chapter 2 infrared spectroscopy ir

8/19/2019 Chapter 2 Infrared Spectroscopy IR

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Infrared Spectroscopy-IR

Common Applications

• Identification of compounds by matching spectrum ofunknown compound with reference spectrum

(fingerprinting)

• Identification of functional groups in unknown substances

• Identification of reaction components and kinetic studiesof reactions

• Identification of molecular orientation in polymer films

• Detection of molecular impurities or additives present in

amounts of 1% and in some cases aslow as 0.01%• Identification of polymers, plastics, and resins

• Analysis of formulations such as insecticides and

copolymers


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Complementary or Related Techniques

Nuclear magnetic resonance providesadditional information on detailed molecular

structure

Mass spectrometry provides molecular mass

information and additional structural

information

Raman spectroscopy provides complementary

information on molecular vibration. (Somevibrational modes of motion are IR-inactive

but Raman-active and vice versa.)- NOT

INCLUDED IN THIS COURSE


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Introduction

Infrared (IR) spectroscopy is one of the

most common spectroscopic techniques

used by organic and inorganic chemists

it is the absorption measurement of

different IR frequencies by a samplepositioned in the path of an IR beam.

The main goal of IR analysis is to

determine the chemical functional groupsin the sample.

Different functional groups absorb

characteristic frequencies of IR radiation.


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IR Frequency Range

Region λ range (μm) Wave number ѵ range,

cm-1

Frequency Range

Near 0.78-2.5 12, 000 to 4000 3.8x1014- 1.2x1014

Middle 2.5-50 4000 to 200 1.2x1014- 6.0x1012

Far 50-1000 200 to 10 6.0x1012- 3.0x1011

Most used 2.5-15 4000 to 670 1.2x1014- 2.0x1013


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FT-IR Instrument

1. The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which

controls the amount of energy presented to the sample(and, ultimately, to the detector).

2. The Interferometer: The beam enters the interferometerwhere the “spectral encoding” takes place. The resultinginterferogram signal then exits the interferometer.

3. The Sample: The beam enters the sample compartmentwhere it is transmitted through or reflected off of thesurface of the sample, depending on the type of analysisbeing accomplished. This is where specific frequencies ofenergy, which are uniquely characteristic of the sample, areabsorbed.

4. The Detector: The beam finally passes to the detector forfinal measurement. The detectors used are speciallydesigned to measure the special interferogram signal.

5. The Computer: The measured signal is digitized and sent tothe computer where the Fourier transformation takesplace. The final infrared spectrum is then presented to the

user for interpretation and any further manipulation.


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FT-IR Instrument- Schematic


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Michaelson Interferometer

Source

Stationary mirror

Moving mirror

Sample

Detector

Beam Splitter

PMT

HeNe laser


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Michaelson Interferometer

Interferometers employ a beamsplitter which takes the incoming infrared beam and divides it

into two optical beams.

One beam reflects off of a flat mirror which is fixed in place. The other beam reflects off of a flat

mirror which is on a mechanism which allows this mirror to move a very short distance(typically a few millimeters) away from the beamsplitter.

The two beams reflect off of their respective mirrors and are recombined when they meet back

at the beamsplitter.

Because the path that one beam travels is a fixed length and the other is constantly changing as

its mirror moves, the signal which exits the interferometer is the result of these two beams

“interfering” with each other. The resulting signal is called an interferogram which has the unique property that every data

point (a function of the moving mirror position) which makes up the signal has information

about every infrared frequency which comes from the source.

This means that as the interferogram is measured, all frequencies are being measured

simultaneously.

Thus, the use of the interferometer results in extremely fast measurements.

Because the analyst requires a frequency spectrum (a plot of the intensity at each individual

frequency) in order to make an identification, the measured interferogram signal can not be

interpreted directly.

A means of “decoding” the individual frequencies is required. This can be accomplished via a well-

known mathematical technique called the Fourier transformation. This transformation is

performed by the computer which then presents the user with the desired spectral informationfor analysis.


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IR Spectrum

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


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Theory of Infrared

Dipole changes During Vibrations and Rotations

IR absorption - a molecule must undergo a net change in dipole momentdue to vibrational or rotational motion.

Homonuclear species such as O2, N2 or Cl2 – no net change in dipolemoment occurs during vibration or rotation.

Eg. The charge distribution around a molecule of HCl is not symetricbecause the Cl has a higher electron density than the hydrogen.

