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CHEMISTRY

PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

Subject

Chemistry

Paper No and Title

Paper 12: Organic Spectroscopy

Module No and Title

Module 4: Basic principles and Instrumentation for IR

spectroscopy

Module Tag

CHE_P12_M4_e-Text

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CHEMISTRY

PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

TABLE OF CONTENTS

1. Learning Outcomes

2. Introduction

3. Origin of Infra-red Spectroscopy

4. Molecular Vibrations

5. Selection Rule

6. Fundamental Vibrations

7. Sample preparation

8. IR-Spectrometer

9. Dispersive Infrared Spectrometer

10. Fourier-Transform Infrared Spectrometer

11. Hands-on Operation of an FTIR Spectrometer

12. Summary

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CHEMISTRY

PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

1. Learning Outcomes

After studying this module, you shall be able to

• To understand the concept of Infra-red spectroscopy

• To predict the number of fundamental modes of vibration of a molecule

• To know how to make samples for recording the spectra of different organic compounds

• To understand the working of IR-spectrophotometer

• Hands-on experience of recording the IR-spectrum

2. Introduction

The most frequent spectroscopic technique used by organic and inorganic chemists is Infrared

(IR) spectroscopy. It deals with the absorption of radiation in the infrared region of the

electromagnetic spectrum. IR spectrum gives sufficient information about the structure

(identification of functional groups) of a compound and can also be used as analytical tool to

assess the purity of a compound. The absorption of infrared radiation by a molecule causes

changes in their vibrational and rotational energy levels and hence IR-spectroscopy is also known

as vibrational-rotational spectroscopy. Unlike UV-spectroscopy which has very few peaks in their

spectrum, IR spectroscopy provides spectrum with a large number of absorption bands and hence

provide plenty of information about the structure of a compound. Different bonds present in the

spectra correspond to various functional groups and bonds present in the molecule.

The infrared spectrum can be divided into three main regions: the far infrared (<400 cm−1), the

mid-infrared (4000–400 cm−1) and the near-infrared (13000–4000 cm−1). The mid IR region is

of greatest practical use to the organic chemist, but the near- and far-infrared regions also provide

important information about certain materials.

Mid IR region: The mid-infrared spectrum extends from 4000 to 400 cm−1 and results from

vibrational and rotational transitions. This region is most useful for the organic chemist since

most of the organic molecules absorb in this region. The mid-infrared can be divided into two

regions viz functional group region (4000-1300) and finger print region (1300-600).

Functional group region (4000-1300): Most of the functional groups present in organic

molecules exhibits absorption bands in the region 4000-1300 cm-1, hence this is known as

functional group region. In this region each band can be assigned to a particular deformation of

the molecule, the movement of a group of atoms, or the bending or stretching of a particular

bond.

Finger print region (1300-600): The region from 1300 cm

-1

to 600 cm

-1

usually contains a very

complicated series of absorptions. These are mainly due to molecular vibrations, usually bending

motions that are characteristic of the entire molecule or large fragments of the molecule. Except

enantiomers, any two different compounds cannot have precisely the same absorption pattern in

this region. Thus absorption patterns in this region are unique for any particular compound that is

why this is known as finger print region.

It is very difficult to assign individual bands in this region. Two molecules having the same

functional group may show similar spectra in the functional group region but their spectra differ

in the finger print region. Therefore both the regions are very useful for confirming the identity of

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PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

a chemical substance. This is generally accomplished by comparing the spectrum of an authentic

sample. When two compounds show a good match between the IR spectra in all frequency

ranges, mainly in the fingerprint region, strongly indicates that they have the same molecular

structures.

Both the compounds are alcohols and contain exactly the same bonds. Now if you compare the

infra-red spectra of these compounds, the functional group region is very similar for both the

compounds but the fingerprint region is totally different. Therefore fingerprint region could be

crucial to identify the compound.

To understand the importance of finger print region to identify a compound, we can take the

example of propan-1-ol and propan-2-ol.

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CHEMISTRY

PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

Near-infrared region (13000–4000 cm−1): The absorptions observed in the near-infrared region

(13000–4000 cm−1) are overtones or combinations of the fundamental stretching bands. Bands in

the near infrared are usually weak in intensity. They are often overlapped and hence are less

useful than the bands in mid-infrared region.

