CYN008 - IR Spectroscopy (Full)
CYN008 - IR Spectroscopy (Full)
CYN008 - IR Spectroscopy (Full)
2-Pentanone
2-Pentanone
Wavenumbers (cm-1)
IR Spectroscopy-Basics and principle
• Molecules are flexible, atoms and groups of atoms can rotate about single
covalent bonds.
• Bonds can stretch and bend just if their atoms were joined by flexible springs.
• Infrared spectroscopy, also called IR spectroscopy, probes stretching and
bending vibrations of molecules.
• Atoms joined by bonds are not fixed in one position but rather undergo
continual vibrations relative to each other. The energies associated with
these vibrations are quantized, which means that, within a molecule, only
specific vibrational energy levels are allowed.
• The energies associated with these transitions between vibrational energy
levels in organic molecules correspond to frequencies in the infrared region
which stretches from 4000 to ~600 cm-1.
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IR Spectroscopy-Basics and principle
• IR Spectroscopy is not used for complete structural characterization of
compounds. It is used to determine the various functional groups present in
the molecule.
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Presentation of the IR Spectra
Designation Abbreviation Wavelength
Near-Infrared NIR 0.78–3 μm
Mid-Infrared MIR 3–50 μm
Far-Infrared FIR 50–1000 μm
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(a) High Frequency Region: (4000 to 1650 cm-1)
• This region is especially useful for identification of various functional groups.
• This region shows absorption arising from stretching modes and often contains only a
few bands.
(b) Fingerprint Region: (1650 to 600 cm-1)
• It is called fingerprint region because the pattern of absorption in this region are unique
to any particular compound, just as a person’s fingerprint are unique.
• Many of the vibrational modes in the fingerprint region depend on complex vibrations
involving the entire molecule, it is impossible for any two different compounds (except
enantiomers) to have precisely the same IR spectrum.
• Both stretching and bending modes give rise to absorptions here. One therefore, cannot
correlate an individual band with a specific functional group with accuracy in this
region. 6
Molecular Vibrations
HCl molecule as an anharmonic
oscillator vibrating at energy level E 3
• For the excitation of a molecule from one vibrational energy level to another, the
molecule has to absorb IR radiation of a particular energy (i.e, radiation of particular
wavelength or frequency).
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Molecular Vibrations
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Symmetric and Asymmetric Vibration
The fundamental vibrations for water, H2O (n= 3; non linear; fundamental
vibrations = 9 – 6 = 3), are shown in picture. Water, which is nonlinear, has
three fundamental vibrations (two stretching + one bending).
Stretching vibrations require higher energy than bending vibrations and occur
at higher frequency!!
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Stretching and Bending
Carbon dioxide, CO2, is linear and hence has four (9-5) fundamental
vibrations.
The asymmetrical stretch of CO2 gives a strong band in the IR at 2350 cm–1.
You may notice this band in samples which you run on the instruments in the
labs, since CO2 is present in the atmosphere.
The two scissoring or bending vibrations are equivalent and therefore, have
the same frequency and are said to be degenerate, appearing in an IR
spectrum at 666 cm–1.
Stretching Vibrations
Bending Vibrations 11
Different stretching modes in acetic acid
• Apart from these characteristic absorptions (stretching vibrations) which throw light
on the general nature and the presence of functional groups in the compound there
are other absorption bands (bending vibrations) as well.
• Neither there is necessity nor time for a skilled chemist to interpret these
absorptions because of the complexity of vibrational modes!
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Role of Dipole Moment in IR
• The reason for this involves the mechanism by which the photon transfers
its energy to the molecule, which is beyond the scope of this discussion.
• Carbon monoxide (CO) and iodine chloride (I-Cl) absorb IR radiation, but
hydrogen (H2), nitrogen (N2), Chlorine (Cl2) and other symmetrical
diatomics do not.
• In general, the larger the dipole change, the stronger the intensity of the
band in an IR spectrum.
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Trans dichloroethylene shows no C=C stretching around 1640 cm-1, whereas, the cis
isomer shows this band. It may be noted that both isomers show bands for C-H and
C-Cl stretchings.
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Calculation of Vibrational Frequencies
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• The actual range for C–H absorptions is 2850–3100 cm–1.
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From the knowledge of bond dissociation energies, one knows that a C-H bond (104
kcal/mole, i.e., 435 kJ/mol) is only slightly stronger than a C-C bond ( 88 kca/mol, i.e.,
368.2 kJ/mol, ethane). Even then, the difference is nicely reflecting in the stretching
frequencies of these two bonds (2950 and 1200 cm-1, respectively).
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SIMPLIFIED CORRELATION CHART
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SIMPLIFIED CORRELATION CHART
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• It is easy to remember these values if you start simply and
then slowly increase your familiarity with and ability to
interpret the finer details of an infrared spectrum.
