Four common spectroscopic techniques used to determine the structure of organic compounds: Infrared Spectroscopy (IR) Mass Spectrometry (MS or Mass Spec) Nuclear Magnetic Resonance Spectroscopy (NMR) Ultraviolet Spectroscopy
Infrared Spectroscopy
Infrared spectroscopy:
Used to determine the functional groups present (or absent) in a molecule
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 1/5/11)
Infrared Spectroscopy
Infrared spectroscopy is a type of absorption spectroscopy in which:
Sample is irradiated by an infrared light source
The amount of light transmitted (or absorbed) at various wavelengths is measured by a detector
A spectrum is obtained. Graph of light transmitted (or absorbed) as a function of wavelength The
position of an absorption peak in the spectrum is reported in wavenumbers (u) the number of wavelengths per cm u = 10,000 where l = mm l
Infrared Spectroscopy
The covalent bond between two atoms acts like a spring, allowing the atoms to vibrate (stretch and bend) relative to each other.
Absorption of IR radiation increases the amplitude of the bond vibrations. Stretching Symmetric Asymmetric Bending
Infrared Spectroscopy
Since energy is quantized, covalent bonds can vibrate/stretch only at certain allowed frequencies.
The position of a peak in an IR spectrum correlates with the type of chemical bond.
Infrared Spectroscopy
The frequency of an absorption band in an IR spectrum depends primarily on:
Type of vibration Stretching vibrations: higher frequency Bending vibrations: lower frequency
Masses of the atoms in a bond AW
Freq
Strength of the bond or bond order BO
Freq
Infrared Spectroscopy
The polarity of a bond has a significant impact on the intensity of an IR absorption band.
Vibrations that cause a significant change in the dipole moment of a chemical bond lead to strong absorption bands.
Vibrations that result in no change/very little change in dipole moment lead to very weak or no absorption band. Symmetrical
bonds often exhibit very weak or no absorption band.
Infrared Spectroscopy
Each molecule has a unique IR spectrum. The IR spectrum is a “fingerprint” for the molecule.
IR spectrum results from a combination of all possible stretching and/or bending vibrations of the individual bonds and the whole molecule. Simple stretching: ~1600-4000 cm-1. Complex vibrations: 600-1400 cm-1, called the “fingerprint region.”
Infrared Spectroscopy
IR Spectrum of n-octane
Infrared Spectroscopy
An IR spectrum is used to identify functional groups that are present (or absent).
Cannot conclusively identify a structure by IR alone unless an IR spectrum of an authentic (known) sample of the compound is available.
Absorptions from specific functional groups are found in certain regions of the IR spectrum.
Carbon-Carbon Bonds
Increasing bond order leads to higher frequencies: C-C 1200 cm-1 (fingerprint region) C=C 1600 - 1680 cm-1 CC 2200 cm-1 (weak or absent if internal)
C=C peaks are generally weak to moderate in intensity.
Carbon-Hydrogen Bonds
Bonds with more s character absorb at a higher frequency. sp3 (alkane) C-H just
below 3000 cm-1 (to the right)
sp2 (alkene or aromatic hydrocarbon) C-H
sp (alkyne) C-H
just
at
above 3000 cm-1 (to the left)
3300 cm-1
O-H Bonds
The peak for an alcohol O-H stretch is commonly centered at around 3000 cm-1. broad with rounded tip when hydrogen bonding is present sharp in the absence of hydrogen bonding
The peak for a carboxylic acid O-H is very broad and spans the region from ~2500-3500 cm-1. Acid Acid O-H
O-H
N-H Bonds
Peak(s) for the N-H stretch also appear around 3300 cm-1, but they usually look different from the alcohol O-H peak. Primary amine/amide (RNH2 or RCONH2) Broad (usually) with two sharp spikes.
Secondary amine/amide (R2NH or RCONHR’) Broad (usually) with one sharp spike
No signal for a tertiary amine/amide (R3N or RCONR2’)
NH stretch
NH stretch
NH2 stretch
NH Bend
A broad, round peak may be observed around 1600 cm-1 for the N – H bend, especially with primary amines. NH2 stretch
N-H bend has a different shape than an aromatic ring or C=C
N-H bend
Carbonyls
Carbonyl stretches are generally strong:
Conjugation shifts all carbonyls to lower frequencies.
Ring strain shifts carbonyls to higher frequencies. H3C
-1
1745 cm
O
Aldehydes
overtone
1710-1725 cm-1
Carboxylic Acids
Ketones overtone
Esters
C=O stretch at ~ 1730-1760 cm-1
C-O stretch at 1000-1300 cm-1 (broad)
and
strong
(Note: other functional groups may have peaks in the 1000-1300 cm-1 region too!)
O O
1743
1245
Amides
C=O stretch at 1640-1680 cm-1
N-H stretch (if 1o or 2o) around 3300 cm-1
double peak)
(sometimes a
Nitriles
C N absorbs just above 2200
cm-1 (med – strong)
The alkyne C C signal is much weaker and is just below 2200 cm-1
IR Spectroscopy Example: Interpret the following IR spectrum by assigning each of the major peaks. Identify what functional group(s) are present.
3403 cm-1
1604 cm-1
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 12/30/09)
IR Spectroscopy Example: Interpret the following IR spectrum by assigning each of the major peaks. Identify what functional group(s) are present.
2814 cm-1
2733 cm-1
1691 cm-1
1642 cm-1
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 12/30/09)
Infrared Spectroscopy Example: Which one of the following compounds is the most reasonable structure for the IR spectrum shown below? OCH3
O
O
O
OH O
1721
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 12/30/09)
Infrared Spectroscopy Example: Which of the following compounds is the most reasonable structure for the IR spectrum shown below? O NH2
OH
OH
O
OH O
O OH
1603 1689 http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 12/30/09)
Infrared Spectroscopy (IR)
Infrared Spectroscopy
Four common spectroscopic techniques used to determine the structure of organic compounds: Infrared Spectroscopy (IR) Mass ...
