INFRARED SPECTROSCOPY (IR) M.pharm PDF Downlaod

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SPECT ROS
PRESENTED BY

UTTAM PRASAD PANIGRAHY
M.PHARM

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CONTENTS
1. INTRODUCTION
2.PRINCIPLE
3. THEORY-MOLECULAR
VIBRATION
4. INSTRUMENTATION
>. IMPORTANT FEATURES
6.APPLICATIONS

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Definition:

e It is the study of absorption of infrared radiation
which results in vibrational transitions.
e IR spectrum is an important record which gives
sufficient information about the structure of a
compound and also determine the functional group.

IR spectroscopy is an useful tool to
identify functional groups in organic molecules
IR spectroscopy is a result of molecular vibrational
transitions that occur when light interacts with matter
Molecules are always vibrating For a molecule to be IR
active, the vibrations should give rise to a net change
in dipole moment Infrared spectroscopy

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The absorption of IR radiations can be expressed either
in terms of wavelength()) or in wave number ( v ).
Relationship between wavelength(,) and wave number
(v).

wave number( v )= 1/ wavelength(A) in cm

suppose wavelength()) is 2.5 » = 2.5x 10-4 cm, then

wave number( v )= 1/ 2.5x 10°* cm=4000 2.5 10-4 cm

ium =10°m;

cm+= no. of waves per cm of path
= 1/[ (cm)]
o energy of wave

E=hv= hco
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Infrared region

LIMIT OF RED LIGHT: 800 nm, 0.8 um,
12500 cm:!

NEAR INFRARED: 0.8 -2.5 um, 12500 – 4000
cm-!

MID INFRARED: 2.5 – 25 um, 4000 – 400 cm*

FAR INFRARED: 25 – 1000 um, 400 – 10 cm?

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Figure 4-7: Typical Infrared spectrum

4400 3900 3400 2900 2400 1900 1400 300 400

Energy, cm-1

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% Transmittance

 

my
~— co

ae

IR radiation does not have enough energy to induce
electronic transitions as seen with UV.

Absorption of IR is restricted
to compounds with small energy differences in the
possible vibrational and rotational states. For a molecule
to absorb IR, the vibrations or rotations within a
molecule must cause a net change in the dipole moment
of the molecule.

The alternating electrical field
of the radiation (remember that electromagnetic
radiation consists of an oscillating electrical field and an
oscillating magnetic field, perpendicular to each other)
interacts with fluctuations in the dipole moment of the
molecule. If the frequency of the radiation matches the
vibrational frequency of the molecule then radiation will
be absorbed, causing a change in the amplitude of
molecular vibration.

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Theory of infra red absorption

IR radiation does not have enough energy to induce electronic
transitions as seen with UV. Absorption of IR is restricted to
compounds with small energy differences in the possible
vibrational and rotational states. For a molecule to absorb IR,
the vibrations or rotations within a molecule must cause a net
change in the dipole moment of the molecule.

The alternating electrical field of the radiation (remember that
electromagnetic radiation consists of an oscillating electrical field
and an oscillating magnetic field, perpendicular to each other)
interacts with fluctuations in the dipole moment of the molecule.
If the frequency of the radiation matches the vibrational
frequency of the molecule then radiation will be absorbed,
causing a change in the amplitude of molecular vibration.

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Molecular rotations

Rotational transitions are of little use to the spectroscopist. Rotational levels
are quantized, and absorption of IR by gases yields line spectra. However, in
liquids or solids, these lines broaden into a continuum due to molecular
collisions and other interactions.

Molecular vibrations

The positions of atoms in a molecules are not fixed; they are subject to a
number of different vibrations. Vibrations fall into the two main catagories of
stretching and bending.
Stretching: Change in inter-atomic distance along bond axis

Stretching vibrations

x
Symmetnc Asymmetric

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Bending: Change in angle between two bonds. There are four types of
bend:
*Rocking
*Scissoring
*-Wagging
* Twisting Bending vibrations

Near Near Near

RAR
in-plane rocking = —In-plane scissonng © Out-of-plane wagging + Out-of-plane twisting

