1. Infrared Spectroscopy and Mass Spectroscopy
CH12
Leroy Wade
Mohammed Ali

2. Introduction
Absorption Spectroscopy: Measurement of the amount of light absorbed by a compound as a function of the wavelength of light.
In general, a sample is irradiated by a light source, and the amount of light transmitted at various wavelenght is measured by a detector and plotted on a graph.
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3. Spectroscopic Techniques:
Infrared (IR) Spectroscopy: vibrations of bonds – functional groups.
Mass (MS) Spectroscopy: molecules are bombarded with electrons to break into small fragments – MW, MF, clues to the FG and structure.
Nuclear Magnetic Resonance (NMR) Spectroscopy: observes chemical environments – structure of molecules.
Ultraviolet (UV) Spectroscopy: observes electronic transitions – electronic bonding in the sample.
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4. 12-2 The Electromagnetic Spectrum
Fig 12-1
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5. 12-3 The Infrared Region
The position of IR band is specified by its wavelength (l), measured in micron, one millionths of a meter.
A more common unit is wave number,u , number of cycles (wavelength) of the wave in a centimeter.
The wavenumber is the reciprocal of the wavelength (in centimeter)
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6. Molecuar Vibrations
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8. An Infrared (IR) Spectrum
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9. 12-5 IR Active and IR Inactive Vibrations
Vibrations of bonds with dipole moment
absorbs IR and are said to be IR active.
Vibrations of bonds (symmetric bonds)
without a dipole moment does not absorb
IR and are said to be IR inactive.
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10. IR frequency range 600-4000 cm-1
Fingerprint region: 600 – 1400 cm-1 frequencies of two different molecules are different in this region.
cm-1 where the characteristic frequencies of various FG shows.
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11. 12-6 Measurement of IR Spectrum
Fig 12-4 Block diagram of an interferometer in an FT-IR
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12. Carbon-Hydrogen Bond Stretching Frequencies
12-7 Infrared Spectroscopy of Hydrocarbons
Carbon-Hydrogen Bond Stretching Frequencies
Note: C-H bending frequencies have lower wave numbers
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13. Carbon-Carbon Bond Stretching Frequencies
Note: C-C bending frequencies have lower wave numbers
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16. Infrared Spectrum of tert-butylbenzene
Monsubstituted benzene
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17. 12-8 Characteristic Absorptions of Alcohols and Amines
An Alcohol:
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18. An amine:
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19. 12-9 Characteristic Absorptions of Carbonyl Compounds
(Esters and carbonates have slightly higher C=O absorption value)
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21. Carboxylic acid
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22. Conjugated C=O groups
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25. Mass Spectroscopy
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26. GC-MS
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27. Mass spectrum of 2-butanone72 57 43
Intensity
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28. Mass spectrum of 2-butanone
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29. Other important data we can obtain from mass spectrometry.
“m/z” means mass to charge ratio. Most ions have only one charge, so m/z equals the molecular weight of the ion. The ion produced after the first electron is removed is called the “molecular ion”, and sometimes abbreviated as M+.
Other important data we can obtain from mass spectrometry.
A compound with an odd number of nitrogens will have an odd molecular weight, usually. This is always true if the compound only contains C, H, N, and O.
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30. Compounds containing chlorine or bromine show two peaks two mass units apart for fragments that contain a chlorine or bromine. This is because chlorine is composed of two major isotopes, 35Cl and 37Cl. About 75% of Cl is 35Cl, and 25% is 37Cl. Therefore, the size of the two peaks will have a ratio of about 3:1. The two isotopes of Bromine occur in about equal amounts, so the two peaks will be about the same size.
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32. Isotope Peaks: 35Cl : 37Cl = 3:1
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33. Isotope peaks: 79Br : 81Br = 1:1
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37. Benzyl bromide: base peak m/z 91
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