Ir spectroscopy sud mpharm pdf

By

Sudheerkumar Kamarapu

pharmaceutical analysis

Sri Shivani College of Pharmacy

INFRARED SPECTROSCOPY

INFRARED SPECTROPHOTOMETER

1.Separation techniques →Chromatography

2.Spectrophotometric → SPECTROSCOPY

3. Electro analytical → Potentiometry,conductometry

4. Titrimetric analysis→ Titrations

Classification of analytical techniques

Spectroscopy is the branch of science deals with the study of interaction of electromagnetic radiation with matter.

Electromagnetic radiation is a type of energy that is transmitted through space at enormous velocities.

EMR→ANALYTE→SPECTROPHOTOGRAPH

↓

concentration should be lower

Spectroscopy

“seeing the unseeable”.

Using electromagnetic radiation as a probe to obtain

information about atoms and molecules that are too

small to see.

Electromagnetic radiation is propagated at the speed

of light through a vacuum as an oscillating wave.

electromagnetic relationships:

λυ = c λ 1/υ

E = hυ E υ

E = hc/λ E 1/λ

λ = wave length

υ = frequency

c = speed of light

E = kinetic energy

h = Planck’s constant

λ

c

Two oscillators will strongly interact when their energies are equal.

E1 = E2

λ1 = λ2

υ1 = υ2

If the energies are different, they will not strongly interact!

We can use electromagnetic radiation to probe atoms and molecules to

find what energies they contain.

some electromagnetic radiation ranges

Approx. freq. range Approx. wavelengths

Hz (cycle/sec) meters

Radio waves 104 - 1012 3x104 - 3x10-4

Infrared (heat) 1011 - 3.8x1014 3x10-3 - 8x10-7

Visible light 3.8x1014 - 7.5x1014 8x10-7 - 4x10-7

Ultraviolet 7.5x1014 - 3x1017 4x10-7 - 10-9

X rays 3x1017 - 3x1019 10-9 - 10-11

Gamma rays > 3x1019 < 10-11

INTRODUCTION

Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the functional groups.

The infrared region of the spectrum encompasses radiation with wave numbers ranging from about 12,500 to 50cm-1 (or) wave lengths from 0.8 to 200µ.

Infrared region lies between visible and microwave region.

IR SPECTROSCOPY

The infrared region constitutes 3 parts

a) The near IR (0.8 -2.5µm) (12,500-4000cm-1)

b) The middle IR (2.5 -15µm) (4000-667cm-1)

i) Group frequency Region (4000-1500cm-1)

ii) Finger print Region (1500-667cm-1)

c) The far IR (15-200µm) (667-50cm-1)

most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region.

Wave number is mostly used measure in IR absorption because wave numbers are larger values & easy to handle than wave length which are measured in µm.

E = hν = hc/λ = hcν¯

It gives sufficient information about the structure of a compound.

In any molecule it is known that atoms or groups of atoms are connected by bonds.

These bonds are analogous to springs and not rigid in nature.

Because of the continuous motion of the molecule they maintain some vibrations with some frequency characteristic to every portion of the molecule. This is called the natural frequency of vibration.

When energy in the form of infrared radiation is applied and when,

Applied infrared frequency= Natural frequency of vibration

PRINCIPLE

There are 2 types of vibrations.

1) Stretching vibrations

2) Bending vibrations

• 1)Stretching vibrations: in this bond length is altered.

• They are of 2 types

• a) symmetrical stretching: 2 bonds increase or decrease in length.

MOLECULAR VIBRATIONS

http://en.wikipedia.org/wiki/File:Symmetrical_stretching.gif

b) Asymmetrical stretching: in this one bond length is increased and other is decreased.

2)Bending vibrations:

•These are also called as deformations.

•In this bond angle is altered.

•These are of 2 types

•a) in plane bending→ scissoring, rocking

•b) out plane bending→ wagging, twisting

http://en.wikipedia.org/wiki/File:Asymmetrical_stretching.gif

Scissoring:

This is an in plane bending.