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Vibrational coupling
In addition to the vibrations mentioned above, interaction between
vibrations can occur (coupling) if the vibrating bonds are joined to a
single, central atom. Vibrational coupling is influenced by a number of
factors;

1.Strong coupling of stretching vibrations occurs when there is a
common atom between the two vibrating bonds

2.Coupling of bending vibrations occurs when there is a common bond
between vibrating groups

3.Coupling between a stretching vibration and a bending vibration occurs
if the stretching bond is one side of an angle varied by bending vibration

4.Coupling is greatest when the coupled groups have approximately
equal energies

5.No coupling is seen between groups separated by two or more bonds

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Factors Affecting Frequency of Absorption
Bond strength
C=O stretching (1700 cm-1) vs C-O stretching (1200 cm-1)
C=C stretching (1650 cm-1) vs C-C stretching (1200 cm-1)
It takes more IR energy to stretch short strong bonds than it
does to stretch long weak bonds
It also takes more energy to stretch a bond between two heavy
atoms
than it does if the atoms are less massive
Atomic Size
C-H (3000 cm-1)
C-C (1200 cm-1)
C-Cl (800 cm-1)
C-Br (550 cm-1)
Bigger masses vibrate at lower energy

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¢ As a bond stretches, the atoms are moved
apart from each other
¢ If the bond elongation changes the net
dipole moment of the molecule, an IR peak
is manifested
Examples of large and small peaks
¢ Large peaks are observed for C=O bonds
¢ Small peaks are observed for C=C bonds
¢ If the atoms that stretch have different
electro negativities, you are likely to see
larger peaks

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ple few double

tri
ee

eg.0-H bonds |

eg.C:C bands | bonds single bonds

egC-C C0

NH CN ¥ C=N CN CX
CH =(

3000 2500 2000 1500 1000 500

Wavenwnbers / cm’! Bond strength
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3jodip ul asuey)

 

Base Values for
major bond stretching

absorptions
4000 2500 1800 1600 400

*~—H tiple bond double bond | fingerprint region

O—H —C¢ =C — —cC=0 —¢ —-O—

N—H + c=n | ae ec «Cc —N —

* | 7 | |
2250 Lf iS |

1100
3600 1650

3500 21430
3000

Ranges are broad, not exact
e Peaks are generally broad, not sharp
e Exact frequency depends upon
— conjugation
—- proximity effects

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Instrumentation

2 Types of IR spectrometer:
» Dispersive Grating Spectrophotometers
» Multiplex Instruments employing Fourier Transform (FTIR)

As infrared is an absorption technique, infrared radiation passes through the
sample. and is detected. A dispersive spectrometer works as follows:

pee pce JS A sutdida > _mon ochromator
a i

o

IR > > -A- wha www www nw ene n enna =,

@::! reference ‘. ee »
source ~~ * =| chopper sae

Mer anram – > – —-1.——- J Pigs

sample | S@mple —@}——- gins |

compartment IR detector
@1905 CHP

NOTE: It is the same configuration as a UV/Vis. absorption spectrometer except
the light passes through the sample before reaching the monochromator.

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Simplified Infrared pectrophotomety

focusing plates

mirror :
/ Detection Electronics

Es & and Computer

> 3
: K
Determines Frequencies

of Infrared Absorbed and
Infrared plots them on a chart
Source v

|

Infrared
Sample | Spectrum

arate | aman intensity of
Absorption — | eevee

“peaks”
>

| equa cy
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Sources
e Tungsten incandescent lamp —- black body source for
measurements in NIR
e Nichrome (or rhodium) wire — Coiled, heated by resistance to
incandescence.
Black oxide layer forms on surface. Temperature 1100°C. Requires little
maintenance and no cooling required. Emits in Mid-IR but less power
than other
sources. Cheaper instruments
e Nernst Glower (rare earth oxides) — More intense emitted
radiation.
Constructed from mixture of fused oxides of Zr, Th and Cs. Non-
conducting at ambient temperatures but at temperatures >800 °C it is
electrically conducting, maintains high temperature by resistive heating.
Good energy output (intensity 2x nichrome wire or globar)
e Globar — A rod of silicon carbide 6-8 mm in diameter.
Characteristics between
nichrome wire and Nernst Glower. Self starting and operates at 1300
°C. Globar
must be water cooled – brass jacket surrounds globar.
e Carbon Dioxide Laser — Useful for narrow radiation bands

