Infrared Spectrometry

PHARMACEUTICAL ANALYSIS

Infrared Spectrometry

Dr. Hinna Hamid Lecturer Dept. of Chemistry

Faculty of Science Jamia Hamdard Hamdard Nagar New Delhi- 110062

(10.10.2007) CONTENTS

Introduction Spectrophotometry IR Spectroscopy Molecular Vibrations

Sample Preparation and Handling Instrumentation

Keywords

Spectrometry, vibrations, Fermi resonance, overtones, fingerprint region

Introduction

Different regions of electromagnetic spectrum have different kinds of spectroscopic techniques associated with them.

Radio waves : ESR and NMR spectroscopy Microwaves : Rotational spectroscopy.

Infrared waves : IR spectroscopy UV/ Vis waves : UV/Vis spectroscopy

NMR spectroscopy - is used to determine the carbon-hydrogen framework of a molecule and works with even the most complex molecules.

Infra-red spectroscopy - is used to determine the presence of distinct functional groups in organic molecules. IR radiation causes them to vibrate at various frequencies, allowing us to identify them.

Ultra-violet/Visible spectroscopy- is used primarily to identify the extent of conjugation in a molecule; it can also be used to identify how much (quantitative analysis) of a substance is present, usually in a solution. The color absorbances are used with the Beer-Lambert Law.

Principle: Molecules absorb energy and this energy can bring about translational, rotational or vibrational motion or ionization of the molecules depending upon the frequency of the electromagnetic radiation they receive. Excited molecules are unstable and quickly drop down to ground state again giving off the energy they have received as electromagnetic radiation.

The wavelength and intensity of the electromagnetic radiation absorbed or emitted can be recorded to get a spectrum. Spectral analysis yields qualitative and quantitative information about the matter under study.

Spectrophotometry

Beer – Lambert Law is central to spectrophotometry and for many current applications a spectrometer is increasingly becoming the measurement device of choice. This law forms the basis of nearly all the colorimetric methods for spectroscopic data. Simply stated, the law claims that when a sample is placed in the beam of a spectrometer, there is a direct and linear relationship between the amount (concentration) of its constituent(s) and the amount of energy it absorbs. In mathematical terms:

Po P

b

Absorption (A) = -log(P/Po) = εbc

Where A is the sample’s Absorbance value at specific wavelength (or frequency), P is the light intensity, ε is the absorptivity coefficient of the material (constituent) at that wavelength, b is the path length through the sample and C is the concentration.

The absorptivity coefficient for every material is different, but for a given compound at a selected wavelength, this value is a constant.

IR Spectroscopy

For organic molecules, molecular vibrations have energies that correspond to that of the infrared region of the electromagnetic spectrum. These molecular vibrations are typically measured using infrared spectroscopy.

For organic chemistry, the most useful range of the infrared spectrum encompasses approximately 2.5 to 15 µm. Functional groups present in the molecules have characteristic vibrational frequencies, and the presence of an absorption band in the infrared spectrum is strong suggestive evidence that the molecule contains that functional group. Likewise, for many

“reliable” bands, the absence of an absorbance is also strong suggestive evidence that a particular functional group is absent in the molecule.

Infrared spectroscopy can be used for:

Chemical analysis: Spectra can be matched to known databases for identification of an unknown compound.

Monitoring of chemical reactions in situ.

To determine which chemical groups are present in a specific compound.

Identification of reaction components and kinetic studies of reactions Identification of compounds, polymers, plastics, and resins.

Detection of molecular impurities or additives present in amounts of 1% and in some cases as low as 0.01%

Analysis of formulations such as insecticides and copolymers.

IR causes polar molecules to undergo bond vibration.

Molecular Vibrations

There are two major types of vibrations:

Stretching: Involve change in bond length

Bending: Involve change in bond angle, it requires lesser energy and occurs at lower frequency.

Stretching vibrations are of two types: symmetrical and asymmetrical stretching, with asymmetric vibration taking place at a higher frequency than the stretching frequency. During a symmetrical stretch, both the atoms involved move in and out simultaneously. In asymmetrical stretching, one atom moves in and the other atom moves out.

Bending vibrations are of two types;

• In plane bending: Atoms remain in the same plane as the nodal plane of the system.

These vibrations include scissoring & rocking. During a scissoring vibration the atoms

swing in opposite direction and during rocking deformation, both the atoms swing to the same side.

• Out of plane bending: Atoms bend out of the nodal plane. Two types of out of plane vibrations are observed, namely, wagging and twisting. In a wagging vibrational motion, the atoms involved swing up and down out of the nodal plane and during twisting one atom swings up and the other swings down related to the plane.

