SpectroscopyInfrared Spectra

INTRODUCTION

• 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).

THE ELECTROMAGNETIC SPECTRUMTHE ELECTROMAGNETIC SPECTRUM

Atkins & de Paula 2002

© Atkins and de Paula 2002

THE ENERGIES OF ELECTROMAGNETIC WAVES

THE ENERGIES OF ELECTROMAGNETIC WAVES

hcNc

hNhNE

hcc

hhE

c

AAA

1

hcNc

hNhNE

hcc

hhE

c

AAA

1

(nu-bar) represents wavenumber, the number of wavelengths in 1 cm

Theory of IR

• Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states.

• Molecular rotations: No use.

• Molecular vibrations:

• Stretching: Change in inter-atomic distance along bond axis

Bending: Change in angle between two bonds. There are four types of bend:Rocking Scissoring Wagging Twisting

The stretching frequency of a bond can be approximated by Hooke’s Law.

Vibrational coupling

interaction between vibrations can occur (coupling ) if the vibrating bonds are joined to a single central atom. Vibrational coupling is influenced by a number of factors; Strong coupling of stretching vibrations occurs when there is a common atom between the two vibrating bonds .Coupling of bending vibrations occurs when there is a common bond between vibrating groups Coupling between a stretching vibration and a bending vibration occurs if the stretching bond is one side of an angle varied by bending vibration Coupling is greatest when the coupled groups have approximately equal energies

A molecule consisting of N atomshas a total of 3n degrees of freedom, corresponding to the coordinates of each atom in the molecule. a nonlinear molecule,3 of these degrees are rotational and 3 are translational and the remaining correspond to fundamental vibrations; in a linear molecule, 2 degrees are rotational and 3 are translational. The net number of fundamental vibrations for nonlinear and linear molecules is therefore:

molecule degrees of freedomnonlinear 3n– 6linear 3n– 5

propane,C3H8, has 27 fundamental vibrations, and therefore 27 bands in an IR spectrum. Water, which is nonlinear, has three fundamental vibrations.

Factors affecting Vibrational Frequencies:1. Coupled Vibrations and Fermi Resonane: one stretching absorption

frequency for an isolated C-H bond but in methylene (-CH2) – two absorptions i.e. symmetric and assymetric (higher wave number).Fermi Resonance: energy of an overtone level coincide with fundamental mode of different vibrations i.e. transfer of energy from fundamental to overtone and back again.

2. Electronic Effects: Inductive:

+I: lengthening or weakening of bond, absorption at lower wave number. E.g. alkyl group.HCHO : 1750cm-1, CH3CHO : 1745cm-1

-I: increases wave number. E.g. CH3COCH3:1715cm-1, ClCH2COCH3: 1725cm-1

Mesomeric Effects: Conjugation lowers wave number.E.g. Methylvenyl ketone: CH3COCH=CH2; 1706 cm-1

Acetohenone: C6H5COCH3: 1693 cm-1

Hydrogen Bonding: Downward frequency shift. Stronger bonding, greater absorption shift towards lower wave number.Intermolecular: broad bands, concentration dependent-band disappear on dilution.Intramolecular: sharp bands, concentration independent.

Detection Electronics and Computer

InfraredSource

Determines Frequenciesof Infrared Absorbed andplots them on a chart

Sample

Simplified Infrared SpectrophotometerSimplified Infrared SpectrophotometerNaClplates

Absorption “peaks”

Infrared Spectrum

frequency

intensity ofabsorption

(decreasing)

focusingmirror

IR Radiation Source:

• Incandescent lamp: far IR, low emissivity

• Nernst Glower: Hollow rod: 2mm dia, 30 mm length, earth oxides of Zirconia, yttria and thoria

• Non-conducting at room temperature.

• Heated between 1000-18000C

• GLOBAR SOURCE:sintored silicon carbide rod: 50mm length, 4mm dia.

Cont…

• Self-starting

• MERCURY ARC:

2. Monochromators:

Prism

Grating

3. Sampling of Substances:

Solids:Nujol Mull:

Potassium Bromide disk

Thin Films

Liquids

Gases

DETECTORS

• Bolometers• Thermocouple• Thermistors• Golay cells• Photoconductivity cell• Semiconductor Detectors• Pyroelectric• Fourier Transform

Single Beam

Instrumentation for Spectroscopy

SourceWavelength Selector

Sample Detector Data Readout

Absorption Spectroscopy

• Double Beam Spectrophotometer (figure above)– Analysis of Reference and

Sample Simulataneously• Single Beam

– Only one detector – reference and sample cuvettes move to come in line to the path of light.

Capabilities of Infrared Analysis

Identification and quantitation of organic solid, liquid or

gas samples.

Analysis of powders, solids, gels, emulsions, pastes, pure

liquids and solutions, polymers, pure and mixed gases.

Infrared used for research, methods development, quality

control and quality assurance applications.

