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