Electron Spin Resonance (ESR)
Electron spin resonance spectroscopy/ESR is a magnetic resonance technique, which is based on the interaction of unpaired electron spins with an external magnetic field. In ESR, the essential aspects are may be illustrated by considering the hypothetical case of a single isolated electron.
It has been used for over 50 years to study a variety of paramagnetic species. Although it is supposed to be a mature field with a fully developed theory, there have been some surprises as organometallic problems have explored new domains in ESR parameter space.
ESR spectroscopy is the spin change at the electron level when a microwave frequency is absorbed in the presence of a magnetic field. It is also called as Electron Paramagnetic Resonance (EPR). It is a non-destructive technique.
It is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. There is a variety of it, each with its own advantages.
In CW-ESR (Continuous Wave ESR), the sample is subjected to a sustained beam of microwave irradiation of fixed frequency, and then the magnetic field is swept. There are many different microwave frequencies that may be used and are denoted as S-band (3.5 GHz), X-band (9.25 GHz), K-band (20 GHz), Q-band (35 GHz) and W-band (95 GHz).
Other techniques, such as ENDOR (Electron-Nuclear Double Resonance) and ESEEM (Electron Spin Echo Envelope Modulation) spectroscopies, record, in essence, the NMR spectra of paramagnetic species.
History Of Electron Spin Resonance–
ESR is discovered by the Russian scientist Zavoisky in 1945, has become one of the most powerful local probes for the study of condensed matter. It trials with systems with unpaired electrons. Most molecules have all their electrons paired.
However free radicals, biradicals, triplet state systems, systems with three or more unpaired electrons, point defects in solids or localized crystal imperfections, and most transition metal ions, and rare earth ions have unpaired electrons which the technique of ESR can be applied. It deals with ESR studies of systems with free radicals generated by irradiation with X-rays.
Principle Of Electron Spin Resonance–
ESR spectra are given only by molecules that have unpaired electrons. If paired electrons are present, they have a spin quantum number of +1/2 and -1/2.Hence the magnetic moment caused by the spin of one electron cancels that of the other electron. Therefore, molecules with unpaired electron only give ESR spectra.
Unpaired electrons are present in free radicals, transition metals, rare earth ions, odd molecules, and triplet electronic states. Analogous to NMR, when no magnetic field is applied, there is only one spin state of the electron. But when the magnetic field is applied, there are two states viz. Ground state and Excited-state.
The energy required to undergo this transition from Ground-state to Excited-state is absorbed in the form of Microwave radiation.
Energy required= ∆H=hv= gβH0
where h=Plank’s constant
v=frequency of radiation
Ho=strength of magnetic field
g= spectroscopic splitting factor (depends on electronic environment)
A magnetic field strength of 500 to 3400 gauss is applied and exiting frequency (Microwave) in a range of 9500MHz to 35,000 MHz is used in it.
You may read- Spectroscopy.
Theory of Electron Spin Resonance–
ESR spectroscopy is similar to NMR spectroscopy also known as the small-big sister of NMR. ESR is the absorption spectroscopy that involves the absorption of radiation in the microwave region between 104-106 MHz by the substances containing one or more unpaired electrons.
That absorption of microwave radiation occurs under the impact of the applied magnetic field. The substances with one or more unpaired electrons are paramagnetic and exhibit ESR, and also known as electron paramagnetic resonance (EPR) spectroscopy or electron magnetic resonance spectroscopy.
Substances containing unpaired electrons, paramagnetic substances are of two types-
–Stable Paramagnetic Substances include simple molecules like NO, 0 2, and N02, and the ions of transition metals and their complexes, e.g. Fe3+, [Fe (CN)6] 3- etc. Such stable paramagnetic substances can be easily studied by ESR spectroscopy.
-Unstable Paramagnetic Substances are generally called free radicals or radical ions and are formed either as intermediates in chemical reactions or by irradiation of a stable molecule with UV or X-ray radiation or by a beam of nuclear particles. If the lifetimes of such radicals are greater than 10-6 s, they may be studied by ESR spectroscopy.
