Fourier Transform Infrared (FT-IR) Spectroscopy

1. Infrared
(FT-IR) Spectroscopy
Presented by:
NIVEDITHA G
1st M.Pharm,
Dept. of Pharmaceutics.
Nargund College of
Pharmacy.
1

2. Contents :
1. FTIR
2. Interferometer
3. Advantages
4. Disadvantages
5. Interpretation of IR.
2

3. Dispersive I.R. Fourier Transform I.R.
(With Interferometer)
(With Monochromator)
Light source
Monochromator
Sample compartment
Detector
Light source
Interferometer
Sample compartment
Detector
9

4. Albert Michelson (1852-1931)
Michelson wanted to measure the speed the
the earth moves through the ether (the
medium in which light travels). By
measuring the interference between light
paths at right angles, one could find the
direction & speed of the ether.
Michelson’s
first
interferometer
(1881)
10

5. H
V
MIRROR2
BEAM
SPLITTER
SOURCE
DETECTOR
V
H
H
H
V
H
MIRROR 1
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6. INTERFEROMETER…
• In the FT-IR instrument, the sample is placed between the output
of the interferometer and the detector. The sample absorbs
radiation of particular wavelengths.
• An interferogram of a reference is needed to obtain the spectrum
of the sample.
• After an interferogram has been collected, a computer performs
a Fast Fourier Transform, which results in a frequency domain
trace (i.e. intensity vs. wave number).
15

7. Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
Detector
(DLATGS)
Beam splitter
FT Optical System Diagram
16

8. Fixed mirror
Ｂ
Movable
mirror
Fixed mirror
Ａ
Movable
mirror
Fixed
mirror
Ｃ
Movable
mirror
Same-phase interference
wave shape
Opposite-phase
interference
wave shape
Same-phase
interference
wave shape
l
0 Ｄ Interference pattern of
light manifested by the
optical-path difference
Signal
strength
Ｉ(X)
-2l -l 0 l 2l
Interference of two beams of light
17

9. 28

10. Interferometerconsistsof……….
• Drive mechanism
• Beam splitter
• Source and transducers.
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11. 30

12. Drive mechanism..
• Speed and planarity- CONSTANT.
A motor driven micrometer screw measures displacement of the
mirror.
31

13. Beam splitter
• Constructed of transparent materials
• 50% radiation is reflected and 50% is transmitted.
• Mylar sandwiched between two plate of a low refractive index
solid.
Germanium or silicon deposited on cesium iodide or bromide ,
NaCl, KBr - MID IR REGIONS.
• Iron(III) oxide is deposited on calcium fluoride- NEAR IR
REGION.
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14. SOURCES AND Transducers
SOURCES:
• Tungsten- Halogen lamp.
• High Pressure Mercury Lamp
• Globar (or) Nernst sources
TRANSDUCERS:
• Pyroelectric transducers.
Ex:- Deuterated triglycine sulfate (DCT) pyroelectric
Transducer
• Photoconduction transducers:
Ex:- Mercury cadmium telluride(MCT) or indium antimonite, lead
sulfide photoconductive transducers.
33

15. TGS
Operates at room temperature
MCT
Operates at the temperatur
of liquid nitrogen
1010
109
108
4000 600
Transducer Properties
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16. • Most common pyro-electric compound is Tri-glycerine sulfate
(T.G.S). Different types are:
1. Lithium niobate
2. Lead zircobate
3. Triglycerine sulfate:
a) Tri-glycerine sulfate (only upto 450c)
b) Deuterium tri-glycerine sulfate (deuterated)
• These substances have permanent dipole moment.
Advantages:
i. First choice of detector
ii. Response is fast and used for multiple scanning.
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17. Advantages
1.Better sensitivity and brightness.
2.High wavenumber accuracy.
3. Resolution.
4. Stray light.
5. Wavenumber range flexibility.
Disadvantages
1. CO2 and H2O sensitive
2. Single-beam, requires collecting blank
3. Can’t use thermal detectors – too slow
4. Destructive
5. Too sensitive that it would detect the smallest contaminant
FT-IR Advantages and Disadvantages
36