Only bonds, which have significant dipole moments, will absorb infrared radiation and the intensity of absorption depends on the change in dipole on vibration. Bonds, which do not absorb infrared, include:

Symmetrically substituted alkenes and alkynes Many types of C-C Bonds.

Symmetric diatomic molecules

ν

1isIR inactive

ν

2 is IR active

Fig: CO2-Linear molecule

The intensity of a particular fundamental absorption depends on the difference between the dipole moments of a molecule in the ground state and the vibrational excited state. The greater the difference in these dipole moments, the more intense the absorption is. The carbonyl group is one of the strongest absorbers of IR. O-H and C-O bonds also absorb strongly

δ- δ+

C O -

-

+ +

Fig: The oscillating dipole of the electromagnetic radiation couples with the dipole of the carbonyl group

In IR the intensity of the bands cannot be measured as accurately as in UV. The bands are marked as weak (50-30% relative intensity), medium (60-50% relative intensity), strong (80- 60% relative intensity) or of variable intensity with respect to the most intense peak (base peak), to which a relative intensity of 100% is assigned.

Principle

Mathematical Description of the Vibration Molecule as a Hooke’s Law device

Picture molecules as two (or more) nuclei held together by ‘non-totally rigid’ bonds (i.e. like springs).

K

R R

compres s

stretc h K

∆ x m2

m1

Bo

Fig : Molecule as a Hooke’s Law device

Restoring Force = - F F – K (R- Re)

Using Hooke’s Law and the Simple Harmonic Oscillator approximation, the following equation can be derived to describe the motion of a bond.

Vibrational energy levels of a harmonic oscillator: Ev = (v + ½) hν

Vibrational quantum number, v = 0, 1, 2, ..

1 2π c

K

= µ

ν

Larger K, higher the frequency.

Where

m

₁

m

₂

m

₁

+ m

₂

requency Larger atomic masses, lower the f

µ =

ν = frequency in cm ^–1 C = velocity of light K = force constant µ = Reduced mass

From this relation it is clear that the frequency absorbed depends on K and µ

Increasing K

C=C > C=C > C-C 2150 = 1650 1200

Increasing µ

C-H > C-C > C-O > C-Cl > C-Br 3000 1200 1100 750 650

Fig: Factors affecting absorption

Fig: Potential energy curve for a diatomic molecule

Factors influencing absorption frequency

Masses of attached atoms: As masses increase, wavenumber decreases.

Strength of chemical bond: As bond strength increases, wavenumber increases.

C-H C-C C-O C-Cl C-Br C-I

3000 cm^-1 1200 cm^-1 1100 cm^-1 750 cm^-1 600 cm^-1 500 cm^-

C=C triple bond ^{C=C C-C}

2150 cm^-1 1650 cm^-1 1200 cm^-1

=

rav distanc

e n e r g y

rmi rma

zero point vibrational energy Harmonic

ACTUAL MOLECULE

(average bond

Hybridization: Bonds are stronger in the order

sp > sp2 > sp3.

Resonance: Conjugation lowers the energy to vibrate bond.

C-H (sp²) C-H (sp³)

2900 cm^-1 3100 cm^-1

C-H (sp)

3300 cm^-1

isolated ketones

α,β-unsaturated ketone

α,β,γ,δ-unsaturated ketone

Frequency shifts by change of phase and solvents

1715 cm^-1 1690 cm^-1 1675 cm^-1

Variations in the physical state influence the infrared frequencies by means of association effects. Shifts of absorption bands occur as a result of changes of solvents or phase can take place. Carbonyl stretching of acetone occurs at 1742 cm^-1 in the vapour phase, in liquid phase frequency is lowered due to dipole-dipole association, in hexane it absorbs at 1726 cm^-1, at 1713 cm^-1 in chloroform and at 1709 cm^-1 in ethanol.

Selection Rule 2 for an IR transition is ∆v = ±1.

The transition frequency thus occurs at:

∆E = Ev+1-Ev = ((v + 1) +½) hv- (v +½) hv =hv

Thus we might predict the following appearance for an IR absorption spectrum

Real molecules are not truly harmonic oscillators.

• At high enough levels of vibrational excitation the bond must break → two atoms.

Anharmonicity relaxes the selection rules, allowing ∆v = ±1, ±2, ±3, etc

fundamental band

overtone transitions

(These exhibit progressively weaker intensities)

The frequencies of the v = 2 ←v = 1, v = 3←v = 2 etc transitions are red-shifted relative to the fundamental.

In IR, most overtones are found in the near IR region beyond 4000 cm^-1. Aromatic compounds exhibit characteristic overtone bands in the region from 2000-1667 cm^-1.

A number of weak absorptions are seen in the IR spectra which may be either due to sum of two or more fundamental vibrational frequencies or difference between two vibrational frequencies.

