Introduction

The term "infra red" covers the range of the electromagnetic spectrum between

0.78 and 1000 mm. In the context of infra red spectroscopy, wavelength is measured

in "wavenumbers", which have the units cm-1. wavenumber = 1 / wavelength in

centimeters

• It is useful to divide the infra red region into three sections; near, mid and far infra red;

• RegionWavelength range (mm)Wavenumber range (cm-1)Near0.78 - 2.512800 - 4000Middle2.5 - 504000 - 200Far50 -1000200 - 10

• The most useful I.R. region lies between 4000 - 670cm-1.

• Theory of infra red absorptionTheory of infra red absorption

• IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states.

• For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. The alternating electrical field of the radiation (remember that electromagnetic radation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration

• Molecular rotations

• Rotational transitions are of little use to the spectroscopist. Rotational levels are quantized, and absorption of IR by gases yields line spectra. However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions.

• Molecular vibrations

• The positions of atoms in a molecules are not fixed; they are subject to a number of different vibrations. Vibrations fall into the two main catagories of stretching and bending.

• Stretching: Change in inter-atomic distance along bond axis

• Bending: Change in angle between two bonds. There are four types of bend:

• Rocking

• Scissoring

• Wagging

• Twisting

•

• Vibrational coupling In addition to the vibrations mentioned above, interaction between vibrations can occur (coupling) if the vibrating bonds are joined to a single, central atom. Vibrational coupling is influenced by a number of factors;

• Strong coupling of stretching vibrations occurs when there is a common atom between the two vibrating bonds

• Coupling of bending vibrations occurs when there is a common bond between vibrating groups

• Coupling between a stretching vibration and a bending vibration occurs if the stretching bond is one side of an angle varied by bending vibration

• Coupling is greatest when the coupled groups have approximately equal energies

• No coupling is seen between groups separated by two or more bonds

• SELECTION RULE :• SELECTION RULE 1) ABSORPTION OF CORRECT

WAVELENGTH OF RADIATION (MATCHING OF FREQUENCY A molecule will absorb suitable radiation when its natural frequency of vibration matches with the frequency of incident radiation (i.e. a net transfer of energy takes place and it results in a change in the amplitude of molecular vibration) Natural frequency of HCl molecule is 8.7 X 1013Hz(vib/sec)or(2890 cm-1) When IR radiation will allow to pass on HCl sample and transmitted radiation is analyzed .It is observed that part of radiation which has same frequency 8.7X1013 HZ is absorb. Thus remaining has been transmitted so gives characteristic value of HCl

• 2) DIPOLE MOMENT :• 2) DIPOLE MOMENT “A molecule will

absorb IR radiation if the change in the vibrational state is associated with the change in the dipole moment of the molecule” μ = q x r A dipole moment arises from a separation of charges in a molecule : μ =dipole moment (Coulomb ·meters) q =magnitude of charges r =vector going from –ve charge to +ve charge

• SELECTION RULE OF VIBRATIONAL QUANTUM NUMBER :

• 3)SELECTION RULE OF VIBRATIONAL QUANTUM NUMBER Based on the harmonic oscillator model (Δv = ±1) Here + absorption & - emission or Vibrational quantum number change by unity

• Harmonic oscillator Anharmonic oscillator Overtones and fundamental band So far as harmonic oscillator model is consider the lower vibration state can be explain (v - v´=1) but for higher vibrational state the selection rule may fail because vibrational energy levels are less separated as the vibration state increases (But for harmonic oscillator model it is not true ) Overtones and fundamental band can not be explain by Harmonic oscillator model. So vibrational diatomic molecule can be explain well using anharmonic oscillator model Anharmonic oscillator model can explain the selection rule for all transition . For real molecule they don’t obey simple harmonic motion. Because the bonds are also real Therefore anharmonic oscillator model consider Therefore energy difference goes on decreasing as quantum no. increasing Δv = ±1, ±2, ±3…… (overtone lines) is possible but they are having extremely low intensity. This are known as overtones (they are weaker)

• they found that when a ball was suspended on a spring from a horizontal wall, the frequency of vibration or oscillation, n, depended only on the mass of the ball and the stiffness of the spring. The term A is a constant of the proportionality. By varying the mass of the ball and the stiffness of the spring, they were able to uncover the following simple relationship between frequency, mass and force constant:

