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Infrared Spectroscopy Infrared Spectroscopy and Mass Spectroscopyand Mass Spectroscopy
CH12CH12
Leroy WadeLeroy Wade
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12-1 Introduction12-1 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.
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Spectroscopic TechniquesSpectroscopic 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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12-2 The Electromagnetic Spectrum12-2 The Electromagnetic Spectrum
Fig 12-1
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12-3 The Infrared Region12-3 The Infrared Region
The position of IR band is specified by its wavelength (), measured in micron, one millionths of a meter.
A more common unit is wave number, , number of cycles (wavelength) of the wave in a centimeter.
The wavenumber is the reciprocal of the wavelength (in centimeter)
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12-4 Molecuar Vibrations12-4 Molecuar Vibrations
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An Infrared (IR) SpectrumAn Infrared (IR) Spectrum
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12-5 IR Active and IR Inactive12-5 IR Active and IR InactiveVibrationsVibrations
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.
CH3CH3
CH3H
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IR frequency range 600-4000 cm-1IR 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.
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12-6 Measurement of IR Spectrum
Fig 12-4 Block diagram of an interferometer in an FT-IR
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Carbon-Hydrogen Bond Stretching FrequenciesCarbon-Hydrogen Bond Stretching Frequencies
C C H C C
H
C C H
2800 - 3000 cm-1 3000 - 3100 cm-1
(SP)
(SP3) (SP2)
3300 cm-1
12-7 Infrared Spectroscopy of Hydrocarbons12-7 Infrared Spectroscopy of Hydrocarbons
Note: C-H bending frequencies have lower wave numbers
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Carbon-Carbon Bond Stretching Frequencies
C C C C
C C
1660 cm-1
< 2200 cm-1
1200 cm-1
SP3 SP2
SP
Note: C-C bending frequencies have lower wave numbers
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Infrared Spectrum of tert-butylbenzeneInfrared Spectrum of tert-butylbenzene
H
C
CH3CH2
CH3
H
MonsubstitutedMonsubstitutedbenzenebenzene
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12-8 Characteristic Absorptions 12-8 Characteristic Absorptions of Alcohols and Aminesof Alcohols and Amines
An Alcohol:An Alcohol:
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An amine:An amine:
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12-9 Characteristic Absorptions of 12-9 Characteristic Absorptions of Carbonyl CompoundsCarbonyl Compounds
(Esters and carbonates have slightly higher C=O absorption value)(Esters and carbonates have slightly higher C=O absorption value)
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Carboxylic acidCarboxylic acid
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Conjugated C=O groupsConjugated C=O groups
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C N
C N
C N
1200 cm-1
1660 cm-1
2200 cm-1
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Mass SpectroscopyMass Spectroscopy
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GC-MSGC-MS
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Mass spectrum of 2-butanoneMass spectrum of 2-butanone
72
57
43
Intensity
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Mass spectrum of 2-butanone
CH3 C
O
CH2CH3
CH3 C
O
CH2CH3
CH3CH3 C
O
C
O
CH2CH3CH2CH3
Electron beamknocks offan electron
loss of ethyl radical
loss of methylradical
MW = 72
m/z = 72, M+
(molecular ion)
m/z = 57
m/z = 43
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“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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• 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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Isotope Peaks: Isotope Peaks: 3535Cl : Cl : 3737Cl = 3:1Cl = 3:1
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Isotope peaks:Isotope peaks: 79 79Br : Br : 8181Br = 1:1Br = 1:1
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Benzyl bromide: base peak m/z 91Benzyl bromide: base peak m/z 91
