4. Infrared (IR) spectroscopy

Molecular Structure and Organic Chemistry

• The structure of a molecule refers to the arrangement of atoms within

the molecule. The structure of a molecule is critical to the chemical and

physical properties of a substance. In fact so vital is structure to

molecular identity that the same molecular formula may represent

more than one substance based upon their differing structures.

• Example: Two totally different substances share the same molecular

formula, C2H6O, but are different because of their differing molecular

structures.

Ethyl Alcohol C2H6O Dimethyl Ether C2H6O

Isomers

• Molecules that have the same molecular formulas but different

structures are called isomers

• There are 2 isomers corresponding to the molecular formula C4H10 ,

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And 3 corresponding to C5H12 and 39 corresponding to the molecular

formula C9H20

The IR Region

• Just below red in the visible region.

• Wavelengths usually 2.5-25 mm.

• More common units are wavenumbers, or cm-1, the reciprocal of the

wavelength in centimeters.

Wavenumbers are proportional to frequency and energy.

Infrared Spectrophotometry

• using molecular vibrations as a key to structure

• Although ball and stick models of molecules are very effective at

approximating the actual shapes of molecules, they do have one

major flaw they leave you with the false impression that molecules

are rigid objects

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Some Vibrational Modes

Covalent bonds vibrate at only certain allowable frequencies.

Each type of vibration has a frequency that depends upon the: the mass of

the vibrating atoms and the nature of the bond between them.

• For a constant bond type (single, double or triple) the frequency of

the vibration is low for a bond between heavy atoms. Conversely,

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for a given bond type the frequency of vibrations is high for light

atoms.

• Multiple bonds vibrate at a higher frequency than do single bonds.

Stretching Frequencies

Frequency decreases with increasing atomic weight.

Frequency increases with increasing bond energy.

Infrared Absorptions

Vibrations as a key to structure- the entire range of vibrations for all

organic molecules falls within the Infrared Region of the

Electromagnetic Spectrum (2500nm – 25000nm). If a beam of IR

radiation is directed at a molecular sample and if the beam has the

same frequency as one of the vibrational modes of the molecule then

the molecule will absorb the energy of the IR radiation and the

molecular vibration will increase in intensity.

If in order for absorption to occur, the IR frequency must match the

frequency of the vibrating atoms, and if the frequency of the vibrating

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atoms is dependent upon the mass of the atoms and the bond type;

then the frequency at which absorbance occurs is dependent upon the

mass of the atoms and the bond type. Therefore, the same two bonded

atoms, regardless of the molecule that they are in, will have the same

absorbance frequency in the IR region. This is the major strength of IR

Spectrophotometry. IR Spec identifies the functional groups present in

an organic molecule.

Functional groups

Functional Group- this is an atom or group of atoms that imparts a unique

set of chemistry to whatever organic molecule it is bonded to. If the same

functional group is attached to two different organic molecules then the

two organic molecules will have similar chemistry’s and have similar

absorbance values of the IR Specs

THE FINGERPRINT REGION OF AN INFRA-RED SPECTRUM

What is the fingerprint region

This is a typical infra-red spectrum:

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The region to the right-hand side of the diagram (from about 1500 to 500

cm-1) usually contains a very complicated series of absorptions. These are

mainly due to all manner of bending vibrations within the molecule. This is

called the fingerprint region.

The importance of the fingerprint region is that each different compound

produces a different pattern of troughs in this part of the spectrum.

Using the fingerprint region

Compare the infra-red spectra of propan-1-ol and propan-2-ol. Both

compounds contain exactly the same bonds. Both compounds have very

similar troughs in the area around 3000 cm-1 - but compare them in the

fingerprint region between 1500 and 500 cm-1.

The pattern in the fingerprint region is completely different and could

therefore be used to identify the compound.

So . . . to positively identify an unknown compound, use its infra-red

spectrum to identify what sort of compound it is by looking for specific

bond absorptions. That might tell you, for example, that you had an

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alcohol because it contained an -OH group. You would then compare the

fingerprint region of its infra-red spectrum with known spectra measured

under exactly the same conditions to find out which alcohol (or whatever)

you had.

INTERPRETING AN INFRA-RED SPECTRUM

The infra-red spectrum for a simple carboxylic acid (Ethanoic acid)

You will see that it contains the following bonds:

carbon-oxygen double, C=O

carbon-oxygen single, C-O

oxygen-hydrogen, O-H

carbon-hydrogen, C-H

carbon-carbon single, C-C

The carbon-carbon bond has absorptions which occur over a wide range of

wavenumbers in the fingerprint region - that makes it very difficult to pick

out on an infra-red spectrum.

The carbon-oxygen single bond also has an absorbtion in the fingerprint

region, varying between 1000 and 1300 cm-1 depending on the molecule it

is in. You have to be very wary about picking out a particular trough as

being due to a C-O bond.

The other bonds in ethanoic acid have easily recognised absorptions

outside the fingerprint region.

The C-H bond (where the hydrogen is attached to a carbon which is singly-

bonded to everything else) absorbs somewhere in the range from 2853 -

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2962 cm-1. Because that bond is present in most organic compounds, that's

not terribly useful! What it means is that you can ignore a trough just under

3000 cm-1, because that is probably just due to C-H bonds.