In this bond angles are decreased.2 atoms approach each other.

Rocking:

•In this movement of atoms takes place in same direction.

http://en.wikipedia.org/wiki/File:Scissoring.gif

http://en.wikipedia.org/wiki/File:Modo_rotacao.gif

Wagging:

It is an out of plane bending.

In this 2 atoms move to one side of the plane. They move up and down the plane.

Twisting:

•In this one atom moves above the plane and the other atom moves below the plane.

http://en.wikipedia.org/wiki/File:Wagging.gif

http://en.wikipedia.org/wiki/File:Twisting.gif

NUMBER OF VIBRATIONAL MODES

A molecule can vibrate in many ways, and each way is called a vibrational mode.

If a molecule contains ‘N’ atoms, total number of vibrational modes

For linear molecule it is (3N-5)

For non linear molecule it is (3N-6)

Eg: H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.

occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion.

The frequency of the periodic motion is known as a vibration frequency.

The value of stretching vibrational frequency of a bond can be calculated by the application of hooke’s law.

ν/c = ν¯ = 1/2пc[k/m1m2/m1+m2]1/2

= 1/2пc√k/µ

Where, µ→reduced mass

m1&m2 →masses of the atoms

k →force constant

c →velocity of radiation

VIBRATIONAL FREQUENCY

http://en.wikipedia.org/wiki/Atoms

http://en.wikipedia.org/wiki/Molecule

http://en.wikipedia.org/wiki/Periodic_function

Calculated value of frequency of absorption for a particular bond is never exactly equal to its experimental value.

There are many factors which are responsible for vibrational shifts

1) Vibrational coupling:

• it is observed in compounds containing –CH2 &

-CH3.

EG. Carboxylic acid anhydrides

amides

aldehydes

Factors influencing vibrational frequencies

2) Hydrogen bonding: Hydrogen bonding brings about remarkable downward frequency

shifts.

Stronger the hydrogen bonding, greater is the absorption shift towards lower wave length than the normal value.

There is 2 types of hydrogen bonding

a) inter molecular→broad bands

b) intra molecular → sharp bands

•hydrogen bonding in O-H and N-H compounds deserve special

attention.

•Eg: alcohols&phenols

enols & chelates

3) Electronic effects:

In this the frequency shifts are due to electronic effects which include conjugation, mesomeric effect, inductive effect.

a) conjugation: conjugation lowers the absorption frequency of C=O stretching whether the conjugation is due to α,β- unsaturation or due to an aromatic ring.

b) mesomeric effect: a molecule can be represented by 2or more structures that differ only in the arrangement of electrons.

c) inductive effect: depends upon the intrinsic tendency of a substituent to either release or withdraw electrons.

There are 2 types of infrared spectrophotometer, characterized by the manner in which the ir frequencies are handled.

1) dispersive type (IR)

2) interferometric type(FTIR)

In dispersive type the infrared light is separated into individual frequencies by dispersion, using a grating monochromator.

In interferometric type the ir frequencies are allowed to interact to produce an interference pattern and this pattern is then analyzed, to determine individual frequencies and their intensities.

TYPES OF INSTRUMENTATION

DISPERSIVE INSTRUMENTS

These are often double-beam recording instruments, employing diffraction gratings for dispersion of radiation.

These 2 beams are reflected to a chopper which consists of rotating mirror.

It sends individual frequencies to the detector thermopile.

Detector will receive alternately an intense beam & a weak beam.

This alternate current flows from detector to amplifier.

INTERFEROMETRIC INSTRUMENTS THE MICHELSON INTERFEROMETER:

It is used to produce a new signal of a much lower frequency which contains the same information as the original IR signal.

The output from the interferometer is an interferogram.

Radiation leaves the source and is split.

Half is reflected to a stationary mirror and then back to the splitter.