NIR shows some similarities to UV-visible spectrophotometry and some to mid-IR spectrometry.

Indeed the spectrometers used in this region are often combined UV-visible-NIR ones.

Usually Hydrogen-stretching vibrations that occur in the region 3 to 6 µm, are the absorption

bands due to overtones (or combination) of fundamental bands

NIR is generally used for quantitative organic functional-group analysis. The NIR region has also

been used for qualitative analyses and studies of hydrogen bonding, solute-solvent interactions,

organometallic compounds, and inorganic compounds

Far-infrared region (600-100 cm

-1

): The far-infrared spectrum extends from 600 to 100 cm

−1

.

Organometallic and inorganic molecules contain heavy atoms and have weak bonds , therefore

the fundamental vibrations of these molecules fall in this reigon. Lattice vibrations of crystalline

materials occur in this region.

3. Origin of Infra-red Spectroscopy

IR-spectroscopy gives the information about molecular vibrations or more precisely on transitions

between vibrational and rotational energy levels. Since the absorption of infrared radiation leads

to transition between vibrational and rotational energy levels, it is also vibrational-rotational

spectroscopy.

When a molecule absorb IR-radiation below 100 cm

-1

, transitions between rotational energy

levels takes place. Since these energy levels are quantized, a rotational spectrum consists of

discrete lines. If a molecule absorbs radiation in the range 100-10,000 cm

-1

, it causes transitions

between vibrational energy levels. These energy levels are also quantised but vibrational spectra

appear as bands rather than discrete lines. Each vibrational energy level is associated with a large

number of closely spaced rotational energy levels or we can say that the energy difference

between various rotational energy levels is very short than the energy difference between various

vibrational energy levels. Therefore the vibrational spectra appear s vibrational-rotational bands

instead of discrete lines. Organic chemists are mainly concerned with these transitions especially

with those occur in the range 4000-667 cm

-1

.

4. Molecular Vibrations

The atoms in a molecule do not remain fixed at certain positions. The two nuclei can vibrate

backwards and forwards or towards and away from each other around an average position. There

are two types of fundamental molecular vibrations viz stretching (change in bond length) and

bending (change in bond angle).

Stretching vibrations: It involves a rhythmic movement along a bond axis with a subsequent

increase and decrease in bond length. Stretching vibrations are of two types viz Symmetrical

Stretching and asymmetrical stretching.

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CHEMISTRY

PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

Bending vibrations: It involves a change in bond angle or movement of a group of atoms with

respect to the rest of the molecule. Bending vibrations are of four types.

i) Rocking

ii) Scissoring

iii) Wagging

iv) Twisting

All the bonds in a molecule are not capable of absorbing infrared radiation but only those bonds

which are accompanied by a change in the dipole moment will absorb in the infra-red region.

Thus, vibrations which are associated with the change in the dipole moment of the molecule are

called infra-red active transitions otherwise the vibration is said to be IR-inactive and do not show

any absorption band in the IR-spectrum. Generally, larger the change in the dipole moment, the

higher is the intensity of absorption. Hence the vibrational absorption bands in simple

hydrocarbons are weak while bands associated with bonds connecting atoms with considerable

electronegativity difference give strong bands.

5. Selection Rule

IR-radiation is absorbed only when a change in dipole moment of the molecule takes place.

Complete symmetry about a bond may eliminate certain absorption bands. Therefore number of

absorption bands observed is not exactly equal to the fundamental vibrations, some of the

fundamental vibrations are IR-active while others are not. This is governed by selection rule

described below.

1) In a molecule with a centre of symmetry, the vibrations symmetrical about the centre of

symmetry are IR-inactive.

2) The vibrations which are not symmetrical about the centre of symmetry are IR-active.

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PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

Here are some examples which could explain the selection rule.

a) All the symmetrical diatomic molecules such as H

2

, N

2

and Cl

2

etc. are IR-inactive.

b) The symmetrical stretching of the C=C bond in ethylene (centre of symmetry) is IR-inactive.

c) The symmetrical stretching in CO2 is IR-inactive, whereas asymmetric stretching is IR-active.

d) Cis-dichloro-ethylene molecule shows C=C stretching bands whereas trans molecule does not

show this band.

