chapter 9

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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

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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)..

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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

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

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Wavelength and frequency are inversely proportional.

• In IR ,frequency is expressed as wavenumbers

Wavenumbers have units of reciprocal cm (cm-1)

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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.

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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

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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

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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

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

1) Non-polar bonds weak absorption

2) Polar bonds strong absorption

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The infrared spectrum

The instrument used to measure absorption of infrared radiation

infrared spectrophotometer

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Interpretation of IR spectrum

Correlation Chart

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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)

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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

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Aromatic Compounds

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

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Haloalkanes: C-X 500-1430cm-1

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

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Alcohol:O-H 3000-3600cm-1

(strong) + C-O

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

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Amines: RNH23000-3600cm-1

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

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Amines: R2NH3000-3600cm-1

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

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

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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

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

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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

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

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

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Esters: C=O 1735-1760 cm-1

• Also shows C-O 1100-1300 cm-1

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Conclusion

• IR is used for functional groups identification.

• Not all the peaks can be analyzed.

• Example

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