The other half of the radiation from the source passes through the splitter and is reflected back by a movable mirror. Therefore, the path length of this beam is variable. The two reflected beams recombine at the splitter, and they interfere .

interference alternates between constructive and destructive phases.

The accuracy of this measurement system means that the IR frequency scale is accurate and precise.

FOURIER TRANSFORM IR SPECTROMETER

In the FT-IR instrument, the sample is placed between the output of the interferometer and the detector. The sample absorbs radiation of particular wavelengths.

An interferogram of a reference is needed to obtain the spectrum of the sample.

After an interferogram has been collected, a computer performs a Fast Fourier Transform, which results in a frequency domain trace (i.e intensity vs wavenumber).

The detector used in an FT-IR instrument must respond quickly because intensity changes are rapid .

Pyroelectric detectors or liquid nitrogen cooled photon detectors must be used. Thermal detectors are too slow.

To achieve 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 dispersive 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)

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)

Opaque or cloudy samples

Energy limiting accessories such as diffuse reflectance or FT-IR

microscopes

High resolution experiments (as high as 0.001 cm-1 resolution)

Trace analysis of raw materials or finished products

Depth profiling and microscopic mapping of samples

Kinetics reactions on the microsecond time-scale

Analysis of chromatographic and thermogravimetric sample

fractions

To separate IR light, a grating is used.

Grating

Light source

Detector

Sample

Slit

To select the specified IR light,

A slit is used.

Dispersion

Spectrometer In order to measure an IR spectrum,

the dispersion Spectrometer takes

several minutes.

Also the detector receives only

a few % of the energy of

original light source.

Fixed CCM

B.S.

Moving CCM

IR Light source

Sample

Detector

An interferogram is first made

by the interferometer using IR

light.

The interferogram is calculated and transformed

into a spectrum using a Fourier Transform (FT).

FTIR In order to measure an IR spectrum,

FTIR takes only a few seconds.

Moreover, the detector receives

up to 50% of the energy of original

light source.

(much larger than the dispersion

spectrometer.)

Comparison Beetween Dispersion Spectrometer and FTIR

FTIR seminar

Interferometer

He-Ne gas laser

Fixed mirror

Movable mirror

Sample chamber

Light

source

(ceramic)

Detector

(DLATGS)

Beam splitter

FT Optical System Diagram

Applications of Infrared Analysis

Pharmaceutical research

Forensic investigations

Polymer analysis

Lubricant formulation and fuel additives

Foods research

Quality assurance and control

Environmental and water quality analysis methods

Biochemical and biomedical research

Coatings and surfactants

Etc.

PARTS OF INSTRUMENTATION

• I R Radiation Source – Incandescent lamp

– Nernst Glower

– Globar Source

– Mercury Arc

• Sample Cells & Sampling Substances

– Sampling of solids • Solids run solution

• Solid films

• Mull technique

• Pressed pellet technique

– Sampling of Liquids

– Sampling of Gases

• Detectors – Bolometers

– Thermocouple

– Thermistors

– Golay Cells

– Photoconductivity cell

– Semiconductor

– Pyroelectric detectors

Monochromators

sudheerkumar kamarapu 2/5/2013 32

I R Radiation Sources

Incandescent Lamps

• ordinary lamp used

• glass enclosed

Disadv.

• fails in far infrared

• low spectral emissivity


Nernst Glower

Composed of rare earth oxides such as Zirconia, Yttria & Thoria

Non conducting at room temperature

Heating

Conducting state

Provides radiation of about 7100 cm-1

Disadv.

Emitts I R radiation over wide wavelength range

Frequent mechanical failure

Energy concentrated in visible & near I R region of spectrum

WO

RK

ING


Globar Source

• Self starting, Controlled conveniently with variable

transformer

Works at wavelength longer than 650 cm-1 (0.15µ)

5200 cm-1 radiation given at 1300 – 1700 OC

Disadv.

