Chapter_2_9 - Chapter 2 9 Infrared Spectroscopy(IR Mass...

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Chapter 2, 9 Infrared Spectroscopy (IR) , Mass Spectroscopy (MS) , Nuclear Magnetic Resonance (NMR)
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2 Infrared Spectroscopy: An Instrumental Method for Detecting Functional Groups Electromagnetic radiation in the infrared (IR) frequency range is absorbed by a molecule at certain characteristic frequencies Energy is absorbed by the bonds in the molecule and they vibrate faster The bonds behave like tiny springs connecting the atoms The bonds can absorb energy and vibrate faster only when the added energy is of a particular resonant frequency The frequencies of absorption are very characteristic of the type of bonds contained in the sample molecule The type of bonds present are directly related to the functional groups present A plot of these absorbed frequencies is called an IR spectrum
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3 Infrared Spectrometer An infrared spectrometer detects the frequencies absorbed by the sample molecule Light of all the various IR frequencies is transmitted to the molecule and the frequencies absorbed are recorded The absorption frequencies are specified as wavenumbers in units of reciprocal centimeters (cm -1 ) Alternatively the wavelength (λ29 in units of microns ( μ m) can be specified The spectrum is a plot of frequency on the horizontal axis versus strength of absorption on the vertical axis
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4 There are different types of stretching and bending vibrations induced by the absorption of infrared energy The actual relative frequency of vibration can be predicted Bonds with lighter atoms vibrate faster than those with heavier atoms
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5 Triple bonds (which are stiffer and stronger) vibrate at higher frequencies than double bonds Double bonds in turn vibrate at higher frequencies than single bonds The IR spectrum of a molecule usually contains many peaks These peaks are due to the various types of vibrations available to each of the different bonds Additional peaks result from overtone (harmonic) peaks which are weaker and of lower frequency The IR is a “fingerprint” of the molecule because of the unique and large number of peaks seen for a particular molecule
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6
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7 Interpreting IR Spectra Generally only certain peaks are interpreted in the IR Those peaks that are large and above 1400 cm -1 are most valuable Hydrocarbons The C-H stretching regions from 2800-3300 cm -1 is characteristic of the type of carbon the hydrogen is attached to C-H bonds where the carbon has more s character are shorter, stronger and stiffer and thus vibrate at higher frequency C-H bonds at sp centers appear at 3000-3100 cm -1 C-H bonds at sp 2 centers appear at about 3080 cm -1 C-H bonds at sp 3 centers appear at about 2800-3000 cm -1 C-C bond stretching frequencies are only useful for multiple bonds C-C double bonds give peaks at 1620-1680 cm -1 C-C triple bonds give peaks at 2100-2260 cm -1 These peaks are absent in symmetrical double and triple bonds
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8 Example: octane Example: 1- hexyne
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Chapter_2_9 - Chapter 2 9 Infrared Spectroscopy(IR Mass...

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