InfraRed Spectroscopy


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INTRODUCTION Spectroscopy Spectroscopy is the measurement and interpretation of electromagnetic radiation absorbed or emitted when the molecules or atoms or ions of a sample move from one energy state to another energy state. These changes may be from ground state to exited state or excited state to ground state.

Definition :

Definition The technique is based up on the simple fact that a chemical shows marked selective absorption of IR region. After absorption of IR radiations the molecules of a chemical vibrate at many different rate of vibration giving rise to close packed absorption bands called Infrared absorption spectrum which extend over wide wave length range. 1

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Basically it is divided into following regions Near IR region: - 0.8 - 2.5µ Middle IR region: - 2.5 - 15 µ Far IR region: - 15 - 200 µ

Principle :

Principle The molecules of the substances contain group of atom which are connected by bonds these bonds are in continuous motion they maintain some vibration/frequency which is characteristic to ever portion of molecule. This is the called natural frequency of vibration. When the IR radiation is applied and when applied IR frequency is equal to natural frequency of vibration absorption of IR radiation takes place and the peak is observed. In IR spectra we use wave numbers instead of wave length for mentioning the characteristic peak (wave numbers are larger in number and easy to handle) 1,2

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Wave number: it is number of waves present per cm which can be calculated from wave length. The entire IR region is divided into Group frequency region - 4000cm 1 to 1500cm 1 Finger print region – 1500cm 1 to 400cm 1 In the group frequency region the peaks corresponding to different functional group can be observed ( eg :- Amino group, Alcoholic group) 2

Finger print region:

Finger print region Every part of the molecule has different atoms and are connected by bonds each bond requires different IR region for absorption and so characteristic peaks are observed. Hence this region of IR spectrum is called as the finger print region of a molecule Nitro group have detected in the region 1500-1350cm 1 Alcohols, Esters, lactons , acid anhydrides 1350-1000cm 1 Cis trans alkenes identified in region 1000cm 1 .

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Types of Vibrations There are different vibrations Stretching vibrations Bending vibrations Stretching vibrations Symmetrical stretching In this symmetrical stretching vibration the two bonds may increase or decrease in length. Asymmetrical stretching In which one bond length increases and the other one decreases.

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Symmetric Stretch Asymmetric Stretch

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Bending vibrations In plane bending In these vibrations there is change in bond angle, bending of bond takes place within the same plane. Scissoring In which the bond angle decreases. Rocking In which bond angle is maintained but both bonds moves within the plane.

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Scissoring Rocking

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Out of plane bending Wagging In which both atoms move to one side of the plane. Twisting In which one atom is above the plane and the other is below the plane. 3

Factors affecting the frequency of the IR absorption:

Factors affecting the frequency of the IR absorption The relative mass of the atoms heavier the atoms lower is the vibration frequency of the bond between them eg : C-H bond vibrate at a lower frequency than O-H bond. The force constant of the body Stronger the bond higher is the vibration frequency. Hence a double bond vibrates at higher frequencies than single bonds between like pair of atoms. Stronger bonds such as O-H, N-H & C-H vibrates at higher frequency than weaker bonds such as C-C & C-O bond. 4

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The arrangement of the atoms within the molecules Requirement of IR absorption Correct wave length of radiation Change in Dipole moment

  Correct wavelength of radiation :

Correct wavelength of radiation A molecule absorbs radiation only when the natural frequency of vibration of the molecule or some part of the molecule is equal to the frequency of incident radiation. It has been found that no two compounds expect the enantiomer can have similar infrared spectra. The charge distribution around a molecule such as HCL is not symmetric because the cl has higher electron density than hydrogen. Thus HCL has significant dipole movement and it is said to be polar. 5

Effects of bonds :

Effects of bonds C=C stretching is expected to absorb at a higher frequency than C-C stretching. It is due to strength of bonds Eg : C≡C 2200cm 1 C=C 1650cm 1 C-C 1200cm 1

Requirement of IR absorption :

Requirement of IR absorption Natural frequency of vibration of HCL molecules is 8.7×10 13 sec -1 When IR radiation is passed through the sample of HCL and the transit radiation is analyzed it shows that part of the radiation which has a frequency of 8.7×10 13 sec -1 has been absorbed by HCL solution. The HCl molecule vibrates at a increased amplitude after absorbing the correct wave length.