Less intense source than Nernst Glower


Mercury Arc

Special high pressure mercury lamps are used in far I R

Beckman devised the Quartz Mercury Lamps in unique manner

shorter wavelength ------- heated quartz envelope provides radiation

longer wavelength -------- mercury plasma provides radiation


MONOCHROMATORS

• They convert polychromatic light into mono chromatic

light.

• They must be constructed of materials which transmit

the IR.

• They are of 3 types.

• a) metal halide prisms

• b) NaCl prisms

• c) gratings


a) metal halide prisms:

• prisms which are made up of KBr are used in the

wavelength region from 12-25µm.

• LiF prisms are used in the wavelength region from

0.2-6µm.

• CeBr prisms used in wavelength region from 15-38µm.

b) NaCl prisms:

• Used in the whole wave length region from 4000-

650cm-1.

• they have to be protected above 20•c because of

hygroscopic nature.

c) gratings:

• They offer better resolution at low frequency than prisms.


• Sample cells made up of alkali halides like NaCl or KBr are

used.

• Aqueous solvents cannot be used as they dissolve alkali halides.

• Only organic solvents like chloroform is used.

• IR spectroscopy has been used for the characterization of solid,

liquid, gas samples.


Solids run in solution

Solids dissolved in a aqueous solvent

Placed over the alkali metal disk

Solvent is allowed to evaporate

Thin film of solute formed

Entire solution is run in one of the cells for liquids

Note :

This method not used because suitable number of solvents available are

less.

Absorption due to solvent has to be compensated by keeping the solvent

in a cell of same thickness as that containing the reference beam of

double beam spectrometer. sudheerkumar kamarapu 2/5/2013 40

Solid Films

• Technique used for Amorphous sample.

• Deposited on the KBr / NaCl cell by evaporation of solution.

• Only useful for rapid qualitative analysis.

Mull Technique

• Finely ground solid sample is used.

• Mixed with Nujol (mineral oil)

• Thick paste is made.

• Spread between I R transmitting windows

• Mounted in path of I R beam &

• The spectrum is run.

Disadv. Nujol has the absorption maxima at 2915, 1462, 1376 & 719 cm-1


Pressed Pellet Technique

• Finely ground sample used

• Potassium Bromide is mixed (100 times more)

• Passed through a high pressure press

• Small pellet formed (1-2 mm thick, 2cm diameter)

• The pellet is transparent to I R radiation & is run as such

Adv.

• Pellet can be stored for long period of times.

• Concentration of sample can be adjusted in KBr pellet hence used

for quantitative analysis.

• Resolution of spectrum is superior.

Disadv.

• Always has a band at 3450 cm-1 (moisture OH-)

• At high pressure polymorphic changes occur

• Unsuccessful for polymer which are difficult to grind with KBr.


Diffuse Reflectance

• Sometimes referred to as DRIFTS (diffuse reflectance infrared

Fourier transform spectroscopy)

• Involves irradiation of the powdered sample by an infrared

beam.

• The incident radiation undergoes absorption, reflection, and

diffraction by the particles of the sample.

• Only the incident radiation that undergoes diffuse reflectance

contains absorptivity information about the sample.


Microspectroscopy

• The ultimate sampling technique, since only one particle

is required for analysis.

• Particles of interest must be greater in size than 10 X 10 μm.

• Sample placed on an IR optical window and the slide is

placed onto the microscope stage and visually

inspected

• Once the sample of interest is in focus, the field of view

is apertured down to the sample.

• Depending on sample morphology, thickness, and

transmittance properties, a reflectance and/or

transmittance IR spectrum may be acquired by the IR

microscope accessory.


Attenuated Total Reflectance

• The basic premise of the technique involves

placing the sample in contact with an infrared

transmitting crystal with a high refractive index.

• The infrared beam is directed through the

crystal, penetrating the surface of the sample,

and displaying spectral information of that

surface.

