An Introduction to Infrared and An Introduction to Infrared and UV-Visible SpectroscopyUV-Visible Spectroscopy
LEARNING OBJECTIVES
Describe the principal regions of the electromagnetic spectrum.
Describe the principles of infrared spectroscopy. Describe the principles of UV-Vis spectroscopy. Describe and explain the principal factors that govern the
vibrational frequencies of bonds. Describe and explain the principal factors that govern the
electronic absorption process in UV-Vis spectroscopy. Experimental and instrumental
THE ELECTROMAGNETIC SPECTRUM
WHAT IS SPECTROSCOPY? Atoms and molecules interact with electromagnetic
radiation (EMR) in a wide variety of ways. Atoms and molecules may absorb and/or emit EMR. Absorption of EMR stimulates different types of
motion in atoms and/or molecules. The patterns of absorption (wavelengths absorbed and
to what extent) and/or emission (wavelengths emitted and their respective intensities) are called ‘spectra’.
The field of spectroscopy is concerned with the interpretation of spectra in terms of atomic and molecular structure (and environment).
INFRARED SPECTROSCOPY
Infrared radiation stimulates molecular vibrations.
Infrared spectra are traditionally displayed as %T (percent transmittance) versus wave number (4000-400 cm-1).
Useful in identifying presence or absence of functional groups.
INFRARED SPECTROSCOPY
OC
O
CH3
C
O
HO
Wavenumber = 1/ wavelengthWavenumber = 1/ wavelength
INFRARED SPECTROSCOPY
In the IR region of In the IR region of the electromagnetic the electromagnetic spectrum, the spectrum, the absorption of absorption of radiation by a sample radiation by a sample is due to changes in is due to changes in the vibrational the vibrational energy states of a energy states of a molecule.molecule.
INFRARED SPECTROSCOPY
MethaneMethane
Rocking or in plane bending
HH HH HH
C
HH
H H
H H
H
H H
C
H H
C
H
H
C
HH
C
HH
C
H
Asymmetrical stretching
Symmetrical stretching
Bending or scissoring
Twisting or out-of-plane bending
Wagging or out-of-plane
bending
INFRARED SPECTROSCOPY
Only vibrations that cause a change in ‘polarity’ give rise to bands in IR spectra – which of the vibrations for CO2 are infrared active?
O C O
O C O
O C O O C O
Symmetric stretch
Asymmetric stretch
Bending (doubly degenerate)
INFRARED SPECTROSCOPY
What is a vibration in a molecule? Any change in shape of the molecule- stretching of bonds,
bending of bonds, or internal rotation around single bonds
What vibrations change the dipole moment of a molecule?
Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active.
Symmetrical stretching/bending does not. Not IR active
INFRARED SPECTROSCOPY
Human BreathHuman Breath
0
20
40
60
80
100
5001000150020002500300035004000wavenumber/cm -1
%T
OH H
OH H
O C O
O C O
O C O
INFRARED SPECTROSCOPY
How much movement occurs in the vibration of a C-C bond?
10 pm
154 pmstretching vibration For a C-C bond with a bond length of 154 pm, the variation is about 10 pm.
bending vibration4o 10 pm
For C-C-C bond angle a change of 4o is typical. This moves a carbon atom about 10 pm.
INFRARED SPECTROSCOPY
What wavelength of electromagnetic radiation is involved in causing vibrations in molecules?