Change in Dipole moment :

Change in Dipole moment Dipole moment is determined by the magnitude of the charge difference & the distance between the two centers of charge. For a molecule to absorb IR the vibration or vibrations with in a molecule must cause a net change in the dipole moment. If the frequency of the IR radiation will be absorbed causing a change in the amplitude of the molecule then radiation will be absorbed causing a change in the amplitude of molecular vibration.

Hydrogen bonding :

Hydrogen bonding Hydrogen bonding brings about remarkable downward frequency shifts stronger the hydrogen bonding greater is the absorption shift towards lower wave number than the normal value. Two types of hydrogen bonding is observed Intermolecular hydrogen bonding Intramolecular hydrogen bonding Intermolecular hydrogen bonding are concentration dependent they takes place in same or different molecules, it gave rise to broad band’s means shows absorption towards lower frequency. Eg : amines show N-H stretching at 3500cm 1 in dilutions while condensed phase spectra absorption occurs at 3300cm 1 .

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Intramolecular hydrogen bonding Takes place between different groups within the molecule Eg : Salycylic acid bonding takes place between COOH & OH groups Hydrogen bonding alters force constants of both the groups (proton donor & proton acceptor) as result the vibrational frequencies of these groups are also altered. Eg : Groups such as OH & NH 2 shows characteristic of vibrational frequencies when exposed to IR radiation. However these frequencies are altered when the above exhibit hydrogen bonding.

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Hydrogen bonding in alcohols (OH) is stronger than that of amines (NH 2 ) because of oxygen in alcohol is more electronegative than nitrogen and amines. Thus vibrational frequency of (OH) group is lowered more in IR spectrum of alcohol when compared to (NH 2 ) group of amines. 6


FT I R Why FT – IR Fourier transform infrared (FT-IR) Spectrometer was developed in order to over come the limitation encountered with dispersive instrument’s the main difficulty was the slow scanning process. A method for measuring all of the infrared frequencies simultaneously rather than individually so it was developed which employed a very simple device called an “interferometer”. The interferometer produces a unique type of signal which has all of the infrared frequencies “encoded” into it. The signal can be measured very quickly, usually on the order of one second. 7



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The Michelson interferometer functions in the following manner. Radiation from the source is emitted & directed toward a beam splitter. Half of the radiation is reflected by the beam splitter to a mirror that reflects the radiation back toward the beam splitter. The remaining portion of the radiation passes through the beam back towards the beam splitter the remaining portion of the radiation passes through the beam splitter & strikes a mirror that is continuously moved back of forth over a distance of as much as 21 cm.

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After striking the movable mirror, the radiation is refelected back to the beam splitter. A portion of the radiation that was reflected from the stationary mirror & a portion of the radiation from the movable mirror combine at the beam splitter. The signal which exit’s the interferometer is the result of these two beams “interfering” with each other. The resulting signal is called a n interferogram , which has the unique property that every data point ( a function of the moving mirror position which make up the signal) has information about every infrared frequency which comes from the source.

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This means that as the interferogram will measure; all frequencies are being measures simultaneously. The use of the interferometer results in extremely fast measurements. Advantages All energy from the source reaches the sample improving signal to noise rates then to the detector hence intensity is high. Since the sample is exposed to entire radiation beam at same time and absorptions effect at all wavelength modify final signal multiplex advantage. Rapid scan speed permits monitoring of samples undergoing rapid changes. Here we don’t use the energy wasting slits in dispersive equipment it is know jacquinot advantage. Addition and subtraction can be done on digitized spectra.

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Disadvantages It is a single beam instrument it can’t use the reference sample correction ratio. The IR active atmospheric components like carbon dioxide & water will appear in the spectrum. 8

References :

References 1 Instrumental method of Analysis by B.K. Sharma Page No.1 to 260. 2 Elementary Organic Spectroscopy by Y.R. Sharma Page No. 65 to 79. 3 Spectrometric Identification of organic compounds. Sixth Edition by Silverstein Page No. 71 to 75. 4 Methods of Analysis of Willard & Merit Page No. 287 to 293. 5 Introduction to spectroscopy IIIrd Edition by Pavia Lampman Page No. 13 to 21. 6 Principles of Instrumental Analysis, Fifth Edition by Skoog , Holler, Mieman Page 380 to 389. 7 Organic Spectroscopy by William Kemp IInd Edition Page No.20 to 29. 8

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