• Advantage of this technique is that it requires

very little sample preparation,

• Simply place the sample in contact with the

crystal sudheerkumar kamarapu 2/5/2013 45

Photoacoustic

• The PAS phenomenon involves the selective absorption

of modulated IR radiation by the sample.

• Once absorbed, the IR radiation is converted to heat

and subsequently escapes from the solid sample and

heats a boundary layer of gas.

• The increase in temperature produces pressure changes

in the surrounding gas.

• The pressure changes in the coupling gas occur at the

frequency of the modulated light, as well as the acoustic

wave.

• This acoustical wave is detected by a very sensitive

microphone and the subsequent electrical signal is

Fourier processed and a spectrum produced.


• Liquid samples taken.

• Put it into rectangular cells of KBr, NaCl etc.

• I R spectra obtained.

• Sample thickness … such that transmittance lies

between 15 – 20 % i.e., 0.015 – 0.05 mm in thickness.

• For double beam, matched cells are generally employed

• One cell contains sample while other has solvent used

in sample.

• Matched cells should be of same thickness, protect

from moisture. sudheerkumar kamarapu 2/5/2013 47

• Small size particles hence the cells are large.

• 10 cm to 1m long

• Multiple reflections can be used to make the effective path

length as long as 40 cm

• Lacks sensitivity


DETECTORS

They convert the radiation into electrical signal. Two Types Of Detectors

Thermocouple Bolometers Thermistors Golay Detectors Pyroelectric Detectors

Photon Detectors Thermal Detectors

Semiconductors Photovoltaic Intrinsic

Detectors Photoconductive

Intrinsic Detectors


Thermocouples

Based upon the fact that

an electrical current will flow when two dissimilar metal wires

are connected together at both ends and a temperature

differential exists between the two ends

Example : Bismuth & Antimony


Bolometer

• It consists of thin metallic conductor, its resistance changes due to increase in temperature when IR radiation falls on it.

• It is a electrical resistance thermometer which can detect and measure feeble thermal radiation.

• The electrical resistance increases approx 0.4% for every celsius degree increase of temperature .

• The degree of change in resistance is regarded as the measure of the amount of IR radiation falling on it.

• A bolometer is made of two platium strips, covered with lamp black, one strip is sheilded from radiation and one exposed to it. The strips formed two branches of wheatstone bridge


Working – The circuit thus effectively operating as resistance temperature detector. When IR radiations falling on the exposed strip would heat it, and change the resistance, this causes current to flow, the amount of current flowing is a measure of intensity of IR radiation The response time is 4 secs.


Golay Cells

• It consist of a small metal detector closed by a rigid blackened metal plate (2 mm), flexible silvered diaphragm at the other end filled with Xenon gas.

• Its response time is 20 msec, hence faster than other thermal detectors

• It is suitable for wavelengths greater then 15 u


Working • The radiation falls on the blackened metal plate, this heats

the gas which lead to deformation of flexible silvered diaphragm.

• The light from a lamp inside the detector is made to fall on the diaphragm which reflects the light on to a phototube.

• The signal seen by the phototube / photocell is modulated in accordance with the power of the radiation beams incident on the gas cell.


Thermistors • It is made up of metal oxides. • It functions by changing resistance when heated. • It consists of two closely placed thermistor flakes, one of the 10

um is an active detector, while the other acts as the compensating / reference detector.

• A steady voltage is applied, due to the temperature increase there is change in resistance which is measured and this gives the intensity of the IR radiation


Pyroelectric Detectors • It consist of a thin dielectric flake on the face of which

an electrostatic charge appears. When the temperature of the flake changes upon exposure to IR radiations, electrodes attached to the flake collect the charge creating a voltage.

• The most common is TGS (Triglycine Sulfate) however its response deteriorates above 45 C and is lost above the 49 C

• Detureated triglycine Sulfated are available and can be used at room temperature.


PHOTON DETECTORS

• These detectors convert photons directly into free current carriers by photo exciting electrons across the energy band gap of the semiconductor to the conduction band. This produces a resistance change in the detectors.

