Q.3. (c)Discuss principle and quantitative and qualitative applications of ESR
Ans.3. (c)Principle of Electron Spin Resonance:
Electron Spin Resonance is a branch of absorption spectroscopy in which radiation having frequency in the microwave region is absorbed by paramagnetic substances to induce transitions between magnetic energy levels of electrons with unpaired spins. The magnetic energy splitting is done by applying a static magnetic field. The electron spin resonance phenomenon is shown by atoms having an odd number of electrons, ions having partly filled inner electron shells and other molecules that carry angular momentum of electronic origin.
The main interest of electron spin resonance lies in the study of free radicals having unpaired electrons. These electrons remain after the hemolytic fission of a covalent bond which is brought about by the irradiation of the sample with the ultraviolet or gamma radiation.
The electron spin resonance (ESR) is another type of magnetic resonance. It is concerned with the magnetic behavior of spinning electrons. The ESR spectrum like NMR, results from transition from one spin state to the other state of an electron. Each spin state has its energy level, the transition occurs under resonance condition in a magnetic field. However ESR differs from NMR in that the transition in ESR is induced by radiation of microwave frequency rather than by radio wave.
Applications of Electron Spin Resonance:
Quantitative Applications
1. Study of Free Radicals
Free radicals can be readily studied by ESR, even in very low concentration. For example, a signal for DPPH radical can be detected even if there is 10-12gm of material in the spectrometer. The most important application of ESR is in the determination of the structure of organic and inorganic free radicals. In order to obtain a good spectrum, the free radical must be produced in a concentration of about 10-13mol dm-3. The intensity of ESR signal is directly proportional to the number of the free radicals present. Hence, using ESR we can measure relative concentrations of free radicals.
2. Reaction Velocities and Mechanisms
A large no. of organic reactions is known which proceed by a radical mechanism. Most of the radicals formed during organic reactions are not stable but are very reactive. In order to maintain a high and enough steady concentration for ESR studies, the rapid flow system is generally utilized. With this method, radicals of lifetimes of about 0.01 second have been characterized.
The ESR spectroscopy can be used to study very rapid electron exchange reactions. An interesting example is that when naphthalene is added to a solution of naphthalene radical anion, an electron exchange reaction will take place. This causes the broadening of the hyperfine component of the ESR resonance line. This broadening can be employed to calculate the rate constant for the exchange between naphthalene and naphthalene radical anion.
3. Analytical applications:
(a) Determination of Mn2+ When the ESR spectrum of Mn2+ ions in solution is recorded, it shows six lines. The multiplicity is given by 2I+1 where I is 5/2for Mn2+ ions. This accounts for the six peaks. This ion can be measured and detected even when present in trace quantities
(b) Determination of vanadiumESR method has proved to be a rapid and convenient method for the determination of vanadium in petroleum products. Vanadyl (IV) etioporphyrin dissolved in heavy oil distillate is used as a standard. By this method, vanadium can be measured over the range from 0.1 to 50 µg/ml.
(c) Determination of Polynuclearhydrocarbons: The ESR spectroscopy has been used to estimate polynuclear hydrocarbons which are first converted into radical cations and then absorbed in the surface of an activated silica-alumina catalyst.
Qualitative application
1. Structural Determination
The ESR technique cannot be applied to determine molecular structure because the information obtained from the superfine structure is mostly about the extent of electron delocalization and Fermi contact interaction. It does not tell us about the arrangement of the atoms in the molecule although the symmetry of the molecule can be sometimes deduced from the sets of equivalent nuclei. However, in certain cases, ESR is able to provide useful information about the shape of the radicals. Eg- determination of methyl radical, it may have one of the following structures: planar or tetrahedral.
2. Study of Inorganic Compounds
ESR is very successful in the study of inorganic compounds. Some examples are asfollows:
(a) An interesting example is that [NO(SO3)2]2- yields a triplet in its ESR spectrum in chloroform. This arises from the interaction between the spin of the unpaired electron and the spin of a 14N nucleus (I = 1), confirming that this electron is mainly localized on the nitrogen atom.
(b) Another interesting example is sodium trimesitylborate whose ESR spectrum in tetrahydrofuran shows four peaks. This indicates that the odd electron in the [(BC6H2Me3)3]- ion couples with the spin of 11B for which I = 3/2.
(c) Determination of oxidation state of a metal: the ESR spectrum of Cu (II) complex gives a spectrum signal whereas that of Cu(I) complex system gives a much reduced signal. This difference between the signals for two oxidation states of copper has been successfully used to determine the state the copper in many complexes and biological compounds. For eg: copper is found to be divalent in copper protein complexes whereas it is found to be monovalent in some biologically active copper complexes.
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