INFRARED
SPECTROMETRY
Infrared radiation

Position:

The portion of the IR region most useful for
analysis of organic compounds has a
wavelength range from 2,500 to 16,000 nm,
with a corresponding frequency range of 19120 THz (1.9x1013 -1.2x1014 Hz)
Wavenumber


Band positions in IR spectra are usually expressed
as wavenumbers, n , in cm-1
4
10
n(cm- 1 ) =
l (mm)
wavenumbers are sometimes called "frequency",
but it is formally incorrect
•

frequency: ν = c/λ
In most cases IR spectra are recorded as a
function of n , and very rarely as a function
of λ
• they differ significantly
Polystyrene
Band intensity
Can be expressed as transmittance (T) or
absorbance (A)
 transmittance: T= I/I0
I – intensity of the radiation passing through the
sample
I0 – intensity of the incident radiation
 absorbance: A = log10 (1/T)
 for intensity of bands very often are in use semi
quantitative terms: strong (s), medium (m), weak
(w)

What happens with the molecule
when irradiated by IR?



IR radiation of the mentioned wavelength (2.5
to 16 µm) does not have enough energy (only
from 1 to 15 kcal/mol) for electronic transitions
but, it may induce vibrational excitation of
covalently bonded atoms and groups
The covalent bonds in molecules are not rigid
sticks or rods, but are more like stiff springs
that can be stretched and bent
Vibrational motions of
molecules


virtually all organic compounds will absorb infrared
radiation that corresponds in energy to a wide
variety of vibrational motions, characteristic of their
component atoms
A molecule composed of n-atoms has 3n degrees
of freedom
•
•
6 are translations and rotations of the molecule itself
the rest (3n-6) are degrees of vibrational freedom (3n-5 if the
molecule is linear) – fundamental vibrations.
The two main types of vibrations:
stretching and bending


stretching: rhythmic movement of atoms along
the bond axis, so the interatomic distance is
increasing and decreasing
bending: changing bond angle between two
atoms or groups
•
•
•
•
twisting
rocking
scissoring
wagging
Observable vibrations in IR



only those resulting from a rhythmical change
in the dipole moment of the molecule
vibrations cause the change in charge
distribution in a molecule → alternating electric
field
alternating electric field couples the molecule
vibration with the oscillating electric field of the
electromagnetic radiation
Fundamental vibrations


they involve no change in the center of
gravity of the molecule
three fundamental vibrations of the
threeatomic nonlinear water molecule
Vibrations of the CO2 molecule



threeatomic, linear molecule
3n-5=3x3-5=4 fundamental vibrations
symmetrical stretching (1) are inactive in IR
(no change in dipole moment of the molecule)
Number of fundamental
vibrations


observed number of absorption bands is bigger than the
theoretical number
•
•
multiples of a given frequency (overtones)
combination tones (sum of two vibrations)
•
•
•
•
•
bands fall outside of the 4000-400 cm-1 region
too weak bands to be observed
close vibration bands which coalesce
the occurrence of a degenerate band in highly symmetrical
molecules
not observable vibrations because of no change in
molecular dipole
observed number of absorption bands is lower than the
theoretical number
Hydrogen bonding

H-bonds – in any system containing a proton
donor group (X-H) and a proton acceptor (Y:)
• the common proton
donor groups in organic
molecules:
-COOH
-OH
-NH2
-CONH- (amido)
• the common proton
acceptor groups:
-O
-N
- Halogens
- C=C linkage
Instrumentation: Dispersion IR
spectrometer
Instrumentation:
FTIR Spectrometer

advantages
•
•
•
•
no monochromator –
the whole range of
radiation is passed
through the sample
simultaneously
high resolution
easier data
manipulation (digital)
convenient for coupling
with other analytical
instruments (HPLC, GC)
Sample handling


Gases
•
•
in special gas cells
paths: few cm to 40 m (by multiple reflection optics)
•
as neat or in solution
•
solutions:
Liquids
• neat: thin layer between two flat plates
• 1-10 mg required
• 0.1 – 1 mL volume, 0.05 – 10% concentration
• reference cell for solvent
• interferences of solvent
• combination of solvents
Sample handling: solids

usually examined as a mull
•
•

preparation: by thoroughly grinding 2-5 mg of solids in
a smooth, agate mortar
particles must not exceed 2 µm (to avoid scattering of
radiation)
in the form of pressed-disk (pellet)
•
•
the sample is mixed with powdered KBr and usually
homogenized in a small vibrating ball mill
then it's pressed in a nut portion
Interpretation of spectra

for correct interpretation - no general
rule, but still:
• the spectrum must be adequately resolved
•
•
•
and of adequate intensity
the compound must be reasonably pure
the spectrophotometer should be calibrated
the method of sample handling must be
specified
Preliminary examination areas