Samples range in size from single fibers only 20 microns

in length to atmospheric pollution studies involving large

areas.

Applications of Infrared Analysis

Pharmaceutical research Forensic investigations Polymer analysis Lubricant formulation and fuel additives Foods research Quality assurance and control Environmental and water quality analysis

methods Biochemical and biomedical research Coatings and surfactants Etc.

A Functional Group Chart

O-H str

NH str

COO-H

=C-H str

Csp3-H

C-H

-(C=O)-H

CN

CC

C=O

-C=N

-C=C

phenyl

C-O

C-N

F C-X

4000 3600 3200 2800 2400 2000 1600 1200 800 group

IBrCl

Infrared SpectroscopyFor isopropyl alcohol, CH(CH3)2OH, the infrared absorption bands identify the various functional groups of the molecule.

Interferogram is made by an interferometer.

Interferogramis transformedinto a spectrum using a FT.

BKG

SB

3000 2000 1000

[cm-1]

Sample

SB

Sample

3000 2000 1000

[cm-1]

Sample/BKG

IR spectrum

%T

3000 2000 1000 [cm-1]

The Principles of FTIR Method

FTIR seminar

Interferometer

He-Ne gas laser

Fixed mirror

Movable mirror

Sample chamber

Light source

(ceramic)

Detector

(DLATGS)

Beam splitter

FT Optical System Diagram

Fixed mirror

Ｂ Movable mirror

Fixed mirror

Ａ Movable mirror

Fixed mirrorＣ Movable mirror

Same-phase interference wave shape

Opposite-phase interference wave shape

Same-phase interference wave shape0

Movable mirror

Ｄ Interference pattern of light manifested by the optical-path difference

Continuous phase shift

Sig

na

l s

tre

ng

th

Ｉ (X)

-2 - 0 2

-2 - 0 2

FTIR seminarInterference of two beams of light

1.Better sensitivity and brightness- Allows simultaneous measurement over the entire wavenumber range- Requires no slit device, making good use of the available beam2.High wavenumber accuracy- Technique allows high speed sampling with the aid of laser light interference fringes- Requires no wavenumber correction- Provides wavenumber to an accuracy of 0.01 cm-13. Resolution- Provides spectra of high resolution4. Stray light- Fourier Transform allows only interference signals to contribute to spectrum. Background light effects greatly lowers.- Allows selective handling of signals limiting intreference5. Wavenumber range flexibility- Simple to alter the instrument wavenumber range

CO2 and H2O sensitive

FT-IR Advantages and Disadvantages

FT-IR Advantages• Fellgett's (multiplex) Advantage

• FT-IR collects all resolution elements with a complete scan of the interferometer. Successive scans of the FT-IR instrument are coadded and averaged to enhance the signal-to-noise of the spectrum.

• Theoretically, an infinitely long scan would average out all the noise in the baseline.

• The dispersive instrument collects data one wavelength at a time and collects only a single spectrum. There is no good method for increasing the signal-to-noise of the dispersive spectrum.

FT-IR Advantages• Connes Advantage

• an FT-IR uses a HeNe laser as an internal wavelength standard. The infrared wavelengths are calculated using the laser wavelength, itself a very precise and repeatable 'standard'.

• Wavelength assignment for the FT-IR spectrum is very repeatable and reproducible and data can be compared to digital libraries for identification purposes.

FT-IR Advantages• Jacquinot Advantage

• FT-IR uses a combination of circular apertures and interferometer travel to define resolution. To improve signal-to-noise, one simply collects more scans.

• More energy is available for the normal infrared scan and various accessories can be used to solve various sample handling problems.

• The dispersive instrument uses a rectangular slit to control resolution and cannot increase the signal-to-noise for high resolution scans. Accessory use is limited for a dispersive instrument.

FT-IR Application Advantages• Opaque or cloudy samples

• Energy limiting accessories such as diffuse reflectance or FT-

IR microscopes

• High resolution experiments (as high as 0.001 cm-1

resolution)

• Trace analysis of raw materials or finished products

• Depth profiling and microscopic mapping of samples

• Kinetics reactions on the microsecond time-scale

• Analysis of chromatographic and thermogravimetric sample

fractions

To separate IR light, a grating is used.

Grating

Light source

Detector

Sample

Slit

To select the specified IR light, A slit is used.

Dispersion Spectrometer

In order to measure an IR spectrum,the dispersion Spectrometer takesseveral minutes.Also the detector receives onlya few % of the energy oforiginal light source.

Fixed CCM

B.S.

Moving CCM

IR Light source

Sample

Detector

An interferogram is first made by the interferometer using IR light.

The interferogram is calculated and transformedinto a spectrum using a Fourier Transform (FT).

FTIRIn order to measure an IR spectrum,FTIR takes only a few seconds.Moreover, the detector receivesup to 50% of the energy of originallight source.(much larger than the dispersionspectrometer.)

Comparison Beetween Dispersion Spectrometer and FTIR