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 => Slide 5: 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 Slide 9: F = force, restoring
back to equilibrium position
k = characteristic
x = displacement from
position Slide 10: 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? Slide 11: 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. Slide 13: Quantum mechanics: The frequency () depends on the energy gap between vibrational levels
E = h= hc/(cm-1) Only the natural frequency will be absorbed Slide 14: The natural frequency (8.67 x 1013 s-1) is absorbed selectively Slide 15: 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? Slide 16: The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon Slide 17: iii.) Types of Molecular Vibrations Bond Stretching symmetric asymmetric Slide 18: In-plane rocking In-plane scissoring Out-of-plane wagging Out-of-plane twisting Bond Bending Slide 19: 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. Slide 23: 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 Slide 24: 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 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
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
Hg arc (Far IR), Tungsten filament (Near IR) Michaelson Interferometer : Michaelson Interferometer Beam splitter
Moving mirror at constant velocity
He/Ne laser; sampling interval, control mirror velocity Slide 30: Schematic of a Michelson Interferometer. Sample : Sample Sample holder must be transparent to IR- salts
Neat, 1 drop
Samples dissolved in volatile solvents- 0.1-10%
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. Slide 34: 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 Slide 35: 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 Slide 36: 1) Examine what functional groups are present by looking at group frequency region - 3600 cm-1 to 1200 cm-1 Slide 37: 2) compare spectrum of compound to IR library
- looking at functional group and fingerprint region
- small differences in structure results in large differences in
- 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. Slide 38: 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 THANK YOU : THANK YOU