Instructions

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Instructions

• Run as a slide show

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UV Infrared and Raman Spectroscopy

• Derek J Gardiner

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Molecular energy

• Electronic
• Vibrational
• Rotational
• All molecular energies are quantised and give
  rise to discrete energy levels in the molecule

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Molecular orbital bonding theory
Electronic energy
Atomic orbitals combined to form molecular
orbitals Allowed orbital combinations are
determined by symmetry, spatial distribution and
energy. Only valance orbitals are involved in
bonding.
Example O2
?
?
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Example H2O
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Example CH4
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Example C6H6
energy
LUMO
HOMO
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Possible electron transitions in organic compounds

• ? to ? Large ?E. Generally beyond the UV
  visible range, 150 nm
• n to ? Large ?E (less than above) but rarely
  seen in UV
• n to ? and ? to ? ?E in the 200-700 nm
  range
• CT (charge transfer) strong molar
  absorbtivities, often near UV and may result in
  strong colours
• Allowed transitions are excited by incident
  light with energy equal to the energy gap between
  ground state and excited state. Measurement of
  the light energies absorbed from a continuous
  source of light provides a UV visible spectrum.

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Molecular vibrations
A molecule with N atoms will have 3N-6 (or 3N-5
linear) normal vibrations. Example H2O
N3, 3 vibrations
?3
?2
?1
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Molecular vibrations

• A molecule will have 3N-6(5) sets of vibrational
  energy levels each characterised by a vibrational
  quantum number vi

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Absorption of infrared radiation
If ?rad ?vib IR radiation will be absorbed
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IR selection rule
There must be a change in dipole moment
(?) during a vibration (qv) if it is to absorbe
IR radiation and thus be IR active.
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Vibrational transitions

• Erad h?rad

Evib h?vib
To excite a vibrational transition
h?rad h?vib
The radiation frequency must equal the vibration
frequency to excite a transition.
Vibrational frequencies are 1013 Hz The
infrared region of the spectrum.
If we measure the infrared frequencies (normally
expressed in cm-1) absorbed by a molecule then we
will measure its vibrational frequencies
and obtain its Infrared spectrum.
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Molecular energy levels
e4
e3
energy
e2
e1
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Spectroscopy

• Spectroscopy measurement of wavelength
  (frequency) and Intensity of light
• Light (electromagnetic radiation) radiates from a
  source with a speed of 3x108 m s-1
• Light travels as a waveform but may behave as
  quantised energy packets (photons) depending upon
  detection method
• Energy Planks constant x frequency E h?

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Wavelength ?
Frequency ? number of wavelengths propagating
per second
Speed C ? ? 1/? ?/C
1/? ? ?
When ? is in cm, 1/? wavenumber ? (cm-1)
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Interference
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Diffraction grating
2
1
N
N
?
?
?
?
a
For constructive interference between rays 1 and
2
K order of diffraction (normally 1 is used)
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Spectrometer principle
slit
detector
grating
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UV visible spectrometer
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Absorbance (A) and Molar Absorbtivity (?)
A log I0/ I
? A / c I
c sample concentration in moles/liter l
length of light path through the sample in cm.)
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Absorbance (A) and Molar Absorbtivity (?)
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Measurement of infrared spectra

• Dispersion instrument spectrometer similar to
  UV visible but using an IR source, IR detector.
  Chopper alternates IR through reference and
  sample paths, an attenuator is adjusted to
  balance the beams and its movement is a measure
  of absorbance.
• FTIR instrument (Fourier Transform IR)

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Dispersive IR spectrometer
attenuator
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Simplified FTIR instrument
sample
detector
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Interference due to path difference between fixed
and moving mirrors
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Individual wavelengths
Mirror
position (time)
intensity
Summed
wavelengths interferogram
Path difference 0
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Fourier transform (FT)
FT will transform time dependent data to
frequency dependent data
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Apodization function

• In theory an interferogram spans - ? to ?
• A limiting (apodization function) is used to deal
  with this problem which in turn imparts shape to
  the spectral bands and removes sidebands which
  arise if a simple boxcar truncation function is
  used .