HCl has a significant dipole moment and known as polar molecule

Dipole moment: determined by the magnitude of the charge differenceand the distance between the two centre of centre of charge.

as a HCl vibrates, a regular fluctuation in dipole moment occurs, an afield is established that can interact with the electrical field associated

with radiation. If the frequency of the radiation exactly matches a naturalvibrational frequency of the molecule, a net transfer of energy takesplace that results in a change in the amplitude of the molecular vibration,absorption of the radiation is the consequence. Similarly, the rotation ofasymmetric molecules around their centres of mass result in a periodicdipole fluctuation that can interact with radiation.


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Theory of Infrared

Rotational Transitions

The energy required to cause a change in rotational is minute (small)and corresponds to radiation of 100 cm-1 or less (>100μm)

Rotational levels are quantized;

i) Absorption by gases in the far IR region is characterized bydiscrete, well-defined line

ii) Absorption by liquids or solids intramolecular collisions andinteractions cause broadening of the lines in to a continuum.

Vibrational/Rotational Transitions

Vibrational energy levels are also quantized for most molecules theenergy differences between quantum states correspond to the mid-IRregion.

The IR spectrum of a gas usually consists of closely spaced lines,because there are several rotational energy state for each vibrationalstate.

On the other hand, rotation is highly restricted in liquids and solids, insuch sample, discrete vibrational/rotational lines disappear, leaving only

somewhat broadening vibrational peaks.


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Types of molecular vibration

i) Stretching: involves a continues changein the inter atomic distance along the

axis of the bond between two atoms. 2

types: Symetric stretching and asymetric

stretching

ii) Bending: characterized by a change in

the angle between two bonds. 4 types of

bending vibrations: scissoring(bending),

rocking, wagging and twisting.


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Stretching and Bending

Symmetric Asymmetric

In plane rocking In plane scissoring

Out of plane wagging Out of plane twisting


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The IR spectrum ( Dudley Williams pg 30)

A complex molecule-many vibrate modes involve in the

whole molecule

However, these molecular vib. are largely associated with thevib. of individual bonds and called localized vibrations.

These localized vibrations are useful for the identification offunctional groups, such as the stretching vibs of:

Single bond (O-H & N-H) and all kinds of triple and doublebonds, all of which occur with frequency greater than1500cm-1

The stretching vib. of other single bonds, most bendingvibrations and the soggier vibration of the molecule as awhole give rise to a series of absorption bands at lowerenergy, below 1500cm-1, the position of which arecharacteristic of that molecule (Fingerprint region).


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Full spectrum of cortisone acetate , pg 31


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Full spectrum of cortisone acetate , pg 31

Strong absorption from the stretchingvibrations above 1500cm-1, showing that

the presence of each of the functional

groups:

O-H group

3 diff C=O groups

The weaker absorption of the C=C

double bond

“finger print region” below 1500cm-1


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Identification of IR spectrum (organic comp)

Two step processes:

Determine what functional groups are present-examining thegroup frequency region (3600 – 1200cm-1)

Detailed comparison of the spectrum of the unknown with

the spectra of pure compounds that contain all of thefunctional groups found in the first step. The fingerprintregion (1200-600cm-1) is useful due to small differences inthe structure and constitution of a molecule result insignificant changes in the appearance and distribution ofabsorption peaks in this region. Consequently, a close match

between two spectra in the fingerprint region (as well asothers) constitutes almost certain evidence for the identityof the compound yielding the spectra.


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Table of group frequency for organic group

Bond Type of compound Frequency range,

cm-1

Intensity

C-H Alkanes 2850-2970

1340-1470

Strong

StrongC-H Alkenes (C=C) 3010-3095

675-995

Medium

Strong

C-H Alkynes (C≡C) 3300 Strong

C-H Aromatic ring 3010-3100

690-900

Medium

Strong

O-H Monomeric alcohols, phenols

Hydrogen-bonded alcohols, phenols

Monomeric carboxylic acids

H-bonded carboxylic acids

3590-3650

3200-3600

3500-3650

2500-2700

Variable

Variable

(sometimes broad)

Medium

Broad

N-H Amines, amides 3300-3500 Medium

C=C Alkenes 1610-1680 Variable

C=C Aromatic rings 1500-1600 Variable

C≡C Alkynes 2100-2260 Variable

C-N Amines, amides 1180-1360 Strong

C≡N Nitriles 2210-2280 Strong

C-O Alcohols, ethers, carboxylic acids, esters 1050-1300 Strong

C=O Aldehydes, ketone,-COOH, esters 1690-1760 Strong

NO2 Nitro compounds 1500-15701300-1370

StrongStrong

IR S t T t i l H t l IR t


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IR Spectroscopy Tutorial: How to analyze IR spectra