18. Fellgett's (multiplex) Advantage
• FT-IR collects all resolution elements with a complete scan of
the interferometer. Successive scans of the FT-IR instrument are
coadded and averaged to enhance the signal-to-noise of the
spectrum.
• Theoretically, an infinitely long scan would average out all the
noise in the baseline.
• The dispersive instrument collects data one wavelength at a time
and collects only a single spectrum. There is no good method
for increasing the signal-to-noise of the dispersive spectrum.
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19. Connes Advantage
• An FT-IR uses a HeNe laser as an internal wavelength
standard. The infrared wavelengths are calculated using the
laser wavelength, itself a very precise and repeatable 'standard'.
• Wavelength assignment for the FT-IR spectrum is very
repeatable and reproducible and data can be compared to
digital libraries for identification purposes.
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20. Jacquinot Advantage
• FT-IR uses a combination of circular apertures and
interferometer travel to define resolution. To improve signal-to-
noise, one simply collects more scans.
• More energy is available for the normal infrared scan and
various accessories can be used to solve various sample
handling problems.
• The dispersive instrument uses a rectangular slit to control
resolution and cannot increase the signal-to-noise for high
resolution scans. Accessory use is limited for a dispersive
instrument.
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21. 40

22. • A 100 % transmittance in the spectrum implies no absorption of IR
radiation.
•When a compound absorbs IR radiation, the intensity of transmitted
radiation decreases.
•This results in a decrease of per cent transmittance and hence a dip
in the spectrum. The dip is often called an absorption peak or
absorption band.
FEATURES OF AN IR SPECTRUM
• Different types of groups of atoms (C-H, O-H, N-H, etc…) absorb
infrared radiation at different characteristic wavenumbers.
41

23. 1. No two molecules will give exactly the same IR
spectrum (except enantiomers).
2. Simple stretching: 1600-3500 cm-1
3. Complex vibrations: 400-1400 cm-1, called the
“fingerprint region”
I.R SPECTRUM
42

24. Describing IR Absorptions
IR absorptions are described by their frequency and appearance.
• Frequency (n) is given in wavenumbers (cm-1)
• Appearance is qualitative: intensity and shape
• Conventional abbreviations:
Vs very strong
S strong
M medium
w weak
Br broad
Sh sharp OR shoulder
43

25. •In general, the IR spectrum can be split into four
regions for interpretation:
1. 4000  2500 cm-1: e.g. OH, NH, CH
2. 2500  2000 cm-1: e.g. C≡C, C≡N
3. 2000  1500 cm-1: e.g. C=C, C=O
4. 1500  600 cm-1: This region often consists of many different,
complicated bands and is called the fingerprint region.
44

26. FINGER PRINT REGION
• 1500 and 600 cm-1 .
• Absorption in this fingerprint region is characteristic of the
molecule as a whole.
• C-O-C stretching vibration in ethers and esters at about 1200
cm-1
• C-Cl stretching vibration at 700 to 800 cm-I.
• Sulfate, Phosphate, Nitrate, and Carbonate also absorb at
wavenumbers below 1200 cm-I
45

27. Finger print region can be subdivided into three regions
as follows:
1. 1500 – 1350 cm-1
2. 1350 – 1000 cm-1
3. Below 1000 cm-1
• Region 1500 – 1350 cm-1 :
• doublet near 1380 cm-1(m) and 1365 cm-1(s)→ tertiary butyl group in
compound.
• nitro compound, one bond → near 1350 cm-1.
• Region 1350 - 1000 cm-1 :
• alcohols, esters, acid anhydrides
• Region below 1000 cm-1 : cis and trans alkenes. The higher value (970 – 960
cm-1) indicates that hydrogen atoms in alkenes are trans and viceversa.
. E.g.: a band in the region 750 – 700 cm-1 show mono sunstituted benzene.
46

28. Specific groups
1. Alkanes – combination of C-C and C-H bonds
• Show various C-C stretches and bends between 1360-1470 cm-1 (m)
• C-C bond between methylene carbons (CH2’s) 1450-1470 cm-1 (m)
• C-C bond between methylene carbons (CH2’s) and methyl (CH3)
1360-1390 cm-1 (m)
• Show sp3 C-H between 2800-3000 cm-1 (s) cm -1
Infrared Spectroscopy
Octane
49

29. Specific groups
2. Alkenes – addition of the C=C and vinyl C-H bonds
• C=C stretch occurs at 1620-1680 cm-1 and becomes weaker as substitution
increases
• vinyl C-H stretch occurs at 3000-3100 cm-1
• Note that the bonds of alkane are still present!
• The difference between alkane and alkene or alkynyl C-H is important! If the
band is slightly above 3000 it is vinyl sp2 C-H or alkynyl sp C-H if it is below
it is alkyl sp3 C-H
1-Octene
Infrared Spectroscopy
50