Infrared (IR) electromagnetic radiation causes vibrations in molecules (wavelengths of 2500-15,000 nm or 2.5 – 15 mm)
How does the mass influence the vibration? The greater the mass - the lower the wavenumber
H2 I2
INFRARED SPECTROSCOPY
Ethane Chloroethane
INFRARED SPECTROSCOPY
POSITIONPOSITION REDUCED MASSREDUCED MASS
BOND STRENGTHBOND STRENGTH(STIFFNESS)(STIFFNESS)
LIGHT ATOMS HIGH LIGHT ATOMS HIGH FREQUENCYFREQUENCY
STRONG BONDS STRONG BONDS HIGH FREQUENCYHIGH FREQUENCY
STRENGTHSTRENGTH CHANGE IN CHANGE IN ‘‘POLARITY’POLARITY’
STRONGLY POLAR STRONGLY POLAR BONDS GIVE BONDS GIVE
INTENSE BANDSINTENSE BANDSWIDTHWIDTH HYDROGEN BONDINGHYDROGEN BONDING STRONG HYDROGEN STRONG HYDROGEN
BONDING GIVES BONDING GIVES BROAD BANDSBROAD BANDS
INFRARED SPECTROSCOPY
In generalIn general
BondBond
C-HC-H C-DC-D C-OC-O C-ClC-Cl
/cm/cm-1-1
30003000 22002200 11001100 700700
BondBond
CCOO C=OC=O C-OC-O
/cm/cm-1-1
21432143 17151715 11001100
INFRARED SPECTROSCOPY
4000-30004000-3000cmcm-1-1
3000-20003000-2000cmcm-1-1
2000-15002000-1500cmcm-1-1
1500-10001500-1000cmcm-1-1
O-H O-H N-HN-HC-HC-H
CCCCCCNN
C=CC=CC=OC=O
C-OC-OC-FC-FC-ClC-Cl
deformationsdeformations
Increasing energy
Increasing frequency
INTERPRETATION OF INFRARED SPECTRA An element of judgement is required in interpreting IR spectra but you
should find that it becomes relatively straightforward with practice. It is often possible to assign the peaks in the 1600-3600 cm-1 region by
consulting tables or databases of IR spectra. When making an assignment, give both the type of bond and the type of vibration, e.g. O-H stretch or C-H bending vibration.
The most useful regions are as follows: 1680-1750 cm-1:C=O stretches feature very strongly in IR spectra and
the type of carbonyl group can be determined from the exact position of the peak.
2700-3100 cm-1: different types of C-H stretching vibrations. 3200-3700 cm-1: various types of O-H and N-H stretching vibrations. Too many bonds absorb in the region of 600-1600 cm-1 to allow confident
assignment of individual bands. However, this region is useful as a fingerprint of a molecule, i.e. if the spectrum is almost identical to an authentic reference spectrum then the structure can be assigned with some confidence.
INTERPRETATION OF INFRARED SPECTRA
Ethanoic acid
Infrared Instrumentation
Sample compartmentIR Source Detector
Infrared Instrumentation
All modern instruments are Fourier Transform All modern instruments are Fourier Transform instruments. instruments.
In all transmission experiments radiation from a In all transmission experiments radiation from a source is directed through the sample to a detector.source is directed through the sample to a detector.
The measurement of the type and amount of light The measurement of the type and amount of light transmitted by the sample gives information about the transmitted by the sample gives information about the structure of the molecules comprising the sample.structure of the molecules comprising the sample.
Infrared Instrumentation
Infrared Experimental
To obtain an IR spectrum, the sample must be To obtain an IR spectrum, the sample must be placed in a “container” or cell that is transparent placed in a “container” or cell that is transparent in the IR region of the spectrum. in the IR region of the spectrum.
Sodium chloride or salt plates are a common Sodium chloride or salt plates are a common means of placing the sample in the light beam of means of placing the sample in the light beam of the instrument.the instrument.
InfraredExperimental
IR transparent Salt PlatesIR transparent Salt Plates
Infrared Experimental
These plates are made These plates are made of salt and must be of salt and must be stored in a stored in a water freewater free environment such as the environment such as the dessicatordessicator shown here. shown here.
Infrared Experimental
The plates must also be The plates must also be handled with gloves to handled with gloves to avoid contact of the avoid contact of the plate with moisture plate with moisture from one’s hands.from one’s hands.
Infrared Experimental
To run an IR spectrum of a To run an IR spectrum of a liquid sample, a drop or two liquid sample, a drop or two of the liquid sample is of the liquid sample is applied to a salt plate.applied to a salt plate.
A second salt plate is placed A second salt plate is placed on top of the first one such on top of the first one such that the liquid forms a thin that the liquid forms a thin film “sandwiched” between film “sandwiched” between the two plates.the two plates.
Infrared Experimental
The cell holder is then The cell holder is then placed in the beam of the placed in the beam of the instrument.instrument.
The sample is then scanned The sample is then scanned by the instrument utilizing by the instrument utilizing predestinated parameters.predestinated parameters.
