IR Spectroscopy.pptx

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IR Spectroscopy

Principles, Instrumentation and application

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INFRA RED SPECTROSCOPY

Introduction

• _{The region from 0.8 µ to 200 µ of the electromagnetic spectrum}

is called infrared region and the study of the spectra in this

region is referred to as infrared spectroscopy.

• _{This is an important tool for determination of structure of a}

compound.

• _{The absorption in Infra-red radiations can be expressed either in}

terms of wavelength (λ) or in wave number (ν).

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INFRA RED SPECTROSCOPY

Introduction

• _{Infra-red spectra of organic compounds are plotted as %}

transmittance against wave number.

• _{The following names are given in connection with infra-red}

spectroscopic work.

Region Wavelength range (µ)

1. Near IR 0.8 to2.5

2. Ordinary IR2.5 to 15

3. Far IR 15 to 200

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INFRA RED SPECTROSCOPY

INSTRUMENTATION

• _{Infra-red absorption spectra are generally obtained by placing the sample in}

one of the beam of double-beam spectrophotometer.

• _{The light source is Nernst glower.}

• _{Prism and grating is used as monochromator. It is used for dispersing the light.}

• Thermocouple or bolometer is used as detector.

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PRINCIPLES OF INFRA-RED SPECTROSCOPY

• Absorption in the infra-red region is due to the changes in the vibrational and rotational levels.

• Only selected frequencies of infra-red radiation will be absorbed by a molecule.

• It has been found that no two compounds except the enantiomers can have similar Infra-red spectra.

• Only those bonds which have a dipole moment are capable of absorbing infra-red radiation.

• _{Symmetric bonds like those of H}₂_{or Cl}₂_{, will not absorb infra-red radiation.}

• _{The bonds most likely to be affected by this restrain are those of symmetric}

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Types of molecular vibrations

1. Stretching (Valence bond vibrations)

• _{In this type of vibration, the distance between the two atom increases or}

decreases but the atoms remain in the bond axis. These vibrations are of two types:

(a) Symmetric stretching: are those vibrations in which the bonds stretch and compress simultaneously i.e., movement of the atoms with respect to a particular atom in the same direction.

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Types of molecular vibrations

2. Bending or (Deformation vibrations)

• _{Atoms or the group as a whole oscillates perpendicular to the bond axis.}

• The positions of the atoms are changed with respect to the original bond axis.

• Stretching absorption of a bond appears at high frequencies (higher energy) as compared to the bending absorptions of the same bond.

• Four important types of bending vibrations are:

(a) Scissoring: In this type, two-atoms approach each other.

(b) Rocking: In this type, the movement of the atoms takes place in the same direction.

(c) Wagging: Two atoms move up and below the plane with respect to the central atom.

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Number of fundamental vibrations

The number of normal modes of vibration = 3N-6 (for non-linear molecules).

= 3N-5 (for linear molecules)

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Useful Tips for interpreting an infra-red spectrum

1. The absence of a band in a particular region is a sure indication of the

absence of group/groups absorbing in that region. For example, if there is no absorption in the region 1900-1600 cm-1, the

carbonyl group (> C = O) must be absent in the compound.

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3. For easy detection of the various groups present in the compound, the infra-red region (4000 to 667 cm-1) may be visualized as consisting of the following portions:

(a) 3600-3200cm-1_:_{The appearance of bands in this region shows the}

presence of (-OH, -NH₂, =NH) group in the compound.

(b) 3200-3000 cm-1_:_{Absorptions due to = C-H stretching and Ar-H stretching}

occur in this region.

(c) 3000-2500 cm-1_{: Shows the presence of acids (-COOH group). Two}

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3. (d) 2300-2100 cm-1_:

• _{This is the region in which alkynes, cyanides, cyanates, Isocyantes absorb.}

• The bands observed are weak and variable.

• _{–C = C stretching occurs between 2140-2100 cm}-1

• _{–C = N stretching between 2260-2200 cm-1,}

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3. (e) 1900-1650 cm-1_:

• _{Strong bands due to C=O stretching occur in this region.}

• _{Anhydrides show two strong bands in the region 1850-1740 cm-1. Examples are}

Esters, aldehydes, ketones, lactones, carboxylic acids, amides.

• Imides around 1700 cm-1.

• Following points regarding C=O stretching may be helpful:

(i) α, β-unsaturation lowers the frequency of absorption by 15-40 cm-1. But in amides, a

small, absorption shift towards lower frequency is observed.

(ii) Increase in the ring strain in case of cyclic ketones raises υC=O absorption.

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(f) 1600-1000 cm-1_:

• This region is very important for identifying nitro compounds and also confirming the

presence of ethers, esters, primary, secondary, secondary and tertiary alcohols.

(g) Below 1000 cm-1_:

• This region is very useful in identifying the type of substitution on the aromatic ring:

(i) a strong band, at 770-730 cm-1_{(s) shows monosubstitution.}

(ii) ortho and para disubstituted compounds show one band each.

• _{The latter absorbs at a higher wave number.}

• _{Meta-disubstituted compounds are usually recognized by two medium bands in}