30. Specific groups
3. Alkynes – addition of the C≡C and vinyl C-H bonds
• C≡C stretch occurs between 2100-2260 cm-1; the strength of this band
depends on asymmetry of bond, strongest for terminal alkynes, weakest for
symmetrical internal alkynes (w-m)
• C-H for terminal alkynes occurs at 3200-3300 cm-1 (s)
• Remember internal alkynes ( R-C≡C-R ) would not have this band!
1-Octyne
Infrared Spectroscopy
51

31. Specific groups
4. Aromatics
• Due to the delocalization of electrons in the ring, where the bond order
between carbons is 1 ½, the stretching frequency for these bonds is slightly
lower in energy than normal C=C
• These bonds show up as a pair of sharp bands, 1500 (s) & 1600 cm-1 (m),
where the lower frequency band is stronger
• C-H bonds off the ring show up similar to vinyl C-H at 3000-3100 cm-1 (m)
Ethyl benzene
Infrared Spectroscopy
52

32. Specific groups
4. Aromatics
• If the region between 1667-2000 cm-1 (w) is free of interference (C=O
stretching frequency is in this region) a weak grouping of peaks is observed
for aromatic systems
• Analysis of this region, called the overtone of bending region, can lead to a
determination of the substitution pattern on the aromatic ring
Monosubstituted
1,2 disubstituted (ortho or o-)
1,3 disubstituted (meta or m-)
1,4 disubstituted (para or p-)
G
G
G
G
G
G
G
Infrared Spectroscopy
53

33. Specific groups
6. Ethers – addition of the C-O-C asymmetric band and vinyl C-H bonds
• Show a strong band for the antisymmetric C-O-C stretch at 1050-1150 cm-1
• Otherwise, dominated by the hydrocarbon component of the rest of the
molecule
Diisopropyl ether
O
Infrared Spectroscopy
55

34. Specific groups
7. Alcohols
• Show a strong, broad band for the O-H stretch from 3200-3400 cm-1 (s, br)
this is one of the most recognizable IR bands
• Like ethers, show a band for C-O stretch between 1050-1260 cm-1 (s)
• This band changes position depending on the substitution of the alcohol: 1°
1075-1000; 2° 1075-1150; 3° 1100-1200; phenol 1180-1260
• The shape is due to the presence of hydrogen bonding
1-butanol
HO
Infrared Spectroscopy
56

35. Specific groups
8. Amines - Primary
• Shows the –N-H stretch for NH2 as a doublet between 3200-3500 cm-1
(s-m); symmetric and anti-symmetric modes
• -NH2 group shows a deformation band from 1590-1650 cm-1 (w)
• Additionally there is a “wag” band at 780-820 cm-1 that is not diagnostic
2-aminopentane
H2N
Infrared Spectroscopy
57

36. Specific groups
9. Amines – Secondary
• -N-H band for R2N-H occurs at 3200-3500 cm-1 (br, m) as a single sharp
peak weaker than –O-H
• Tertiary amines (R3N) have no N-H bond and will not have a band in this
region
pyrrolidine
NH
Infrared Spectroscopy
58

37. Specific groups
10. Aldehydes
• Show the C=O (carbonyl) stretch from 1720-1740 cm-1(s)
• Band is sensitive to conjugation, as are all carbonyls (upcoming slide)
• Also displays a highly unique sp2 C-H stretch as a doublet, 2720 & 2820
cm-1 (w) called a “Fermi doublet”
Cyclohexyl carboxaldehyde
C
H
O
Infrared Spectroscopy
59

38. Specific groups
11. Ketones
• Simplest of the carbonyl compounds as far as IR spectrum – carbonyl only
• C=O stretch occurs at 1705-1725 cm-1 (s)
3-methyl-2-pentanone
O
Infrared Spectroscopy
60

39. Specific groups
12. Esters
1. C=O stretch occurs at 1735-1750 cm-1 (s)
2. Also displays a strong band for C-O at a higher frequency than ethers or
alcohols at 1150-1250 cm-1 (s)
Infrared Spectroscopy
Ethyl pivalate
O
O
61

40. Specific groups
13. Carboxylic Acids:
• Gives the messiest of IR spectra
• C=O band occurs between 1700-1725 cm-1
• The highly dissociated O-H bond has a broad band from 2400-3500 cm-1
(m, br) covering up to half the IR spectrum in some cases
4-phenylbutyric acid
OH
O
Infrared Spectroscopy
62