• This photon excitation is caused by radiation interacting directly with the lattice sites.


Semiconductors

• These act as insulators but when radiation fall on them, they become conductors.

• Exposure to radiation causes a rapid response to the IR signal. • Working – An IR photon displaces an electron in the detector

which excites electrons to move from the valence band to the higher energy conduction band.

• Semiconductor materials are Telluride, Indium, Antimonide & Germanium.


Photovoltaic intrinsic detectors • Under IR radiation, the potential barrier of the P N junction leads

to the photovoltaic effect. An incident photon with the energy greater the energy band gap of the junction generates electron hole pairs and the photocurrent is excited.

• The amount of the photon excited current is denoted by photocurrent.

• The highest performance PV detectors are fabricated from Si, Ge, As, In & Sb.


Photoconductive Intrinsic Detectors

• This is non thermal detector of greater sensitivity. • It consists of a thin layer of lead sulfide supported on gas

envelope. When IR radiation is focused on the lead sulfide its conductance increases and causes more current to flow.

• It has high sensitivity and good speed of about 0.5 msec • Upon drastic cooling the range can be broadened. • PC detectors include Germanium and Silicon detectors.


COMPARISON


Identification of organic and inorganic compounds

by IR Spectroscopy (Interpretation of Spectra)

2/5/2013 sudheerkumar kamarapu 62

IR source sample prism detector

graph of % transmission vs. frequency

=> IR spectrum

4000 3000 2000 1500 1000 500

v (cm-1)

100

%T

0


toluene


Intensity: Transmittance (T) or %T

T = I

I0

Absorbance (A)

A = log I

I0

Intensity in IR

IR : Plot of %IR that passes through a sample (transmittance) vs Wavelenght


Infrared

• Position, Intensity and Shape of bands gives clues on Structure of molecules

• Modern IR uses Michelson Interferometer => involves computer, and Fourier Transform

Sampling => plates, polished windows, Films … Must be transparent in IR

NaCl, KCl : Cheap, easy to polish

NaCl transparent to 4000 - 650 cm-1

KCl transparent to 4000 - 500 cm-1

KBr transparent to 400 cm-1


Infrared: Low frequency spectra of window materials


Bond length and strength vs

Stretching frequency

Bond C-H =C-H -C-H

Length 1.08 1.10 1.12

Strenght 506 kJ 444 kJ 422 kJ

IR freq. 3300 cm-1 3100 cm-1 2900 cm-1


Calculating stretching frequencies Hooke’s law :

n = 1

2pc K m

n : Frequency in cm-1

c : Velocity of light => 3 * 1010 cm/s

K : Force constant => dynes /cm

m:masses of atoms in grams

m=m1 m2

m1 + m2 =

M1 M2

M1 + M2 (6.02 * 1023)

n = 4.12 K m

C—C K = 5* 105 dynes/cm

C=C K = 10* 105 dynes/cm

CC K = 15* 105 dynes/cm


Calculating stretching frequencies

C=C K = 10* 105 dynes/cm n = 4.12 K m

m=M1 M2

M1 + M2 =

(12)(12)

12 + 12 =6 n= 4.12 10* 105

6= 1682 cm-1

n Experimental 1650 cm-1

C—H K = 5* 105 dynes/cm n= 4.12 5* 105

.923= 3032 cm-1

m=M1 M2

M1 + M2 =

(12)(1)

12 + 1 =0.923

n Experimental 3000 cm-1

C—D K = 5* 105 dynes/cm n= 4.12 5* 105

.923= 2228 cm-1

m=M1 M2

M1 + M2 =

(12)(2)