• Suspending a ball and spring from a horizontal surface is a special case of the more general situation when you have two more comparable masses attached to each other. Under these circumstances, when two similar masses are attached to a spring, the relationship between frequency of vibration, mass and force constant is given by:

• where m, represents the product of the masses divided by their sum (m1m2)/(m1+m2). This latter term is found in other physical relationships and has been given the name, the reduced mass. It can easily be seen that equation 2 is a special case of the more general relationship given by equation 3. If we consider m1to be much larger than m2, the sum of m1+ m2 m1 and substituting this approximation into (m1m2)/(m1+m2) m2. Substituting m2 into equation 3 where m2 is the smaller of the two masses gives us exactly the same relationship as we had above when the ball was suspended from a horizontal wall. The horizontal wall is much more massive than the ball so that the vibration of a smaller ball has very little effect on the wall. Despite their simplicity, equations 2 and 3 play an important role in explaining the behavior of molecular systems. However, before we discuss the important role these equations play in our understanding of infrared spectroscopy, we need to review some of the properties of electromagnetic radiation, particularly radiation in the infrared range.

• Number of possible vibrational modes :Number of possible vibrational modes CAN WE KNOW THE POSSIBLE VIBRATION? YES, but how? 3N-5 for linear molecules 3N-6 nonlinear molecules N: number of atoms in a molecules BUT WHAT IS 3N, 5 & 6. HOW’S IT COME?

• Atoms are never fixed in the space but move about continuously Each atom may be said to posses three degrees of freedom of movement, and thus in N-atom (polyatomic molecule) Molecule there will be 3N degrees of freedom (That means 3 coordinates are needed to locate a point in space. And each coordinate corresponds to one degree of freedom for one of the atom ,so a molecule containing N-atom is said to have 3N degree of freedom )

• From this 3 types of motion 1) TRANSLATION MOTION Corresponds to the movement of the entire molecule through space while the position of the atoms relative to each other remain fixed. In this sence, the molecule may be considered as a single particle with the mass of the molecule located at its center of gravity and possesses three degree of translation freedom OR Defination of translation motion require 3 coordinates and thus this common motion requires 3 of the 3N degree of freedom

• 2)ROTATIONAL MOTION :Another 3 degree of freedom are needed to describe the rotational motion of the molecule. (i.e. poly atomic molecule has generally 3 degree of rotation freedom) But in the special case of linear molecule all the atom lie on a straight line and only 2 rotation can be define (because rotation around the bond axis is not possible) 2)ROTATIONAL MOTION

• 3)VIBRATIONAL MOTION :Now the rest i.e. substrate transition & rotation motion from 3N degree of freedom i.e. 3N-6 ( for non linear molecule) 3 of translation motion + 3 of rotational motion=6 & 3N-5(for linear molecule) 3 of translation motion + 2 of rotational motion=5 3)VIBRATIONAL MOTION

• Now out of this normal vibration no. of stretching and bending vibration can be calculated For stretching vibration =N -1 For bending vibration =2N -5(non linear) i.e. [(3N - 6)-(N -1)]=2N -5 =2N-4(linear) i.e. [(3N - 5)-(N -1)] =2N – 4 Examples: O2, N2, Cl2: 3N-5 =3x2-5 = 1, only one vibration mode CO2 3N-5 =3x3-5 = 4(linear molecule) H2O 3N-6 =3x3-6 = 3

• FACTOR THAT PRODUCED LESS NO. OF PEAKS THAN EXPECTED :FACTOR THAT PRODUCED LESS NO. OF PEAKS THAN EXPECTED The symmetry of a molecular vibration result in no change in the dipole moment (i.e. in co2 only 2 peak out of 4) The energy of two or more vibrations are identical (same or degenerate) The absorption intensity is so low as to be undetectable by means (detector) The vibration energy is in the wL region beyond the range of the instrument

• INSTRUMENTATION

SOURCE SAMPLE REFRENCE TRANSDUCER MC INFRARED SOURCE :INFRARED SOURCE IR source consist of an inert solid that is heated electrically to a temperature between 1500 and 2200 K The material is chosen so that its emission approximates As closely as possible to that of black body radiator. (in candescent solid-the glow due to the great heat) We get continuum radiation approximating that of black body results

What is this black body radiation? It is truly continuum radiation is produced when solid are heated incandescence Thermal radiation of this kind, which is called black body radiation, is characteristic of temp. of emitting surface rather than the material of which the surface is composed.

