Infrared Spectroscopy and Mass Spectroscopy CH12 Leroy Wade Mohammed Ali

12-1 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

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

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

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

12-4 Molecuar Vibrations Mohammed Ali

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An Infrared (IR) Spectrum Mohammed Ali

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

IR frequency range 600-4000 cm-1 Fingerprint region: 600 – 1400 cm-1 frequencies of two different molecules are different in this region. 1600-3500 cm-1 where the characteristic frequencies of various FG shows. Mohammed Ali

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

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

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

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Infrared Spectrum of tert-butylbenzene Monsubstituted benzene Mohammed Ali

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

An amine: Mohammed Ali

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

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Carboxylic acid Mohammed Ali

Conjugated C=O groups Mohammed Ali

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Mass Spectroscopy Mohammed Ali

GC-MS Mohammed Ali

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

Mass spectrum of 2-butanone Mohammed Ali

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

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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Isotope Peaks: 35Cl : 37Cl = 3:1 Mohammed Ali

Isotope peaks: 79Br : 81Br = 1:1 Mohammed Ali

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Benzyl bromide: base peak m/z 91 Mohammed Ali 1