These absorptions are referred to as combination bands. When an overtone band or a combination band interacts with a fundamental band, it causes a decrease in the intensity of the fundamental band and a large increase in the intensity of the overtone or the combination band.

Such an interaction is called as ‘Fermi resonance’

In aldehydes appearance of two moderately intense bands in the region 2830-2695 cm^-1 is due to the interaction between the aldehydic C-H stretch and the first overtone of aldehydic C-H in plane bending which appears near 1390 cm^-1.

Fig: A typical infrared spectrum

10 8 6 4 2 0

C H₃C H C H₂C C H₃ O C H₃

WAVENUMBER (cm-1)

350 300 250 200 150 100 50

10 8 6 4 2 0

% T R A N S M I T T A N

Interpreting Infrared Spectra

Most functional groups absorb at about the same energy and intensity independent of the molecule they are in.

Fig: Typical Infrared Absorption Regions

The Region below 1500 cm-1 is called as the “fingerprint” region. It is characteristic of a compound and can be used for it identification. The region above is called as the “functional group region” and gives information about the presence of functional groups in the compound under study.

propan-1-ol

propan-2-ol

6

Fig: Using the fingerprint region

Fingerprint region can be very efficiently used to confirm the identity of a compound.

Hydrocarbons (saturated and unsaturated)

C-H spstretch ~ 3300 cm^-1

C-H sp² stretch > 3000 cm^-1

C-H sp³ stretch < 3000 cm^-1

SATURATED UNSATURATED

3000 divides

Methylene group C-H stretching vibrations Asymmetric Stretch ~2926 cm-1

Symmetric Stretch ~2853 cm-1

C

H

H

C

H

H In phase

Out of phase

Methyl group stretching vibrations Symmetric Stretch ~2872 cm-1 Asymmetric Stretch ~2962 cm-1

C

H H H

C

H H H

in-phase

out-of-phase

Aromatic compounds commonly exhibit multiple weak bands in the region 3100-3000cm^-1. C-H stretching frequencies in cyclic alkanes with unstrained rings are almost the same as acyclic alkanes. Increasing the ring strain moves absorption to higher frequencies.

The C-C stretching region

The C-C stretching vibrations appear as weak bands in the region from 1200 to 1800 cm-1 and are of little value for structural study.

C=C: : weak peak at 1650 cm-1

C=C aromatic ring : shows peak(s) near 1600 and 1400 cm-1 one or two at each value

Methylene group C-H bending vibrations

Scissoring Wagging

C

H H C

H H

C

H H

C

H

H

^{~1250 cm-1}

~1250 cm^-1

~1465 cm^-1

~720 cm^-1

Rocking Twisting

^Bending

Vibrations

in-plane out-of-plane

FFiigg :: Methylene group C-H bending vibrations

geminal dimethyl (isopropyl)

CH₃

1460 1375 asym sym

1370

1380 _C ^{C H 3}

C H ₃ C C H ₃

The sym methyl peak splits when more than one CH3 is attached to a carbon.

one peak

two peaks

t-butyl 1390 1370

C C H ₃ C H₃

C H₃

two peaks

Fig.: Methyl C-H bending vibrations

C-H in plane bending vibrations in olefins

Vinyl and vinylidine groups display a medium intensity band near 1420 m ^–1 due to =CH2

scissoring vibration , CH2 rocking appears at 1075 cm^-1, and C-H rocking gives rise a weak band at 1300 cm^-1 for vinyl, trans and trisubstituted double bands.

C-H in plane bending vibrations in aromatic compounds

C-H in-plane ring bending vibrations are observed as many m-w intensity sharp bands in the region 1300-1000 cm^-1.

Out-of-plane bending

Alkenes can easily be distinguished from the saturated systems by means of =C-H and C=C stretching vibrations as well as bending vibration of the olefinic =C-H bonds.

FiFigg..:: OOuutt ooff ppllaannee CC--HH bbeennddiinngg iinn aallkkeenneess

Above

ALKENES

PLANE

H H

H

Below

C-H bending bands in linear olefins

In alkynes the wagging vibration in acetylenes occurs as a strong broad band near 680- 610 cm

=C-H

-1. The first overtone of this band appears as a weak broad band near 1375-1225cm= ^-1.

Aromatic C-H out of plane bending (900-667 cm^-1)

The assignment of structure based upon the out of plane bending vibrations is most reliable for alkyl substituted aromatic compounds. Overtones and combination band is of C-H out of plane bending vibration form a group of weak bands which appear between 2000-1667cm^-1and depends on the substitution pattern on the ring.