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Sources

Nernst Glower emits most IR radiation in Mid-IR, tungsten lamp effective
for NIR. Beyond 50 mm black body radiators lose effectiveness.
High pressure arc sources, such as Hg lamp housed in a quartz jacket
are used

W filament lamp 1100 K 0.78 – 2.5 um

Globar Heated SiC rod (—1300 KF) 1-10 um

bs : Heated rare ea rth oxide
Nermst Glower rod (~1500 K) 1-10 .1m

He arc lamp Plasma >S50O pum

CO, laser Stimulated emission lhnes 9-11 um

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Instrumentation-Components
¢ Sample Cells and Preparation
e Solids
e Mull – suspend ground solid in oil of similar refractive index
(Nujol, perfluorocarbon)
e KBr Pellet – few mg sample + 0.5 to 1 g dry KBr ground +
compressed at very high pressure
e Disposable polyethylene film strips (dissolve solid in volatile
solvent,
“paint” on the film or on a salt plate)
e Liquids
e Gases
¢Optics – dessicated salts such as NaCl, CsBr, LiF, KBr and front
Surface mirrors. Glass and quartz lenses cannot be used because
they absorb IR radiation
¢Chopper (modulation and tuned amplifier)

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Most flexible system for analyzing all 3 states
of matter (solid, liquid, gas)
“Neat” (analysis of liquids/oils)
Pellet (analysis of solids)
Thin Cell (analysis of dissolved solid samples –
solutions)
Long Cell (analysis of gases)

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Lhe el Le

 

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Sample cells

To obtain an IR spectrum, t
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These plates are made of salt
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onment

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Preparing a Sample to Measure an IR Spectrum

 

 

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Instrumentation – Detectors
¢ Thermal Detectors

IR radiation impinging on detector — thermal change. Active element is
small as possible to maximise temperature change. When radiation
ceases the element returns to the ambient temperature. Decay time is
determined by the finite thermal conductance of the insulator.

» |hermocouple — 2 dissimilar materials (Bismuth/Antimony). Hot
junction produces voltage change «= incident radiation.

» Thermopile — Combination of 6 thermocouples in series so that
outputs are additive. ‘Hot’ junctions act as active elements, ‘cold’
junctions act as reference. Simplest device for converting thermal
energy into electrical signal

»s |hermistor (Bolometer) — Functions by changing resistance on
heating

=» Golay detector — Pneumatic detector, uses expansion of s, |, g as
the means of measurement. (Not often used)

s Pyroelectric detector — Most sensitive, thermal detector of choice.
Certain crystals possess temperature senstivie dipole moments;
when placed between metal plates have a temperature sensitive
capacitor (e.g. Triglycine Sulfate TGS). Measures rate of change of
temperature.

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¢ Photon Detectors
More sensitive detectors rely on quantum interaction between incident photons and a

semiconductor. Results in production of electrons + holes — internal photoelectric
effect. Energy from the photon striking e- in detector raises electron from non-
conducting (valence) band to a conductance state (conduction band). Energy
required to promote e is a definite minimum energy photon — hence sharp cut-
off towards far IR

‘ Indium antimony (InSb) — p-n junction type photodiodes
. PbSnTe — operated at liquid N, temp, extended sensitivity
° Mercury cadmium telluride (MCT) detector — more efficient than PbSnTe. 5
times more sensitive than pyroelectric detector. Response times as low as 20 ns

Thermoelectric effect –
Thermocouple dissimilar metal junction Cheap, slow, insensitive

Bolometer a Highly sensitive (<400cem”)
at1SK

7 islycine sulfat =
Pyroelectric bs cee Fast and sensitive (mid IR)

piezoelectric

PbSnTe, HgCdTe light
Photoconducting sensitive resistance Fast and sensitive

thermometer 77 K
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b) Nernst glower

ceramic holder
5 are Ys5O3;
———————— ._, FnOsz,

ZrO,
aux. heated up
heate to 1500°C

2 -S cm

———————— t leads

cement
—<—— >

1 – 3 mm dia.

