Detection Of Free Radical By Different Methods 1. Magnetic Susceptibility Measurement. 2. ESR ( Electron Spin Resonance) Technique. 3. Spin Trapping Technique. 4. NMR (Nuclear magnetic resonance) Spectra by CIDNP effect. 5. X-Ray Technique

1. Detection Of Free Radical
1 Written By : AMIR HASSAN OF BS – CHEMISTRY DEPTT GPGC MARDAN, KPK, PAKISTAN
Email : amirhassan741@gmail.com
Detection Of Free Radical : -
The single electron have spinning motion and can produced magnetic field and magnetic
moment. While the single paired electron can cancel the effect of each other and cannot
produced any magnetic field and magnetic moment. Therefore a free radical ( a species that
contain one or, more unpaired electron) can be detected by the following ways:
1. Magnetic Susceptibility Measurement.
2. ESR ( Electron Spin Resonance) Technique.
3. Spin Trapping Technique.
4. NMR (Nuclear magnetic resonance) Spectra by CIDNP effect.
5. X-Ray Technique.
1) Magnetic Susceptibility Measurement.
 Detection of free radical associated with spin of an electron is a magnetic moment which
can be expressed by a quantum number.
Quantum number of + ½ and - ½
 According to Pauli principle: any two electron occupying the same orbital must have
opposite spin so that their magnetic moment is zero for any species in which all the
electron are paired.
 In free radical one or more electron are unpaired so there is a net magnetic moment and
the species is paramagnetic.
 So, therefore a free radical can be detected by a magnetic susceptibility.
2) ESR ( Electron Spin Resonance) Technique.
 A much more important technique is ESR this method is similar to that of NMR
technique except that electron spin is involved rather than nuclear spin.
 The two electron spin states : ms = + ½ and ms = - ½
are ordinarily of equal energy but in a magnetic field the energies are different as in NMR
a strong magnetic field is applied and electron are caused to flip from lower sates to a
higher state by the application of appropriate frequency signals.
 If two electron are paired in orbital have opposite spin cancel the effect of other and
hence ESR spectrum arises only for a species that have one or more unpaired electron.
 Since free radical give only ESR spectrum.
 The ESR spectrum can detect both the presence of free radical, and also determine
their concentration.
 Furthermore ESR spectrum (ESR Peaks) structure of free radical can be obtained.
 Electron spin resonance spectra have been observed for free radical with life time
considerably less than one second 1 < sec.
3) Spin Trapping Technique.
 In case of failure of ESR technique another technique is used called spin trapping
technique for detection and involvement of free radical.

2. Detection Of Free Radical
2 Written By : AMIR HASSAN OF BS – CHEMISTRY DEPTT GPGC MARDAN, KPK, PAKISTAN
Email : amirhassan741@gmail.com
R - N = O + R`.
RR`N – O.
 SPIN TRAPPING AGENT: in this technique a compound is added to the reaction
medium which combine to reactive free radical to produced more persistent radical which
can be observed by ESR spectrum.
 The free radical are trapped by trapping agent and then detected by ESR spectrum.
Trapping agent + Free radical reaction ESR Detector
 EXAMPLE : azulenyl Nitrone is used as spin trapping reagent the most important spin
trapping agents are nitroso compounds nitrosyl compounds which react with radical to
give stable nitroxide radicals.
4) NMR (Nuclear magnetic resonance) Spectra by CIDNP effect.
 Another magnetic technique for detection free radical is NMR.
 The NMR spectrum is taken during the course of reaction certain signals may be
enhanced either in a positive or, negative direction other may be reduced this
phenomenon is called CIDNP(chemically induced nuclear polarization).
 When CIDNP is formed in the reaction product it mean that at least a portion of product
formed via the intermediacy of free radical.
 During reaction some ethyl iodide signals are
enhanced other go below the base line. Thus ethyl iodide
formed in exchange show CIDNP and hence was formed
via free radical intermediate.
 The CDINP results when proton in a reacting
molecule couple to an unpaired electron while leading
from reactants to products.
 Thus CIDNP is much useful technique the
presences of which always mean that a free radical is
involved.
The End !!!!!!!!