INFRARED SPECTROSCOPY

BYDr. Suman PattanayakAssociate ProfessorDepartment of Pharma Analysis & QA.

Vijaya Institute of Pharmaceutical Sciences for Women

IV B. Pharm/ I SemPharmaceutical Analysis

CONTENTS1. INTRODUCTION2.PRINCIPLE3. THEORY-MOLECULAR VIBRATION4. INSTRUMENTATION5.IMPORTANT FEATURES6.APPLICATIONS

Definition:

● It is the study of absorption of infrared radiation which results in vibrational transitions.● 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 moleculesIR spectroscopy is a result of molecular vibrationaltransitions that occur when light interacts with matterMolecules are always vibrating For a molecule to be IR active, the vibrations should give rise to a net change in dipole moment Infrared spectroscopy

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

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

suppose wavelength() is 2.5 = 2.5× 10-4 cm, then wave number( )= 1/ 2.5× 10-4 cm=4000 2.5× 10-4 cm

  1m = 10-6 m ; cm-1 = no. of waves per cm of path

= 1/[ (cm)] energy of wave

E = h = hc

Infrared region LIMIT OF RED LIGHT: 800 nm, 0.8 m, 12500 cm-1

NEAR INFRARED: 0.8 -2.5 m, 12500 - 4000 cm-1

MID INFRARED: 2.5 - 25 m, 4000 - 400 cm-1

FAR INFRARED: 25 - 1000 m, 400 - 10 cm-1

Principle: 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.

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.

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

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

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

Factors Affecting Frequency of AbsorptionBond strengthC=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 atomsthan it does if the atoms are less massiveAtomic SizeC-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

♦ 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 manifestedExamples 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

Ranges are broad, not exact• Peaks are generally broad, not sharp• Exact frequency depends upon– conjugation– proximity effects

Detection Electronics and Computer

InfraredSource

Determines Frequenciesof Infrared Absorbed andplots them on a chart

Sample

Simplified Infrared SpectrophotometerNaClplates

Absorption “peaks”

Infrared Spectrum

frequency

intensity ofabsorption

(decreasing)

focusingmirror

Sources• Tungsten incandescent lamp – black body source for measurements in NIR• Nichrome (or rhodium) wire – Coiled, heated by resistance to incandescence.Black oxide layer forms on surface. Temperature 1100°C. Requires littlemaintenance and no cooling required. Emits in Mid-IR but less power than othersources. Cheaper instruments• 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)• Globar – A rod of silicon carbide 6-8 mm in diameter. Characteristics betweennichrome wire and Nernst Glower. Self starting and operates at 1300 °C. Globarmust be water cooled – brass jacket surrounds globar.• Carbon Dioxide Laser – Useful for narrow radiation bands

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

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)

Preparing a “Neat” IR Sample

Preparing a KBr Disk

Apparatus for KBr Disk

Sample cells

To obtain an IR spectrum, the sample must be placed in a “container "or cell that is transparent in the IR region of the spectrum. Sodium chloride or salt plates are a common means of placing the sample in the light beam of the instrument.These plates are made of salt (NaCl or KBr) and must bestored in a water free environment

b) Nernst glower

aux.heater

2 - 5 cm

Has - temp coefficient.of resistance.

1 - 3 mm dia.

ceramic holderY2O3, ThO2, ZrO2heated up to 1500oC

Pt leads

cement

c ) G l o b a r

5 c m

6 - 8 m m d i a .

S i C r o d h e a t e d t o 1 3 0 0 o C

w a t e r -c o o l e d b r a s s t u b e w i t h s l o t

+ t e m pc o e f f . o f r e s i s t a n c e

G 1 5 m6 5 0 c m - 1

N GG

1 0 m1 0 0 0 c m - 1

N G 1 m1 0 0 0 0 c m - 1

e - p r o m o t e d f r o m v a le n c e b a n d to u n f i l le d c o n d u c t io n b a n d , c a u s in g e - h o le p a ir f o r m a t io n . N o . o f p a i r s d e p e n d s o n l ig h t in t e n s i t y . p h o t o v o l t a ic : p d c a u s e d b y s e p a r a t io n o f e - h o le p a ir s b e tw e e n n , p la y e r . p h o t o c o n d u c t iv e : R c h a n g e s w ith r a d ia t io n p o w e r , fo r s e m ic o n d u c to r . p h o t o e le c t r o m a g n e t ic : u t i l is e H a ll E f fe c t in s e m ic o n d u c to r .

http://www.chem.vt.edu/chem-ed/scidex.html

Photon detectors

Analyzing IR Spectra

Look for C=O peak (1820-1660 cm-1)If C=O check for OH (3400-2400 cm-1)

◦indicates carboxylic acidIf C=O check for NH (3500 cm-1)

◦indicates amideIf C=O check for C-O (1300-1000 cm-1)

◦indicates esterIf no OH, NH or C-O then ketone

Analyzing IR Spectra

If no C=O check for OH (3600-3300 cm-1)◦indicates alcohol

If no C=O check for NH (3500 cm-1)◦indicates amine

If no C=O & no OH check C-O (1300 cm-1)◦indicates ether

Look for C=C (1650-1450 cm-1) then aromatic

IR Characteristic Vibrations

Sample IR Spectrum #1

A - CO-OH stretch (3000)

B - CH stretch (2800)

C - C=O ester (1757)

D - C=O carboxy (1690)

E - C=C aromatic (1608)

F - C=C aromatic (1460)

Sample IR Spectrum #2

A

BC

C CH3

O

Acetophenone

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

ApplicationsQualitative “fingerprint” check for

identification of drugsUsed for screening compounds and

rapid identification of C=O groupsCan be used to characterize samples in

solid states (creams and tablets)Can detect different crystal isoforms

(polymorphs)Water content measurement

ApplicationsAnalysis of urine and other biofluids

(urea, creatinine, protein)

ApplicationsUsed in non-invasive measurement of

glucose

Applications of Near IR (NIR)

Quality control of pharmaceutical formulations

Determination of particle sizeDetermination of blend uniformityDetermination or identification of

polymorphic drugs