12 + 2 =1.71

n Experimental 2206 cm-1 2/5/2013 sudheerkumar kamarapu 70

Vibrations www.cem.msu.edu/~reusch/Virtual/Text/Spectrpy/InfraRed/infrared.htm

Modes of vibration

C—H Stretching Bending C

O

H

H

H

Symmetrical 2853 cm-1

H

H

Asymmetrical 2926 cm-1

H

H

H

H

Scissoring 1450 cm-1

Rocking 720 cm-1

H

H

H

H

Wagging 1350 cm-1

Twisting 1250 cm-1

Stretching frequency

Bending frequency


Vibrations www.cem.msu.edu/~reusch/Virtual/Text/Spectrpy/InfraRed/infrared.htm

General trends:

•Stretching frequencies are higher than bending frequencies

(it is easier to bend a bond than stretching or compresing them)

•Bond involving Hydrogen are higher in freq. than with heavier atoms

•Triple bond have higher freq than double bond which has higher freq than single bond


Structural Information from Vibration Spectra

• Spectrum can be treated as finger print to recognize the product of a reaction as a known compound. (require access to a file of standard spectra)

• At another extreme , different bands observed can be used to deduce the symmetry of the molecule and force constants corresponding to vibrations.

• At intermediate levels, deductions may be drawn about the presence/absence of specific groups

The symmetry of a molecule determines the number of bands expected

Number of bands can be used to decide on symmetry of a molecule

Tha task of assignment is complicated by presence of low intensity bands and presence of forbidden overtone and combination bands. There are different levels at which information from IR can be analyzed to allow identification of samples:


Methods of analyzing an IR spectrum The effect of isotopic substitution on the observed spectrum Can give valuable information about the atoms involved in a particular vibration

1. Comparison with standard spectra : traditional approach

2. Detection and Identification of impurities

if the compound have been characterized before, any bands that are

not found in the pure sample can be assigned to the impurity (provided that the 2 spectrum are recorded with identical conditions: Phase,

Temperature, Concentration)

3. Quantitative Analysis of mixture

Transmittance spectra = I/I0 x 100 => peak height is not

lineraly related to intensity of absorption

In Absorbance A=ln (Io/I) => Direct measure of intensity 2/5/2013 sudheerkumar kamarapu 74

Analyzing an IR spectrum

In practice, there are similarities between frequencies of molecules containing similar groups.

Group - frequency correlations have been extensively developed for organic compounds and some have also been developed for inorganics


Some characteristic infrared absorption frequencies BOND COMPOUND TYPE FREQUENCY RANGE, cm-1 C-H alkanes 2850-2960 and 1350-1470 alkenes 3020-3080 (m) and RCH=CH2 910-920 and 990-1000 R2C=CH2 880-900 cis-RCH=CHR 675-730 (v) trans-RCH=CHR 965-975 aromatic rings 3000-3100 (m) and monosubst. 690-710 and 730-770 ortho-disubst. 735-770 meta-disubst. 690-710 and 750-810 (m) para-disubst. 810-840 (m) alkynes 3300 O-H alcohols or phenols 3200-3640 (b) C=C alkenes 1640-1680 (v) aromatic rings 1500 and 1600 (v) C≡C alkynes 2100-2260 (v) C-O primary alcohols 1050 (b) secondary alcohols 1100 (b) tertiary alcohols 1150 (b) phenols 1230 (b) alkyl ethers 1060-1150 aryl ethers 1200-1275(b) and 1020-1075 (m) all abs. strong unless marked: m, moderate; v, variable; b, broad


IR spectra of ALKANES

C—H bond ―saturated‖

(sp3) 2850-2960 cm-1

+ 1350-1470 cm-1

-CH2- + 1430-1470

-CH3 + ― and 1375

-CH(CH3)2 + ― and 1370, 1385

-C(CH3)3 + ― and 1370(s), 1395 (m)


n-pentane

CH3CH2CH2CH2CH3

3000 cm-1

1470 &1375 cm-1

2850-2960 cm-1

sat’d C-H


CH3CH2CH2CH2CH2CH3

n-hexane


2-methylbutane (isopentane)


