1. Infra-red Spectroscopy
CHM 504
Infra-red Spectroscopy

2. Electromagnetic Spectrum

3. Objectives of IR spectroscopy
To identify the functional groups present in molecules
To identify compounds by comparison with spectra of known compounds in a database.

4. Molecular vibrations Natural frequency of vibration depends on
Mass of each atom; light atoms (e.g., H) vibrate faster.
Strength of bond; strong bonds vibrate faster.

5. Modes of vibration
Molecules with more than two atoms can vibrate in several different ways. Each way is called a MODE.
Every mode of vibration has a natural frequency.
Each mode of vibration involves distortions of
Bond length : STRETCHING
Bond angle : BENDING

6. Vibrations of H2O
Symmetric stretching
Asymmetric stretching
Bending

7. Interaction of matter with infra-red energy
Molecules can absorb energy in the form of infra-red radiation.
The radiation being absorbed must have the same frequency as one of the modes of vibration of the molecule.
When a molecule absorbs one photon of IR radiation of appropriate frequency, the corresponding mode of vibration increases its amplitude.
The frequency of vibration does not change.

8. Vibrations and infra-red radiation
Amplitude of vibration increases
Frequency of vibration unchanged

9. Energy of IR radiation E = hn l n = c
IR radiation is a form of electromagnetic radiation.
Every photon of electromagnetic radiation has a quantum of energy, given by the equation
E = hn
where h is Planck’s constant,  J s.
The frequency n is related to the wavelength l by the equation
l n = c
where c is the speed of light.

10. Measurement of IR radiation
Can be measured as either frequency (cycles per second, Hz, s-1) or wavelength (micrometers, mm).
For historic reasons, usually measured as wavenumber (cycles per centimeter, cm-1).
Wavenumber is a kind of frequency; the greater the wavenumber, the higher the energy.
Symbol for wavenumber is .

11. Wavenumber and wavelength
Absorption of IR radiation due to vibrations occur in the range
cm-1 ( mm)
Range of real interest in IR spectroscopy:
cm-1 ( mm)

12. Infra-red Spectrum Graph Horizontal axis: Vertical axis:
Wavelength ( mm) OR
Wavenumber ( cm-1) – usually.
Vertical axis:
Absorbance (A) OR
Transmittance (T) – usually ( %).

13. IR spectrum of 3-hydroxyacetophenone

14. Some definitions
Transmittance:
Absorbance:

15. Spectral bands or peaks
A complex molecule has many modes of vibration.
Each mode has a characteristic frequency.
The molecule can absorb IR radiation of each characteristic frequency.
Each such absorption appears as a band or peak in the IR spectrum.

16. Identification of molecules
Each molecule has a unique set of modes of vibration.
Therefore each molecule has a unique spectrum.
Unknown molecule can be identified by comparing spectrum with spectra of known molecules. Exact one-to-one matching of peaks sufficient to identify molecule.
Computer required to search database of known spectra.

17. Qualitative information from IR spectra
Many modes of vibration principally involve specific bonds or functional groups.
Peaks corresponding to those vibrations reveal the presence of those functional groups.
Each peak has 3 characteristics that can provide information:
Wavenumber
Intensity (strong, medium, or weak)
Shape (sharp, normal, or broad)
Not every peak provides useful information.

18. Shapes of peaks
sharp
normal
broad

19. Analysis of IR spectrum
Spectrum can be divided into 4 regions:
Region 1: cm-1
Region 2: cm-1
Region 3: cm-1
Region 4: cm-1
Different types of vibrations, corresponding to different functional groups, are found in different regions.

20. cm-1
C–H, N–H, O–H, and (rarely) S–H stretching vibrations.
Can be distinguished based on wavenumber, intensity, and shape.
Very important in the identification of
Alcohols / phenols
Carboxylic acids
Primary / secondary amines
Amides
Terminal alkynes (R–CC–H)

21. 2500 - 2000 cm-1 Triple bond stretching (R–CC & R–CN)
Stretching in cumulative pairs of double bonds (X=C=Y, where X and Y could be C, N, or O).
In most spectra, this area is blank.

