INTRODUCTION TO SPECTROSCOPY

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INTRODUCTION TO SPECTROSCOPY
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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.
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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.
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THE ELECTROMAGNETIC SPECTRUM
Important As the wavelength gets shorter, the
energy of the radiation increases.
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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.
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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?
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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
Absorbs below 200 nm not seen in typical UV
spectra
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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.
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Wavelengths Absorbed by Functional Groups
What is the absorbance max?
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Example of a Method to Determine the Absorption
Spectra of an Organic Compound
Woodwards 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
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Other Concepts Important to UV/Vis Spectroscopy

• 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.

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INFRARED SPECTROSCOPY
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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
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Many of these bands can be assigned to the
vibration of particular chemical groups in the
molecule
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Absorption Frequencies of Functional Groups
See Appendix B (2 tables) and Table 2
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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 1710
cm-1 strong carbonyl absorbance 1610 cm-1 no
absorbance to suggest carbon-carbon double bonds
Structure IUPAC Name 3-pentanone
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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 2200 cm-1 no unsymmetrical triple
bonds 1690 cm-1 strong carbonyl
absorbance 1610 cm-1 weak absorbance bands
consistent with carbon-carbon double bonds
Structure IUPAC Name acetophenone
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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
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C7H6O
Structure IUPAC Name benzaldehyde
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 2750-2600 cm-1 moderate peaks
strongly suggesting aldehydic CH 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
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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
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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 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
Structure IUPAC Name ethyl acetate
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C4H8O2
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NMR SPECTROSCOPY
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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
gt100,000 J/mol
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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.
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Relative Sensitivity of NMR Techniques
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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.
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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.
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Benzene, C6H6 has only one sort of hydrogen atom,
so that the NMR spectrum shows a single peak
(the TMS peak is omitted)
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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).
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Ethanol CH3CH2OH has methyl hydrogen, methylene
hydrogen, and hydroxyl hydrogen. It therefore has
three peaks in its spectrum
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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.