6. Fundamental Vibrations

The IR spectrum of a compound may show more than one vibrational absorption bands. The

number of these bands corresponds to the number of fundamental vibrations in the molecule

which can be calculated from the degree of freedom (DOF) of the molecule. A molecule

comprising of n atoms has a total of 3n DOF. In a nonlinear molecule, three of these degrees of

freedom are rotational and three are translational and the remaining (3n-6) correspond to

vibrational degree of freedom or fundamental vibrations. Whereas in a linear molecule, only two

degrees of freedom are rotational (because rotation about its axis of linearity does not change the

positions of the atoms) and three are translational. The remaining (3n-5) degrees of freedom are

vibrational degree of freedom or fundamental vibrations.

Simple diatomic molecules have only 1 bond and only 1 vibrational band. If the molecule is

symmetrical such as hydrogen, nitrogen, and chlorine, the band is not observed in the IR

spectrum. Asymmetrical diatomic molecules, e.g. CO and iodine chloride absorb in the IR

spectrum..

It has been observed that in actual IR spectrum, the theoretical number of fundamental bands is

seldom observed because there are certain factors which may increase or decrease the number of

bands. Some fundamental vibrations lie outside the IR region (4000-400 cm

-1

), whereas some are

too weak to be observed. Few fundamental vibrations are too close that they merge into one

another. The occurrence of degenerate bands (bands of same frequency) also cause decrease in

the fundamental vibrational bands.

For example: Carbon dioxide, CO

2

is linear and has four fundamental vibrations but actually it

shows only two bands (666 cm

–1

and 2350 cm

–1

). The symmetrical stretching vibration of CO

2

is

inactive in the IR because this vibration produces no change in the dipole moment of the

molecule. The two scissoring or bending vibrations are equivalent and therefore have the same

frequency (degenerate) at 666 cm

–1

. The asymmetrical stretch of CO

2

gives a strong band in the

IR at 2350 cm

–1

.

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CHEMISTRY

PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

The appearance of certain types of non-fundamental (overtones, combinations of fundamental

vibrations or difference of fundamental vibrations) bands increases the number of bands as

compared to that expected from the theoretical number of fundamental bands. All these bands

have very weak intensity than the fundamental vibration bands.

Overtones bands: In addition to the fundamental vibrations, other frequencies can be generated

by modulations of the fundamental bands. An overtone band occurs when the molecule makes a

transition from the ground state (v=0) to the second excited state (v=2), where v is the vibrational

quantum number. The intensity of the overtone band is very low as compared to the fundamental

band and they are usually found in the near infrared region. Based on the harmonic oscillator

approximation it has been found that the energy of the overtone transition is about n times of the

fundamental vibration associated with that particular transition. Suppose a compound shows

strong absorptions at x and y cm

-1

then it may also give rise to weaker absorptions at 2x, 2y, 3x

and 3y cm-1, respectively. The intensity of overtones bands decreases as the order of the overtone

increases, i.e. the intensity of 3x or 3y overtones will be less than the 2x and 2y. Therefore second

and third overtones are rarely observed.

Combination Bands: Combination bands are observed when two or more than two fundamental

vibrations are excited simultaneously. If there are two fundamental vibrations at x and y cm

-1

then

it may also give rise to absorption bands at (x+y), (x+2y), (2x+y) cm

-1

.

Difference bands: It is also possible to have a difference band where the frequencies of two

fundamental bands are subtracted, i.e. (x-y), (x-2y), (2x-y) cm

-1

.

Fermi bands/resonance: When a fundamental vibration couples with an overtone or

combination band, the coupled vibration is called Fermi resonance. As a result, two strong bands

are observed in the spectrum, instead of the expected strong and weak bands. Fermi resonance is

often observed in carbonyl compounds.

