IR SPECTROSCOPY & ITS APPLICATIONS

Amit AgnihotriDefence Reserach Laboratory Tezpur

EMR ANALYTE SPECTROPHOTOGRAPH

1.UV-Visible radiations--------excitation of electrons---------UV-visible spectrum

2.IR-radiations-----------------------vibration changes --------------------IR spectrum

3.Radio frequency---------------spin rotational changes-------------N.M.R spectrum

Spectroscopy is a technique used to determine the structure of a compound by the study of the interaction between matter and electromagnetic radiation

Nondestructive (destroys little or no sample).

THE ELECTROMAGNETIC SPECTRUM

3

IR REGIONS RANGE

Near infrared region 0.8-2.5 µ (12,500 - 4000 cm-1) Mid infrared region 2.5-15 µ (4000 - 667cm-1) Far infrared region 15-200 µ (667 - 50 cm-1)

TYPES OF SPECTROSCOPY

UV- Vis spectroscopy uses electronic transitions to determine bonding patterns

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

Nuclear magnetic resonance (NMR) spectroscopy analyzes the environment of the hydrogens in a compound. This gives useful clues as to the alkyl and other functional groups present.

Mass spectrometry (MS) fragments the molecule and measures their mass. MS can give the molecular weight of the compound and functional groups

PRINCIPLE OF IR SPECTROSCOPY

Molecules are made up of atoms linked by bonds. The movement of atoms and the chemical bonds like

spring and balls (vibration)

This characteristic vibration are called Natural vibration. The  energy  of molecular  vibration  is  quantized.

When EMR (IR) is applied then it causes the vibration between the atoms of the molecules when,

Applied infrared frequency = Natural frequency of vibration

Then, Absorption of IR radiation takes place and a peak is observed.

Different functional groups absorb characteristic frequencies of IR radiation. Hence gives the characteristic peak value.

Therefore, IR spectrum of a chemical substance is a finger print of a molecule for its identification.

Like a fingerprint no two unique molecular structures produce the same infrared spectrum. This makes infrared spectroscopy useful for several types of analysis.

CRITERIA FOR A COMPOUND TO ABSORB IR RADIATION Change in dipole momentA molecule can only absorb IR radiation when its absorption cause a change in its electric dipoleA polar bond is usually IR-active.A nonpolar bond in a symmetrical molecule will absorb weakly or not at all.

MOLECULAR VIBRATIONSThere are 2 types of vibrations:1. Stretching vibrations2. Bending vibrations

1.Stretching vibrations: Vibration or oscillation along the line of bond Change in bond length Occurs at higher frequency: 4000-1250 cm-1

2 types:a)Symmetrical stretchingb)Asymmetrical stretching

A) SYMMETRICAL STRETCHING:Both bonds increase or decrease in length simultaneously.

H

H

C

B) ASYMMETRICAL STRETCHING in this, one bond length is increased and other is

decreased.

H

H

C

2. BENDING VIBRATIONS• Vibration or oscillation not along the line of bond• Also called as deformations• In this vibrations bond angle is altered• Occurs at low frequency : 1400-666 cm-1

• 2 types:a) In plane bending: E.g. scissoring, rockingb) Out plane bending: E.g. wagging, twisting

A) IN PLANE BENDINGi. Scissoring: This is an in plane blending 2 atoms approach each other Bond angles are decrease

H

H

CC

ii. Rocking: Movement of atoms take place in the same

direction.

H

H

CC

B) OUT PLANE BENDING i. Wagging: 2 atoms move to one side of the plane. They move

up and down the plane.

ii. Twisting: One atom moves above the plane and another atom

moves below the plane.

H

H

CC

H

H

CC

IR STRETCHING FREQUENCIES ( DEPEND ON?

= frequency= frequencyk = spring strength (bond stiffness)k = spring strength (bond stiffness) = reduced mass (~ mass of largest atom) = reduced mass (~ mass of largest atom)

Directly on the strength of the bonding between the Directly on the strength of the bonding between the two atoms (two atoms ( ~ k) ~ k)

Inversely on the reduced mass of the two atoms (v ~ Inversely on the reduced mass of the two atoms (v ~ 1/m)1/m)

STRETCHING FREQUENCIES

isolated C=C 1640-1680 cm-1

conjugated C=C 1620-1640 cm-1

aromatic C=C approx. 1600 cm-1

Conjugation lowers the frequency Frequency decreases with increasing atomic weight. Frequency increases with increasing bond energy.

19SUMMARY OF IR ABSORPTIONS

CLASSIFICATION OF IR BANDSThree types : strong (s), medium (m), or weak (w)

Depending on their relative intensities in the IR spectrum.

