Table of IR Absorptions
Functional
Group
|
Characteristic
Absorption(s) (cm-1)
|
Notes
|
Alkyl烷基
C-H Stretch
|
2950 - 2850 (m or s)
|
Alkane C-H bonds are fairly
Ubiquitous 普遍存在 and therefore
usually lessuseful in determining
structure.
|
Alkenyl C-H Stretch
Alkenyl C=C Stretch
烯烃
|
3100 - 3010 (m)
1680 - 1620 (v)
|
Absorption peaks above 3000 cm-1
are frequently diagnostic判断 of
unsaturation不饱和
|
Alkynyl C-H Stretch
Alkynyl C三C Stretch
|
~3300 (s)
2260 - 2100 (v)
|
炔烃
|
Aromatic C-H Stretch
Aromatic C-H Bending
Aromatic C=C Bending
|
~3030 (v)
860 - 680 (s)
1700 - 1500 (m,m)
|
芳烃
|
Alcohol/Phenol 醇/酚
O-H Stretch
|
3550 - 3200 (broad, s)
|
See
|
Carboxylic Acid 羧酸
O-H Stretch
|
3000 - 2500 (broad, v)
|
|
Amine 氨
N-H Stretch
|
3500 - 3300 (m)
|
Primary amines produce two N-H
stretch absorptions, secondary
amides 氨基化合物only one, and
tetriary none.
|
Nitrile C=N Stretch
|
2260 - 2220 (m)
|
睛基
|
Aldehyde C=O Stretch
Ketone C=O Stretch
Ester C=O Stretch
Carboxylic Acid
C=O Stretch
Amide C=O Stretch
|
1740 - 1690 (s)醛
1750 - 1680 (s) 酮
1750 - 1735 (s) 酯
1780 - 1710 (s) 羧酸
1690 - 1630 (s) 酰羰基
|
The carbonyl stretching absorption is
one of the strongest IR absorptions,
and is very useful in structure
determination as one can determine both
the number of carbonyl groups 羰基
(assumingpeaks do not overlap)
but also an estimation of which types.
|
Amide N-H Stretch
|
3700 - 3500 (m)
|
As with amines, an amide produces zero
to two N-H absorptions depending on
its type.
|
All figures are for the typical case only -- signal positions and intensities may vary depending on the particular bond environment.
v - variable, m - medium, s - strong, br - broad, w - weak
These trends in aborption can be further summarized into the following categories 类别
3600 - 2700 cm-1
|
X-H
|
2700 - 1900 cm-1
|
X=Y
|
1900 - 1500 cm-1
|
X=Y
|
1500 - 500 cm-1
|
X-Y
|
Upon first inspection, a typical infrared spectrum can be visually divided into two regions. The left half, above 2000 cm-1, usually contains relatively few peaks, but some very diagnostic information can be found here.
First, alkane C-H stretching absorptions just below 3000 cm-1 demonstrate the presence of saturated carbons, and signals just above 3000 cm-1 demonstrate unsaturation. A very broad peak in the region between 3100 and 3600 cm-1 indicates the presence of exchangeable protons可交换的质子, typically from alcohol, amine, amide or carboxylic acid groups (see further discussion of this below).
The frequencies from 2800 to 2000 cm-1 are normally void of other absorptions, so the presence of alkyne 炔 or nitrile groups can be easily seen here.
In contrast, the right half of the spectrum, below 2000 cm-1, normally contains many peaks of varying intensities, many of which are not readily identifiable. Two signals which can be seen clearly in this area is the carbonyl group, which is a very strong peak around 1700 cm-1, and the C-O bond with can be one or two strong peaks around 1200 cm-1. This complex lower region is also known as the "fingerprint region" because almost every organic compound produces a unique pattern in this area -- Therefore identity can often be confirmed by comparison of this region to a known spectrum.
Quick Procedures for Infrared Analysis
Infrared spectra
/span> It is important to remember that the absence of an absorption band can often provide more information about the structure of a compound than the presence of a band. Be careful to avoid focusing on selected absorption bands and overlooking others. Use the examples linked to the table to see the profile and intensity of bands. Remember that the absence of a band may provide more information than the presence of an absorption band. Look for absorption bands in decreasing order of importance:
- the C-H absorption(s) between 3100 and 2850 cm-1. An absorption above 3000 cm-1 indicates C=C, either alkene or aromatic. Confirm the aromatic ring by finding peaks at 1600 and 1500 cm-1 and C-H out-of-plane 面外 bending to give substitution patterns below 900 cm-1. Confirm alkenes with an absorption at 1640-1680 cm-1. C-H absorption between 3000 and 2850 cm-1 is due to aliphatic 脂肪族 hydrogens. 脂肪族氢
- the carbonyl (C=O) absorption between 1690-1760cm-1; this strong band indicates either an aldehyde醛, ketone酮, carboxylic acid, ester酯, amide, anhydride 酸酐 or acyl halide卤化酰基. The an aldehyde may be confirmed with C-H absorption from 2840 to 2720 cm-1.
- the O-H or N-H absorption between 3200 and 3600 cm-1. This indicates either an alcohol, N-H containing amine or amide, or carboxylic acid. For -NH2 a doublet 成对的 will be observed.
- the C-O absorption between 1080 and 1300 cm-1. These peaks are normally rounded like the O-H and N-H peak in 3. and are prominent突出的. Carboxylic acids, esters, ethers 醚, alcohols and anhydrides all containing this peak.
- the C三C and C三N triple bond absorptions at 2100-2260 cm-1 are small but exposed 暴露.
- a methyl group may be identified with C-H absorption at 1380 cm-1. This band is split into 分成a doublet for isopropyl(gem-dimethyl) groups异丙基.
- structure of aromatic compounds may also be confirmed from the pattern of the weak overtone 含义 and combination tone bands found from 2000 to 1600 cm-1.
CHARACTERISTIC INFRARED ABSORPTION FREQUENCIES
|
Bond
|
Compound Type
|
Frequency range, cm-1
|
C-H
|
|
2960-2850(s) stretch
|
1470-1350(v) scissoring and bending
|
|
1380(m-w) - Doublet – isopropyl异丙基, t-butyl 丁基
|
C-H
|
|
3080-3020(m) stretch
|
1000-675(s) bend
|
C-H
|
|
3100-3000(m) stretch
|
|
870-675(s) bend
|
|
2000-1600(w) - fingerprint region
|
C-H
|
|
3333-3267(s) stretch
|
700-610(b) bend
|
C=C
|
|
1680-1640(m,w)) stretch
|
CºC
|
|
2260-2100(w,sh) stretch
|
C=C
|
|
1600, 1500(w) stretch
|
C-O
|
|
1260-1000(s) stretch
|
C=O
|
|
1760-1670(s) stretch
|
O-H
|
Monomeric -- Alcohols, Phenols
|
3640-3160(s,br) stretch
|
|
3600-3200(b) stretch
|
|
3000-2500(b) stretch
|
N-H
|
|
3500-3300(m) stretch
|
1650-1580 (m) bend
|
C-N
|
|
1340-1020(m) stretch
|
CºN
|
|
2260-2220(v) stretch
|
NO2
|
|
1660-1500(s) asymmetrical stretch
|
1390-1260(s) symmetrical stretch
|
v - variable, m - medium, s - strong, br - broad, w - weak
For steps to follow in the analysis of an infrared spectrum select this link