C H₃ C H₂ C H₂ C H₂ C H₂ C H₃

^CH2

rocking CH₃

CH2 CH

Carbonyl Compounds

This functional group is present in many classes of organic compouds like ketones, aldehydes, carboxylic acids, acid halides, anhydrides, amides, lactones, esters etc. Carbonyl stretching region stretches from about 1800 to 1650 cm-1 - Right in the middle of the spectrum. The base value is 1715 cm-1 (ketone). The bands are very strong due to the large C=O dipole moment.

C=O is sensitive to its environment

acid chloride

carboxylic acid

ester aldehyde ketone amide

C R

O H

C O

O C R O C

O

C l R C O

O R ' R C

O

R R C

O

N H₂ C

R O R O H

1800 1735 1725 1715 1710 1690

BASE VALUE anhydride

R

1810 and 1760 THESE VALUES ARE

WORTH LEARNING all are +/- 10 cm^-1 ( two peaks )

Factors that influence the C=O absorption

Inductive and resonance effects significantly affect the carbonyl absorption.

• Ketones absorb at a lower frequency than Aldehydes because of the second electron- donating alkyl group.

• Acid chlorides are seen absorbing at higher frequency than ketones because of the electron-withdrawing halide.

• Esters absorb at higher frequencies than ketones due to the electron-withdrawing oxygen atom. This is more important than resonance with the electron pair on the oxygen.

• Amides absorb at lower frequencies than ketones due to resonance involving the unshared pair on nitrogen. The electron-withdrawing effect of nitrogen is less important than the resonance.

• Acids have a lower frequency of absorption than ketones due to H-bonding.

Conjugation of C=O with C=C

• Conjugation of a carbonyl with a C=C bond shifts values to lower frequencies

• For aldehydes, ketones and esters, a about shift of about 25-30 cm-1 is seen for conjugation with C=O

• Conjugated ketone absorbs from1690 to 1680 cm-1

• Conjugated ester absorbs from 1710 to 1700 cm-1

Angle strain raises the carbonyl frequency Conformation of functional groups

R C O

H

C=O at 1710 cm^-1 C=O at 1725 cm^-1

Also look for aldehyde CH stretch at 2850 and 2750 cm-¹

R C O

O H Also look for OH (H- bonded) and C-O ~1200 cm^-1

R C O

N H H

C=O at 1690 cm^-1

Also look for two NH peaks at 3400 cm^-1

R C O

O R'

C=O at 1735 cm^-1

Also look for two C-O stretch absorptions at 1200 and 1000 cm^-1

Ketones have C=O at 1715 cm^-1 and no NH, OH, C-O or -CHO Anhydrides have two C=O peaks near 1800 cm^-1 and two C-O

Hydroxy Compounds The O-H stretching region

Alcohols and phenols, in dilute solutions using non-polar solvents (<. 005 molar) or in vapour state exhibit a sharp peak due to non bonded or free OH groups at 3650 cm^-1 and 3600 cm^-1, respectively. Inter molecular hydrogen bonding increases as the concentration increases and additional bands start to appear at lower frequencies near 3550-3200^-1. Hydrogen bonding is predominant for these hydroxyl compounds in solid and pure liquid states also.

• O-H 3600 cm-1 (alcohol, free)

• O-H 3300 cm-1 (alcohols & acids: H-bonding)

Strong hydrogen-bonding in the dimer weakens the O-H and C=O bonds and leads to broad peaks at lower frequencies.

C O O

H C R

O

O H

R

OH bending vibrations

The in-plane O-H bending vibrations in hydroxyl compounds absorb in the region from 1420- 1260cm^-1, depending on the type of the hydroxyl group.

Tertiary Alcohols and Phenols: 1420 -1330 cm^-1. Primary and Secondary Alcohols: ∼ 1330 and 1260 cm^-1.

A broad band due to bonded O-H out-of-plane bending vibrations can be seen from 770 – 650 cm^-1, in the spectra of alcohols and phenols in liquid state

C-O stretching vibration

In alcohols and phenols a strong band near 1260-1000 cm^-1 is observed for C-O stretching vibrations. By examining the C-O stretching vibrations a distinction can be made between primary, secondary and tertiary alcohols.

Primary Alcohols: 1050 cm^-1 Secondary Alcohols: 1100 cm^-1 Tertiary Alcohols: 1150 cm^-1

Phenols: 1200 cm^-1

Ethers

Ethers are characterized by strong C-O-C stretching vibrations in the range 1300-1000 cm^-1. Presence of ether is confirmed only if there is no absorption in carbonyl and hydroxyl regions.

