Esr For Ty As Handout

INDEX: 1. 2. 3. 4. 5. 6. 7.

What is ESR? Basis of ESR spectroscopy Energy states of electron ESR spectrum Field for resonance Hyperfine coupling Hyperfine splitting pattern (ESR spectra of methyl free radical

and isobutyl free radical) 8. Advantages and Disadvantages.

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WHAT IS ESR SPECTROSCOPY? Electron Spin Resonance Spectroscopy, also referred to as Electron Paramagnetic Resonance Spectroscopy is a versatile and non-destructive analytical technique which can be used for variety of application including oxidation and reduction processes, reaction kinetics as well as additional application in biology, medicine and physics. However, this technique can only be applied to samples having one or more unpaired electrons.

Spectroscopy is the measurement and interpretation of the energy difference between atomic or molecular states. According to Plank’s Law, electromagnetic radiation will be absorbed if

ΔE=h γ Where ΔE – difference in energy of the two states And h – Plank’s constant γ – Frequency of the radiation.

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The absorption of this energy causes a transition of an electron from the lower energy state to the higher energy state.

In ESR spectroscopy, the radiation used is in the gigahertz range. Unlike most traditional spectroscopy technique, in ESR spectroscopy the frequency of the radiation is held constant while the magnetic field is varied in order to obtain an absorption spectrum

BLOCK DIAGRAM FOR A TYPICAL ESR SPECTROMETER

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BASIS OF ESR SPECTROSCOPY – The basis of ESR spectroscopy lies in the spin of an electron and its associated magnetic moment. When an electron is placed within an applied magnetic field B0, the two possible spin state of the electron have different energies. This energy difference is a result of the Zeeman Effect.

Zeeman Effect: It’s the effect of splitting a spectral line into several components in the presence of magnetic field.

The lower energy occurs when the magnetic moment of the electron, µ, is aligned with the magnetic field and higher energy state occurs when the µ is aligned against the magnetic field.

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The two states are labelled by the projection of the electron spin, Ms, on the direction of the magnetic field, Where Ms =-1/2 is the parallel state Ms =+1/2 is the anti-parallel state

An unpaired electron can move between the two energy levels by either absorbing or emitting a photon of energy h γ, such that the resonance condition, h γ=E, is obeyed.

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ENERGY STATES OF THE ELECTRONSo for a molecule with one unpaired electron in a magnetic field, the energy state of the electron can be defined as E = g µ B B0 M s

= ±1/2 g µB B0

since Ms =±1/2

Where g- the proportionality factor µB- Bohr Magneton B0- magnetic field Ms- Electron spin quantum number

From the relationship ie E = g µB B0 Ms There are two important factors to NOTE:  The two spin state have the same energy when there is no applied magnetic field.  The energy difference between the two spin states increase linearly with the increasing magnetic field strength.

ESR SPECTRUM –

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Like most spectroscopic technique, when the radiation is absorbed, a spectrum is produced similar to the one shown above (left).

But in ESR spectrometer, a phase-sensitive detector is used and this results in the absorption signal being present as the first derivative as shown above (right). So the absorption maximum corresponds to the point where spectrum passes through zero. This is the point that is used to determine the centre of the signal.

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FIELD FOR RESONANCE – We know that, ESR spectrum is obtained by holding the frequency of radiation constant and varying the magnetic field “tunes” the two spins so that their energy difference is equal to the radiation. This is known as field for resonance. As spectra can be obtained at variety of frequencies, the field for resonance does not provide unique identification of compound.

The proportionality factor, however, can yield more useful information. g = h γ/ µBB0 For a free electron, the proportionality factor is 2.00232

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For an organic radical, the value is typically quite close to that of a free electron with values ranging from 1.99-2.01 For transition metal compounds, large variations can occur due to spin orbit coupling and zero-field splitting, resulting in values ranging from 1.4-3.0 NOTE: 1. In addition to the applied magnetic field unpaired electrons are also sensitive to their local environment. 2.The ESR spectrum of a free radical with one unpaired electron is the simplest of all forms of spectroscopy.

