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Infrared Spectroscopy
Chapters 16-17
Chapter 7-I
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IR Selection Rules
• Change in dipole moment during vibration
• Δv = ±1 (for harmonic oscillator)
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http://www.lsbu.ac.uk/water/vibrat.html
Water Vibrations
http://chemmac1.usc.edu/Bruno/java/Vibrate.html
Carbon Dioxide Vibrations
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FT-IR
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Retardation – difference in path length of two beams
1 22 d d
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Interferograms
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Infrared Sources
• Glowbar• Nernst glower
• Patented sources
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Detectors
• DLaTGS (pyroelectric)• MCT (semiconductor/quantum well)
• Thermodetectors (not used for FT-IR, response time is too slow)– Thermocouple– Bolometer– Thermister
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FT Method Advantages1. Fellget Advantage (Multiplex Adv)
– Collect all at once– Faster, don’t need to scan axis
2. Jaquinot Advantage (Throughput Adv)– No slits or optical elements to limit (attenuate)
radiation
3. Connes Advantage– Internally calibrated w/ HeNe laser– Wavenumber calibration is:
• More accurate• Better stability
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Black Body Radiation• Why don’t we use a light bulb as IR source? According to Black Body
Radiation, it should give more intensity in IR region of spectrum.
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ATR / Evanescent wave
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Sat
urat
ed v
s. U
nsat
urat
ed
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Sat
urat
ed v
s. U
nsat
urat
ed
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Alc
ohol
s
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Ket
ones
and
Ald
ehyd
es
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Car
boxy
lic A
cids
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Eth
ers
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Est
ers
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Unknowns
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Unknowns
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Unknowns
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Unknowns
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Unknowns
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Unknowns
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Raman Spectroscopy
Chapters 18
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Elastic scatteringPhoton scattered at same as incident photon
Inelastic scatteringPhoton scattered at different as incident photon
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• e- cloud of molecule can be polarized by E.M. radiation
• It is possible to induce dipole moment using elect field of E.M.
• Size of induced dipole depends on polarizability of e- cloud
m E
Induced dipole moment polarizability
Oscillating electric field
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Raman Selection Rule
• with vibration
• Polarizability changes with bond length/vibration
0 eqr rr
Change in bond length
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Gaussian Calculations
• CO2
• SF6
• Chloroform
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1 1
0 0
1 1
0 0
# of states at E
# of states at E
N g Eexp
N g kT
g
g
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-1 11 m .694 cm
100 cm
hc hcE hc
hc
kkT K
hc
1 1
0 0
N g Eexp
N g kT
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kTImportant concept in Thermo and Stat Mech
23
21
-1
k 1.38 10 J/K
if T=298K kT=4.1 10 J
=2470 J/mol
=2.5 kJ/mol
=200 cm
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Vibrational Selection Rules
Raman Infrared
1
0
v
r
1
0
v
r
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90° Scattering Instrumentation
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Advantages and Disadvantages of Raman
Advantages
• Water is weak scatterer
• Little sample prep
• Sharper peaks than IR
• Use of fiber optics
Disadvantages
• Insensitive (LOD ~ 0.1%)
• Fluorescence interference– Trade off of / Int / fluor.
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Resonance Raman
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Surface Plasmons – electron oscillations at surface of a metal.
Surface Plasmon Resonance – when freq of light photons matches the natural freq of oscillating surface electrons.
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FT-Raman
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Fiber-Optic Raman SpectrometerBP=band pass filterBR=band rejection filter
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Imaging Spectrograph
HoloPlex
TE CooledCCD
Detector
Slit Notch
ControlElectronics
ProbeHead
Invictus NIR Laser
Kaiser Optical Systems Inc Rxn1 Raman system operating at 785 nm
Instrumentation
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Improving Raman Collection Efficiency
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Crystal Hydrate Structure
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Calibration
1500 1550 1600 1650 1700 1750 1800
Raman shift [cm-1]
AT
MT
- RAT
- RMT
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Phase Transformation upon Water Addition
1838.7 1700 1600 1500 1400 1300 1200 1100 977.4
Raman Shift / cm-1
Int
immediately
3 min
6 min
9 min
12 min
15 min
18 min
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Apply Calibration to get Kinetic Transformation Profile
0
20
40
60
80
100
0 2 4 6 8 10 12
Time (min)
% Hydrate
Typical Transformation
No Tranformation