Fias – temp coefficient.
of resistance.

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cc) Globar

Si@WC rod
Heated

S c t@
m 2
1300°C

jw ater-

Cool
+ i
t e
e c i
m
p

coe j
ff o
. r
ass

of
<—_=> ‘tube

r
esistance So – S mm dia. VWith slot

Cs 1S ptm IN G LO ptm ING
6SO 1

cm pu
! m

Cc LOOO cm! LOOOO cm!

 

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Photon detectors Electrical leads

|

T+)

e- promoted from valence band
to unfilled conduction band,
causing e- hole pair formation. Liquid Ny ~_ a
No. of pairs depends on light
intensity.
photovoltaic:
pd caused by separation of e-
hole pairs between n,p layer. vessel

photoconductive:
R changes with radiation power,
for semiconductor. Det c

e tor
on heat | R-transmitting
sink window

photoelectromagnetic:
utilise Hall Effect in
semiconductor. FAR Oe Oe eww Oe Oe EEO ES OE OEE ES ES OE REESE ES OEE ER EE OE OE RES ES REE ew EN ee rrr tet iet t ttt te tt it

‘Detectortype A (1!)
1d CETTE EERE EET PE RTE ET EE EERE ET ET EE RETET TREE ESET ET EEN ET ET ETE ET ES EE RERES FEET OES ES EE RERET ES EERE ET ET RENEE TET EER ET ET RE RERET ET ETRETET ET ETTET ET PERET ET ET ETRES ET ET RES ES ES DER EN ES

 

Schematic of semiconductor defector ‘si 0.2-1.1
Ge 0.4-1.8

Radiation |

hy ‘|inAs 1.0 – 3.8

Wm | mre Cv InSb 1.0-7.0

Vt ae “—p-n junction

Wem | p-type
i
InSb(77K)—/1. 0- 5.6

HgCdTe (77K) 1.0 -25.0

rrr ir i Fn bebe os Re eee es he eee be es ee eseee es esetees eee

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Look for C=O peak (1820-1660 cm‘)
If C=O check for OH (3400-2400 cm!)

indicates carboxylic acid

If C=O check for NH (3500 cm‘)
indicates amide

If C=O check for C-O (1300-1000 cm?)
indicates ester

If no OH, NH or C-O then ketone

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hin =m e =) AF – SAR res
yy ee & So & awe oS & =S s

If no C=O check for OH (3600-3300 cm?)
indicates alcohol

If no C=O check for NH (3500 cm?)
indicates amine

If no C=O & no OH check C-O (1300 cm?)
indicates ether

Look for C=C (1650-1450 cm!) then
aromatic

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Wave Number, cm

4000 $000 2900 2000 1500 1300 1200 1100

Wavelength, microns

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100

‘ A-CO-OH stretch (3000)

B – CH stretch (2800)

i C – C=O ester (1757)

| D – C=O carboxy (1690)

E – C=C aromatic (1608)

– | F – C=C aromatic (1460)

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Wvave Number, om’

4900 3000 2500 2000 190) «61901801100 1000 S00

Wavelength microre

A)C O (1730) B) C= C aromatic (1590) C) C-H aromatic (3050)

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Qualitative “fingerprint” check for
identification of drugs
Used for screening compounds and
rapid identification of C=O groups
Can be used to characterize samples in
Solid states (creams and tablets)
Can detect different crystal isoforms
(polymorphs)
Water content measurement

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Ah PRA A ~~ Ns e ~ ~ € —~
~~

» Analysis of urine and other biofluids
(urea, creatinine, protein)

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/ a“ ~«~ ~{_ 7 – ~ _ —

– oe
a — — = SS —_— = ——— – _-_—

Used in non-invasive measurement of
glucose

 

FT/IR-550

 

CHALCOGENIDE OPTICAL FIBER

YY om . ate

CHALCOGENIDE®.
ATR PROSE

CHALCOGENIDE OPTICAL FIBER = [—6 —_dee— —t—- |

 

CHALCOGENIDE ATR PRISM

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Quality control of pharmaceutical
formulations
Determination of particle size
Determination of blend uniformity
Determination or identification of
polymorphic drugs

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