The distinctive bands of the common functional bands:

3500-3300 cm-1

N –

H stretch amines

3500-3200 cm-1 O – H stretchalcohols, a broad, strong

band

3100-3000 cm-1 C – H stretch alkenes

3000-2850 cm-1 C – H stretch alkanes

1760-1665 cm-1 C=O stretchketones, aldehydes,

esters

1680-1640 cm-1 C=C stretch alkenes


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i) Begin by looking in the region from 4000-1300. Look at the C – H

stretching bands around 3000:

ii) Look for a carbonyl in the region 1760-1690. If there is such a band:

Indicates:

Is an O – H band also present? a carboxylic acid group

Is a C – O band also present? an ester

Is an aldehydic C – H band also present? an aldehyde

Is an N – H band also present? an amide

Are none of the above present? a ketone

Indicates:

Are any or all to the right of 3000?alkyl groups (present in most organicmolecules)

Are any or all to the left of 3000?a C=C bond or aromatic group in the

molecule


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IR S T i l Alk


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IR Spectroscopy Tutorial: Alkanes

The spectra of simple alkanes are characterized by absorptions due to C – H stretching and bending (the C –

C stretching and bending bands are either too weak or of too low a frequency to be detected in IR

spectroscopy). In simple alkanes, which have very few bands, each band in the spectrum can be assigned.

• C – H stretch from 3000 – 2850 cm-1

• C – H bend or scissoring from 1470-1450 cm-1

• C – H rock, methyl from 1370-1350 cm-1

• C – H rock, methyl, seen only in long chain alkanes, from 725-720 cm-1

IR SpectroscopyTutorial:Alkenes


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IR Spectroscopy Tutorial: Alkenes

Alkenes are compounds that have a carbon-carbon double bond, – C=C – . The stretching vibration of the

C=C bond usually gives rise to a moderate band in the region 1680-1640 cm-1.

Stretching vibrations of the – C=C – H bond are of higher frequency (higher wavenumber) than those of

the – C – C – H bond in alkanes.

C=C stretch from 1680-1640 cm-1

=C – H stretch from 3100-3000 cm-1

=C – H bend from 1000-650 cm-1

IR Spectroscopy Tutorial: Aromatics


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IR Spectroscopy Tutorial: Aromatics

The =C – H stretch in aromatics is observed at 3100-3000 cm-1. Note that this is at slightly higher frequency than is the – C – H

stretch in alkanes.

Aromatic hydrocarbons show absorptions in the regions 1600-1585 cm-1 and 1500-1400 cm-1 due to carbon-carbon stretching

vibrations in the aromatic ring.

C – H stretch from 3100-3000 cm-1

overtones, weak, from 2000-1665 cm-1

C – C stretch (in-ring) from 1600-1585 cm-1

C – C stretch (in-ring) from 1500-1400 cm-1


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IR Spectroscopy Tutorial: Alcohols

Alcohols have characteristic IR absorptions associated with both the O-H and the C-O stretching vibrations.

O – H stretch, hydrogen bonded 3500-3200 cm-1

C – O stretch 1260-1050 cm-1 (s)


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IR Spectroscopy Tutorial: Ketones

The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears at 1715 cm-1.

Conjugation of the carbonyl group with carbon-carbon double bonds or phenyl groups, as in alpha, beta-

unsaturated aldehydes and benzaldehyde, shifts this band to lower wave numbers,1685-1666 cm-1

C=O stretch:

◦ aliphatic ketones 1715 cm-1

◦ α,β-unsaturated ketones 1685-1666 cm-1


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IR Spectroscopy Tutorial: Aldehydes

The carbonyl stretch C=O of saturated aliphatic aldehydes appears from 1740-1720 cm-1. As in ketones, if the carbons

adjacent to the aldehyde group are unsaturated, this vibration is shifted to lower wavenumbers, 1710-1685 cm -1

H – C=O stretch 2830-2695 cm-1

C=O stretch:

◦ aliphatic aldehydes 1740-1720 cm-1

◦ alpha, beta-unsaturated aldehydes 1710-1685 cm-1

chapter 2 infrared spectroscopy ir

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