The carbon-oxygen double bond, C=O, is one of the really useful

absorptions, found in the range 1680 - 1750 cm-1. Its position varies slightly

depending on what sort of compound it is in.

The other really useful bond is the O-H bond. This absorbs differently

depending on its environment. It is easily recognised in an acid because it

produces a very broad trough in the range 2500 - 3300 cm-1.

The infra-red spectrum for ethanoic acid looks like this:

The possible absorption due to the C-O single bond is queried because it

lies in the fingerprint region. You couldn't be sure that this trough wasn't

caused by something else.

The infra-red spectrum for an alcohol (Ethanol) .

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The O-H bond in an alcohol absorbs at a higher wavenumber than it does in

an acid - somewhere between 3230 - 3550 cm-1. In fact this absorption

would be at a higher number still if the alcohol isn't hydrogen bonded - for

example, in the gas state. All the infra-red spectra on this page are from

liquids - so that possibility will never apply.

Notice the absorption due to the C-H bonds just under 3000 cm-1, and also

the troughs between 1000 and 1100 cm-1 - one of which will be due to the

C-O bond.

The infra-red spectrum for an ester (Ethyl ethanoate)

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This time the O-H absorption is missing completely. Don't confuse it with

the C-H trough fractionally less than 3000 cm-1. The presence of the C=O

double bond is seen at about 1740 cm-1.

The C-O single bond is the absorption at about 1240 cm-1. Whether or not

you could pick that out would depend on the detail given by the table of

data which you get in your exam, because C-O single bonds vary anywhere

between 1000 and 1300 cm-1 depending on what sort of compound they

are in. Some tables of data fine it down, so that they will tell you that an

absorption from 1230 - 1250 is the C-O bond in an ethanoate.

The infra-red spectrum for a ketone (Propanone)

You will find that this is very similar to the infra-red spectrum for ethyl

ethanoate, an ester. Again, there is no trough due to the O-H bond, and

again there is a marked absorption at about 1700 cm-1 due to the C=O.

Confusingly, there are also absorptions which look as if they might be due

to C-O single bonds - which, of course, aren't present in propanone. This

reinforces the care you have to take in trying to identify any absorptions in

the fingerprint region.

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Aldehydes will have similar infra-red spectra to ketones.

The infra-red spectrum for a hydroxy-acid (lactic acid).

This is interesting because it contains two different sorts of O-H bond - the

one in the acid and the simple "alcohol" type in the chain attached to the -

COOH group.

The O-H bond in the acid group absorbs between 2500 and 3300, the one in

the chain between 3230 and 3550 cm-1. Taken together, that gives this

immense trough covering the whole range from 2500 to 3550 cm-1. Lost in

that trough as well will be absorptions due to the C-H bonds.

Notice also the presence of the strong C=O absorption at about 1730 cm-1.

The infra-red spectrum for a primary amine

1-aminobutane

Primary amines contain the -NH2 group, and so have N-H bonds. These

absorb somewhere between 3100 and 3500 cm-1. That double trough

(typical of primary amines) can be seen clearly on the spectrum to the left

of the C-H absorptions.

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Table of IR Absorptions

Functional Group Characteristic Absorption(s) (cm-1)

Alkyl C-H Stretch 2950 - 2850 (m or s)

Alkenyl C-H Stretch Alkenyl C=C Stretch

3100-3010(m). 1680 - 1620 (v)

Alkynyl C-H Stretch Alkynyl C=C Stretch

~3300(s) 2260 - 2100 (v)

Aromatic C-H Stretch Aromatic C-H Bending Aromatic C=C Bending

~3030(v) 860-680(s) 1700 - 1500 (m,m)

Alcohol/Phenol O-H Stretch 3550 - 3200 (broad, s)

Carboxylic Acid O-H Stretch 3000 - 2500 (broad, v)

Amine N-H Stretch 3500 - 3300 (m)

Nitrile C=N Stretch 2260 - 2220 (m)

Aldehyde C=O Stretch Ketone C=O Stretch Ester C=O Stretch Carboxylic Acid C=O Stretch Amide C=O Stretch

1740 - 1690 (s) 1750 - 1680 (s) 1750 - 1735 (s) 1780 - 1710 (s) 1690 - 1630 (s)

Amide N-H Stretch 3700 - 3500 (m)

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IR Spectrophotometers

• Because every molecule has a unique set of atoms and bonds that

compose it, each molecule will absorb IR radiation only at certain

frequencies. These frequencies are related to the types of bonds and

arrangements of atoms in a molecule.

• An IR Spectrophotometer is an instrument that measures the

absorbance of IR radiation by a sample as a function of frequency.

Instruments

• IR spectrometer consist of

1. Source of IR light (radiation) Nernst Glowers and Globars.

2. Monochromator: Using either prism or Grating system

3. Sample-compartment (liquid o solid with KBr)

4. Light-detector Thermal transducer

5. Signal processor The same as in spectrophotometer

Dispersive (Double Beam)

IR Spectrophotometer

Prismor

DiffractionGrating

Slit

Photometer

IR Source Recorder

Split

Beam Air

Lenz Sample

Dispersive (Double Beam)

IR Spectrophotometer

Prismor

DiffractionGrating

Slit

Photometer

IR Source Recorder

Split

Beam Air

Lenz SampleSample