INTERPRETING AN INFRA-RED SPECTRUM

This page explains how to use an infra-red spectrum to identify the presence of a few simple bonds in organic compounds.
 
The infra-red spectrum for a simple carboxylic acid
Ethanoic acid
Ethanoic acid has the structure:

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 - 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

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

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.
Aldehydes will have similar infra-red spectra to ketones.
The infra-red spectrum for a hydroxy-acid
2-hydroxypropanoic 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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Instrumental Techniques for Analytical Chemistry

Introduction
Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and
inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a sample
positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the
chemical functional groups in the sample. Different functional groups absorb characteristic frequencies
of IR radiation. Using various sampling accessories, IR spectrometers can accept a wide range of sample
types such as gases, liquids, and solids. Thus, IR spectroscopy is an important and popular tool for
structural elucidation and compound identification.

IR Frequency Range and Spectrum Presentation
Infrared radiation spans a section of the electromagnetic spectrum having wavenumbers from roughly
13,000 to 10 cm–1, or wavelengths from 0.78 to 1000 μm. It is bound by the red end of the visible region
at high frequencies and the microwave region at low frequencies.
IR absorption positions are generally presented as either wavenumbers ( ) or wavelengths (l).
Wavenumber defines the number of waves per unit length. Thus, wavenumbers are directly proportional
to frequency, as well as the energy of the IR absorption. The wavenumber unit (cm–1, reciprocal centimeter)
is more commonly used in modern IR instruments that are linear in the cm–1 scale. In the
contrast, wavelengths are inversely proportional to frequencies and their associated energy. At present,
the recommended unit of wavelength is μm (micrometers), but μ (micron) is used in some older literature.
Wavenumbers and wavelengths can be interconverted using the following equation:
n in cm–1 ( ) 1
l (in μm)
------------------------ 104
(15.1)
IR absorption information is generally presented in the form of a spectrum with wavelength or
wavenumber as the x-axis and absorption intensity or percent transmittance as the y-axis (Fig. 15.1).

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Infrared Spectroscopy Chemical Analisys

General Uses
• Identification of all types of organic and many types of inorganic compounds
• Determination of functional groups in organic materials
• Determination of the molecular composition of surfaces
• Identification of chromatographic effluents
• Quantitative determination of compounds in mixtures
• Nondestructive method
• Determination of molecular conformation (structural isomers) and stereochemistry (geometrical
isomers)
• Determination of molecular orientation (polymers and solutions)
Common Applications
• Identification of compounds by matching spectrum of unknown compound with reference
spectrum (fingerprinting)
• Identification of functional groups in unknown substances
Identification of reaction components and kinetic studies of reactions
• Identification of molecular orientation in polymer films
• Detection of molecular impurities or additives present in amounts of 1% and in some cases as
low as 0.01%
• Identification of polymers, plastics, and resins
• Analysis of formulations such as insecticides and copolymers

Samples
State
Almost any solid, liquid or gas sample can be analyzed. Many sampling accessories are available.
Amount
Solids 50 to 200 mg is desirable, but 10 μg ground with transparent matrix (such as KBr) is the minimum
for qualitative determinations; 1 to 10 μg minimum is required if solid is soluble in suitable solvent.
Liquids 0.5 μL is needed if neat, less if pure.
Gases 50 ppb is needed.
Preparation
Little or no preparation is required; may have to grind solid into KBr matrix or dissolve sample in a
suitable solvent (CCl4 and CS2 are preferred). Many types of sample holders and cells are available.
Water should be removed from sample if possible.
Analysis Time
Estimated time to obtain spectrum from a routine sample varies from 1 to 10 min depending on the type
of instrument and the resolution required. Most samples can be prepared for infrared (IR) analysis in
approximately 1 to 5 min.
Limitations
General
• Minimal elemental information is given for most samples.
• Background solvent or solid matrix must be relatively transparent in the spectral region of interest.
• Molecule must be active in the IR region. (When exposed to IR radiation, a minimum of one vibrational
motion must alter the net dipole moment of the molecule in order for absorption to be
observed.)
Accuracy
In analysis of mixtures under favorable conditions, accuracy is greater than 1%. In routine analyses, it
is ± 5%.
Sensitivity and Detection Limits
Routine is 2%; under most favorable conditions and special techniques, it is 0.01%.
Complementary or Related Techniques
• Nuclear magnetic resonance provides additional information on detailed molecular structure
• Mass spectrometry provides molecular mass information and additional structural information
• Raman spectroscopy provides complementary information on molecular vibration. (Some vibrational
modes of motion are IR-inactive but Raman-active and vice versa.) It also facilitates
analysis of aqueous samples. Cell window material may be regular glass.

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