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sample
detector
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FT
time
Continuous (all wavelengths) IR interferogram
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Background subtraction
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Characteristic IR Band Positions and Group
Frequency Ranges (cm-1)
OH stretching vibrations Free OH 3610-3645
(sharp) Intramolecular H bonds 3450-3600
(sharp) Intermolecular H Bonds 3200-3550
(broad) Chelate Compounds 2500-3200 (very
broad) NH Stretching vibrations Free NH
3300-3500 H bonded NH 3070-3350 CH
Stretching vibrations -C-H 3280-3340
C-H 3000-3100 C-CH3 2862-2882, 2652-2972
O-CH3 2815-2832 N-CH3 (aromatic)
2810-2820 N-CH3 (aliphatic) 2780-2805
CH2 2843-2863,2916-2936 CH 2880-2900
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SH Stretching Vibrations Free SH 2550-2600
C-N Stretching Vibrations Nonconjugated
2240-2260 Conjugated 2215-2240 C-C
Stretching Vibrations C-CH (terminal)
2100-2140 C-C-C-C 2190-2260
C-C-C-C-CH 2040-2200 CO Stretching
Vibrations Nonconjugated 1700-1900
Conjugated 1590-1750 Amides 1650 CC
Sretching Vibrations Nonconjugated
1620-1680 Conjugated 1585-1625
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CH Bending Vibrations CH2 1405-1465
CH3 1355-1395, 1430-1470 C-O-C Vibrations in
Esters Formates 1175 Acetates 1240,
1010-1040 Benzoates 1275 C-OH Stretching
Vibrations Secondary Cyclic Alcohols
990-1060 CH out-of-plane bending vibrations
in substituted ethylenic systems -CHCH2
905-915, 985-995 -CHCH-(cis) 650-750
-CHCH-(trans) 960-970 CCH2 885-895
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Mercury cadmium telluride detector
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Raman effect
Classical Theory
When a molecule is placed in an electric field,
its electrons are displaced relative to the
nuclei thus developing an electric dipole moment.
For small fields, the induced moment (?i)
is proportional to the field strength (F)
(1)
?i ?F
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? polarisability (the ease with which the
electron cloud of a molecule can be distorted)
A fluctuating field will produce a fluctuating ?i
of the same frequency.
Electromagnetic radiation will induce a
fluctuating dipole (?i) in the molecule of
frequency ?0. This in turn will emit radiation of
frequency ?0.
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RAYLEIGH SCATTERING
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Consider a diatomic molecule vibrating with
a frequency ?v. If it performs simple
harmonic vibrations, then a coordinate along the
axis of vibration (bond length) at time t is
given by qv q0Cos2??vt
(3)
If the polarisability changes during the
vibration, its value for a small vibrational
amplitude will be
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qv q0Cos2??vt (3)
(4)
Substitute (3) into (4)
Consider incident radiation of frequency ?0 from
(1) and (2) ?i ?F ?F0Cos2??0t (6)
Substitute (5) into (6)
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Cos A Cos B ½(Cos (AB) Cos (A-B))
Therefore
The induced dipole moment will scatter light at
different frequencies
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Raman selection rule
There must be a change in polarisability
(?) during a vibration if it is to scatter Raman
shifted radiation and thus be Raman active.
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Raman effect
Quantum mechanical theory
The quantum mechanical theory of Raman
scattering takes account of the fact that the
energies of molecular vibrations are quantised.
Perturbation theory is used to generate the
new vibrational wave function which exists after
the scattering event.
Perturbation theory adds parts of the existing
electronic wave functions of the molecule to the
initial vibrational wave function until the new
vibrational wave function is obtained.
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The perturbing wave function can be considered
as giving rise to an enabling energy level -
Virtual Level
Rayleigh scatter
Stokes Raman scatter
Anti-Stokes Raman scatter
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Raman shift
By convention the exciting frequency (?0) is
regarded as zero and a shift to lower frequency
is assigned a positive value. RAMAN SHIFT
Stokes Raman shifts (0 - ?v) are thus positive
Anti-Stokes Raman shifts (0 ?v) are thus
negative
Raman shift Vibrational frequency (cm-1)
Raman shift corresponds to frequencies in the
infrared spectrum.
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Raman spectrum of carbon tetrachloride
Rayleigh
Stokes
Anti-Stokes
X 100
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Raman spectrum of carbon tetrachloride
intensity
0 1000
Raman shift cm-1
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Why is anti-Stokes scatter less intense than
Stokes scatter ?
The anti-Stokes transition starts from the less
populated v1 state
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Raman effect summary
Sample vibrations ?v
Incident laser light visible region h?o
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The fluorescence problem

• If the laser wavelength falls within the
  fluorescence absorption band of the sample or an
  impurity then a strong fluorescence emission
  background will result which may obscure the
  relatively weaker Raman spectrum.

