12 Infrared Spectroscopy and Mass Spectrometry.ppt

Chapter 12 Infrared Spectroscopy and Mass Spectrometry
Jo Blackburn
Richland College, Dallas, TX
Dallas munity College District
ã 2003, Prentice Hall
Organic Chemistry, 5th EditionL. G. Wade, Jr.
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Chapter 12
Introduction
Spectroscopy is an analytical technique which helps determine structure.
It destroys little or no sample.
The amount of light absorbed by the sample is measured as wavelength is varied.  =>
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Chapter 12
Types of Spectroscopy
Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group.
Mass spectrometry (MS) fragments the molecule and measures the masses.
Nuclear ic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers.
Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns. =>
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Chapter 12
ic Spectrum
Examples: X rays, microwaves, radio waves, visible light, IR, and UV.
Frequency and wavelength are inversely proportional.
c = ln, where c is the speed of light.
Energy per photon = hn, where h is Planck’s constant. =>
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Chapter 12
The Spectrum and Molecular Effects
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Chapter 12
The IR Region
Just below red in the visible region.
Wavelengths usually -25 mm.
mon units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters.
Wavenumbers are proportional to frequency and energy. =>
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Chapter 12
Molecular Vibrations
Covalent bonds vibrate at only certain allowable frequencies.
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Chapter 12
Stretching Frequencies
Frequency decreases with increasing atomic weight.
Frequency increases with increasing bond energy. =>
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Chapter 12
Vibrational Modes
Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational modes.
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Chapter 12
Fingerprint of Molecule
Whole-molecule vibrations and bending vibrations are also quantitized.
No two molecules will give exactly the same IR spectrum (except enantiomers).
Simple stretching: 1600-