ELECTRON SPIN RESONANCE SPECTROSCOPY

BYDr. Suman PattanayakAssociate ProfessorDepartment of Pharma Analysis & QA.

Vijaya Institute of Pharmaceutical Sciences for Women

IV B. Pharm/ I SemPharmaceutical Analysis

Introduction

Also known as….        Electron Paramagnetic

Resonance (EPR) Electron Magnetic

Resonance (EMR)

The molecule which contain unpaired electrons are paramagnetic species or free radical species discovered by ZAVOISKII in 1944.

What Is ESR???

It is a branch of absorption spectroscopy in which   radiation having frequency in microwave region is absorbed by paramagnetic substance to induce transition between magnetic energy level of electron with unpaired spin.

Magnetic energy splitting is done by applying a static  magnetic field.

ESR Phenomenon Is Shown By... Atoms having an odd number of electrons. Ions having partly filled inner electron shells. Molecule that carry angular momentum of

electronic   origin. Free radicals having unpaired electrons. Molecule with paired electrons and zero

magnetic field Diamagnetic Molecules with unpaired electrons and

magnetic moment-

Paramagnetic

Basic Principle Of ESR……………

The unpaired electrons are excited to a high energy state under the magnetic field by the absorption of microwave. The excited electron changes its direction of spin and relaxes into the ground state by emitting phonons.Microwave absorption is measured as a function of the magnetic field by ESR spectroscopy.

Spin Spin Splitting (Hyperfine Splitting) Magnetic resonance can also occur without an

external magnetic field from interaction of the electron and nuclear spin. Such resonance produces fine and hyperfine structure of atomic spectra.

The nuclei of atoms in a molecule or complex often have magnetic moment which produces a local magnetic field at the electron. The interaction between an unpaired electron an nuclei with non zero nuclear spin is called the hyperfine  interaction. This leads to splitting of the ESR line and is known as hyperfine spectrum

Diagrammatic presentation of hyperfine splitting due to one proton.

The dipole-dipole interaction For a large concentration of electronic spins,

the electronic magnetic moments also interact appreciably with each other, and this can alter considerably theESR spectra.

The interaction is mediated by the dipolar field associated with the magnetic moment

The dipolar interaction induces therefore a broadening of the resonance line, which increases with the concentration of dipole moments.

ESR spectrometerWe need four essential components to build an

ESR spectrometer: A monochromatic microwave source A waveguide for guiding the microwave

power to the sample A cavity designed to ensure a proper

coupling between the sample and the incoming wave.

A detector for microwave power to detect the response of the sample to microwave irradiation.

ESR spectrometer

Microwave parts

A gun oscillator is a monochromatic source of microwave. Tuning of the frequency is achieved by slowly turning the screw on the top of the metallic case of the oscillator. The frequency can be read out with the frequency counter located next to the source.

A calibrated attenuator is use to control the level of microwave power from the source.

T-hybrid

"A T-hybrid" is the 4-ports device . A wave entering from the source at input 3

splits equally into two waves travelling to 1 and 2.

The port 4 being orthogonal, no transmission from port 3 to port 4 is allowed. Also, no reaction occurs at port 3 and 4 owing to the presence of the source and the detector.

The relative phase and amplitude of inputs 1 and 2 can be controlled with an attenuator and a moveable short located on the right arm of the hybrid.

The detector is a crystal rectifier (diode) which consists of a semi-conducting material. The incident microwave power causes the current to flow.

The waveguide is a rectangular opened-ended metallic tube delimiting a dielectric media in which electromagnetic waves propagate according to Maxwell equations.

Boundary conditions have to be fulfilledby the electrical and magnetic components of the wave on the metalliwalls. Consequently the propagation is restricted to a set of modes occurring at well-defined frequencies which are the characteristic values other wave equation.

The cavity is a closed metallic box with an iris to allow the microwave to couple in and out

Any cavity possesses resonant frequencies at which the energy stored reaches large values.

These frequencies are related to the dimensions of the cavity.

Applications:

All application of ESR is based on three aspects, which are,

Study of free radicals, Investigation of molecules in the

triple state, and Study of inorganic compounds.

Study of free radicals

Even in very low concentration of sample ESR can study via free radicals.

It is also applied in determination of structure of organics and inorganics free radicals.

The intensity of ESR signal is directly proportional to the no. of free radicals present.

Hence using ESR we can measure relative concentration of free radicals.

Investigation of molecules in the triple state A triple state molecule has a total spin S=1

so that, its multiplicity can be given as 2S+1=3. 

While free radicals with S=½ has an odd no. of unpaired electrons. 

A triple state molecule has an even no. of electrons two of them unpaired. 

In triple state molecule the unpaired electrons must interact whereas in diradical, the unpaired electrons do not interact for they are a great distance apart.

Study of inorganic compounds ESR is very successful in the study of inorganic

compounds. The ESR studies may be used in knowing the exact

structures of solvated metal ions. ESR is used in the study of catalysts. ESR is used

in the determination of oxidation state of metal. eg. Copper is found to be divalent in copper protein

complexes whereas it is found to be monovalent in some biologically active copper complexes.  

The information of unpaired electrons is very useful in various aspects in applications of ESR.

Like, Spin labels, Structural determination, and Reaction velocities and reaction mechanisms.

The various applications of ESR spectroscopy are grouped in to two categories.

1.Analytical applications2.Biological applications

Analytical applications: Mn+2 ions can be measured and detected even

when present in trace quantities. The method is very rapid and can be measured

in aqueous solution over the range from 10-6 M to 0.1M.

ESR method has proved to be a rapid and convenient method for determination of Vanadium in petroleum products.

ESR can also be used to estimate Cu(II), Cr(II), Gadolinium(III), Fe(III) and Ti(III).

The ESR spectroscopy has been used to estimate polynuclear hydrocarbons, which are first, converted in to radical cations and then absorbed in the surface of an activated silica-alumina catalyst.

Biological systems: From the ESR studies of variety of biological system

such as, leaves, seeds, and tissue preparation, it is found that a definite, correlation exists between the concentration of free radicals and the metabolic activity of the plant material.

ESR has studied the presence of free radicals in healthy and diseased tissues.

Most of the oxidative enzymes function via one electron redox reaction involving the production of either enzyme bound free radicals or by a change in the valence state of transition metal ion. This has been conformed by ESR studies.

Much of the ESR work on photosynthesis has been carried out with photosynthetic bacteria. The oxidation of bacteriochlorophyll in vitro produces an ESR signal.

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