FTIR AND ITS INTERPRETATION BY KARAN AJAY GUPTA

PRASENTATION ON FTIR AND ITS INTERPRETATIONPRESENTED BY –KARAN GUPTA

HOW THE FTIR WORKS

WHAT IS IR RADIATION

VIBRATIONAL SPECTROSCOPY STUDIES OF GLASS STRUCTURE : IR spectroscopy

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What does this technique measure?

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IR spectroscopy measures the absorption of IR radiation by materials as the atoms vibrate about their bonds

IR spectroscopy is primarily used to identify bond types, structures, and functional groups in organic and inorganic compoundsWhy are IR absorption spectra important? Molecules absorb IR radiation at

frequencies related to their unique compositions, structures and the numbers, types, strengths and position of their bands.


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Infrared refers to that part of the electromagnetic

spectrum between the visible and

microwave regions.


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Introduction

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The energy of a molecule consists partly of translational energy, partly of rotational energy, partly of vibrational energy and partly of electronic energy.

- Electronic energy transitions normally give rise to absorption or emission in the UV and visible regions of the electromagnetic spectrum

- Pure rotation gives rive to absorption in the microwave region or the far infrared

- Molecular vibrations give rise to absorption bands throughout most of the IR region of the spectrum


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Introduction

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The near-IR can excite overtone or harmonic vibrations.

The mid- infrared may be used to study the fundamental vibrations and associated rotational-vibrational structure

The far-infrared, may be used for rotational spectroscopy.


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Definition

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Wavelength and frequency are inversely related:

Energy is related to wavelength and frequency :

Note that energy is directly proportional to frequency and inversely proportional to wavelength.


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IR spectroscopy

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IR radiation does not have enough energy to induce electronic transitions as seen with UV.

Infrared spectroscopy works because chemical bonds have specific frequencies at which they vibrate corresponding to energy levels.

Infrared radiation is absorbed by molecules and converted into energy of molecular vibration.

When the radiant energy matches the energy of a specific molecular vibration, absorption occurs.

Weaker bonds require less energy, as if the bonds are springs of different strengths.


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Degrees of Freedom of Molecular Motion

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The vibration of any structure is analyzed in terms of the degrees of freedom which the structure possesses.

Each mass required 3 coordinates to define its position such as x, y and z in a cartesian coodinate . 3 independent degrees of freedom of motion in the x, y and z direction.

For example:

A sphere has 3 degrees of transitional freedom and 0 degrees of rotational freedom since rotation does not result in a perceptibly different state. Note that a single sphere or a single atom does not have vibrational states.


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Degrees of Freedom of Molecular Motion

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What about the grouping of 2 spheres?

When two spheres are bonded, the group has 3*2 degrees of translational freedom. As a unit, it possesses 3 degrees of translational freedom and 2 degrees of rotational freedom, since rotation about the axis of the two spheres does not result in a perceptible change.When considering vibrational states, the degrees of freedom for the grouping are subtracted from the total number of translational degrees of freedom for the individual spheres. 3*2 - (3+2) = 1 degree of vibrational freedom.


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Degrees of Freedom of Molecular Motion: Classical model

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If there are N atomic nuclei in the molecule, there will be a total of 3N degrees of freedom of motion for all the nuclear masses in the molecule

The center of gravity of the molecule requires 3 coordinates to define its position. It has 3 independent degrees of freedom of motion which are translation of the center of gravity of the molecule.


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Degrees of Freedom of Molecular Motion: Classical model

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When a non-linear molecule is in its equilibrium configuration, it requires 3 rotational coordinates to specify the molecular orientation about the center of gravity.

A linear molecule only has 2 independent rotational degrees of freedom about 2 mutually perpendicular axes, perpendicular to the molecule axis.

After subtracting the translational and rotational degrees of freedom from the total 3N degrees of freedom, there are 3N-6 internal degrees of freedom for a nonlinear molecule and 3N-5 internal degrees of freedom for a linear molecule


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Molecule such as water?

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Vibrational states?

3N-6 3*3-6 = 3 vibration states which results in three absorption bands in IR and Raman. The number of stretching vibrations is N-1

and the number of bending vibrations is 2N-5.Stretching vibrations 3-1 = 2: symmetric stretching of the H-0 bonds and asymmetric stretching of the H-O bonds Bending vibration 2*3-5 = 1: 1 scissors bending of the HOH structure.