N-H Stretch in amides

In non-polar solvents two medium intensity bands can be seen for primary amides near 3500 and 3400 cm^-1 due to symmetric and asymmetric stretches, respectively. However in solid state due to hydrogen bonding, the bands are shifted to 3350 and 3180 cm^-1, respectively. Secondary amides absorb at ∼ 3450 cm^-1 in dilute solutions, and due to hydrogen bonding in solid state multiple bands are observed near 3330-3100 cm^-1.

Amines

N-H stretch: Primary amines as dilute solutions in non-polar solvents absorb at ∼ 3550-3330 cm^-1 and ∼ 3450-3250 cm^-1 due to asymmetric and symmetric stretching vibrations. Secondary amines display one band in the region from 3500-3300 cm^-1.

N-H in-plane bend: Strong to medium bands could be observed for primary amines in the region between 1650-1850 cm^-1 which could shift to higher values in hydrogen bonded molecules. C-N-H in plane bending vibration at ∼1515 cm^-1 can be easily detected for aromatic secondary amines in comparison to the aliphatic counterparts.

N-H out of plane bending: Liquid aliphatic amines display multiple absorptions from 909-666 cm^-1 and secondary amines display a strong band near 750-700 cm^-1 . .

C-N stretching vibrations

Medium to weak band can be observed at ∼1250-1020 cm^-1 in primary and secondary aliphatic amines, for C-N stretching vibration.

Aryl amines absorb in the region from 1360-1250 cm^-1 Primary aryl amines: 1340-1250 cm^-1

Secondary aryl amines: 1350-1280 cm^-1 Tertiary aryl amines: 1360-1310 cm^-1

Nitro compounds

Strong bands at 1565-1515 cm-1 and 1385-1335 cm-1 can be observed for asymmetric and symmetric stretchings, respectively, often the former is stronger than the latter.

C-N stretching andC-N-O bending vibrations occur near 870 and 610 cm^-1. Nitriles and Isonitriles

C=N group in an aliphatic nitrile absorbs at ∼ 2250 cm^-1 and in an aromatic amine the absorption can be seen at ∼ 2230 cm^{= -1}. Isonitriles absorb strongly near 2185-2121cm^-1.

CHARACTERISTIC INFRARED ABSORPTION FREQUENCIES

Frequency range, cm^-1 Compound Type

Bond

2000-1600(w) - fingerprintregion

Phenyl Ring Substitution Overtones

870-675(s) bend Phenyl Ring Substitution Bands

3100-3000(m) stretch Aromatic Rings

C-H

1000-675(s) bend 2040-2200 C-C=C-C=CH

2190-2260 C-C=C-C

2100-2140 C=CH (terminal)

3080-3020(m) stretch Alkenes

C-H

1380(m-w) - Doublet - isopropyl, t-butyl CH Umbrella Deformation3

1355-1395, 1430-1470 bend CH3

1405-1465 bend CH2

1470-1350(v) scissoring and bending 2960-2850(s) stretch

Alkanes C-H

2880-2900 CH

3333-3267 (s) stretch Alkynes

C-H

700-610 (b) bend 2815-2832 O-CH3

2810-2820 N-CH3 (aromatic)

2815-2832 N-CH3 (aliphatic)

2780-2805 N-CH3 (aliphatic)

2843-2863, 2916-2936 CH2

1200 Phenols

Frequency range, cm^-1 Compound Type

Bond

1760-1670 (s) stretch Aldehydes, Ketones, Carboxylic acids, Esters

C=O

990-1060 Secondary Cyclic Alcohols

1150 Tertiary alcohols

1100 Secondary alcohols

1050 Primary alcohols

1260-1000 (s) stretch Alcohols Ethers Carboxylic acids

,

^Esters

C-O

1600, 1500 (w) stretch Aromatic Rings

2260-2100 (w,sh) stretch Alkynes

1680-1640 (m,w)) stretch Alkenes

C=C

3640-3160 (s,br) stretch Monomeric -- Alcohols, Phenols

O-H

3600-3200 (b) stretch Hydrogen-bonded -- Alcohols, Phenols

3450-3600 (sharp) Intramolecular H bonds

2500-3200 (very broad) Chelate Compounds

3000-2500 (b) stretch Carboxylic acids

1310-1410 Bending Vibrations

3200-3550 (broad) Intermolecular H Bonds

3500-3300 (m) stretch Amines

N-H

3300-3500 Free NH

3070-3350 H bonded NH

1650-1580 (m) bend

1390-1260 (s) symmetrical stretch

Frequency range, cm^-1 Compound Type

Bond

1660-1500 (s) asymmetrical stretch Nitro Compounds

NO2

1260-1000 (s) stretch Nitriles

C=N

1340-1020 (m) stretch Amines

C-N

=

Minimum values needed to remember

-CHO C-H =C-H -C-H

EXPANDED C=O acid

EXPANDED

benzene C=C : between 1400 and 1600 BASE VALUES

anhydride : 1810 and 1760

3300 3100 2900

ketone ester

acid chloride

aldehyde

amide CH2 and CH3 bend: 1465 and 1365

1800 1735 1725 1715 1710 1690 1710 1690

300

OH 3600 NH 3400 CH 3000

C N 2250

C C 2150 C=O 1715 C=C 1650 C-O 1100

Know also the effects of H-bonding, conjugation and ring size.

