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Infrared Spectroscopy and Mass Spectroscopy

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Presentation on theme: "Infrared Spectroscopy and Mass Spectroscopy"— Presentation transcript:

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. Mohammed Ali

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. Mohammed Ali

4 12-2 The Electromagnetic Spectrum
Fig 12-1 Mohammed Ali

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) Mohammed Ali

6 Molecuar Vibrations Mohammed Ali

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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. Mohammed Ali

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. Mohammed Ali

11 12-6 Measurement of IR Spectrum
Fig 12-4 Block diagram of an interferometer in an FT-IR Mohammed Ali

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 Mohammed Ali

13 Carbon-Carbon Bond Stretching Frequencies
Note: C-C bending frequencies have lower wave numbers Mohammed Ali

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16 Infrared Spectrum of tert-butylbenzene
Monsubstituted benzene Mohammed Ali

17 12-8 Characteristic Absorptions of Alcohols and Amines
An Alcohol: Mohammed Ali

18 An amine: Mohammed Ali

19 12-9 Characteristic Absorptions of Carbonyl Compounds
(Esters and carbonates have slightly higher C=O absorption value) Mohammed Ali

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21 Carboxylic acid Mohammed Ali

22 Conjugated C=O groups Mohammed Ali

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25 Mass Spectroscopy Mohammed Ali

26 GC-MS Mohammed Ali

27 Mass spectrum of 2-butanone
72 57 43 Intensity Mohammed Ali

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. Mohammed Ali

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. Mohammed Ali

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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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