15Vibrations

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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

iii.) Types of Molecular Vibrations

Bond Stretching Bond Bending

symmetric

asymmetric

In-plane rocking

In-plane scissoring

Out-of-plane wagging

Out-of-plane twisting

symmetric asymmetric

In-plane rocking

In-plane scissoring

Out-of-plane waggingOut-of-plane twisting

Another Illustration of Molecular Vibrations

iv.) Number of Vibrational Modes:- for non-linear molecules, number of types of vibrations: 3N-6 - for linear molecules, number of types of vibrations: 3N-5- why so many peaks in IR spectra- observed vibration can be less then predicted because • symmetry ( no change in dipole)• energies of vibration are identical• absorption intensity too low• frequency beyond range of instrument

http://www.chem.purdue.edu/gchelp/vibs/co2.htmlSee web site for 3D animations of vibrational modes for a variety of molecules

Examples:1) HCl: 3(2)-5 = 1

mode

2) CO2: 3(3)-5 = 4 modes - -moving in-out of plane+

v.) IR Active Vibrations:- In order for molecule to absorb IR radiation: • vibration at same frequency as in light• but also, must have a change in its net dipole moment

as a result of the vibration

Examples:1) CO2: 3(3)-5 = 4

modes

- -+

m = 0; IR inactive

m > 0; IR active

m > 0; IR active

m > 0; IR actived- d-

2d+

d- d-2d+

d- d-2d+

d- d-2d+

degenerate –identical energy single IR peak

MODES OF VIBRATIONS


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Vibrations

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The symmetric stretch/bend is an easier deformation than the asymmetric stretch/bend

The bending vibration is much easier than stretching


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Can you find the absorption band related to:

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Gas Phase Infrared Spectrum of Formaldehyde, H2C=O The symmetric stretch is an easier

deformation than the asymmetric stretch

occurs at lower wavenumbers The bending vibration is much

easier than stretching so it occurs at lower wavenumber


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Dipole moment

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A dipole moment is a vector quantity - it has both size and direction.

where μ is the bond dipole moment in coulomb-meters, δ is the amount of charge at either end of the dipole, given in coulombs, and d is the distance between the charges in meters


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Dipole moment

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Any molecule where the overall centre of the positive charges and the overall centre of the negative charges coincide will have zero dipole moment. This happens in molecules with sufficiently symmetric shape.

So it is the shape of a molecule that is important in determining whether it has a dipole moment.

For a complete molecule the overall dipole moment is calculated as the vector sum of individual dipole moments


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Example: what is the dipole moment of BF3?

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Shape: trigonal planarEach BF bond has a dipole moment with a

partial negative charge on the fluorine (it is much more electronegative than the boron), but the shape of the molecule is such that the bond dipole moments add up to zero.


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Molecule such as water?

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3 absorption bands in IR and Raman

1 symmetric stretching1 asymmetric stretching1 Bending vibration.

Are they IR or Raman active?


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Symmetrical stretch

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In this mode the dipole moment for the molecule does not change in direction, but it does change in magnitude. As the molecule stretches, the dipole moment increases. So the dipole moment changes and it does so along the z-axis. This vibration is IR active


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Asymmetric stretch

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In this mode, both the direction and magnitude of the dipole moment are changing. The dipole moment switches from left to right. This mode is also IR active.


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Bending mode

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In this bending (scissoring) mode, the dipole does not change direction. It is still pointed along the z-axis, but it does change in magnitude (increasing with the bend). Thus, this mode is also IR active.


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What about CO2?

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Linear Molecule! O=C=O Vibrational states: 3n-5 = 3*3-5= 4


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Theory: Hooke’s law

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the frequency of the vibration of the spring is related to the mass and the force constant of the spring,k, by the following formula:

where k is the force constant, m is the mass, ν is the frequency of the vibration


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Theory: Hooke’s law

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The stretching frequency of a bond can be approximated by Hooke’s Law.

In this approximation, two atoms and the connecting bond are treated as a simple harmonic oscillator composed of 2 masses (atoms) joined by a spring:


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Theory: Classical vibrational Frequency for a diatomic molecule

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Diatomic molecule is represented by 2 masses: m1 and m2, connected by a massless spring. For simplicity, the masses may be allowed to move only along the molecular axis

The displacement of each mass from equilibrium along the axis is X1 and X2. In this case, (X2-X1) is the amount the bond length differs from the equilibrium length.


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Theory: Classical vibrational Frequency for a diatomic molecule

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Each mass will experience a force equal to a constant F.

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2

112 dtXdmXXF 2

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212 dtXd

mXXF

)cos( tX )sin( tdtdX )cos(2

2

2

tdtXd

Acceleration along the X axis


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Source

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All the manufacturers use a heated ceramic source.

The composition of the ceramic and the method of heating vary but the idea is always the same, the production of a heated emitter operating at as high a temperature as with a very long life.

The manufacturers tend to go for either a conducting ceramic or a wire heater coated with ceramic.

C) Instrumentation

1.) Basic Design- normal IR instrument similar to UV-vis- main differences are light source & detector

2.) Fourier Transfer IR (FTIR) – alternative to Normal IR- Based on Michelson Interferometer

Principal:1) light from source is split by central mirror into 2 beams of

equal intensity2) beams go to two other mirrors, reflected by central mirror,

recombine and pass through sample to detector3) two side mirrors. One fixed and other movable

a) move second mirror, light in two-paths travel different distances before recombined

b) constructive & destructive interferencec) as mirror is moved, get a change in signal

Destructive Interference can be created when two waves from the same source travel different paths to get to a point.