A practical use for understanding overtones and combination bands is applied to organic solvents

used in spectroscopy. Most organic liquids have strong overtone and combination bands in the

mid-infrared region, therefore, acetone, DMSO, or acetonitrile should only be used in very

narrow spectral regions. Solvents such at CCl

4

, CS

2

and CDCl

3

can be used above 1200 cm

-1

.

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PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

7. Sample Preparation

IR spectrum of a compound can be recorded in many different forms, such as liquid, solid, gas

and solution. Some of the materials are opaque to infrared radiation, so in order to obtain spectra

they must be dissolved or diluted in a transparent matrix. For recording IR spectra, the sample

should be properly dry as water absorb near 3710 and 1630 cm

-1

. The samples should be perfectly

dried, since cell materials (NaCl, KBr) are usually spoiled by the moisture.

1) Solid samples: There are several methods by which an IR spectrum of a solid sample can be

recorded.

a) As a pressed disc: The first common method involves the mixing of finely ground solid

sample with powdered potassium chloride. A translucent pellet of this powder mixture is formed

by pressing it in a mechanical pressure. The main advantage of using KBr is that it does not

interfere with the bands due to compound since KBr is transparent to IR radiation 4000-650 cm

-1

and thus gives better spectra. The disadvantage of this method is that KBr absorbs water quickly

which may interfere with the spectra that is obtained.

b) As a mull or paste: Finely ground compound is mixed with an oily mulling agent (usually

Nujol) using a pestle and mortar. A thin film of the mull is placed between two flat plates of NaCl

and the spectrum is measured. The main disadvantages of this method is that nujol has absorption

bands at 2924-2860, 1462, 1380 cm

-1

, therefore no information about the observed compound can

be obtained in this region.

c) As a film: The third method is to dissolve the soild sample in a suitable, non-hygroscopic

solvent usually methylene chloride or carbon tetra chloride. A drop of this solution is deposited

on surface of Potassium bromide or Sodium chloride plate. The solution is then evaporated to

dryness and the film thus formed on the KBr disc is analysed directly to obtain the IR spectrum.

The most important thing is that the film should not be too thick otherwise light cannot pass

through it. This method gives good results with dilute solution of the compound in a non-polar

solvent.

2) Liquid samples: Liquids are studied neat or in solution. A drop of neat liquid sample or a

solution of the sample in an appropriate solvent is placed between two plates of a salt (sodium

chloride or potassium bromide) to give a thin film and analysed to obtain the spectrum. The plates

are transparent to the infrared light and do not introduce any lines onto the spectra. Salt plates

break easily and are water soluble therefore compounds analysed by this method should be free

from water. Spectrum obtained by this method is known as neat spectrum since no solvent is used

in recording the spectrum.

3) Gaseous samples: The gas is introduced into a special cell with a long path length and the

walls of its both the ends are normally made up of NaCl. Gases have very less densities compared

to liquids, and hence path lengths should be correspondingly greater, usually 10 cm or longer.

The vapor phase technique is limited because most of the organic compounds have too low vapor

pressure to produce a useful absorption spectrum.

It is important to note that spectra obtained from different sample preparation methods

will look slightly different from each other due to differences in the samples' physical states.

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PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

8. IR-spectrometer

Traditionally, dispersive infrared spectrophotometers, developed in 1940s, were used to obtain

infrared spectra. In 1960s, a new method was developed known as Fourier-transform infrared

(FT-IR) spectrometers. But due to high cost of the instrument, this was tended to be used for

advanced research only at that time. Gradually, technology advancements in computers and

instruments have reduced the cost and enhanced the capabilities of an FT-IR spectrophotometer.

Today they are predominantly used and have improved the acquisition of infrared spectra

dramatically.

9. Dispersive Infrared Spectrometer

The basic components of a dispersive IR spectrometer include a radiation source, Sample and

reference cells, monochromator, detector, amplifier and recorder. A schematic diagram of a

typical dispersive spectrometer is shown in Fig.

Radiation source: The common IR radiation source are inert solids that are heated electrically to

1000 to 1800 °C to promote thermal emission of radiation in the infrared region of the EM

spectrum. The most common sources are Nernst filament (composed of rare-earth oxides such as

zirconium, cerium and thorium), Globar (composed of silicon carbide), and Nichrome coil. They

all produce continuous radiations, but with different radiation energy profiles. The beam from the

source is divided into two equivalent beams, one passing through the sample and the other as

reference beam.