A satisfactory spectrum has A satisfactory spectrum has well defined peaks-but not well defined peaks-but not so intense as to cause so intense as to cause flattening on the bottom of flattening on the bottom of the peaks.the peaks.
Infrared Experimental
Benzoic acid
Infrared Experimental
The salt plates are cleaned The salt plates are cleaned by rinsing into a waste by rinsing into a waste container with a suitable container with a suitable organic solvent-commonly organic solvent-commonly cyclohexane - cyclohexane - NEVER NEVER WATER!WATER!
Cloudy plates must be Cloudy plates must be polished to return them to a polished to return them to a transparent condition.transparent condition.
To polish cloudy windows, To polish cloudy windows, rotate salt plate on polishing rotate salt plate on polishing cloth.cloth.
UV-Visible Spectroscopy
Ultraviolet radiation stimulates molecular vibrations and electronic transitions.
Absorption spectroscopy from 160 nm to 780 nm
Measurement absorption or transmittance Identification of inorganic and organic species
UV-Visible Spectroscopy
CO
O
N C
H3C
O
H3CO
Cocaine
UV-Visible Spectroscopy
Electronic transitions occur when the molecule Electronic transitions occur when the molecule absorbs energyabsorbs energy
Electronic Transitions in Electronic Transitions in FormaldehydeFormaldehyde
Electronic transitions: p, s, and n electrons d and f electrons Charge transfer
UV-Visible Spectroscopy
UV-Visible Spectroscopy
UV-Visible Spectroscopy
Electronic transitionsElectronic transitions Molecular Orbital TheoryMolecular Orbital Theory
UV-Visible Spectroscopy
d-d Transitionsd-d Transitions 3d and 4d 1st and 2nd 3d and 4d 1st and 2nd
transitions seriestransitions series Partially occupied d Partially occupied d
orbitalsorbitals Transitions from lower Transitions from lower
to higher energy levelsto higher energy levels
UV-Visible Spectroscopy
Charge TransferCharge Transfer Electron donor and acceptor characteristicsElectron donor and acceptor characteristics
Absorption involves e- transitions from donor to acceptorAbsorption involves e- transitions from donor to acceptor SCNSCN-- to Fe(III) to Fe(III)
Fe(II) and neutral SCNFe(II) and neutral SCN Metal is acceptor Metal is acceptor
Reduced metals can be exceptionReduced metals can be exception
UV-Visible Spectroscopy
THE BEER-LAMBERT LAW
For a light absorbing medium, the light intensity falls exponentially with sample depth.
For a light absorbing medium, the light intensity falls exponentially with increasing sample concentration.
100xIIT%
IIT
o
t
o
t
Io It
l
cuvettelight
inte
nsity
(I)
Sample depth
Io
It
l
UV-Visible SpectroscopyA
bsor
banc
e
Concentration
TAclA 10log
The negative logarithm of T is called the absorbance (A) and this is directly proportional to sample depth (called pathlength, l) and sample concentration (c). The equation is called the Beer-Lambert law.
is called the molar absorption coefficient and has units of dm3 mol-1 cm-1
UV-Visible Spectroscopy
Beer-Lambert Law limitations Polychromatic LightPolychromatic Light Equilibrium shiftEquilibrium shift SolventSolvent pHpH
UV-Visible Instrumentation
Several types of spectrometerSeveral types of spectrometer
UV-Visible Instrumentation
Light sourceLight source Deuterium and hydrogen lampsDeuterium and hydrogen lamps W filament lampW filament lamp Xe arc lampsXe arc lamps
Sample containersSample containers CuvettesCuvettes
PlasticPlastic GlassGlass QuartzQuartz
UV-Visible Spectroscopy
Open-topped rectangular standard cell (a) and apertured cell (b) for limited sample volume
LYCOPENE
UV-Visible Spectroscopy
AcknowledgementsAcknowledgements Bette Kreuz, Ruth Dusenbery at Department of Natural Bette Kreuz, Ruth Dusenbery at Department of Natural
Sciences UM-Dearborn.Sciences UM-Dearborn. Dr David J McGarvey at Keele University Hewlett Packard Andrew Jackson at Staffordshire University