FTIR Forensic Analytical Chemistry

1. IR & Fourier Transform IR
TEJASVI BHATIA

2. ItProvides information about the vibrations
of functional groups in a molecule
Infrared Spectroscopy
Therefore, the functional groups present
in a molecule can be deduced from an IR
spectrum

3. Introduction
►Spectroscopy is an analytical technique
which helps to determine the structure of
the compounds.
►It destroys little or no sample.
►The amount of light absorbed by the sample
is measured as wavelength is varied.

4. The Spectrum and
Molecular Effects
=>

5. The IR Region
►Just below the red in the visible region
usually between the range of 2.5 - 25 mm.
►More common units are wave numbers, or
cm-1, the reciprocal of the wavelength in
centimeters.
►Wave numbers are proportional to
frequency and energy.

6. Molecular Vibrations
Covalent bonds vibrate at only certain
allowable frequencies.

7. Stretching Frequencies
►Frequency decreases with increasing
atomic weight.
►Frequency increases with increasing
bond energy.

8. The absorption intensity depends on how
efficiently the energy of an
electromagnetic wave of frequency  can
be transferred to the atoms involved in
the vibration
What does the absorption intensity
depend on?

9. The greater the change in dipole moment
during a vibration, the higher the intensity
of absorption of a photon

10. iii.) Types of Molecular
Vibrations
Bond Stretching
symmetric
asymmetric

11. In-plane rocking
In-plane scissoring
Out-of-plane wagging
Out-of-plane twisting
Bond Bending

12. iv.) Number of Vibrational Modes:
- for non-linear molecules, number of types of vibrations: 3N-6
- for linear molecules, number of types of vibrations: 3N-5
- observed vibration can be less then predicted because
symmetry ( no change in dipole)
energies of vibration are identical
 absorption intensity too low
frequency beyond range of instrument
Examples:
1) HCl: 3(2)-5 = 1 mode
2) CO2: 3(3)-5 = 4 modes
- -
moving in-out of plane
+

13. Vibrational Modes
Nonlinear molecule with n atoms usually has
3n - 6 fundamental vibrational modes.

14. Fingerprint Region of the Molecule
►Whole-molecule vibrations and bending
vibrations are also quantitized.
►No two molecules will give exactly the
same IR spectrum (except enantiomers).
►Simple stretching: 1600-3500 cm-1.
►Complex vibrations: 600-1400 cm-1,
called the “fingerprint region.”

15. IR-Active and Inactive
►A polar bond is usually IR-active.
►A nonpolar bond in a symmetrical
molecule will absorb weakly or not at all.

16. v.) IR Active Vibrations:
- In order for molecule to absorb IR radiation:
 vibration at same frequency as in light
but also, must have a change in its net dipole moment
as a result of the vibration
Examples:
1) CO2: 3(3)-5 = 4 modes
- -+
m = 0; IR inactive
m > 0; IR active
m > 0; IR active
m > 0; IR active
d-
d-
2d+
d-
d-2d+
d-
d-2d+
d-
d-2d+
9

17. Does O=C=O absorb IR light?
Ans: vibrations of O=C=O which cause a
change in the dipole moment of the molecular
absorb IR light
vibrations of O=C=O which do not cause a
change in the dipole moment of the
molecular DO NOT absorb IR light
No dipole
generated
Dipole
generated

18. FT-IR Spectrometer
►Uses an interferometer.
►Has better sensitivity.
►Less energy is needed from source.
►Completes a scan in 1-2 seconds.
►Takes several scans and averages them
►Has a laser beam that keeps the
instrument accurately calibrated.

