InfraredInfrared

The most useful part of the infrared spectrum The most useful part of the infrared spectrum for the detection and determination of organic for the detection and determination of organic species is from 2.5 to 15 species is from 2.5 to 15 m in wavelength m in wavelength which corresponds to a wavenumber range of which corresponds to a wavenumber range of 4000 to 667 cm4000 to 667 cm-1-1..

FingerprintsFingerprints

Structural InformationStructural Information

Chemists tend to use wavenumbers not Chemists tend to use wavenumbers not wavelengths.wavelengths.

Wavenumbers are directly proportional to Wavenumbers are directly proportional to energy.energy.

A higher wavenumber corresponds to a A higher wavenumber corresponds to a higher energy.higher energy.

InfraredInfrared

Infrared AbsorptionInfrared Absorption Molecules excited to a higher energy stateMolecules excited to a higher energy state A quantised processA quantised process A molecule only absorbs selected frequenciesA molecule only absorbs selected frequencies Corresponds to energy changes of the order 8-Corresponds to energy changes of the order 8-

40 kJ/mole40 kJ/mole

InfraredInfrared

Infrared Absorption Infrared Absorption The absorbed energy corresponds to the The absorbed energy corresponds to the

VIBRATIONALVIBRATIONAL frequencies of the molecule. frequencies of the molecule. Not all bonds in a molecule are capable of Not all bonds in a molecule are capable of

absorbing infrared energy.absorbing infrared energy. Only those bonds which contain a Only those bonds which contain a DIPOLE DIPOLE

MOMENTMOMENT.. A bond must therefore present an electrical A bond must therefore present an electrical

dipole which is changing as a function of time dipole which is changing as a function of time at the same frequency as the incoming radiation.at the same frequency as the incoming radiation.

InfraredInfrared

InfraredInfrared

Vibrational ModesVibrational Modes Many vibrational modesMany vibrational modes

Simplest typesSimplest types StretchingStretching BendingBending

InfraredInfrared

InfraredInfrared

InfraredInfrared

Bond PropertiesBond Properties A diatomic molecule can be considered as two A diatomic molecule can be considered as two

vibrating masses connected by a spring.vibrating masses connected by a spring. The bond distance continually changes but an The bond distance continually changes but an

equilibrium or average bond distance can be defined.equilibrium or average bond distance can be defined. This behaviour is described as harmonic oscillation.This behaviour is described as harmonic oscillation. The natural frequency of vibration of a bond is The natural frequency of vibration of a bond is

derived from Hooke’s law for vibrating springs.derived from Hooke’s law for vibrating springs.

InfraredInfrared Bond PropertiesBond Properties

The total amount of energy is proportional to the The total amount of energy is proportional to the frequency of the vibration and for a harmonic oscillator frequency of the vibration and for a harmonic oscillator is determined by the force constant (K) of the spring is determined by the force constant (K) of the spring and the masses mand the masses m11 and m and m22 of the two bonded atoms. of the two bonded atoms.

K

cv

21

The reduced mass, The reduced mass, of the system is given by: of the system is given by:

= m= m11mm22 / m / m11 + m + m22

InfraredInfrared Stronger bonds have a larger force constant Stronger bonds have a larger force constant

and vibrate at higher frequencies.and vibrate at higher frequencies.

InfraredInfrared

Bonds between atoms of higher masses vibrate Bonds between atoms of higher masses vibrate at lower frequencies.at lower frequencies.

InfraredInfrared

Instrumentation Instrumentation Dispersive SpectrometersDispersive Spectrometers Fourier Transform SpectrometersFourier Transform Spectrometers

Sample PreparationSample Preparation

Correlation Charts and TablesCorrelation Charts and Tables

Spectrum AnalysisSpectrum Analysis

The method of spectral analysis is dependent The method of spectral analysis is dependent upon the information you have available. upon the information you have available.

General RulesGeneral Rules Identification of Functional GroupsIdentification of Functional Groups Molecular Formulae and Hydrogen DeficiencyMolecular Formulae and Hydrogen Deficiency Full Spectral InterpretationFull Spectral Interpretation

Spectrum AnalysisSpectrum Analysis

Spectrum AnalysisSpectrum Analysis

How to approach the analysis of an How to approach the analysis of an infrared spectrum.infrared spectrum.

Or - what you can tell at a glance!Or - what you can tell at a glance!

Looking for functional groups.Looking for functional groups.

Spectrum AnalysisSpectrum Analysis

Look for a few major functional groups - Look for a few major functional groups - C==O, OH, NH, C==C and C==C - C==O, OH, NH, C==C and C==C - which are conspicuous.which are conspicuous.

Do not try to make a detailed analysis of Do not try to make a detailed analysis of the CH absorptions near 3000cmthe CH absorptions near 3000cm-1-1..

DO NOT WORRY ABOUT DO NOT WORRY ABOUT SUBTLETIES.SUBTLETIES.