Black body radiation is produced by the innumerable atomic and molecular oscillation excited in the condensed solid by thermal energy. It is clear that a very high temp. is needed for thermal excitation The maximum radiant intensity at this temp. occur between 5000-5900 cm-1 Problems with Infrared from sun 99% of infrared rays are absorbed by water in our atmosphere At higher wave length the intensity fall is smoothly until it is about 1% of max. intensity But on shorter wavelength side the decrease is much more rapid 10000 5000 670 cm-1 1 2 15 μm

• Nujol mull

• A mull is a suspension of a solid in a liquid. Under these conditions, light can be transmitted through the sample to afford an acceptable infrared spectrum. The commercial sample of Nujol, or mineral oil, which is a long chain hydrocarbon is often used for this purpose. Most solids do not dissolve in this medium but can be ground up in its presence. A small mortar and pestle is used for this purpose. If the grinding process gives rise to small particles of solid with diameters roughly the same as the wavelength of the infrared radiation being used, 2-5 microns, these particles will scatter rather than transmit the light. The effect of poor grinding is illustrated in Figures 29 and 30 for a sample of benzoic acid. If you find this type of band distortion with either a Nujol mull or a KBr pellet (discussed below), simply continue grinding the sample up until the particles become finer.

• The major disadvantage of using a Nujol mull is that the information in the C-H stretching region is lost because of the absorptions of the mulling agent. A spectrum of Nujol is shown in Figure 5. To eliminate this problem, it may be necessary to run a second spectrum in a different mulling agent that does not contain any C-H bonds. Typical mulling agents that are used for this purpose are perfluoro- or perchlorohydrocarbons. Examples include perchlorobutadiene, perfluorokerosene or a perfluorohydrocarbon oil.

• DETECTORS :DETECTORS or TRANSDUCERS There are 3 types of detector (transducer) 1) THERMAL DETECTOR 2) PYROELECRIC DETECTOR 3) PHOTON (QUANTUM) DETECTOR

• THERMAL DETECTOR :THERMAL DETECTOR Thermal detector whose response depend on heating effect. Which in terns alters the physical properties of transducers such as resistance It is a transducer that changes thermal energy in to an electric signal. The electric signal is amplified and routed to the read out device

• THERMOCOUPLE :1)THERMOCOUPLE Most widely used IR detector Consist of 2 pieces of metal such as “Bi” which are joined with dissimilar metal such as “sb” (antimony) and form a pair of junction (2 jn) Jn. Is usually blackened (to improve its heat absorb capacity) With black metallic oxides One jn between 2 dissimilar metal is heated with IR radiation other jn. Kept at const. temp..

• PRINCIPLE “A CHANGE IN THE TEMP. AT THE BOTH DIFF.JN. BETWEEN 2 UNLIKE METAL CAUSES AN ELECTRIC POTENTIAL TO DEVELOPE BETWEEN SPECIES” Or “DUE TO DIFRENCE IN WORK FUNCTION OF METALS WITH TEMP. A SMALL VOLTAGE DEVELOP ACROSS THE THERMOCOUPLE”

• This potential diff. is depend (proportional) to the amt of IR radiation falling on hot jn. Whole assembly is evacuated in still housing with IR transparent KBr window to minimize conductivity heat loss. Capable to responding temp. diff. of 10-6 k corresponding to potential diff. of 6-8 μV/ μW Advantage that independent of response with change in wave length “THERMOPILE” ?

