 18 nm
Calculated:
304 nm
OTHER CONCEPTS IMPORTANT
UV/VIS SPECTROSCOPY
TO
UV/Vis spectra can be used to some extent for
compound identification, however, many compounds
have similar spectra.
 Solvents can cause a shift in the absorbed
wavelengths. Therefore, the same solvent must be
used when comparing absorbance spectra for
identification purposes.
 Many inorganic species also absorb energy in the
UV/Vis region of the spectrum.

INFRARED SPECTROSCOPY
Absorption of electromagnetic energy in the
infrared region causes changes in the vibrational
energy of molecules
Energy changes are typically 6000 to 42,000
J/mol which corresponds to wavelengths of
2.5-40 mm (250-4000/cm)
http://sis.bris.ac.uk/~sd9319/spec/IR.htm
Many of these bands can be assigned to the
vibration of particular chemical groups in the
molecule
Absorption Frequencies of Functional
Groups
See Appendix B (2 tables) and Table 2
1
C5H10O
3400-3200 cm-1:
no OH or NH present
3100 cm-1: no peak to suggest unsaturated CH
2900 cm-1: strong peak indicating saturated CH
2200 cm-1: no unsymmetrical triple bonds
Structure:
IUPAC Name: 3-pentanone
1710 cm-1: strong carbonyl absorbance
1610 cm-1: no absorbance to suggest carbon-carbon double bonds
2
C8H8O
3400-3200 cm-1:
no OH or NH present
3100 cm-1: moderate peak suggesting unsaturated CH
2900 cm-1: weak peak indicating possible saturated CH
Structure:
2200 cm-1: no unsymmetrical triple bonds
IUPAC Name: acetophenone
1690 cm-1: strong carbonyl absorbance
1610 cm-1: weak absorbance bands consistent with carbon-carbon double bonds
3
C7H8O
3400-3200 cm-1: strong peak indicating OH is present
3100 cm-1: weak peak suggesting possible unsaturated CH
2900 cm-1: weak peak indicating possible saturated
CH 2200 cm-1: no unsymmetrical triple bonds
1720 cm-1: no carbonyl absorbance
1450-1500 cm-1: moderate absorbance bands
consistent with aromatic carbon-carbon double bonds
Structure:
IUPAC Name: benzyl alcohol
4
C7H6O
3400-3200 cm-1:
no peak which would indicate OH or NH
3100 cm-1: moderate peak indicating unsaturated CH
2900 cm-1: no peaks to indicate saturated CH
Structure:
2750-2600 cm-1 ; moderate peaks strongly suggesting aldehydic CH
IUPAC Name: benzaldehyde
2250 cm-1:
no absorbance indicating an unsymmetrical triple bonds
1700 cm-1:
strong carbonyl absorbance
1450-1600 cm-1 :moderate absorbance bands consistent with aromatic
carbon-carbon double bonds
5
C3H10NO
3400-3200 cm-1: strong peak suggesting OH or NH
3100 cm-1: minor peak indicating possible unsaturated CH
2900 cm-1: minor peaks indicating saturated CH
2200 cm-1: no unsymmetrical triple bonds
1650 cm-1: strong carbonyl absorbance
1550 cm-1: moderate absorbance band, characteristic of
‘N-H bending’
Structure:
IUPAC Name: N-methylacetamide
6
C4H8O2
3400-3200 cm-1:
no peak to indicate an OH or NH
3100 cm-1: no peak to indicate unsaturated CH
2900 cm-1: minor peaks indicating saturated CH
2200 cm-1: no unsymmetrical triple bonds
1760
cm-1:
strong carbonyl absorbance
Structure:
IUPAC Name: ethyl acetate
1600 cm-1: no peak to indicate a carbon-carbon double bond
1250 cm-1: strong, broad peak consistent with a carbon-oxygen
single bond
C 4 H 8O 2
NMR SPECTROSCOPY
NMR SPECTROSCOPY
Nuclear magnetic resonance spectrometry (NMR)
is based on the absorption of electromagnetic
radiation in the radio-frequency region of the
spectrum resulting in changes in the orientation
of spinning nuclei in a magnetic field
NMR Energies
0.1 J/mol
IR Energies
6000 to 42,000 J/mol
UV/Vis Energies >100,000 J/mol
As the nucleus spins it produces a magnetic
moment or dipole along the axis. The relative
values of the magnetic moment and the angular
momentum determine the frequency at which
energy can be absorbed.
Relative Sensitivity of NMR
Techniques
Table 4. Nuclear Spin Quantum Numbers and Magnetic Properties of Nuclei
Nucleus
Nuclear spin
Quantum
Number
Hydrogen
Deuterium
Carbon 12
Carbon 13
Fluorine 19
1/2
1
0
1/2
1/2
Magnetic
Moment,
(ampere
square meter
x 1027
14.09
4.34
-3.52
13.28
Resonance
Frequency in
MHz at
1.4092
TESLA
60.000
9.211
-15.085
56.446
Relative
Sensitivity at
the Natural
Isotopic
Abundance
1.00
0.00015
-0.00018
0.834
Proton Magnetic Resonance
In PMR the instrument is detecting the energy
difference between protons with a spin of +1/2
(low energy) and -1/2 higher energy.
The application of electromagnetic radiation can
excite the nuclei into the higher energy level. The
frequency that causes the excitation is
determined by the difference in energy between
the energy levels.
Chemical Shift
The NMR spectrum arises because nuclei in
different parts of the molecule experience
different local magnetic fields according to the
molecular structure, and so have different
frequencies at which they absorb. This difference
is called the chemical shift.
This is because the nucleus is shielded from this
field to a greater or lesser extent by the other
atoms in the vicinity and their electrons.
Benzene, C6H6 has only one sort of hydrogen atom,
so that the NMR spectrum shows a single peak
(the TMS peak is omitted):
Ethanal CH3CHO has two sorts of hydrogen atom,
those on the methyl group and the one on the
aldehyde group. It therefore has two peaks in its
spectrum (the TMS peak is omitted).
Ethanol CH3CH2OH has methyl hydrogen,
methylene hydrogen, and hydroxyl hydrogen. It
therefore has three peaks in its spectrum
Spin-spin coupling
In ethanol, the hydrogen atoms on the methyl group interact with those on
the methylene group – their magnetic fields couple. The effect of coupling
on the spectrum is that the lines are split into multiplets. Most coupling
occurs between hydrogen atoms on adjacent carbon atoms, so in the
ethanol spectrum there is splitting of the lines due to the methyl and
methylene hydrogen atoms, but not that of the hydroxyl hydrogen – it is too
far away.