• Memorize a “typical absorption value”- a single number that can be used a pivotal value for
each of the functional groups in this pattern.
• For, e.g., start with a simple aliphatic ketone as a model for all typical carbonyl compounds. The
typical aliphatic ketone has a carbonyl absorption of about 1715 10 cm-1. Without worrying
about the variation, keep in mind 1715 cm-1 as the base value for carbonyl absorption.
• Then slowly, familiarize yourself with the extent of the carbonyl range (1700 – 1800 cm -1) and
intensity.
• Learn how factors such as ring size and conjugation affect the base values (i.e, in which direction
the values are shifted) 21
Normal base values for the C=O stretching vibrations for carbonyl groups
• The C=O frequency of a ketone, which is approximately in the middle of the range, is usually considered the
reference point for comparison of these values. The range of values given above may be explained through
the use of electron-withdrawing effects (inductive effects), resonance effects, and hydrogen bonding.
• An electronegative element may tend to draw in the electrons between the carbon and oxygen atoms
through its electron-withdrawing effect, so that the C=O bond becomes somewhat stronger and a higher
frequency absorption results.
• Since oxygen is more electron electronegative than carbon, this effect dominates in an ester to raise the C=O
frequency above that of a ketone.
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• A resonance effect may be observed when the
unpaired electrons on a nitrogen atom conjugate
with the carbonyl group, resulting in increased
single-bond character and a lowering of the C=O
absorption frequency. This effect is observed in an
amide. Since nitrogen is less electronegative than
an oxygen atom, it can more easily accommodate a
positive charge.
• In acid chlorides, the highly electronegative halogen atom
strengthens the C=O bond through an enhanced inductive effect
and shift the frequency to values even higher than are found in
esters. Anhydrides are likewise shifted to frequencies higher than
are found in esters because of a concentration of electronegative
oxygen atoms. In addition, anhydride give two absorption bands
that are due to symmetric and asymmetric stretching vibrations.
7.) Hydrocarbons:
None of the preceding is found.
Major absorptions are in C-H region near 3000 cm -1.
Very simple spectrum; the only other absorptions appear near 1460 and 1375 cm -1. 25
OVERTONE, COMBINATION AND DIFFERENCE BANDS
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IMPORTANT NOTE
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Alkanes
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Alkenes
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Alkane vs. Alkenes
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Alkynes
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Alkyl Halides
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Aromatics
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Alcohols
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Ketones
• C=O stretch:
◦ aliphatic ketones 1715 cm-1
◦ alpha, beta-unsaturated ketones 1685-1666 cm-1
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Aldehydes
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Aromatic Aldehydes
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Carboxylic Acids
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Esters
• C=O stretch
◦ aliphatic from 1750-1735 cm-1
◦ α, β-unsaturated from 1730-1715 cm-1
• C–O stretch from 1300-1000 cm-1
Aromatic Esters
• C=O stretch
◦ aliphatic from 1750-1735 cm-1
◦ α, β-unsaturated from 1730-1715 cm-1
• C–O stretch from 1300-1000 cm-1
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Amines
• N–H stretch 3400-3250 cm-1
◦ 1° amine: two bands from 3400-3300 and 3330-3250 cm-1
◦ 2° amine: one band from 3350-3310 cm-1
◦ 3° amine: no bands in this region
• N–H bend (primary amines only) from 1650-1580 cm-1
• C–N stretch (aromatic amines) from 1335-1250 cm-1
• C–N stretch (aliphatic amines) from 1250–1020 cm-1
• N–H wag (primary and secondary amines only) from 910-665 cm-1
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Amines
• N–H stretch 3400-3250 cm-1
◦ 1° amine: two bands from 3400-3300 and 3330-3250 cm-1
◦ 2° amine: one band from 3350-3310 cm-1
◦ 3° amine: no bands in this region
• N–H bend (primary amines only) from 1650-1580 cm-1
• C–N stretch (aromatic amines) from 1335-1250 cm-1
• C–N stretch (aliphatic amines) from 1250–1020 cm-1
• N–H wag (primary and secondary amines only) from 910-665 cm-1
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Amines
• N–H stretch 3400-3250 cm-1
◦ 1° amine: two bands from 3400-3300 and 3330-3250 cm-1
◦ 2° amine: one band from 3350-3310 cm-1
◦ 3° amine: no bands in this region
• N–H bend (primary amines only) from 1650-1580 cm-1
• C–N stretch (aromatic amines) from 1335-1250 cm-1
• C–N stretch (aliphatic amines) from 1250–1020 cm-1
• N–H wag (primary and secondary amines only) from 910-665 cm-1
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Nitro Groups
• N–O asymmetric stretch from 1550-1475 cm-1
• N–O symmetric stretch from 1360-1290 cm-1
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Aromatic Nitro Groups
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