Infrared Spectroscopy—Mid-infrared, Near-infrared, and Far ...
Infrared Spectroscopy Chapters 16-17 Chapter 7- I.
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Transcript of Infrared Spectroscopy Chapters 16-17 Chapter 7- I.
Infrared Spectroscopy
Chapters 16-17
Chapter 7-I
IR Selection Rules
• Change in dipole moment during vibration
• Δv = ±1 (for harmonic oscillator)
http://www.lsbu.ac.uk/water/vibrat.html
Water Vibrations
http://chemmac1.usc.edu/Bruno/java/Vibrate.html
Carbon Dioxide Vibrations
FT-IR
Retardation – difference in path length of two beams
1 22 d d
Interferograms
Infrared Sources
• Glowbar• Nernst glower
• Patented sources
Detectors
• DLaTGS (pyroelectric)• MCT (semiconductor/quantum well)
• Thermodetectors (not used for FT-IR, response time is too slow)– Thermocouple– Bolometer– Thermister
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
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.
ATR / Evanescent wave
Sat
urat
ed v
s. U
nsat
urat
ed
Sat
urat
ed v
s. U
nsat
urat
ed
Alc
ohol
s
Ket
ones
and
Ald
ehyd
es
Car
boxy
lic A
cids
Eth
ers
Est
ers
Unknowns
Unknowns
Unknowns
Unknowns
Unknowns
Unknowns
Raman Spectroscopy
Chapters 18
Elastic scatteringPhoton scattered at same as incident photon
Inelastic scatteringPhoton scattered at different as incident photon
• 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
Raman Selection Rule
• with vibration
• Polarizability changes with bond length/vibration
0 eqr rr
Change in bond length
Gaussian Calculations
• CO2
• SF6
• Chloroform
1 1
0 0
1 1
0 0
# of states at E
# of states at E
N g Eexp
N g kT
g
g
-1 11 m .694 cm
100 cm
hc hcE hc
hc
kkT K
hc
1 1
0 0
N g Eexp
N g kT
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
Vibrational Selection Rules
Raman Infrared
1
0
v
r
1
0
v
r
90° Scattering Instrumentation
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.
Resonance Raman
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.
FT-Raman
Fiber-Optic Raman SpectrometerBP=band pass filterBR=band rejection filter
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
Improving Raman Collection Efficiency
Crystal Hydrate Structure
Calibration
1500 1550 1600 1650 1700 1750 1800
Raman shift [cm-1]
AT
MT
- RAT
- RMT
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
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