19. Components
►Source
►Michelson Interferometer
►Sample
►Detector

20. Sources
► Black body radiators
► Inert solids resistively heated to 1500-2200 K
► Max radiation between 5000-5900 cm-1 (2-1.7
mm), falls off to about 1 % max at 670 cm-1 (15
mm)
► Nernst Glower – cylinder made of rear earth
elements
► Globar- SiC rod
► CO2 laser
► Hg arc (Far IR), Tungsten filament (Near IR)

21. Michaelson Interferometer
►Beam splitter
►Stationary mirror
►Moving mirror at constant velocity
►He/Ne laser; sampling interval, control
mirror velocity

22. FIXED POSITION MIRROR
MOVABLE MIRROR
SINGLE BEAMSPLITTER
FREQUENCY
SOURCE ()
d = 0 d =/2 d = d =3/2
SAMPLE
POSITION
DETECTOR
THE MICHELSON INTERFEROMETER

23. Schematic of a Michelson Interferometer.

24. Sample
► Sample holder must be transparent to IR- salts
► Liquids
 Salt Plates
 Neat, 1 drop
 Samples dissolved in volatile solvents- 0.1-10%
► Solids
 KBr pellets
 Mulling (dispersions)

25. FT-IR detectors
► Pyroelectric tranducers (PTs)
► Pyroelectric substances act as temperature-
dependent capacitors
► Triglycine sulfate is sandwiched between two
electrodes. One electrode is IR transparent
► The current across the electrodes is Temperature
dependent
► PTs exhibit fast response times, which is why most
FT instruments use them

26. Sequence for Obtaining Spectrum
• Interferogram of Background is obtained
(without sample)
• System uses Fourier Transform to create
single beam background spectrum.
• Interferogram of Sample is obtained.
• System uses Fourier Transform to create
single beam spectrum of sample.
• System calculates the transmittance or
absorbance spectrum.

27. Advantages of FTIR compared to Normal IR:
1) much faster, seconds vs. minutes
2) use signal averaging to increase signal-to-noise (S/N)
3) higher inherent S/N – no slits, less optical equipment,
higher light intensity
4) high resolution (<0.1 cm-1)
Disadvantages of FTIR compared to Normal IR:
1) single-beam, requires collecting blank
2) can’t use thermal detectors – too slow

28. D) Application of IR
1.) Qualitative Analysis (Compound Identification)
- main application
- Use of IR, with NMR and MS, in late 1950’s
revolutionized organic chemistry

29. 1) Examine what functional groups are present by looking
at group frequency region
- 3600 cm-1 to 1200 cm-1

30. 2) compare spectrum of compound to IR library
- looking at functional group and fingerprint region
- small differences in structure results in large differences in
fingerprint region
- close match in fingerprint and group frequency regions 
strong evidence of good match
ii.) Group Frequency Region
- approximate frequency of many functional groups (C=O,C=C)
can be calculated from atomic masses & force constants
-
- serves as a good initial guide to compound identity, but not
positive proof.

31. iii.) Fingerprint Region (1200-700 cm-1)
- region of most single bond signals
- many have similar frequencies, so affect each other & give pattern
characteristics of overall skeletal structure of a compound
- exact interpretation of this region of spectra seldom possible because of
complexity

32. Bond Type of Compound Frequency Range, cm-
1
Intensity
C-H Alkanes 2850-2970 Strong
C-H Alkenes 3010-3095
675-995
Medium
strong
C-H Alkynes 3300 Strong
C-H Aromatic rings 3010-3100
690-900
Medium
strong
0-H Monomeric alcohols, phenols
Hydrogen-bonded alchohols, phenols
Monomeric carboxylic acids
Hydrogen-bonded carboxylic acids
3590-3650
3200-3600
3500-3650
2500-2700
Variable
Variable, sometimes broad
Medium
broad
N-H Amines, amides 3300-3500 medium
C=C Alkenes 1610-1680 Variable
C=C Aromatic rings 1500-1600 Variable
Alkynes 2100-2260 Variable
C-N Amines, amides 1180-1360 Strong
Nitriles 2210-2280 Strong
C-O Alcohols, ethers,carboxylic acids, esters 1050-1300 Strong
C=O Aldehydes, ketones, carboxylic acids, esters 1690-1760 Strong
NO2 Nitro compounds 1500-1570
1300-1370
Strong
C C
H
C C H
C C
C N
Abbreviated Table of Group Frequencies for Organic Groups

33. THANK YOU