Sample and reference cells: Like UV sample tubes (cuvettes) glass or quartz cannot be used to

make the sample cells for IR-spectroscopy, because they absorb strongly in most of the IR region.

Alkali metal halides such as KCl, NaCl are commonly used as they are transparent to the IR-

region.

Monochromator: The monochromator is a device used to disperse or separate a broad spectrum

of IR radiation into a continuous spectrum of frequencies of Infrared light. The monochromator

consists of rapidly rotating chopper that passes the two beams alternately to a diffraction grating.

The slowly rotating diffraction grating varies the frequency or wavelength of radiation and sends

it the individual frequency to the thermocouple detector which generates an electrical signal as a

response.

Detectors and Amplifier: Detectors are devices that convert the radiant energy into an electrical

signal. The detector determines the ratio between the intensities of the reference and sample

beams. Due to the difference in the intensities of the two beams falling on the detector, an

alternating current starts flowing from the detector to the amplifier, where it is amplified and

relayed to the recorder.

The detectors used in IR spectrometers can be categorized into two classes: thermal detectors and

photon detectors. Thermal detectors consists of several thermocouples connected together to

produce greater sensitivity. They measure the heating effect produced by infrared radiation that

causes the flow of current. The current produced is proportional to the intensity of radiation

falling on the thermal detector. Photon detectors rely on the interaction of IR radiation and a

semiconductor material. Non-conducting electrons are excited to a conducting state and therefore

producing a small current or voltage.

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PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

Recorder: It records IR-spectrum as a plot of frequency of absorbed radiation and intensity of

absorption in terms of transmittance. Unlike UV-spectroscopy, here we use the wavenumber unit.

As the detector records the ratio of the intensities of the two beams therefore percent

transmittance is recorded.

Transmittance (T) = I/I

0

Percent transmittance (%T) = I/I

0

X 100

Where I

0

is the intensity of the incident radiation and I is the intensity of the radiation emerging

from the sample.

Figure 1: Schematic diagram of a dispersive IR spectrometer

10. Fourier-Transform Infrared Spectrometer

Fourier transform spectrometers have recently replaced dispersive instruments for most

applications due to their superior speed and sensitivity. They have greatly extended the

capabilities of infrared spectroscopy and have been applied to many areas that are very difficult

or nearly impossible to analyze by dispersive instruments. Instead of viewing each component

frequency sequentially, as in a dispersive IR spectrometer, all frequencies are examined

simultaneously in Fourier transform infrared (FTIR) spectroscopy.

The basic components of an FTIR spectrometer are shown schematically in Figure 2.3. The

radiation emerging from the source is passed through an interferometer to the sample before

reaching a detector. Then the signal is amplified and converted to digital form by an analog-to-

digital converter and transferred to the computer in which Fourier transform is carried out.

Interferometer divides radiant beams, generates an optical path difference between the beams and

then recombines them in order to produce repetitive interference signals measured as a function

of optical path difference by a detector. Thus interferometer produces interference signals, which

contain infrared spectral information generated after passing through a sample.

The most commonly used interferometer is a Michelson interferometer. It consists of three active

components: a moving mirror, a fixed mirror, and a beam splitter (Fig. 15.4). The two mirrors are

perpendicular to each other. The beam splitter is a semi-reflecting device and bisects the plane of

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CHEMISTRY

PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

these two mirrors. The beam splitter is often made by coating a thin film of germanium or iron

oxide onto an ‘infrared-transparent’ substrate such as potassium bromide or cesium iodide.

The energy goes from the source to the beam splitter which splits the beam into two parts. One

part is transmitted to a moving mirror; one part is reflected to a fixed mirror. The moving mirror

moves back and forth at a constant velocity. The two beams are reflected from the mirrors and

recombined at the beam splitter. The beam from the moving mirror has traveled a different

distance than the beam from the fixed mirror. When the beams are combined an interference

pattern is created. Since some of the wavelengths recombine constructively and some

destructively. This interference pattern is called an interferogram. This interferogram then goes

from the beam splitter to the sample, where some energy is absorbed and some is transmitted. The

transmitted portion reaches the detector. The detector reads information about every wavelength

in the infrared range simultaneously.