• 2) THERMISTOR or BOLOMETER When irradiation by IR beam produced an increase in resistance of the metal strip which measured with a whetstone bridge A potential diff. between the 2 elements produced a proportional voltage difference

• 3)PNEUMATIC DETECTOR or GOLAY DETECTOR :3)PNEUMATIC DETECTOR or GOLAY DETECTOR Pneumatic detectors respond to change in vol. of non absorbing gas or liq with temp. change In pneumatic device if gas is used as medium called golay detector Here the absorbing radiation heats an inert gas (usually xenon) in a pneumatic chamber behind the plate and cause the gas to expand As the gas expands the flexible diaphragm at the opposite end of the chamber from the metallic plate is pushed outward

• Applications of IR Spectroscopy to Inorganic Molecules. Many so-called inorganic compounds are in reality largely organic, and for these we look for the same functional group bands in the IR as we do for purely organic compounds. However, the infrared spectra of relatively simple, purely inorganic compounds containing only a few atoms--specifically, inorganic salts containing polyatomic (complex) ions--are quite distinctive and can be used to rapidly identify the ions. Consider a simple inorganic salt, such as KNO2. On the basis of the empirical formula, we might naively expect there to be a total of 3(4)-6 = 6 normal modes of vibration associated with this material. However, this assumes that KNO2 is covalent.

• In fact, KNO2 consists of an ionic lattice of K+ and NO2- ions arranged in an infinite and very regular array. The crystal consists of essentially isolated K+ ions and NO2- ions. Thus we are able to consider the vibrational modes of the cation and anion independently of one another. In this case, since the potassium ions are monatomic, they have no vibrations (3(1)-3 = 0), so we need only consider the nitrite anions. The VSEPR (Valence Shell Electron Pair Repulsion) Theory predicts a bent structure for the nitrite ion. We thus anticipate three normal vibrational modes for NO2-, corresponding to the diagrams drawn earlier for H2O, and they should all be infrared active.

• Indeed, three bands are observed in the IR spectrum of KNO2: the symmetric stretch at 1335 cm-1, the asymmetric stretch at 1250 cm-1, and the bending vibration at 830 cm-1 (bending vibrations occur in general at lower frequencies than stretching vibrations). The frequencies of these vibrations are about the same regardless of counter ion, substantiating the independence of the anion and cation in the crystal.

• C-H aldehydes

• Before concluding the discussion of the carbon hydrogen bond, one additional type of C-H stretch can be distinguished, the C-H bond of an aldehyde. The C-H stretching frequency appears as a doublet, at 2750 and 2850 cm-1. Examples of spectra that contain a C-H stretch of an aldehyde can be found in Figures 14 and 15. You may (should) question why the stretching of a single C-H bond in an aldehyde leads to the two bands just described. The splitting of C-H stretching frequency into a doublet in aldehydes is due to the phenomema we called "Fermi Resonance". It is believed that the aldehyde C-H stretch is in Fermi resonance with the first overtone of the C-H bending motion of the aldehyde. The normal frequency of the C-H bending motion of an aldehyde is at 1390 cm-1. As a result of this interaction, one energy level drops to ca. 2750 and the other increases to ca. 2850 cm-1. Only one C-H stretch is observed for aldehydes that have the C-H bending motion of an aldehyde significantly shifted from 1390 cm-1.

• Primary amines and amides derived from ammoniaThe N-H stretching frequency in primary amines and in amides derived from ammonia have the same local symmetry as observed in CH2. Two bands, a symmetric and an asymmetric stretch are observed. It is not possible to assign the symmetric and asymmetric stretches by inspection but their presence at approximately 3300 and 3340 cm-1 are suggestive of a primary amine or amide. These bands are generally broad and a third peak at frequencies lower than 3300 cm-1, presumably due to hydrogen bonding, is also observed. This is illustrated by the spectra in Figures 18 and 19 for n-butyl amine and benzamide.

• Esters, aldehydes, and ketones

• Esters, aldehydes, and ketones are frequently encountered examples of molecules exhibiting a C=O stretching frequency. The frequencies, 1735, 1725, 1715 cm-1 respectively, are too close to allow a clear distinction between them. However, aldehydes can be distinguished by examining both the presence of the C-H of an aldehyde (2750, 2850 cm-1) and the presence of a carbonyl group. Examples of an aliphatic aldehyde, ester, and ketone are given in Figures 14, 34, 36, and 35, respectively.

• Nitro group

• The final functional group we will include in this discussion is the nitro group. In addition to being an important functional group in organic chemistry, it will also begin our discussion of the importance of using resonance to predict effects in infrared spectroscopy. Let's begin by drawing a Kekule or Lewis structure for the nitro group. You will find that no matter what you do, it will be necessary to involve all 5 valence