4000-1300 cm-1
• the functional group region
• characteristic stretching frequencies for
•
•
important functional groups, like: O-H, C-H
and C=O
attention: very broad signal could easily be
overlooked!
C=O group: strong absorption band in the
region of 1850-1540 cm-1
Region 4000-1300 cm-1



weak bands of S-H and C=C can be
useful
possible overtones and combinationtones
skeletal aromatic and heteroaromatic
bands fall in the 1600-1300 cm-1 region
Region 900-650 cm-1 and "fingerprint" region



Region 900-650 cm-1: generally region
where aromatic and heteroaromatic
bands are located
1300-900 cm-1: "finger-print" region
Spectrum analysis:
• evidences for presence or absence of several
•
characteristic groups
combining information from other
spectroscopy techniques: MS and NMR
Normal alkanes

The most characteristic are C-H stretching and bending
vibrations
•



C-C vibrations either fall out the region of interest (bending
vibrations, 500 cm-1), or are weak and of little value for
identification
those vibrations are common for many organic molecules
C-H stretching and bending frequencies of methyl and
methylene groups remain nearly constant in
hydrocarbons
if CH3 or CH2 groups are attached to atoms other than
carbon, or to a carbonyl group or aromatic ring,
appreciable shifts of the C-H stretching and bending
frequencies are observed



C-H stretching: 3000-2840 cm-1
•
among the most stable in the spectrum
•
two distinct bands: 2962 (asymmetrical) and 2872 cm-1
(symmetrical stretching)
CH3 group
CH2 group
•
•
bands: 2922 (asymmetrical) and 2853 cm-1 (symmetrical
stretching)
the positions do not vary more than ±10 cm-1 in aliphatic and
nonstrained cyclic hydrocarbons
C-H bending vibrations

In CH3 group
• symmetrical: δ CH (I) near 1375 cm
• asymmetrical: δ CH (II) near 1450 cm
s
-1
3
as
3
-1
In CH2 group


The scissoring band in the
spectra of hydrocarbons at a
nearly constant position near
1465 cm-1
rocking vibration - for
straight-chain alkanes of 7
or more carbon atoms – at
~720 cm-1
•

In the lower members of the
n-alkane series, the band
appears at somewhat
higher frequencies
twisting and wagging
vibrations are observed in
the 1350- 1150 cm-1 region
as weak bands
Branched alkanes


the changes in the IR spectrum are due to
skeletal stretching vibrations and methyl
bending vibrations
Occurrence: below 1560 cm-1
Cyclic Alkanes

C-H stretching vibrations
•
•

unstrained (larger) rings: almost the same absorption
bands as in acyclic compds.
Increasing ring strain moves the C-H stretching bands
progressively to high frequencies
C-H bending vibrations
•
CH2 scissoring in
• cyclohexane – 1452 cm ;
• cyclopentane - 1455 cm
• cyclopropane - 1442 cm
-1
-1
-1
n-hexane - 1468 cm-1
Alkenes

Alkene (olefinic) structures introduce
several new modes of vibration into a
hydrocarbon molecule:
• C=C stretching vibration
• =C-H stretching vibrations
• in-plane and out-of-plane bending of the =C-H
bond
Conjugated Systems



two C=C stretching bands
unsymmetrical conjugated diene, (e.g. 1,3pentadiene), shows absorption near 1650 and
1600 cm-1
symmetrical 1,3-butadiene shows only one
band near 1400 cm-1 (asymmetric stretching)
•
the symmetrical stretching band is inactive in the IR
Alkene C-H stretching vibration


any C-H stretching bands above 3000
cm-1 – from aromatic, heteroaromatic,
alkyne or alkene C-H stretching
in the same region: C-H stretching
vibrations in small rings (cyclopropane),
and in halogenated alkyl groups
Alkene C-H bending vibrations





in the same plane as the C=C bond or
perpendicular to it
in phase or out of phase
vinyl group absorbs near 1416 cm-1 (scissoring
vibration of the terminal methylene group)
C-H rocking vibration of a cis-disubstituted
alkene occurs in the same general region
The most characteristic vibrations - out-ofplane C-H bending between 1000 - 650 cm-1
•
the strongest in alkenes' IR spectra
Alkene C-H bending (cont.)