Figure: Schematic Presentation of a typical FTIR spectrometer

The moving mirror produces an optical path difference between the two arms of the

interferometer (the relative position of moving mirror to the fixed mirror). If the two mirrors are

at equal distance from the beam splitter, the two beams travel the same path length. Therefore two

beams are totally in phase with each other and hence they interfere constructively and lead to a

maximum intensity reaching to the detector. When the moving mirror travels in either direction

by the distance λ/4, the optical path is changed by 2 (λ/4), or λ/2. The two beams are 180° out of

phase with each other, and thus interferes destructively resulting minimum intensity reaching the

detector. If the moving mirror further travels by λ/4, then the optical path difference will be 2

(λ/2), or λ. The two beams are again in phase with each other and result in another constructive

interference again giving a maximum response in the detector. Such a maximum will be observed

whenever the path difference is an integral multiple of λ.

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PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

Thus moving mirror is the key component of interferometer, because alternate light and dark

images will reach the detector if the mirror is slowly moved either away from or towards the

beam splitter. When the mirror is moved at a constant velocity, the intensity of radiation reaching

the detector varies in a sinusoidal manner to produce the interferogram output shown in Fig. 15.4.

The interferogram is a complex signal but its wave like pattern contains all the frequencies that

make up the infrared spectrum. It is actually a time domain spectrum and records the detector

response changes versus time. If the sample happens to absorb at this frequency, the amplitude of

the sinusoidal wave is reduced by an amount proportional to the amount of sample in the beam.

Now to obtain the infrared spectrum, the detector signal is sent to the computer, where

mathematical operation known as Fourier transformation converts the interferogram (a time

domain spectrum displaying intensity versus time) to the final IR spectrum, a frequency domain

spectrum showing plot between intensity of signal versus frequency.

11. Hands-on Operation of an FTIR Spectrometer

Step 1: First step is the sample preparation. Sample preparation has already been discussed in the

above section.

Step 2: The second step is to obtain an interferogram of the background which consists of the IR-

active atmospheric gases carbon dioxide (doublet at 2360 cm

–1

and sharp spike at 667 cm

–1

in Fig.

6) and water vapors (two irregular groups of lines at about 3600 cm

–1

and about 1600 cm

–1

in Fig.

6). Nitrogen and oxygen are not IR-active hence do not absorb in IR-region. This interferogram is

subjected to Fourier transform which gives the spectrum of background. Figure 6 shows an

example of an FTIR background spectrum.

Figure 6: (A) Background IR spectrum; (B) Sample IR spectrum

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MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

Step 3: In the third step, spectrum of the sample under

investigation is obtained by the same procedure. This spectrum

contains absorption bands from the sample as well as the background (gaseous or solvent).

Step 4: The ratio between the single-beam sample spectrum and the single beam background

spectrum gives the spectrum of the sample (Figure 7). Computer software automatically subtracts

the spectrum of the background from the sample spectrum.

Step 5: Finally the data obtained is analyzed by assigning the observed absorption frequency

bands in the sample spectrum to appropriate normal modes of vibrations in the molecules.

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PAPER No. 12: ORGANIC SPECTROSCOPY

MODULE No. 4: Basic principles and

Instrumentation for IR spectroscopy

12. Summary

1. Absorption of electromagnetic radiation in infrared region can cause changes in the vibrational

and rotational energy states.

2. A molecule consisting of n atoms has a total of 3n degrees of freedom

3. The number of fundamental vibrational bands in a molecule is equal to the degree of freedom

of a molecule however these numbers of bands is seldom obtained because of the occurrence of

certain non-fundamental bands such as overtones, combinations of fundamental vibrations or

difference of fundamental vibration bands.

4. The IR-spectrum can be obtained in all the three states, solid, liquid and gas.

5. Fourier-transform infrared spectrometers are superior than the traditional dispersive

spectrometers and gives high resolution spectrum in less time.