2024年5月10日发(作者：廉昊苍)
IR Spectroscopy Tutorial: Alkanes
The spectra of simple alkanes are characterized by absorptions due to C–H
stretching and bending (the C–C stretching and bending bands are either
too weak or of too low a frequency to be detected in IR spectroscopy).
In simple alkanes, which have very few bands, each band in the spectrum
can be assigned.
C–H stretch from 3000–2850 cm
-1
-1
•
C–H bend or scissoring from 1470-1450 cm
-1
•
C–H rock, methyl from 1370-1350 cm
•
C–H rock, methyl, seen only in long chain alkanes, from 725-720
-1
cm
•
The IR spectrum of octane is shown below. Note the strong bands in the
3000-2850 cm
-1
region due to C-H stretch. The C-H scissoring (1470), methyl
rock (1383), and long-chain methyl rock (728) are noted on this spectrum.
Since most organic compounds have these features, these C-H vibrations
are usually not noted when interpreting a routine IR spectrum.
The region from about 1300-900 cm
-1
is called the fingerprint region. The
bands in this region originate in interacting vibrational modes resulting
in a complex absorption pattern. Usually, this region is quite complex
and often difficult to interpret; however, each organic compound has its
own unique absorption pattern (or fingerprint) in this region and thus
an IR spectrum be used to identify a compound by matching it with a sample
of a known compound.
IR Spectroscopy Tutorial: Alkenes
Alkenes are compounds that have a carbon-carbon double bond, –C=C–. The
stretching vibration of the C=C bond usually gives rise to a moderate band
-1
in the region 1680-1640 cm.
Stretching vibrations of the –C=C–H bond are of higher frequency (higher
wavenumber) than those of the –C–C–H bond in alkanes.
This is a very useful tool for interpreting IR spectra: Only alkenes and
-1
aromatics show a C-H stretch slightly higher than 3000 cm. Compounds that
do not have a C=C bond show C-H stretches only below 3000 cm
-1
.
The strongest bands in the spectra of alkenes are those attributed to the
carbon-hydrogen bending vibrations of the =C–H group. These bands are
in the region 1000-650 cm
-1
(Note: this overlaps the fingerprint region).
2024年5月10日发(作者：廉昊苍)
IR Spectroscopy Tutorial: Alkanes
The spectra of simple alkanes are characterized by absorptions due to C–H
stretching and bending (the C–C stretching and bending bands are either
too weak or of too low a frequency to be detected in IR spectroscopy).
In simple alkanes, which have very few bands, each band in the spectrum
can be assigned.
C–H stretch from 3000–2850 cm
-1
-1
•
C–H bend or scissoring from 1470-1450 cm
-1
•
C–H rock, methyl from 1370-1350 cm
•
C–H rock, methyl, seen only in long chain alkanes, from 725-720
-1
cm
•
The IR spectrum of octane is shown below. Note the strong bands in the
3000-2850 cm
-1
region due to C-H stretch. The C-H scissoring (1470), methyl
rock (1383), and long-chain methyl rock (728) are noted on this spectrum.
Since most organic compounds have these features, these C-H vibrations
are usually not noted when interpreting a routine IR spectrum.
The region from about 1300-900 cm
-1
is called the fingerprint region. The
bands in this region originate in interacting vibrational modes resulting
in a complex absorption pattern. Usually, this region is quite complex
and often difficult to interpret; however, each organic compound has its
own unique absorption pattern (or fingerprint) in this region and thus
an IR spectrum be used to identify a compound by matching it with a sample
of a known compound.
IR Spectroscopy Tutorial: Alkenes
Alkenes are compounds that have a carbon-carbon double bond, –C=C–. The
stretching vibration of the C=C bond usually gives rise to a moderate band
-1
in the region 1680-1640 cm.
Stretching vibrations of the –C=C–H bond are of higher frequency (higher
wavenumber) than those of the –C–C–H bond in alkanes.
This is a very useful tool for interpreting IR spectra: Only alkenes and
-1
aromatics show a C-H stretch slightly higher than 3000 cm. Compounds that
do not have a C=C bond show C-H stretches only below 3000 cm
-1
.
The strongest bands in the spectra of alkenes are those attributed to the
carbon-hydrogen bending vibrations of the =C–H group. These bands are
in the region 1000-650 cm
-1
(Note: this overlaps the fingerprint region).