41. B. Specific groups
14. Acid anhydrides
• Coupling of the anhydride though the ether oxygen splits the carbonyl band
into two with a separation of 70 cm-1.
• Bands are at 1740-1770 cm-1 and 1810-1840 cm-1 (s)
• Mixed mode C-O stretch at 1000-1100 cm-1 (s)
Propionic anhydride
Infrared Spectroscopy
O
O O
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42. Specific groups
15. Acid halides
• Dominant band at 1770-1820 cm-1 for C=O (s)
• Bonds to halogens, due to their size occur at low frequencies, where their
presence should be used to reinforce rather than be used for diagnosis,
C-Cl is at 600-800 cm-1 (m)
Propionyl chloride
Cl
O
Infrared Spectroscopy
64

43. Specific groups
16. Amides
• Display features of amines and carbonyl compounds
• C=O stretch occurs from 1640-1680 cm-1
• If the amide is primary (-NH2) the N-H stretch occurs from 3200-3500
cm-1 as a doublet
• If the amide is secondary (-NHR) the N-H stretch occurs at 3200-3500
cm-1 as a sharp singlet
pivalamide
NH2
O
Infrared Spectroscopy
65

44. This region stretches from about 1800 to
1650 cm-1 - RIGHT IN THE MIDDLE OF THE SPECTRUM
The base value is 1715 cm-1 (ketone)
The bands are very strong !!! due to the large
C=O dipole moment.
 C=O is often one of the strongest peaks in the
spectrum
THE CARBONYL STRETCHING REGION
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45. C
R
O
H
C
R
O
O C R
O
C
R
O
Cl
C
R
O
OR'
C
R
O
R
C
R
O
NH2
C
R
O
OH
1690
1710
1715
1725
1735
1800
1810 and 1760
BASE
VALUE
acid
chloride ester aldehyde
carboxylic
acid amide
ketone
anhydride
( two peaks )
EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCY
C=O IS SENSITIVE TO ITS ENVIRONMENT
THESE VALUES ARE
WORTH LEARNING
all are +/- 10 cm-1 67

46. Ketones are at lower frequency than Aldehydes.
Acid chlorides are at higher frequency than Ketones.
Esters are at higher frequencies than Ketones.
Amides are at lower frequencies than Ketones.
SUMMARY
Note the electronegativity difference, O versus N, weighs the
two factors (resonance/ e-withdrawal) differently in esters
than in amides.
Acids are at lower frequency than Ketones.
68

47. Specific groups
17. Nitro group (-NO2)
• Proper Lewis structure gives a bond order of 1.5 from nitrogen to each
oxygen
• Two bands are seen (symmetric and asymmetric) at 1300-1380 cm-1 (m-s)
and 1500-1570 cm-1 (m-s)
• This group is a strong resonance withdrawing group and is itself vulnerable
to resonance effects
2-nitropropane
N
O
O
Infrared Spectroscopy
69

48. Specific groups – Hydrocarbons
18. Nitriles (the cyano- or –C≡N group)
• Principle group is the carbon nitrogen triple bond at 2100-2280 cm-1 (s)
• This peak is usually much more intense than that of the alkyne due to the
electronegativity difference between carbon and nitrogen
propionitrile
N
C
Infrared Spectroscopy
70

49. EFFECT OF HYDROGEN BONDING ON I.R.
SPECTRUM
X–H••••••B
• Hydrogen bonding alters the force constant of both groups.
• The X–H stretching bands move to lower frequency.
• The stretching frequency of the acceptor group (B) is also reduced,
but to a lesser degree.
• The X–H bending vibration usually shifts to a shorter wavelength.
• H-bonding can interact with other functional groups to lower
frequencies.
• Intermolecular hydrogen bonds gives rise to broad bands where as
bands arising from intra molecular hydrogen bonds are sharp and
well-defined.
• Hence we can distinguish the types of hydrogen bonding.
72

50. •O-H 3600 cm-1 (alcohol, free)
•O-H 3300 cm-1 (alcohols & acids, H-
bonding)
3600 3300
H-BONDED
FREE
broadens
shifts
The O-H stretching region
74

51. IMPORTANCE OF I.R. SPECTRA:
• Provides more information.
• Identification and structure determination.
• Identification of functional groups on a molecule.
• Quantitative analysis.
• Calculate the bond distance and bond angle.
• Impurities can be detected.
• Distinction between two types of hydrogen bonding.
75

52. REFERENCES:
INSTRUMENTAL METHODS OF CHEMICAL ANALYSIS BY
Gurudeep R. Chatwal, sham K. Anand.
ELEMENTARY ORAGANIC SPECTROSCOPY – PRINCIPLES
AND CHEMICAL APPLICATIONS by S. Chand, Y.R. Sharma.
INFRARED SPECTROSCOPY, FUNDAMENTALS
AND APPLICATONS- Barbara Stuart.
INTERNET ( Wikipedia )
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