Lecture 3 INFRARED SPECTROMETRY Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Lecture 3 INFRARED SPECTROMETRY

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

INTRODUCTION TO INFRARED SPECTROMETRY

INFRARED SPECTRAL REGIONS

INFRARED ABSORPTION SPECTRUM

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

Vibration or rotation causes varying separation

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

INTRODUCTION TO INFRARED SPECTROMETRY

Molecules with permanent dipole moments (μ) are IR active!

INTRODUCTION TO INFRARED SPECTROMETRY

Types of Molecular Vibrations:

Stretch - change in bond lengthsymmetricassymetric

Bend – change in bond anglescissoringwaggingrockingtwisting/tortion

INTRODUCTION TO INFRARED SPECTROMETRY

STRETCH

INTRODUCTION TO INFRARED SPECTROMETRY

GENERAL DESIGN OF OPTICAL INSTRUMENTS

BEND

GENERAL DESIGN OF OPTICAL INSTRUMENTS

Only some modes may be IR active:Example CO2 O=C=O linear

GENERAL DESIGN OF OPTICAL INSTRUMENTS

GENERAL DESIGN OF OPTICAL INSTRUMENTS

GENERAL DESIGN OF OPTICAL INSTRUMENTS

Classical vibrational motion:

GENERAL DESIGN OF OPTICAL INSTRUMENTS

CLASSICAL VIBRATIONAL FREQUENCY

INFRARED SPECTROSCOPY

What about quantum mechanics?

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.

INFRARED SPECTROSCOPY

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

New E-y curve:

IR SPECTROSCOPY

VIBRATIONAL MODES

How many vibrational mode?

HCN, H2CO3

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

IR SPECTROSCOPY

IR SPECTROSCOPY - INSTRUMENTATION

SOURCES

IR SPECTROSCOPY - INSTRUMENTATION

TRANSDUCERS

IR SPECTROSCOPY - INSTRUMENTATION

IR SPECTROSCOPY - INSTRUMENTATION

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.

IR SPECTROSCOPY - INSTRUMENTATION

Fourier Transform Instruments have two advantages:

FOURIER TRANSFORM

FOURIER TRANSFORM

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

IR SPECTROSCOPY - INSTRUMENTATION

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

GENERAL DESIGN OF OPTICAL INSTRUMENTS

GENERAL DESIGN OF OPTICAL INSTRUMENTS

GENERAL DESIGN OF OPTICAL INSTRUMENTS

GENERAL DESIGN OF OPTICAL INSTRUMENTS

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

GENERAL DESIGN OF OPTICAL INSTRUMENTS

GENERAL DESIGN OF OPTICAL INSTRUMENTS

TRANSFORMATION OF ALKENES AND ALKYNES

SAMPLE

SAMPLE

Group Frequencies:• Approximately calculated from masses and spring constants• Variations due to coupling• Compared to correlation charts/databases

Quantitative Analysis:IR more difficult than UV-Vis because• narrow bands (variation in e)• complex spectra• weak incident beam• low transducer sensitivity• solvent absorptionIR mostly used for rapid qualitative but not quantitative analysis