MAKING DECISIONS

anhydride

acid

amide

ester

aldehyde

YES C=O present ?

2 C=O Peaks OH present

? OH present ?

NH present ?

NH present

?

C-O present ?

C-O present

?

CHO present ?

C=N present

? C=C present

? C=C present

?

NO2 present

? C-X present

? (benzene

?)

YES NO

=

=

ketone NO YES

alcoho l amin e ethe r nitril e alkyn e alken e aromati cnitro cpds halide s

Sample Preparation and Handling

Infrared spectroscopy permits the analyst to select from a wide variety of sample preparation techniques. Polystyrene film is commercially used for calibration of wave numbers. (3026, 3002, 2924, 1602, 1495 & 906).

Gaseous samples: Gas phase spectra can be taken at room temperature. What is required is a sample with a vapor pressure of several millimeters and a pathlength of about a decimeter (10 cm). The cells consist of a glass or metal body, two IR-transparent end windows (NaCl or KBr), and valves for filling gas from external sources. KBr is transparent from the wavelength 4000- 250 cm^-1 and sodium chloride from 4000-600 cm^-1.

Liquid Samples: For liquid samples two polished disks of NaCl or KBr are used. A thin film is prepared by depositing a drop of the liquid between the two plates and mounting them in the beam of the spectrometer. Silver chloride plates can be used for samples that dissolve the salt plates.

Solid samples: Neat Spectra (thin film) A thin layer of a solution is used on an infrared cell and the solvent is allowed to evaporate. Solvents such as CS2, CH2Cl2 and CCl4, which are transparent in the region of interest are usually used.

Nujol mull

A suspension of a solid in a liquid is prepared. The commercial Nujol sample often used for this purpose is mineral oil. The major disadvantage of using a Nujol mull is that the information in the C-H stretching region is lost because of the absorptions of the mulling agent. To eliminate this problem, it may be necessary to run a second spectrum in a different mulling agent that does not contain any C-H bonds. Typical mulling agents that are used for this purpose are perfluoro- or perchlorohydrocarbons.

Examples include Florolube, hexachlorobutadiene, perfluorokerosene or perfluorohydrocarbon oil.

KBr pellets

A KBr pellet is a dilute suspension of a solid sample in solid KBr. It is usually obtained by first grinding the sample in anhydrous KBr at a ratio of approximately 1 part sample to 100 parts KBr. The ground up sample mixture is then placed in a hydraulic press and subjected to pressures of 15000 psi for about 20 seconds resulting in a KBr pellet that is reasonably transparent both to visible light and infrared radiation. The only limitation of KBr is that it is hydroscopic.

Instrumentation

Dispersive infrared spectrophotometer

An infrared spectrophotometer comprises of the following:

• A source generates light across the spectrum of interest.

• A monochromater to produce a single spectral line from a broadband (multi-wavelength) source.

• A slit selects the collection of wavelengths that shine through the sample at any given time.

• A beam splitter that separates the incident beam in two; half goes to the sample, and half to a reference.

• The sample absorbs light according to its chemical properties.

• A detector collects the radiation that passes through the sample, and in double-beam operation, compares its energy to that going through the reference.

• The detector puts out an electrical signal, which is normally sent directly to an analog recorder. A link between the monochromator and the recorder allows you to record energy as a function of frequency or wavelength.

Dispersive IR spectrometers use a diffraction grating in a monochromator to disperse the different wavelengths of light.

Diffraction grating consists usually of thousands of narrow, closely spaced parallel slits (or grooves). Because of interference the intensity of the light getting pass through the slits depends upon the direction of the light propagation. There are selected directions at which the light waves

from the different slits interfere in phase and in these directions the maximums of the light intensity are observed.

These selected directions depend upon wavelength, and so the light beams with different wavelength will propagate in different directions to produce a single spectral line from a broadband (multi-wavelength) source.

Fourier-transform infrared (FTIR) spectrometer

• Source

• Beam splitter

• Two mirrors

• laser

• Detecter

• Recorder

Most modern IR absorption instruments use Fourier transform techniques with a Michelson interoferometer. The interferometer is a fundamentally different piece of equipment than a monochromator. Since all wavelengths are passing through the interferometer, an interferogram is a complex pattern. In the interferometer the light passes through a beam splitter, which sends the light in two directions at right angles. One beam goes to a stationary mirror then back to the beam splitter. The other goes to a moving mirror. The motion of the mirror makes the total path length variable versus that taken by the stationary-mirror beam. When the two meet up again at the beam splitter, they recombine, but the difference in path lengths creates constructive and destructive interference, an interferogram.

