an introduction to infrared and uv-visible spectroscopy

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An Introduction to An Introduction to Infrared and UV-Visible Infrared and UV-Visible Spectroscopy Spectroscopy

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Page 1: An Introduction to Infrared and UV-Visible Spectroscopy

An Introduction to Infrared and An Introduction to Infrared and UV-Visible SpectroscopyUV-Visible Spectroscopy

Page 2: An Introduction to Infrared and UV-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

Page 3: An Introduction to Infrared and UV-Visible Spectroscopy

THE ELECTROMAGNETIC SPECTRUM

Page 4: An Introduction to Infrared and UV-Visible Spectroscopy

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).

Page 5: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 6: An Introduction to Infrared and UV-Visible Spectroscopy

INFRARED SPECTROSCOPY

OC

O

CH3

C

O

HO

OC

O

CH3

C

O

HO

Wavenumber = 1/ wavelengthWavenumber = 1/ wavelength

Page 7: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 8: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 9: An Introduction to Infrared and UV-Visible Spectroscopy

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

O C O

O C O

O C O O C O

Symmetric stretch

Asymmetric stretch

Bending (doubly degenerate)

Page 10: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 11: An Introduction to Infrared and UV-Visible Spectroscopy

INFRARED SPECTROSCOPY

Human BreathHuman Breath

0

20

40

60

80

100

5001000150020002500300035004000

wavenumber/cm -1

%T

OH H

OH H

O C O

O C O

O C O

Page 12: An Introduction to Infrared and UV-Visible Spectroscopy

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 vibration

4o 10 pm

For C-C-C bond angle a change of 4o is typical. This moves a carbon atom about 10 pm.

Page 13: An Introduction to Infrared and UV-Visible Spectroscopy

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

H2I2

Page 14: An Introduction to Infrared and UV-Visible Spectroscopy

INFRARED SPECTROSCOPY

Ethane Chloroethane

Page 15: An Introduction to Infrared and UV-Visible Spectroscopy

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 BANDS

WIDTHWIDTH HYDROGEN BONDINGHYDROGEN BONDING STRONG HYDROGEN STRONG HYDROGEN BONDING GIVES BONDING GIVES BROAD BANDSBROAD BANDS

Page 16: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 17: An Introduction to Infrared and UV-Visible Spectroscopy

INFRARED SPECTROSCOPY

4000-30004000-3000

cmcm-1-1

3000-20003000-2000

cmcm-1-1

2000-15002000-1500

cmcm-1-1

1500-10001500-1000

cmcm-1-1

O-H O-H

N-HN-H

C-HC-H

CCCC

CCNN

C=CC=C

C=OC=O

C-OC-O

C-FC-F

C-ClC-Cl

deformationsdeformations

Increasing energy

Increasing frequency

Page 18: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 19: An Introduction to Infrared and UV-Visible Spectroscopy

INTERPRETATION OF INFRARED SPECTRA

Ethanoic acid

Page 20: An Introduction to Infrared and UV-Visible Spectroscopy

Infrared Instrumentation

Sample compartmentIR Source Detector

Page 21: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 22: An Introduction to Infrared and UV-Visible Spectroscopy

Infrared Instrumentation

Page 23: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 24: An Introduction to Infrared and UV-Visible Spectroscopy

InfraredExperimental

IR transparent Salt PlatesIR transparent Salt Plates

Page 25: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 26: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 27: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 28: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 29: An Introduction to Infrared and UV-Visible Spectroscopy

Infrared Experimental

Benzoic acid

Page 30: An Introduction to Infrared and UV-Visible Spectroscopy

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.

Page 31: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 32: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible Spectroscopy

CO

O

N C

H3C

O

H3CO

CO

O

N C

H3C

O

H3CO

Cocaine

Page 33: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 34: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible Spectroscopy

Page 35: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible Spectroscopy

Page 36: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible Spectroscopy

Electronic transitionsElectronic transitions Molecular Orbital TheoryMolecular Orbital Theory

Page 37: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 38: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 39: An Introduction to Infrared and UV-Visible Spectroscopy

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.

100xI

IT%

I

IT

o

t

o

t

Io It

l

cuvetteligh

t in

ten

sit

y (

I)

Sample depth

Io

It

l

Page 40: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible SpectroscopyA

bso

rban

ce

Concentration

TAclA 10log 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

Page 41: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible Spectroscopy

Beer-Lambert Law limitations Polychromatic LightPolychromatic Light Equilibrium shiftEquilibrium shift SolventSolvent pHpH

Page 42: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible Instrumentation

Several types of spectrometerSeveral types of spectrometer

Page 43: An Introduction to Infrared and UV-Visible Spectroscopy

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

Page 44: An Introduction to Infrared and UV-Visible Spectroscopy

UV-Visible Spectroscopy

Open-topped rectangular standard cell (a) and apertured cell (b) for limited sample volume

LYCOPENELYCOPENE

Page 45: An Introduction to Infrared and UV-Visible Spectroscopy

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