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).

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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 UVVISIRRADIO waves Increasing wavelength-Decreasing frequencyDecreasing 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 spectrum An 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 atoms  Different 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. Types of vibrations 1-stretching 2-bending

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

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14 Interpretation of IR spectrum Correlation Chart

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16 A.c-c Bonds C-C single bond  weak absorption (not useful) C=C (sp 2 )  cm -1 C=C( aryl, sp2)  cm -1

17 C-H Bonds (sp3) C-H  cm -1 (sp2) C-H (=C-H)  cm -1

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

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23 Haloalkanes: C-X  cm -1

24 Ether:C-O  cm -1 (strong)

25 Alcohol:O-H  cm -1 (strong) + C-O

26 Hydrogen bonding  O-H broad No H-bonding  O-H sharp

27 Amines: RNH 2  cm -1 (medium or weak double peaks) + C-N ( CM -1 )

28 Amines: R 2 NH  cm -1 (medium or weak one peak) + C-N ( cm -1 )

29 Amines: R 3 N  no N-H peak only C- N at cm -1

30 Carbonyl Functional Groups Generally the carbonyl group gives a strong peak which occurs at cm -1  The exact location depends on the actual functional group present

31 Ketones: C=O  cm -1 (strong)

32 Aldehydes: Carbonyl (C=O) cm -1 Also must show aldehyde C-H bend Two peaks 1) cm -1 2) cm -1

33 Carboxylic acid: carbonyl (C=O)  cm -1 (strong) Also must show O-H stretching  very broad from cm -1

34 Esters: C=O  cm -1 Also shows C-O  cm -1

35 Conclusion IR is used for functional groups identification. Not all the peaks can be analyzed. Example

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