The most reliable bands are those of:
• the vinyl group
• the vinylidene group and
• the trans-disubstituted alkene

In allene structures:
• strong absorption near 850 cm
wagging)
-1
(=CH,
Alkynes

two stretching vibrations in alkynes (acetylenes): C≡C
and C-H stretching
•
•
•
•
•

C≡C: weak, in the region of 2260-2100 cm-1
not observable for symmetrically substituted alkynes
for monosubstituted alkynes, the band appears at 21402100 cm-1
disubstituted alkynes (asymmetric): 2260-2190 cm-1
C-H stretching for monosubstituted alkynes: 3333-3267 cm -1
•
strong absorption and narrower line than the H-bonded OH
and NH groups, whose vibrations appear in the same region
Absorption due to C-H bending is characteristic of
acetylene and mono substituted alkynes
•
strong, broad absorption band at 700-610 cm-1
Aromatic hydrocarbons




The most informative bands occur in the low
frequency range between 900 and 675 cm-1
(out-of-plane C-H bending)
In-plane bending bands appear in the 13001000 cm-1 region
Skeletal carbon-carbon stretching vibrations:
1600-1585 and 1500-1400 cm-1 regions.
The skeletal bands frequently appear as
doublets, depending on the nature of the ring
substituents.
Alcohols and
phenols


The characteristic
bands result from OH and C-O
stretching vibrations
O-H stretching in
alcohols and
phenols:
•
•
"free" (non H-bonded):
strong band in 37003584 cm-1 region
intermolecular Hbond: strong band in
3550-3200 cm-1 region
CH2OH
Alcohols and
phenols (cont.)


intramolecular H-bond:
broad and weak
absorption band
example: ohydroxyacetophenone
O
H
O
A: c = 0.03 moldm-3
CH3
o-hydroxyacetophenone
B: c = 1.00 moldm-3
C-O stretching in alcohols and
phenols

strong band in the region of 1260-1000 cm-1
Ethers



the characteristic stretching vibrations: C-O-C
stretching
relatively intense IR bands – larger changes in
dipole moment
aliphatic ethers:
•
•
•
characteristic strong absorption at 1150-1085 cm-1
(asymmetric C-O-C stretching)
symmetric stretching band is weaker
Branching on the carbon atoms adjacent to the
oxygen usually leads to splitting of the C-O-C band
(diisopropyl ether → triplet in the region 1170-1114
cm-1)
Carbonyl group


Strong C=O stretching vibrations in the region
1870-1540 cm-1
typical for:
•



ketones, aldehydes, carboxylic acids, esters, lactones,
acid halides, anhydrides, amides and lactams
relatively constant position
high intensity
one of the easiest to recognize in IR spectra
The position of the C=O
stretching band

is influenced by:
• the physical state
• electronic and mass effects of neighboring
•
•
•
substituents
conjugation
hydrogen bonding (intermolecular and
intramolecular) and
ring strain
Typical neat, saturated ketone
spectrum

solvent polarity has small effect on the position of
the C=O band (±25 cm-1)
•
•
non-polar: shifting toward higher frequencies
polar: shifting toward lower frequencies (comparing to the
neat compound)
Influence of conjugation

conjugation with the C=C bond leads to
delocalization of the π electrons of both
groups
•
•


double bond character of the C-O bond is reduced
absorption is shifted toward lower wavenumbers
conjugation with an alkene or phenyl group
causes absorption in the 1685-1666 cm-1
region
steric interferences affect coplanarity –
conjugation is hindered
Influence on H-bonding


intermolecular hydrogen bonding between a
ketone and a hydroxylic solvent (e.g.
methanol) causes a slight decrease in the
absorption frequency of the carbonyl group
Example:
•
•
neat ethyl methyl ketone absorbs at 1715 cm-1
10% solution of the ketone in methanol absorbs at
1706 cm-1.
Aldehydes



The carbonyl groups of aldehydes absorb at slightly
higher frequencies than that of the corresponding methyl
ketones
aliphatic aldehydes absorb near 1740 - 1720 cm-1
Aldehydic carbonyl absorption responds to structural
changes in the same manner as ketones
Aldehydes (cont.)



Electronegative substitution on the α-carbon
increases the frequency of carbonyl absorption
Conjugate unsaturation (as in α,β-unsaturated
aldehydes and benzaldehydes) reduces the
frequency of carbonyl absorption (1710-1685
cm-1)
lnternal hydrogen bonding shifts the absorption
to lower wavenumbers.
Carboxylic acids

In the liquid or solid state, and in CCI4
solution at concentrations much over
0.01 mol/L, they exist as dimers (strong
hydrogen bonding)
Carboxylic acids (cont.)






very broad, intense O-H stretching absorption in the
region of 3300-2500 cm-1
The C=O stretching bands of acids are considerably more
intense than ketonic C=O stretching bands
the monomers of saturated aliphatic acids absorb near
1760 cm-1
dimer has a center of symmetry; only the asymmetric
C=O stretching mode absorbs in the IR
Hydrogen bonding and resonance weaken the C-O bond,
resulting in absorption at a lower frequency than the
monomer.
The C=O group in dimerized saturated aliphatic acids
absorbs in the region of 1720-1706 cm-1.
Carboxylic acids (cont.)