O—H AND N—H STRETCHING Both of these occur around 3300 cm-1, but they

look different:

Alcohol O—H is broad with rounded tip.Primary amine (RNH2) is broad with two

sharp spikes Secondary amine (R2NH) is broad with one

sharp spike.No signal for a tertiary amine (R3N) because

there is no hydrogen.

IR SPECTRUM OF ALCOHOLS

IR spectrum of alcohols broad, intense O—H stretching absorption around 3300 cm-1.

The broad shape is due to the hydrogen bonding interactions of alcohol molecules.

IR SPECTRUM OF AMINES

The IR spectrum of amines show a broad N—H stretching absorption centered around 3300 cm-1.

IR SPECTRUM OF AMIDES

strong absorption for the C═O at 1630–1660 cm-1. there will N—H absorptions at around 3300 cm-1.

IDENTIFICATION OF SUBSTANCESIDENTIFICATION OF SUBSTANCES To compare spectrums. No two samples will have identical IR spectrum. Criteria: Sample and reference must be tested in

identical conditions, like physical state, temperature, solvent, etc.

APPLICATIONS

The “Fingerprint” Region (1200 to 600 cm-1) : Small differences in structure & constitution of molecule

result in significant changes in the peaks in this region. Hence this region helps to identify an unknown

compound.

STUDYING PROGRESS OF REACTIONSSTUDYING PROGRESS OF REACTIONS Observing rate of disappearance of characteristic

absorption band in reactants Rate of increasing absorption bands in products of a

particular product. E.g. : O—H = 3600-3650 cm-1 , C=O = 1680-1760 cm-1

Measure the degree of polymerization in chemical compounds.

DETERMINATION OF MOLECULAR STRUCTUREDETERMINATION OF MOLECULAR STRUCTURE Used along with other spectroscopic techniques. Identification is done based on position of absorption

bands in the spectrum.Eg.: C=O at 1717 cm-1. Absence of band of a particular group indicates

absence of that group in the compd.

DETECTION OF IMPURITIESDETECTION OF IMPURITIES Determined by comparing sample spectrum

with the spectrum of pure reference compound. Eg.: ketone impurity in alcohols.

Detection is favored when impurity possess a strong band in IR region where the main substance do not possess a band. Eg :Impurity in bees wax (with petroleum wax)

MONITORING THE STRUCTURAL PLASTICITY OF PLANT CELL WALLS

carbonyl band near 1700 cm−1indicates a high concentration in the older plant. higher protein content in the young plant is suggested by the negative amide I and amide II bands at 1650 and 1545 cm−1, respectively. there is a significant difference in the cellulosic nature of the two samples, with the older plant having a higher relative cellulose content

PROTEIN QUANTITATION

IR spectroscopy is one of the most well established techniques for the analysis of protein structure

In protein amino acids are covalently linked via amide (peptide) bonds. it absorb in multiple regions of the mid-IR spectrum,

By measuring amide bonds in protein chains, we can accurately quantifies an intrinsic component of every protein

Amide A (about 3500 cm-1) is with more than 95% due to the N-H stretching vibrationAmide I   (between 1600 and 1700 cm-1) most intense absorption band in proteins. stretching vibrations of the C=O (70-85%) and C-N groups (10-20%)

In order to determine protein and peptide concentration, the Spectrometer measures the intensity (peak height) of the Amide I band,

NON-INVASIVE BLOOD GLUCOSE MONITORING

Near Infrared  spectroscopy  is used across the ear lobe to measure glucose

 Amount of near infrared light passing  through the ear lobe depends on the amount of blood glucose in that region 

The ear lobe was chosen due to the  absence of bone.tissues and also because of its relatively small thickness

OTHER APPLICATIONSOTHER APPLICATIONS1. Determination of unknown contaminants in industry

using FTIR.2. Determination of cell walls of mutant & wild type

plant varieties using FTIR. 3. Biomedical studies of human hair to identify disease

states (recent approach).4. Identify odor & taste components of food.5. Determine atmospheric pollutants from atmosphere

itself.6. Examination of old paintings

It is also used in forensic analysis in both criminal and civil cases, example in identifying polymer degradation, in determining the blood alcohol content etc.

Chemical Analysis: Testing Pill Quality. According to "Medical News Today," scientists at the University of Maryland have been successful in using the method of near-infrared spectroscopy (NIR) to make a prediction regarding quick dissolution of pills inside the body.

STRENGTHS AND LIMITATIONS IR alone cannot determine a structure. Some signals may be ambiguous. The functional group is usually indicated. The absence of a signal is definite proof that the

functional group is absent. Correspondence with a known sample’s IR

spectrum confirms the identity of the compound.