Ftir final presentation

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 changein 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) CO
2: 3(3)-5 = 4 modes
- -
moving in-out of plane
+

13. Vibrational Modes
Nonlinear molecule with natoms 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) CO
2: 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
►CO
2 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, esters1690-1760 Strong
NO
2 Nitro compounds 1500-1570
1300-1370
StrongC C
H C C H C C C N
Abbreviated Table of Group Frequencies for Organic Groups

33. THANK YOU