2,3-dimethylbutane


cyclohexane

no 1375 cm-1

no –CH3


IR of ALKENES

=C—H bond, ―unsaturated‖ vinyl

(sp2) 3020-3080 cm-1

+ 675-1000

RCH=CH2 + 910-920 & 990-1000

R2C=CH2 + 880-900

cis-RCH=CHR + 675-730 (v)

trans-RCH=CHR + 965-975

C=C bond 1640-1680 cm-1 (v)


1-decene

910-920 &

990-1000

RCH=CH2

C=C 1640-1680

unsat’d

C-H

3020-

3080

cm-1


4-methyl-1-pentene

910-920 &

990-1000

RCH=CH2


2-methyl-1-butene

880-900

R2C=CH2


2,3-dimethyl-1-butene

880-900

R2C=CH2


IR spectra BENZENEs

=C—H bond, ―unsaturated‖ ―aryl‖

(sp2) 3000-3100 cm-1

+ 690-840

mono-substituted + 690-710, 730-770

ortho-disubstituted + 735-770

meta-disubstituted + 690-710, 750-810(m)

para-disubstituted + 810-840(m)

C=C bond 1500, 1600 cm-1


ethylbenzene

690-710,

730-770

mono-

1500 & 1600

Benzene ring

3000-3100 cm-1

Unsat’d C-H


o-xylene

735-770

ortho


p-xylene

810-840(m)

para


m-xylene

meta

690-710,

750-810(m)


styrene

no sat’d C-H

910-920 &

990-1000

RCH=CH2

mono

1640

C=C


2-phenylpropene

mono 880-900

R2C=CH2

Sat’d C-H


p-methylstyrene

para


IR spectra ALCOHOLS & ETHERS

C—O bond 1050-1275 (b) cm-1

1o ROH 1050

2o ROH 1100

3o ROH 1150

ethers 1060-1150

O—H bond 3200-3640 (b)


1-butanol

CH3CH2CH2CH2-OH

C-O 1o

3200-3640 (b) O-H


2-butanol

C-O 2o

O-H


tert-butyl alcohol

C-O 3o O-H


methyl n-propyl ether

no O--H

C-O ether


2-butanone

C=O

~1700 (s)


C9H12

C-H unsat’d &

sat’d

1500 & 1600

benzene

mono

C9H12 – C6H5 = -C3H7

isopropylbenzene

n-propylbenzene? 2/5/2013 sudheerkumar kamarapu 102

n-propylbenzene


isopropyl split 1370 + 1385

isopropylbenzene


C8H6

C-H

unsat’d 1500, 1600

benzene

mono

C8H6 – C6H5 = C2H

phenylacetylene

3300

C-H


C4H8

1640-

1680

C=C

880-900

R2C=CH2

isobutylene CH3

CH3C=CH2

Unst’d


Which compound is this? a) 2-pentanone b) 1-pentanol c) 1-bromopentane d) 2-methylpentane

1-pentanol


What is the compound? a) 1-bromopentane b) 1-pentanol c) 2-pentanone d) 2-methylpentane

2-pentanone



H2C C

HCH2

CH3

CH3CH3CH2CH2CH2CH3

H2C

H2C

CH2CH2CH2CH3

biphenyl allylbenzene 1,2-diphenylethane

o-xylene n-pentane n-butylbenzene

A

B

C

D

E

F

In a ―matching‖ problem, do not try to fully analyze each spectrum. Look

for differences in the possible compounds that will show up in an infrared

spectrum.


1


2


3


4


5


6


References : Lena Ohannesian, Antony J. Streeter; Handbook of Pharmaceutical Analysis; Marcel Dekker, Inc.; Reprint 2002 Chatwal and Anand ; Instrumental methods of chemical analysis;

fifth edition; page no-2.43-46 Spectrometric identification of organic compounds, R M

Silverstein,T.C morril G.C. bassler Fifth edition, p.no.99-100 Internet : www.wikipedia.com www.answers.com www.authorstream.com www.slideworld.com www.google.com



Technology

Ir spectroscopy sud mpharm pdf