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Spectrometer design

• Raman scatter is lt10-5 intensity of Rayleigh
  scatter.
• Need a spectrometer which will separate the
  relatively small number of Raman photons from the
  large number of Rayleigh and incident photons.
• Spectrometer needs to be able to filter out the
  Rayleigh and incident light and to spectrally
  disperse the Raman light to obtain the spectrum.
• A sensitive low noise photon counting detector is
  essential.
• High power monochromatic light source

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Conventional 90 Raman Spectrometry
double monochromator
Lens collects scattered light and focuses it onto
the entrance slit 1
The light is collimated by mirror 1 and
spectrally dispersed by grating 1
Mirror 2 focuses the dispersed light at slit 2
which removes the Rayleigh scattered light.
The second half of the double monochromator
further disperses the light and brings it to a
focus at the exit slit 4.
The gratings rotate to scan the dispersed light
onto the detector. The output (intensity) is
plotted against Raman shift (grating rotation) to
produce the Raman spectrum.
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Infrared and Raman mutual exclusion principal
For molecules with a centre of symmetry,
vibrations which are active in the IR spectrum
will be inactive in the Raman spectrum and vice
versa.
Example CO2
Raman active
Infrared active
?2
?1
?OCO?
Raman spectrum 1 band IR spectrum 2 bands
?3
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cyclohexane C6H12
IR
Raman
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ethanoic acid CH3-(CO)-OH
IR
Raman
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2-butanone CH3CH2-(CO)-CH3
IR
Raman
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phenol C6H5OH
IR
Raman
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methylethanoate CH3-(CO)-OCH3
IR
Raman
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cyanoethene CH2CH-C?N
IR
Raman
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cyanopropane CH3CH2CH2-C?N
IR
Raman
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Application Examples

• Aqueous solutions
• High and low temperatures
• Liquids at high pressures
• Solid state properties

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Aqueous KNO3
Aqueous Ca(NO3)2
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Aqueous KNO3
Aqueous Ca(NO3)2
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High temperatures (liquids lt250)
Heating elements
sample
Metal block
laser
Raman scatter
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Low temperatures gt77K
coolant
Copper block
vacuum
sample
Quartz windows
Raman scatter
laser
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Low temperature cryostat cell
Liquid N2 77K Liquid H2 20K Liquid He 4K
Raman scatter
Sample inlet
laser
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Vacuum furnace to study molten salts lt1500C
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Chlorocyclohexane C6H11Cl
C
l
H
H
C
l
equatorial
axial
110 C
22 C
-40 C
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High pressure Raman cell for liquids lt7kbar
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Phtosensitive samples

• Materials which absorbe the laser light will
  often be raised in temperature and may decompose.
• To counteract this
• Provide a heat sink (good conductor, under water)
• Minimise laser exposure.

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Rotating cell for light sensitive solutions
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Hydraulic press
powder
Spinning cell for light sensitive solids
Compacted crystal powder
Raman scatter
Focussed laser beam
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Raman microscope
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Raman microscope
2 view the magnified sample and position the
focused laser spot
spectrometer
monitor
TV camera
prism
ND filter
beam splitter
monitor
laser
beam splitter
Microscope objective lens
White light
sample
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Raman microscope
3 record the Raman spectrum
spectrometer
monitor
TV camera
prism
monitor
laser
Microscope objective lens
sample
A Raman microscope can record spectra from
samples as small as 0.5?m
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Hot stage (for use with Raman microscope to study
solids)
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Hot stage
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Hot stage
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Temperature / electrical power plot for
polysilicon microheater
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Diamond anvil cell
Pressures up to 400 kbar easily achieved
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Infrared and Raman comparisons
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Infrared and Raman comparisons
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useful websites

• http//www.chemistry.oregonstate.edu/courses/ch361
  -464/ch362/irinstrs.htm
• http//www.forumsci.co.il/HPLC/FTIR_page.html
• http//mmrc.caltech.edu/mmrc_html/FTIR/FTIRintro.p
  df
• http//www.entek.chalmers.se/leam/ftir/ftir_pdf/o
  wn_pub/theory_ftir.pdf
• http//infrared.als.lbl.gov/FTIRinfo.html
• http//www.cem.msu.edu/reusch/VirtualText/Spectrp
  y/UV-Vis/spectrum.htm