HYPERFINE COUPLING: Since the source of an ESR spectrum is a change in an electron’s spin state, it might be thought that all ESR spectra for a single electron spin would consist of just one line. But frequently the nuclei of the atoms in a molecule or complex have magnetic moment which produces a local magnetic field at the electron.

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This results in the interaction between the electron and the nuclei. Thus, the interaction of an unpaired electron by way of its magnetic moment with nearby nuclear spins results in additional allowed energy states and in turn multi-lined spectra.

The spacing between the ESR spectral lines indicate the degree of interaction between the unpaired electron and the perturbing nuclei. The hyperfine coupling constant of a nucleus is directly related to the spectral line spacing and in simplest cases is essentially the spacing itself. Thus, in real system, electrons are not normally alone, but are associated with one or more atoms. There are several important consequences of this: (a) An unpaired electron can gain or lose angular momentum, which can change the value of its proportionality factor. This is especially significant for chemical systems with transition metal ions.

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(b) If an atom with an unpaired electron is associated, has a non-zero nuclear spins, then its magnetic moment will affect the electron and, this leads to the phenomenon of hyperfine coupling which is similar to J-coupling in NMR, also this splits the ESR resonance signal into doublet, triplet and so forth. (c) The interaction of an unpaired electron with its environment influence the shape of an ESR spectral lines. These line shapes can yield information about like for ex: rate of chemical reaction.

HYPERFINE SPLITTING PATTERN: The hyperfine splitting pattern for a radical freely moving in a solution can be predicted. (1) For a radical having M equivalent nuclei, each with spin of l, the number of ESR lines expected is 2Ml + 1. Example: the methyl radical CH3, has three H (protium) nuclei each with l =1/2, and the number of lines

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expected is 2M + l. ie 2(3)(1/2) +1 = 4

Thus 4 lines are expected in the ESR spectrum.and which is actually observed.

ESR SPECTRUM OF METHYL FREE RADICAL x-axis : magnectic field y-axis : first derivative of adsorption.

(2) For a radical having M1 equivalent nuclei each with

l1 and a group of M2 equivalent nuclei, each with a spin of l2, the number of lines expected is (2M1l +1)(2M2l + +1) a spin of

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EXAMPLE: for isobutyl radical the number of ESR lines expected is [2(6)(1/2) +1)] [2(3)(1/2) + 1] = 28 Thus 28 lines are expected in its ESR spectra.

ADVANTAGES AND DISADVANTAGES:  Its helps us know the properties of the electron in the free radical that any compound have.  PROCESS: During any physical activity like the bombardment of some huge amount of energy like light, microwave radiation, or even heat on say ice (water) some free radicals like H ,OH, OH2 will be formed and in the process the ice liquidizes into water. These radicals sometimes get attached to a location and when ESR is applied, using a specific spectroscope the analysis of the cell can be done. This is possible because the radiation of the energy used together with an alternating magnetic field causes the electrons to move between their low and high orbits back and

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forth. They start moving faster and faster as more and more energy is picked up when they move so. Then the gap between their low and high orbits becomes smaller as time passes by. The energy applied gets in between the gap created and the difference in the energy levels can be detected by the sensor used for this purpose.  This application is widely used in the field of biology and also medicine. There are some key advantages. The first being it is the speed with which it can be performed and yet the analysis of the free radicals is found to be near perfect. Moreover, the results of the testing pretty easy to comprehend and understand. Other techniques are used to identify free radicals and also electrons in pairs. This spectroscopy is used for the purpose of identification of only free radicals. This is an important aspect in the study of free radicals.

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Likewise there are some disadvantages too. The foremost is that this process should be performed at low temperature and inly then desired results can be got. So they are performed in the specific laboratories where such temperature can be attained.

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