22. 1900 - 1400 cm-1 C=C, C=O, and C=N stretching peaks.
Stretching of bonds that are intermediate between single and double ( cm-1). For example,
Often the most important part of the spectrum

23. Functional groups visible in 1900 - 1400 cm-1 region
Aldehydes and ketones
Other C=O containing functional groups, including
Carboxylic acids
Acid chlorides and anhydrides
Esters
Amides
C=C double bonds
Aromatic rings

24. cm-1
Stretching of single bonds to atoms other than H, e.g., C–C, C–O, C–N, C–Cl, etc.
C–H bending peaks.
Usually lots of peaks. Most cannot be identified or interpreted.
Region of spectrum that tends to be unique for a given compound.
Sometimes referred to as the fingerprint region.

25. Identification of functional groups
Based on presence of key peaks in spectrum.
Wavenumber, shape, and intensity of peak should be considered.
Some functional groups cannot be easily identified using IR, e.g., those containing only single bonds other than O-H, N-H.
Alkyl halides
Ethers
Tertiary amines

26. Alkanes (alkyl groups)
C–H stretching: cm-1
C–H bending: , cm-1
Many overlapping bands
Presence of these peaks is not informative, since most compounds contain alkyl groups.

27. Alkenes Three types of diagnostic peaks.
1. C=C stretching (1640 – 1670 cm-1, w to m)
2. C–H stretching (3000 – 3100 cm-1)
3. C–H bending

28. IR spectrum of 1-hexene

29. Alkynes 1. CC stretching. 2. C–H stretching. 3. C–H bending.
R–CC–H cm-1 (medium)
R–CC–R’ cm-1 (weak or absent)
2. C–H stretching.
cm-1 (strong, sharp)
3. C–H bending.
A terminal alkyne (R–CCH) is easily recognised; an internal alkyne is very difficult to spot using IR.

30. IR spectrum of 1-hexyne

31. Aromatic hydrocarbons
1. C=C stretching: two sets of peaks.
(i) cm-1 (weak - medium)
(ii) cm-1 (weak - medium)
Each set typically contains two peaks.
The second peak in each set may be weak, absent, or appear as a “shoulder.”
2. C–H stretching cm-1.
(indistinguishable from alkene C-H stretch)
3. C–H bending

32. IR spectrum of toluene

33. Alcohols and phenols
1. O–H stretching cm-1 (strong, broad)
Broad because of H – bonding.
In dilute solutions, sharp peak at ~3600 cm-1.
2. C–O stretching cm-1 (strong)
(Cannot distinguish between alcohols and phenols based on IR)

34. IR spectrum of (CH3)2CHCH2OH

35. The carbonyl group

36. Identifying a carbonyl-containing functional group
C=O stretching peak
Very strong peak - often the strongest in the spectrum
Consider other characteristic peaks
O–H stretching of carboxylic acid
C–H stretching of aldehyde
N–H stretching and bending of amide
C–O stretching of ester
Absent such peaks, probably a ketone

37. Aldehydes C=O stretching. 1680 – 1740 cm-1
C–H stretching cm-1
1-2 peaks, relatively weak
No other peaks appear in this range

38. Ketones C=O stretching : 1670 – 1750 cm-1
Can be distinguished from aldehydes by absence of C–H peaks at cm-1.

39. IR spectrum of CH2CH2CH2CHO

40. IR spectrum of CH3C(O)CH2CH2CH3

41. Carboxylic acids C=O stretching. 1680 – 1720 cm-1
O–H stretching: very broad, distinctive peak, not very strong, stretching from ~3300 to ~2500 cm-1 (centered at ~3050 cm-1); C–H stretching peaks are usually superimposed.

42. IR spectrum of CH3(CH2)4CO2H

43. Esters C=O stretching. 1715 – 1770 cm-1.
A very strong peak.
Absent in ketones; can be used to distinguish between esters and ketones.
However: its presence does not guarantee an ester! Could be a ketone with a C–O single bond elsewhere.

44. IR spectrum of CH3C(O)OCH2CH3

45. Amides C=O stretching: 1680 – 1630 cm-1.
N–H stretching: – 3180 cm-1.
Primary amides: peaks
Secondary amides: 1 peak
Tertiary amides: missing

46. IR spectrum of CH3CONH2

47. Primary and secondary amines
N–H stretching: cm-1
Primary (RNH2) : 2 peaks
Secondary (R2NH) : 1 peak
Relatively weak and sharp. Much weaker than amide N–H stretching.
Easy to distinguish from strong, broad OH stretching.
May be obscured if OH, NH2 in same molecule.

48. IR spectrum of CH3(CH2)3NH2