1

CHAPTER 9

Spectroscopy: the study of the interaction of energy with matter

• Energy applied to matter can be absorbed, emitted, cause a chemical change, or be transmitted

• Spectroscopy can be used to elucidate the structure of a molecule

2

Electromagnetic radiation

• Electromagnetic radiation is the energy that is transmitted through space in the form of waves.

• Types of electromagnetic radiation: Radio waves, Ultraviolet (UV), Infrared(IR), Visible (vis)..

3

Characterization of waves• Waves are characterized by:

1- Wavelength (λ): the distance from the crest of one wave to the crest of the next wave. λ = nm,um, cm

4

2- the waves are also characterized by Frequency (v)= number of complete cycles per second (cps), also called Hertz (Hz).

5

6

Wavelength and frequency are inversely proportional.

• In IR ,frequency is expressed as wavenumbers

Wavenumbers have units of reciprocal cm (cm-1)

7

The relationship between wavelength (or frequency) and energy (E) is well defined

1- Wavelength and frequency are inversely proportional .2- The higher the frequency, the greater the energy of the wave.3- The shorter the wavelength, the greater the energy of the wave.

8

UV VIS IR RADIO wavesIncreasing wavelength- Decreasing frequency Decreasing Energy

Absorption of UV Result of promotion of electron to a higher energy level

Absorption of IR Result in increase of vibration of bonds

9

Features of a spectrumAn infrared spectrum of a compound is a plot of percent transmission (%T) versus either wavelength of frequency changing.

%T = (Intensity/original intensity) x 100

10

Absorption of Infrared Radiation

Cause increase the vibration of bonded atomsDifferent type of bonds ( C-H, C-C, C-O, C=O, O-H)

absorb IR at different λ.Type of vibration absorb at different λ.

R

O

H

at 3330 cm-1

Types of vibrations

1- stretching

2- bending

R

O

H

1250 cm-1

11

The relative amount of absorbed energy depends on the change of bond moment

1) Non-polar bonds weak absorption

2) Polar bonds strong absorption

12

The infrared spectrum

The instrument used to measure absorption of infrared radiation

infrared spectrophotometer

13

14

Interpretation of IR spectrum

Correlation Chart

15

16

A. c-c Bonds

• C-C single bond weak absorption (not useful)

• C=C (sp2) 1600- 1700 cm-1

• C=C( aryl, sp2) 1450-1600cm-1

C C 2100-2250 cm-1(sp)

17

C-H Bonds

• (sp3) C-H 2800-3000cm-1

• (sp2) C-H (=C-H) 3000-3300 cm-1

(sp) C H ( C H ) 3300 cm-1

18

19

20

21

Aromatic Compounds

– Aromatic Compounds The C-C bond stretching gives a set of characteristic sharp peaks between 1450-1600 cm -1

22

23

Haloalkanes: C-X 500-1430cm-1

24

Ether:C-O 1050-1260 cm-1 (strong)

25

Alcohol:O-H 3000-3600cm-1

(strong) + C-O

26

Hydrogen bonding O-H broadNo H-bonding O-H sharp

27

Amines: RNH23000-3600cm-1

(medium or weak double peaks) + C-N (900-1300 CM-1)

28

Amines: R2NH3000-3600cm-1

(medium or weak one peak) + C-N (900-1300cm-1)

29

Amines: R3Nno N-H peak only C-N at 900-1300cm-1

30

Carbonyl Functional Groups

Generally the carbonyl group gives a strong peak which occurs at 1630-1780 cm-1

The exact location depends on the actual functional group present

31

Ketones: C=O 1680-1750 cm-1 (strong)

32

Aldehydes: Carbonyl (C=O) 1720-1740cm-1

Also must show aldehyde C-H bend

Two peaks 1) 2820-2900cm-1

2 (2700-2780 cm-1

33

Carboxylic acid: carbonyl (C=O)1700-1725 cm-1 (strong)

• Also must show O-H stretching very broad from 3330- 2500 cm-1

34

Esters: C=O 1735-1760 cm-1

• Also shows C-O 1100-1300 cm-1

35

Conclusion

• IR is used for functional groups identification.

• Not all the peaks can be analyzed.

• Example

36

37

38