Lecture 3 INFRARED SPECTROMETRY Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
INTRODUCTION TO INFRARED SPECTROMETRY INFRARED SPECTRAL REGIONS INTRODUCTION TO INFRARED SPECTROMETRY
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
INTRODUCTION TO INFRARED SPECTROMETRY Vibration or rotation causes varying separation INTRODUCTION TO INFRARED SPECTROMETRY
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
INTRODUCTION TO INFRARED SPECTROMETRY
INTRODUCTION TO INFRARED SPECTROMETRY Types of Molecular Vibrations: Stretch - change in bond length symmetric assymetric Bend – change in bond angle scissoring wagging rocking twisting/tortion INTRODUCTION TO INFRARED SPECTROMETRY
INTRODUCTION TO INFRARED SPECTROMETRY STRETCH INTRODUCTION TO INFRARED SPECTROMETRY
GENERAL DESIGN OF OPTICAL INSTRUMENTS BEND GENERAL DESIGN OF OPTICAL INSTRUMENTS
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
GENERAL DESIGN OF OPTICAL INSTRUMENTS Classical vibrational motion: GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
CLASSICAL VIBRATIONAL FREQUENCY
INFRARED SPECTROSCOPY What about quantum mechanics? INFRARED SPECTROSCOPY
SAMPLE PROBLEM The 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
= ±1 Vibrational Selection Rule Since levels equally spaced - should see one absorption frequency. SELECTION RULE
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) GENERAL DESIGN OF OPTICAL INSTRUMENTS
New E-y curve: IR SPECTROSCOPY
IR SPECTROSCOPY
VIBRATIONAL MODES
How many vibrational mode? HCN, H2CO3
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
IR SPECTROSCOPY - INSTRUMENTATION SOURCES IR SPECTROSCOPY - INSTRUMENTATION
IR SPECTROSCOPY - INSTRUMENTATION TRANSDUCERS IR SPECTROSCOPY - INSTRUMENTATION
IR SPECTROSCOPY - INSTRUMENTATION
IR SPECTROSCOPY - INSTRUMENTATION
FOURIER TRANSFORM INFRARED SPECTROMETER A 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: IR SPECTROSCOPY - INSTRUMENTATION
FOURIER TRANSFORM
FOURIER TRANSFORM
GENERAL DESIGN OF OPTICAL INSTRUMENTS Unfortunately, no detector can respond on 10-14 s time scale Use Michelson interferometer to measure signal proportional to time varying signal GENERAL DESIGN OF OPTICAL INSTRUMENTS
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
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
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
GENERAL DESIGN OF OPTICAL INSTRUMENTS IR (especially FTIR) very widely used for • qualitative • quantitative analysis of • gases • liquids • solids GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS Most time-consuming part is sample preparation Gases 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
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 absorption IR mostly used for rapid qualitative but not quantitative analysis