Unsaturation in conjugation with the
carboxylic carbonyl group slightly
decreases the frequency of absorption
of both the monomer and dimer forms
Dimers of α,β-unsaturated and aryl
conjugated acids show absorption in the
1710-1680 cm-1 region.
Carboxylic acids (cont.)



electronegative substituent in the α-position
(e.g. halogen), brings about a slight increase in
the C=O absorption frequency (10-20 cm-1)
also observed dual carbonyl bands resulting
from rotational isomerism (field effect)
One of the characteristic bands in the spectra
of dimeric carboxylic acids: out-of-plane
bending of the O-H bond
•
•
•
at ~910 cm-1
broad
medium intensity
IR Spectrum of a typical
carboxylic acid
Carboxylate anion

hybrid structure consisting of two resonance
structures:
O
R C
R C
O


R C
O
O
bonds between C and O have strength intermediate
between C-O and C=O
two bands:
•
•

O
O
a strong asymmetrical stretching band near 1650-1550 cm-1
weaker, symmetrical stretching band near 1400 cm-1
no O-H stretching band
Amides and Lactams

All amides show a carbonyl absorption band known as the
amide I band
•
•

Primary amides:
•


two N-H stretching bands (symmetrical and asymmetrical N-H
stretching, 3520 and 3400 cm-1, respectively )
Secondary amides and lactams:
•

Its position depends on the degree of hydrogen bonding
it occurs at lower frequency than "normal" carbonyl band
(resonance effect)
only one N-H stretching band, at ~3500-3400 cm-1
the frequency of the N-H stretching is reduced by hydrogen
bonding
possible overlapping with O-H stretching vibrations (often
difficult to distinguish)
Amides and Lactams (cont.)

Primary and secondary amides, and a
few lactams:
• display a band or bands in the region of 1650•
1515 cm-1 (NH2, or NH bending) - the amide II
band
a broad band of medium intensity in the 800666 cm-1 region (out-of-plane N-H wagging)
Amines
Amino acids and their salts

Amino acids can exist in three forms:
• "free" acids (zwitter ions)
• salts in acidic medium
• salts in alkaline medium
IR spectra of free amino acids

A broad, strong NH3+ stretching band in the 31002600 cm-1 region
•
•

multiple combination and overtone bands extend the
absorption to about 2000 cm-1
a combination of the asymmetrical NH3+ bending vibration
and the torsional oscillation of the NH3+ group (which occurs
near 500 cm-1) is also possible
A weak asymmetrical NH3+ bending band near
1660-1610 cm-1, and a fairly strong symmetrical
bending band near 1550-1485 cm-1.
IR spectra of free amino
acids (cont.)

The carboxylate ion (-COOө)
• strong absorption near 1600-1590 cm
• weaker absorption near 1400 cm
-1
-1
(asymmetrical and symmetrical stretching of
C-O bonds)
Nitriles

IR absorption of nitriles (R-C≡N)
•
•
•
•
weak to medium band in the triple-bond stretching
region of the spectrum
aliphatic nitriles: near 2260-2240 cm-1
electron-attracting atoms (e.g. O or Cl), attached to
the α−carbon atom to the -C≡N group reduce the
intensity of absorption
conjugation (e.g. in aromatic nitriles), reduces the
frequency of absorption to 2240-2222 cm-1 and
enhances the intensity
Covalent compounds with N-O
bonds (general overview)





Nitro compounds, nitrates, and nitramines
contain an NO2 group
absorption is caused by asymmetrical and
symmetrical stretching of the NO2 group
Asymmetrical absorption:
•
a strong band in the 1661-1499 cm-1 region;
•
in the region 1389-1259 cm-1.
symmetrical absorption:
The exact position of the bands depends on
substitution and unsaturation in the vicinity of
the NO2 group
N-O stretching vibrations in
nitro-compounds



two bands: near 1550 and 1372 cm-1
conjugation lowers the frequency of both bands,
resulting in absorption near 1550-1500 and 13601290 cm-1
attachment of electronegative groups to the αcarbon:
•
•
increase in the frequency of the asymmetrical NO2 band
and
reduction in the frequency of the symmetrical band
• chloropicrin, Cl CNO
3
2
absorbs at 1610 and 1307 cm-1.
Aromatic nitrocompounds


Aromatic nitro groups absorb near the
same frequencies as observed for
conjugated aIiphatic nitro compounds
not reliable low-frequency region for
identification of the substitution pattern
• interactions between the NO
out-of-plane
bending and ring C-H out-of-plane bending
frequencies

2
C-N stretching vibration near 870 cm-1