The recombined beam passes through the sample. The sample absorbs all the different wavelengths characteristic of its spectrum, and this subtracts specific wavelengths from the interferogram. The detector now reports variation in energy versus time for all wavelengths simultaneously. A laser beam is superimposed to provide a reference for the instrument operation.

A mathematical function called a Fourier tranform allows us to convert an intensity-vs.-time spectrum into an intensity-vs.-frequency spectrum.

To acheive a good signal to noise ratio, many interferograms are obtained and then averaged.

This can be done in less time than it would take a dipersive instrument to record one scan.

Advantages of Fourier transform IR over dispersive IR

• Improved frequency resolution

• Improved frequency reproducibility (older dispersive instruments must be recalibrated for each session of use)

• Higher energy throughput

• Faster operation

• Computer based (allowing storage of spectra and facilities for processing spectra)

• Easily adapted for remote use (such as diverting the beam to pass through an external cell and detector, as in GC - FT-IR)

• Results of several scans are combined to average out random absorption artifacts and excellent spectra from very small samples can be obtained in a very short period of time.

Common light sources used are

• Nernst glowers (2200 K)

• glowbars (1500 K)

• tungsten lamps (1100 K)

The Nernst glower is a cylinder (1-2 mm diameter, approximately 20 mm long) of rare earth oxides. Platinum wires are sealed to the ends, and a current passed through the cylinder. The Nernst glower can reach temperatures of 2200 K.

The Globar source is a silicon carbide rod (5mm diameter, 50mm long) which is electrically heated to about 1500 K. Water cooling of the electrical contacts is needed to prevent arcing. The spectral output is comparable with the Nernst glower, except at short wavelengths (less than 5 mm) where it's output becomes larger.

The incandescent wire source is a tightly wound coil of nichrome wire, electrically heated to 1100 K. It produces a lower intensity of radiation than the Nernst or Globar sources, but has a longer working life.

Detectors

There are three categories of detectors

• Thermal

• Pyroelectric

• Photoconducting

Thermocouples consist of a pair of junctions of different metals; for example, two pieces of bismuth fused to either end of a piece of antimony. The potential difference (voltage) between the junctions changes according to the difference in temperature between the junctions

Pyroelectric detectors are made from a single crystalline wafer of a pyroelectric material, such as triglycerine sulphate. In a pyroelectric material when an electric field is applied across it, electric polarisation occurs which when the field is removed, persists. The degree of polarisation is temperature dependant. So, by sandwiching the pyroelectric material between two electrodes, a temperature dependant capacitor is made. The heating effect of incident IR radiation causes a

change in the capacitance of the material. Pyroelectric detectors have a fast response time. They are used in most Fourier transform IR instruments.

Photoelectric detectors such as the mercury cadmium telluride detector comprise a film of semi conducting material deposited on a glass surface, sealed in an evacuated envelope. Absorption of IR promotes non conducting valence electrons to a higher, conducting, state. The electrical resistance of the semiconductor decreases.

These detectors have better response characteristics than pyroelectric detectors and are used in FT-IR instruments - particularly in GC - FT-IR.

Other FTIR Techniques

Attenuated Total Reflectance (ATR)

Attenuated Total Reflectance (ATR) is suitable for studying thick or highly absorbing solid and liquid materials. It can practically eliminate sample preparation for, films, gels, liquids, coatings, powders,threads, adhesives, polymers, and aqueous samples. Pastes and other semi-solids are routinely analyzed with the help of an atenuated total reflectance (ATR) attachment. ATR occurs when a beam of radiation enters from a more-dense (with a higher refractive index) into a less- dense medium (with a lower refractive index). The fraction of the incident beam reflected increases when the angle of incidence increases. All incident radiation is completely reflected at the interface when the angle of incidence is greater than the critical angle (a function of refractive index). The beam penetrates a very short distance beyond the interface and into the less-dense medium before the complete reflection occurs. This penetration is called the evanescent wave and typically is at a depth of a few micrometers (µm). Its intensity is reduced(attenuated) by the sample in regions of the IR spectrum where the sample absorbs.

The sample is normally placed in close contact with a more-dense, high-refractive-index crystal such as zinc selenide, thallium bromide–thallium iodide (KRS-5), or germanium.The IR beam is directed onto the beveled edge of the ATR crystal and internally reflected through the crystal with a single or multiple reflections. For a given angle, the higher length-to-thickness ratio of the ATR crystal gives higher numbers of reflections. A variety of types of ATR accessories are

available, such as 25 to 75° vertical variable-angle ATR, horizontal ATR, and Spectra-Tech Cylindrical Internal Reflectance Cell. The resulting ATR-IR spectrum resembles the conventional IR spectrum, but with some differences.

