Infrared Spectroscopy
紅外線光譜儀
Introduction
• Spectroscopy is an analytical technique
which helps determine structure.
• It destroys little or no sample (nondestructive method).
• The amount of light absorbed by the
sample is measured as wavelength is
varied.
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Types of Spectroscopy
• Infrared (IR) spectroscopy measures the bond
vibration frequencies in a molecule and is used
to determine the functional group.
• Mass spectrometry (MS) fragments the molecule
and measures the masses.
• Nuclear magnetic resonance (NMR)
spectroscopy detects signals from hydrogen
atoms and can be used to distinguish isomers.
• Ultraviolet (UV) spectroscopy uses electron
transitions to determine bonding patterns. =>
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Electromagnetic Spectrum
• Examples: X rays, microwaves, radio waves, visible light, IR, and UV.
• Electromagnetic radiation has the characteristics of both
waves and particles
• The wave nature of electromagnetic radiation is described by
wavelength (l) or frequency (n)
• The relationship between wavelength (or frequency) and
energy (E) is well defined
• Wavelength and frequency are inversely proportional (n= c/l)
• The higher the frequency, the greater the energy of the wave
• The shorter the wavelength, the greater the energy of the
wave
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The Spectrum and
Molecular Effects
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The IR Region
• Just below red in the visible region.
• Wavelengths usually 2.5-25 mm.
• More common units are wavenumbers, or
cm-1, the reciprocal of the wavelength in
centimeters.
• Wavenumbers are proportional to frequency
and energy.
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Molecular Vibrations
Covalent bonds vibrate at only certain
allowable frequencies.
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Stretching Frequencies
• Frequency decreases with increasing
atomic weight.
• Frequency increases with increasing
bond energy.
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Chapter 12
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Vibrational Modes
Nonlinear molecule with n atoms usually has
3n - 6 fundamental vibrational modes.
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Fingerprint of Molecule
• No two molecules will give exactly the
same IR spectrum (except enantiomers).
• Simple stretching: 1600-3500 cm-1has the
most common vibrations, and we can use
it to get information about specific
functional groups in the molecule.
 Complex vibrations (bending): 6001400 cm-1, called the “fingerprint region”
and has the most complex
Chapter 12 vibrations.
12
IR-Active and Inactive
• A polar bond is usually IR-active.
• A nonpolar bond in a symmetrical
molecule will absorb weakly or not at all.
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An Infrared Spectrometer
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FT–IR Spectrometer
• Has better sensitivity.
• Less energy is
needed from source.
• Completes a scan in
1 to 2 seconds.
• Takes several scans
and averages them.
• Has a laser beam that
keeps the instrument
accurately calibrated.
Carbon-Carbon (C-C)
Bond Stretching
• Stronger bonds absorb at higher
frequencies:
C-C
C=C
CC
1200 cm-1
1660 cm-1
2200 cm-1 (weak or absent if internal)
• Conjugation lowers the frequency:
isolated C=C
1640-1680 cm-1
conjugated C=C 1620-1640 cm-1
aromatic C=C
approx. 1600 cm-1
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Carbon-Hydrogen (C-H)
Stretching
Bonds with more s character absorb at a
higher frequency.
sp3 C-H, just below 3000 cm-1 (to the right)
sp2 C-H, just above 3000 cm-1 (to the left)
sp C-H, at 3300 cm-1
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Examples
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An Alkane IR Spectrum
n >3000; d 1465, 1375
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An Alkene IR Spectrum
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An Alkyne IR Spectrum
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O-H and N-H Stretching
• Both of these occur around 3300 cm-1,
but they look different.
Alcohol O-H, broad with rounded tip.
Secondary amine (R2NH), broad with one
sharp spike.
Primary amine (RNH2), broad with two
sharp spikes.
No signal for a tertiary amine (R3N)
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Chapter 12
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Chapter 12
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• The O-H stretching absorption is very characteristic
– In very dilute solutions, hydrogen bonding is absent and there is a
very sharp peak at 3590-3650 cm-1
– In more concentrated solutions, the hydroxyl groups hydrogen bond
to each other and a very broad and large peak occurs at 32003550 cm-1
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An Alcohol IR Spectrum
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An Amine
IR Spectrum
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Carbonyl(n C=O) Stretching
• The C=O bond of simple ketones,
aldehydes, and carboxylic acids absorb
around n 1710 cm-1.
• =>
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• Usually, it’s the strongest IR signal.
• Carboxylic acids will have O-H also.
• Aldehydes have two C-H signals around
2700 and 2800 cm-1.
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A Ketone
IR Spectrum
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An Aldehyde (n HC=O)
IR Spectrum
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O-H Stretch of a
Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm-1,
due to strong hydrogen bonding.
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Variations in
C=O Absorption
• Conjugation of C=O with C=C lowers the
stretching frequency to ~1680 cm-1.
• The C=O group of an amide absorbs at an
even lower frequency, 1640-1680 cm-1.
• The C=O of an ester absorbs at a higher
frequency, ~1730-1740 cm-1.
• Carbonyl groups in small rings (5 C’s or
less) absorb at an even higher frequency.
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習題
•
Portions of the infrared spectra of three cyclic ketones and
three exocyclic alkenes show the influence of ring strain on
the C=O and C=C stretching frequency. Please indicate
your combination and explain your reasons.
•
H2C
CH2
O
CH2
O
O
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習題
• The C=O vibration frequencies are varied to different
molecules which reveals the influence of conjugation
and other factors. Show the order of the carbonyl
absorption of the following molecules and explain
your reasons for full credits.
(III)
(II)
(I)
O
O
O
O
O
O
OH CH3
(IV)
(V)
O
O
CH3
O
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O CH3
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An Amide
IR Spectrum
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習題
• The absorption of carbonyl group in amide is usually in
the range from 1680 to 1630 cm-1. However, the
following compound has the C=O band appearing about
1700cm-1. Please explain this observation.
•
N
C=O:1700cm-1
O
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Carbon - Nitrogen
Stretching (C~N)
• C - N absorbs around 1200 cm-1.
• C = N absorbs around 1660 cm-1 and is
much stronger than the C = C
absorption in the same region.
• C  N absorbs strongly just above 2200
cm-1. The alkyne C  C signal is much
weaker and is just below 2200 cm-1 .
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A Nitrile
IR Spectrum
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Summary of IR Absorptions
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TODAY’S CHEMIST AT WORK
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• In addition to the physical symptoms, diseases cause
changes in the chemical composition of the organs,tissues,
or fluids they affect; these differences are the basis of
everyday clinical chemical tests, tissue staining, and medical
imaging techniques.
• IR spectroscopy not only probes the chemical composition
of a sample but also determines the precise position and
amplitude of IR absorption bands that reflect interactions
among the matrix constituents. Because of its sensitivity to
both molecular structure and molecular interactions, the
spectrum is often referred to as a molecular fingerprint of the
sample;
• the specificity of that fingerprint is the basis for biomedical
applications.
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Strengths and Limitations
•
•
•
•
IR alone cannot determine a structure.
Some signals may be ambiguous.
The functional group is usually indicated.
The absence of a signal is definite proof
that the functional group is absent.
• Correspondence with a known sample’s
IR spectrum confirms the identity of the
compound.
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