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IR & Fourier Transform IR : 

IR & Fourier Transform IR BY SAMEER SAWANT

Slide 2: 

It Provides information about the vibrations of functional groups in a molecule Infrared Spectroscopy Therefore, the functional groups present in a molecule can be deduced from an IR spectrum

Introduction : 

Introduction Spectroscopy is an analytical technique which helps to determine the structure of the compounds. It destroys little or no sample. The amount of light absorbed by the sample is measured as wavelength is varied.

The Spectrum and Molecular Effects : 

The Spectrum and Molecular Effects =>

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Types of Electromagnetic Radiation in nature. E=hn = hc/l

The IR Region : 

The IR Region Just below the red in the visible region usually between the range of 2.5 - 25 mm. More common units are wave numbers, or cm-1, the reciprocal of the wavelength in centimeters. Wave numbers are proportional to frequency and energy.

Molecular Vibrations : 

Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies.

IR: Masses, Atoms and Springs : 

IR: Masses, Atoms and Springs A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds). As a first approximation use Hooke’s Law F = -kx

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F = force, restoring back to equilibrium position k = characteristic stretching constant x = displacement from the equilibrium position

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IR Stretching Frequencies of two bonded atoms: = frequency k = spring strength (bond stiffness) mr = reduced mass (~ mass of largest atom) What Does the Frequency, , Depend On?

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IR Stretching Frequencies: What Do they Depend On? Directly on the strength of the bonding between the two atoms ( ~ k) Inversely on the reduced mass of the two atoms (v ~ 1/m)

Stretching Frequencies : 

Stretching Frequencies Frequency decreases with increasing atomic weight. Frequency increases with increasing bond energy.

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Quantum mechanics: The frequency () depends on the energy gap between vibrational levels E = h= hc/(cm-1) Only the natural frequency will be absorbed

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The natural frequency (8.67 x 1013 s-1) is absorbed selectively

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The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency  can be transferred to the atoms involved in the vibration What does the absorption intensity depend on?

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The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon

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iii.) Types of Molecular Vibrations Bond Stretching symmetric asymmetric

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In-plane rocking In-plane scissoring Out-of-plane wagging Out-of-plane twisting Bond Bending

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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 - 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 Examples: 1) HCl: 3(2)-5 = 1 mode 2) CO2: 3(3)-5 = 4 modes - - moving in-out of plane +

Vibrational Modes : 

Vibrational Modes Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational modes.

Fingerprint Region of the Molecule : 

Fingerprint Region of the Molecule Whole-molecule vibrations and bending vibrations are also quantitized. No two molecules will give exactly the same IR spectrum (except enantiomers). Simple stretching: 1600-3500 cm-1. Complex vibrations: 600-1400 cm-1, called the “fingerprint region.”

IR-Active and Inactive : 

IR-Active and Inactive A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule will absorb weakly or not at all.

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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 active d- d- 2d+ d- d- 2d+ d- d- 2d+ d- d- 2d+ 9

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Does O=C=O absorb IR light? Ans: vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light No dipole generated Dipole generated

FT-IR Spectrometer : 

FT-IR Spectrometer Uses an interferometer. Has better sensitivity. Less energy is needed from source. Completes a scan in 1-2 seconds. Takes several scans and averages them Has a laser beam that keeps the instrument accurately calibrated.

Components : 

Components Source Michelson Interferometer Sample Detector

Sources : 

Sources Black body radiators Inert solids resistively heated to 1500-2200 K Max radiation between 5000-5900 cm-1 (2-1.7 mm), falls off to about 1 % max at 670 cm-1 (15 mm) Nernst Glower – cylinder made of rear earth elements Globar- SiC rod CO2 laser Hg arc (Far IR), Tungsten filament (Near IR)

Michaelson Interferometer : 

Michaelson Interferometer Beam splitter Stationary mirror Moving mirror at constant velocity He/Ne laser; sampling interval, control mirror velocity

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Schematic of a Michelson Interferometer.

Sample : 

Sample Sample holder must be transparent to IR- salts Liquids Salt Plates Neat, 1 drop Samples dissolved in volatile solvents- 0.1-10% Solids KBr pellets Mulling (dispersions)

FT-IR detectors : 

FT-IR detectors Pyroelectric tranducers (PTs) Pyroelectric substances act as temperature-dependent capacitors Triglycine sulfate is sandwiched between two electrodes. One electrode is IR transparent The current across the electrodes is Temperature dependent PTs exhibit fast response times, which is why most FT instruments use them

Sequence for Obtaining Spectrum : 

Sequence for Obtaining Spectrum Interferogram of Background is obtained (without sample) System uses Fourier Transform to create single beam background spectrum. Interferogram of Sample is obtained. System uses Fourier Transform to create single beam spectrum of sample. System calculates the transmittance or absorbance spectrum.

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Advantages of FTIR compared to Normal IR: 1) much faster, seconds vs. minutes 2) 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

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D) Application of IR 1.) Qualitative Analysis (Compound Identification) - main application - Use of IR, with NMR and MS, in late 1950’s revolutionized organic chemistry

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1) Examine what functional groups are present by looking at group frequency region - 3600 cm-1 to 1200 cm-1

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2) compare spectrum of compound to IR library - looking at functional group and fingerprint region - small differences in structure results in large differences in fingerprint region - close match in fingerprint and group frequency regions  strong evidence of good match ii.) Group Frequency Region - approximate frequency of many functional groups (C=O,C=C) can be calculated from atomic masses & force constants - - serves as a good initial guide to compound identity, but not positive proof.

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iii.) Fingerprint Region (1200-700 cm-1) - region of most single bond signals - many have similar frequencies, so affect each other & give pattern characteristics of overall skeletal structure of a compound - exact interpretation of this region of spectra seldom possible because of complexity

Slide 39: 

Abbreviated Table of Group Frequencies for Organic Groups