This may cause a difference in the phase between the two waves. • If the paths differ by an integer multiple of a wavelength, the waves will also be in phase. • If the waves differ by an odd multiple of half a wave then the waves will be 180 degrees out of phase and cancel out.

- observe a plot of Intensity vs. Distance (interferograms)- convert to plot of Intensity vs. Frequency by doing a

Fourier Transform

- resolution Dn = 1/Dd (interval of distance traveled by mirror)

Advantages of FTIR compared to Normal IR:1) much faster, seconds vs. minutes2) use signal averaging to increase signal-to-noise (S/N)

3) higher inherent S/N – no slits, less optical equipment, higher light intensity

4) high resolution (<0.1 cm-1)

Disadvantages of FTIR compared to Normal IR: 1) single-beam, requires collecting blank 2) can’t use thermal detectors – too slow

In normal IR, scan through frequency range. InFTIR collect all frequencies at once.

/increase S N number scans


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Interferometer: heart of the instrument

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This device splits and recombines a beam of light such that the recombined beam produces a wavelength-dependent interference pattern or an interferogram. The Michelson interferometer is most commonly used.The Michelson interferometer

produces interference fringes by splitting a beam of monochromatic light - one beam strikes a fixed mirror -the other a movable mirror.

When the reflected beams are brought back together, an interference pattern results.


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Interferometer: Michelson Interferometer

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The signal is recorded as a function of the optical path difference between the fixed and the movable mirror.

The interferogram contains the basic information on frequencies and intensities characteristic of a spectrum but in a form that is not directly interpretable.

The information is converted to a more familiar form, a spectrum, using Fourier Transform methods.

With an infrared data station, the computer acquires, processes, stores and retrieves spectral data.


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Infrared Detector

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It is a device which measured the infrared energy of the source which has passed through the spectrometer. These devices change radiation energy into electrical energy which can be processed to generate a spectrum.

2 basic types:- Thermal detectors which measure the

heating effect of radiation and respond equally well to all wavelengths

- Selective detectors whose response is markedly dependent on the wavelength


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Infrared detectors

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Examples of thermal detectors: Thermocouples: when incident radiation is

absorbed at the junction, the temperature rise causes an increase in the electromotive potential developed across the junction leads

Bolometers: detecting device which depends on a change of resistance with temperature

Pyroelectric: it consists of a thin pyroelectric crystal. If it is electrically polarized in an electric field, it retains a residual electric polarization after the field is removed.


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Nicolet Continuµm Infrared Microscope

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Acquisition of mid-IR spectra from very small (>10 µm) samples with diffraction limited spatial resolution.

Material science applications including:

• Forensic analysis• Surface analysis• Art conservation• Mineralogy• Biochemistry

SAMPLE PREPARATION


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Measurement of IR spectra

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IR spectra are acquired on a special instrument, called an IR spectrometer.

IR is used both to gather information about the structure of a compound and as an analytical tool to assess the purity of a compound or the structure of the glasses

IR spectra are quick and easy to runCells for holding samples must be made of

infrared transmitting material.


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Infrared transmitting materials

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Infrared transmitting materials

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Some of the most useful window materials for the IR are quite soluble in water: NaCl, KCl, KBr, CsBr and CsI

A typical cell for liquid which is made of NaCl has a transmission that starts to drop at about 700cm-1, is roughly 50% at 600cm-1 and is nearly opaque at 500cm-1.


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Sample Requirements

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The detection limit for routine analysis is ~0.1 wt%; under ideal conditions greater sensitivity can be achieved

Typically only a few milligrams of material are needed for analysis.

Samples may be in liquid, solid, or gaseous form.


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Preparation of the samples: gas

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Gaseous samples require little preparation beyond purification!

To obtain an infrared spectrum of a gas requires the use of a cylindrical gas cell with windows at each end composed of an infrared inactive material such as KBr, NaCl or CaF2. The cell usually has an inlet and outlet port with a tap to enable the cell to be easily filled with the gas to be analyzed.


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Preparation of the samples: liquid

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Sandwiched between two plates of a high purity salt (commonly sodium chloride, or common salt, although a number of other salts such as potassium bromide or calcium fluoride are also used).

The plates are transparent to the infrared light (not introduce any lines onto the spectra).


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Preparation of the samples: solid

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Solid samples can be prepared in two major ways.

1) Crush the sample with a mulling agent (usually nujol) in a marble or agate mortar, with a pestle. A thin film of the mull is applied onto salt plates and measured.

2) Grind a quantity of the sample with a specially purified salt (usually potassium bromide) finely (to remove scattering effects from large crystals). This powder mixture is then crushed in a mechanical die press to form a translucent pellet through which the beam of the spectrometer can pass.

Samples dispersed in powder must be homogenously dispersed, with a particle size small enough not to cause scatter (theoretically < 2 microns).

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FTIR AND ITS INTERPRETATION BY KARAN AJAY GUPTA

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