Spectroscopy

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Spectroscopy

• PART I Introduction and
• Infrared Spectroscopy
• Organic Chemistry 30B

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Spectroscopy

• Spectroscopy is the study of the interaction of
  electromagnetic radiation and matter
• Molecular Spectroscopy is the study of which
  frequencies of radiation are absorbed or emitted
  by a substance and the correlation of those
  frequencies with molecular structure

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Energy, Wavelength, Frequency

• Key relationships for electromagnetic radiation
• E h? hc/?
• E energy
• h Plancks constant
• c Speed of light
• ? Frequency
• ? Wavelength

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Energy, Wavelength, Frequency

• Key relationships E h? hc/?
• Increase Frequency (?) --gt Increase Energy
• Decrease Frequency (?) --gt Decrease Energy
• Increase Wavelength (?) --gt Decrease Energy
• Decrease Wavelength (?) --gt Increase Energy

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Energy, Wavelength, Frequency
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Wavelength Units
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Absorption of Energy by Molecules
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Effects of Electromagnetic Radiation on Molecules

• Radiation Outcome
• hn 109 nm Radiowave Nuclear Spin Flips
• hn 106 nm Microwave Molecular Rotation
• hn 104 nm Infrared Bond Vibrations
• hn 500 nm Visible Electronic Excitation
• hn 200 nm Ultraviolet Electronic Excitation
• hn 10 nm X-ray Molecular Ionization
• hn 0.1 nm Gamma Ray Bond Cleavage

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Infrared Spectroscopy

• Absorption of Infrared light in the wavelength
  range of 2.5x10-6 m to 2.5x 10-5 m (2.5 ?m to 25
  ?m) causes bond vibrations
• Specific wavelengths (or frequencies) can be
  correlated with specific functional groups

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Infrared Spectroscopy

• Infrared Spectrometer

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Infrared Spectrum
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Infrared Spectrum Units
Wavenumber 1/ ? ?/c
2.5 ?m 4000 cm-1
25 ?m 400 cm-1
Increase Wavenumber --gt Increase Energy Decrease
Wavenumber --gt Decrease Energy
Transmittance 100 - Absorbance
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Bond Vibrations for CH2
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Bond Vibrations for CH2
Symmetrical Asymmetrical Scissoring
Twisting Rocking Wagging
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Bond Vibrations for CH2
Symmetrical Asymmetrical Scissoring
Twisting Rocking Wagging
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Theory of Bond Vibrations

• A vibrating bond can be thought of as two masses
  connected by a spring

• The frequency of vibration is given by Hooks Law

k Force Constant m reduced mass m1m2/(m1
m2)
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Absorption Frequencies
Typical absorption frequencies for
organic functional groups
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Functional Group Absorbances
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Functional Group Absorbances
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Absorption Frequencies
Typical absorption frequencies for
organic functional groups
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IR Spectrum of Decane
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IR Spectrum of Cyclohexene
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IR Spectrum of 1-Octyne
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IR Spectrum of Toluene
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IR Spectrum of Anisole
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IR Spectrum of Dibutyl Ether
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IR Spectrum of 1-Hexanol
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IR Spectrum of 1-Hexanol
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IR Spectrum of 1-Butanamine
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IR Spectrum of Menthone
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IR Spectrum of Pentanoic Acid
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IR Spectrum of Ethyl Butanoate
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Comparison of IR Spectra
IR Spectra of t-butanol and n-butyl acetate
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Comparison of IR Spectra
IR spectra, an IR spectra comparison tool and
spectral problems are available
at http//www.chem.ucla.edu/webspectra
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Details of Carbonyl Groups

• CO Stretching Frequency
• Cyclohexanone 1715 cm-1
• Cyclopentanone 1745 cm-1
• Cyclobutanone 1780 cm-1
• Cyclopropanone 1850 cm-1
• Cyclohexenone 1695 cm-1

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Details of Carbonyl Groups - 2

• Functional Group Frequencies
• Ketone CO 1630-1820 cm-1
• Aldehyde CO 1630-1820 cm-1
• C-H 2720 cm-1
• Acid CO 1700-1725 cm-1
• O-H 2500-3300 cm-1
• Ester CO 1735-1800 cm-1
• C-O 1000-1100 (sp3)
• or 1200-1250 cm-1 (sp2)

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Interpreting Infrared Spectra

• Check for signals near 3000 cm-1
• Alkanes vs alkenes vs alkynes
• Check for signals near 3400 cm-1
• OH vs NH vs NH2
• Check for signals near 1700 cm-1
• Aldehyde vs ketone vs ester vs acid vs amide
• The principle goal will be identification of
  functional groups

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End