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Lecture 3 INFRARED SPECTROMETRY
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
INTRODUCTION TO INFRARED SPECTROMETRY
Energy of IR photon insufficient to cause electronic excitation but can cause vibrational or rotational excitation
Molecule electric field (dipole moment) interacts with IR photon electric field (both dynamic)
Magnitude of dipole moment determined by(i) charge(ii) separation of charge
INTRODUCTION TO INFRARED SPECTROMETRY
Molecule must have change in dipole moment due to vibration or rotation to absorb IR radiation!
Absorption causes increase in vibration amplitude/rotation frequency
Types of Molecular Vibrations:
Stretch - change in bond lengthsymmetricassymetric
Bend – change in bond anglescissoringwaggingrockingtwisting/tortion
INTRODUCTION TO INFRARED SPECTROMETRY
SAMPLE PROBLEMThe force constant for a typical triple bond is 1.91 x 103
N/m. Calculate the approximate frequency of the main absorption peak due to vibration of CO.
SELECTION RULE
= ±1 Vibrational Selection RuleSince levels equally spaced - should see one absorption frequency.
GENERAL DESIGN OF OPTICAL INSTRUMENTS
Anharmonic oscillator:Must modify harmonic oscillator potential for
(i) electron repulsion (steeper at small distances)(ii) dissociation (bond breaks at large distances)
IR SPECTROSCOPY
Coupling of different vibrations shifts frequencies
Coupling likely when:(1) common atom in stretching modes(2) common bond in bending modes(3) common bond in bending + stretching modes(4) similar vibrational frequencies
Coupling not likely when(1) atoms separated by two or more bonds(2) symmetry inappropriate
FOURIER TRANSFORM INFRARED SPECTROMETERA photodiode array can measure an entire spectrum at once. The spectrum is spread into its component wavelength and each wavelength is directed onto one detector element.
Fourier Analysis is a procedure in which a spectrum is decomposed into a sum of sine and cosine terms called a Fourier series.
GENERAL DESIGN OF OPTICAL INSTRUMENTS
Unfortunately, no detector can respond on 10-14 s time scaleUse Michelson interferometer to measure signal proportional to time varying signal
GENERAL DESIGN OF OPTICAL INSTRUMENTS
• If moving mirror moves 1/4 (1/2 round-trip) waves are out of phase at beam-splitting mirror - no signal
• If moving mirror moves 1/2 (1 round-trip) waves are in phase at beam-splitting mirror - signal
GENERAL DESIGN OF OPTICAL INSTRUMENTS
• Difference in pathlength called retardation
• Plot vs. signal - cosine wave with frequency proportional to light frequency but signal varies at much lower frequency
• One full cycle when mirror moves distance /2 (round-trip = ) velocity of moving mirror
GENERAL DESIGN OF OPTICAL INSTRUMENTS
APPLICATIONS:FTIR single-beam, dispersive IR double-beam, but FTIR advantages include• High S/N ratios - high throughput• Rapid (<10 s)• Reproducible• High resolution (<0.1 cm-1)• Inexpensive
GENERAL DESIGN OF OPTICAL INSTRUMENTS
IR (especially FTIR) very widely used for• qualitative• quantitativeanalysis of• gases• liquids• solids
GENERAL DESIGN OF OPTICAL INSTRUMENTS
Most time-consuming part is sample preparationGases fill gas cell(a) transparent windows (NaCl/KBr)(b) long pathlength (10 cm) - few molecules
Liquids fill liquid cell(a) solute in transparent solvent - not water (attacks windows)(b) short pathlength (0.015-1 mm) - solvents absorb
Group Frequencies:• Approximately calculated from masses and spring constants• Variations due to coupling• Compared to correlation charts/databases
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