Spectrum AnalysisSpectrum Analysis

1. Is a carbonyl group present?1. Is a carbonyl group present?

A strong absorption in the region 1820-A strong absorption in the region 1820-1660 cm1660 cm-1-1

Often the strongest in the spectrum.Often the strongest in the spectrum.

You You can’tcan’t miss it! miss it!

2. If a carbonyl is present check the 2. If a carbonyl is present check the following types, if it is absent go to 3.following types, if it is absent go to 3.

Spectrum AnalysisSpectrum Analysis

ACIDS:ACIDS: is OH also present?is OH also present?

broad absorption near 3400-2400 cmbroad absorption near 3400-2400 cm-1-1..

AMIDES: AMIDES: is NH also present?is NH also present?

medium absorption near 3400 cmmedium absorption near 3400 cm-1-1..

ESTERS:ESTERS: is C--O also present?is C--O also present?

strong intensity absorption near 1300-strong intensity absorption near 1300-1000 cm1000 cm-1-1..

Spectrum AnalysisSpectrum Analysis

ANHYDRIDES:ANHYDRIDES:two C==O near 1810 and 1760 cmtwo C==O near 1810 and 1760 cm-1-1..

ALDEHYDES: ALDEHYDES: is aldehyde C--H present?is aldehyde C--H present?

two weak absorptions near 2850 two weak absorptions near 2850 andand 2750 cm2750 cm-1-1..

KETONES: KETONES: the proceeding five choices have the proceeding five choices have been eliminated.been eliminated.

Spectrum AnalysisSpectrum Analysis3. If carbonyl absent then check:3. If carbonyl absent then check:

Alcohols and phenolsAlcohols and phenolsbroad absorption near 3400-2400 cmbroad absorption near 3400-2400 cm-1-1 and strong and strong intensity absorption near 1300-1000 cmintensity absorption near 1300-1000 cm-1-1..

AminesAminesmedium absorption near 3400 cmmedium absorption near 3400 cm-1-1..

EthersEthersstrong intensity absorption near 1300-1000 cmstrong intensity absorption near 1300-1000 cm -1-1..

Spectrum AnalysisSpectrum Analysis

4. Double Bonds4. Double Bonds

A weak absorption near 1650 cmA weak absorption near 1650 cm-1-1..

5. Triple Bonds5. Triple Bonds

A weak, sharp absorption near 2150 cmA weak, sharp absorption near 2150 cm-1-1..

6.Hydrocarbons6.Hydrocarbons

None of the preceeding foundNone of the preceeding found

Major absorptions in CH region near 3000 cmMajor absorptions in CH region near 3000 cm-1-1..

Spectrum AnalysisSpectrum Analysis

RESIST THE IDEA OF RESIST THE IDEA OF TRYING TO INTERPRET TRYING TO INTERPRET

EVERY PEAK.EVERY PEAK.

IT IS NOTIT IS NOT

POSSIBLE!POSSIBLE!

Spectrum AnalysisSpectrum Analysis

Molecular FormulaeMolecular Formulae Derived from Empirical FormulaeDerived from Empirical Formulae

EthaneEthane Empirical formula CHEmpirical formula CH33

Molecular mass 30Molecular mass 30 Molecular formula CHMolecular formula CH33CHCH33

Spectrum AnalysisSpectrum Analysis

The Index of Hydrogen DeficiencyThe Index of Hydrogen Deficiency The number of The number of bonds or rings a molecule contains. bonds or rings a molecule contains. From a comparison of the molecular formula and From a comparison of the molecular formula and

that of a corresponding acyclic saturated compound.that of a corresponding acyclic saturated compound. The difference in numbers of hydrogens between The difference in numbers of hydrogens between

these formulae divided by two gives the index of these formulae divided by two gives the index of hydrogen deficiency.hydrogen deficiency.

Spectrum AnalysisSpectrum Analysis

Hydrogen DeficiencyHydrogen Deficiency Alkane: CAlkane: CnnHH2n+22n+2

Alkene or cycloalkane: CAlkene or cycloalkane: CnnHH2n2n

Alkyne: CAlkyne: CnnHH2n-22n-2

Hydrogen DeficiencyHydrogen Deficiency N, P, As, etc: +1N, P, As, etc: +1 O, S, Se, Te: no changeO, S, Se, Te: no change Halides: -1Halides: -1

Spectrum AnalysisSpectrum Analysis

The Index of Hydrogen DeficiencyThe Index of Hydrogen Deficiency OneOne - a double bond or a ring but not both. - a double bond or a ring but not both. TwoTwo - a triple bond, 2 double bonds, two - a triple bond, 2 double bonds, two

rings or a combination of both.rings or a combination of both. FourFour - a ring and three double bonds for - a ring and three double bonds for

example benzene.example benzene.

Spectrum AnalysisSpectrum Analysis

Other examplesOther examples Chloral HydrateChloral Hydrate NicotineNicotine

Spectrum AnalysisSpectrum Analysis

How to approach the analysis of an How to approach the analysis of an infrared spectrum.infrared spectrum.

A more detailed look.A more detailed look.