The absorption band positions are identical in the two spectra, but the relative intensities of corresponding bands are different. Although ATR spectra can be obtained using either dispersive or FT instruments, FTIR spectrometers permit higher-quality spectra to be obtained in this energy-limited situation.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).

Diffuse reflectance technique is mainly used for acquiring IR spectra of powders and rough surface solids such as coal, paper, and cloth. It can be used as an alternative to pressed-pellet or mull techniques. IR radiation is focused onto the surface of a solid sample in a cup and results in two types of reflections:

Specular reflectance, which directly reflects off the surface and has equal angles of incidence and reflectance,

Diffuse reflectance, which penetrates into the sample, then scatters in all directions.

Special reflection accessories are designed to collect and refocus the resulting diffusely scattered light by large ellipsoidal mirrors, while minimizing or eliminating the specular reflectance. The sample can be analyzed either directly in bulk form or as dispersions in IR-transparent matrices such as KBr and KCl. Dilution of analyte in a nonabsorbing matrix increases the proportion of diffuse reflectance in all the light reflected.

Reflection Absorption Infrared Spectroscopy (RAIRS)

The light is directed at the 'Grazing angle' and is reflected at the metal surface. Upon reflection the incident light is inverted and the net electric field is zero parallel to the surface (TE) and doubled perpendicular to the surface (TM) (the Surface Selection Rule).

Using this optical configuration we can obtain structural and orientational information on very thin samples (monolayers).

Photoacoustic spectroscopy (PAS)

Photoacoustic spectroscopy (PAS) is a useful extension of IR spectroscopy and suitable for examining highly absorbing samples that are difficult to analyze by conventional IR techniques.

PAS spectra can be obtained with minimal sample preparation and without physical alteration from a wide variety of samples such as powders, polymer pellets, viscous glues, single crystals, and single fibers.

Typically, the modulated IR radiation from an FTIR interferometer is focused on a sample placed in a small cup inside a small chamber containing an IR-transparent gas such as helium or nitrogen. IR radiation absorbed by the sample converts into heat inside the sample.

The heat diffuses to the sample surface, then into the surrounding gas atmosphere, and causes expansion of a boundary layer of gas next to the sample surface.

Thus, the modulated IR radiation produces intermittent thermal expansion of the boundary layer and generates pressure waves.

A sensitive microphone is used to detect the resulting photoacoustic signal.

PAS spectra are generally similar to conventional IR spectra except for some minor differences;

absorbance peaks appear at the same frequency locations.

FTIR PAS technique offers a unique capability for examining samples at various depths from 1 to 20 µm and thus the multilayer polymers can be studied at various depths.

Infrared microspectroscopy

Infrared microspectroscopy has become a popular technique for analyzing difficult or small samples such as trace contaminants in semiconductor processing, multilayer laminates, surface defects, and forensic samples.

Infrared microscopes are energy-efficient accessories that require the signal-to-noise advantages of FTIR to obtain spectra from submilligram samples. Using a liquid nitrogen cooled mercury cadmium telluride (MCT) detector, samples in the size range of 10 µm can be examined on IR microscopes.

Hyphenated Methods Involving Infrared

Gas chromatography/Fourier transform infrared (GC/FTIR) spectroscopy is a technique that uses a gas chromatograph to separate the components of sample mixtures and an FTIR spectrometer to provide identification or structural information on these components.

Gas chromatography/matrix isolation/Fourier transform infrared (GC/MI/FTIR)

This technique provides subnanogram sensitivity, but is a very expensive technique. The helium carrier gas of a gas chromatograph is mixed with a small amount of argon. While argon is condensed in a track of 300 µm width on a rotating circular gold-coated metal disk cooled at 12

°K, the helium gas is evacuated by pumping.

The components separated by the chromatograph are dissolved and trapped in the argon matrix.

After the GC run is completed, the argon track is rotated into the IR beam and the reflection–

absorption IR spectra are obtained for each component on the cooled surface. Cryogenic temperatures are maintained while the spectra are acquired.

Suggested Readings:

1. Introduction to spectroscopy: Pavia; Lampman, Kriz, Books/cole.

2. Spectrometric identification of organic compounds, R. M. Silverstein, John Wiley and Sons publication.

3. Spectroscopic methods in organic chemistry; H. Williams; I. Fleminig, Tata Mc Grawhills 4. Organic spectroscopy, W. Kemp, Palgrave publications.