Paramagnetic substances with lifetimes shorter than w-6 S may also be studied by ESR spectroscopy. If they are produced at low temperatures in the solid-state, called matrix technique, as this increases their lifetimes. ESR spectroscopy is most useful in the study of free radicals.
Instrumentation of ESR-
ESR spectrometer is used for different purposes that are consists of the following are-
A source of microwave radiation with constant frequency and variable amplitude is used. It contains-
–Klystron– It acts as the source of radiation which offers microwave of 9500 MHz (If 35,000 MHz is used it gives 20 times more resolution). It stabilized against temperature fluctuation by immersion in an oil bath or through forced air cooling.
The frequency of the monochromatic radiation is measured by the voltage that applied to klystron. It is kept a fixed frequency by an automatic control circuit and provides a power output of about 300 milliwatts.
-Wave Meter or waveguide-It is put in between the oscillator and attenuator to measure the radiation. To know the frequency of microwaves produced by the klystron oscillator. The wave meter is usually calibrated in a frequency unit (megahertz) instead of wavelength. The waveguide is a hollow, rectangular brass tube, and used to convey the wave radiation to the sample and crystal.
–Attenuator – It is similar to a filter in a spectrophotometer. The power propagated down the waveguide may be continuously decreased by inserting a piece of resistive material into the waveguide and, the piece is called a variable attenuator. These are used to varying the power of the sample from the full power of klystron to one attenuated by a force 100 or more.
–Isolator– It is a non-reciprocal device that minimizes vibrations in the frequency of microwaves produced by the klystron oscillator. It is used to isolate a narrow range of microwave, and also to prevent the reflection of microwave power back into the radiation source.
It is a strip of ferrite material which allows microwaves in one direction only. It also is stabilizing the frequency of the klystron.
The resonant cavity containing the sample is the heart of the ESR spectrometer. Samples in the form of gas, liquid, or solid can be used. Solvents of low dielectric constant are preferred. A solution concentration of 10-6M to 10-9M is used in sample cells, which are flat and can hold 0.05ml to 0.5ml.
The sample is contained in a resonance cavity. Rectangular TE120 cavity and cylindrical TE011 cavity have widely been used. Mostly in the ESR spectrometers, the dual sample cavities are generally used. This is done for simultaneous observation of a sample and a reference material.
Magnetic field– A magnetic field sweep from O to 500 gausses or O to 3400 gausses is used. Since the magnetic field interacts with the sample to cause spin resonance the sample is placed where the intensity of the magnetic field is greatest.
Also known as Circular-T or Magic-T. In the microwave bridge, the microwave radiations finally enter the circulator through a waveguide by a loop wire which is coupled with the oscillating magnetic fields and setting a corresponding field.
The resonant cavity is placed between the pole’s pieces of an electromagnet. An electromagnet is capable of producing a magnetic field of at least 5000 gausses which is required for ESR.
The stability of the field is achieved by energizing the magnet with a highly regulated power supply. The spectrum of ESR is recorded by the process of slowly varying the magnetic field. Through which the resonance condenses by sweeping the current supplied to the magnet by the power supply.
This sweep is usually accomplished by a variable speed motor drive. In between, both the magnet as well as the power supply may require water cooling.
Modulation of the signal on a frequency compatible with a good signal-noise ratio in the crystal detector which is accomplished by a short alternating variation of the magnetic field. The variation is produced by supplying an A.C. signal to modulation coil oriented with respect to the sample in the same direction as the magnetic field.
If the modulation is of low frequency at 400 cycles/sec or less, then the coils can be mounted outside the cavity and even on the magnet pole pieces. For higher modulation frequencies, it must be mounted inside the resonant cavity or cavities constructed of a non-metallic material. E.g. Quartz with a tin silvered plating, because the metallic penetration is not much effective in case of higher modulation frequencies.
Couplers and matching screws-
These are the various components of the microwave assembly to be coupled together by making use of irises or slots in various sizes.
Amplifier & Recorder-
A Silicon crystal detector, which converts the radiation in D.C., has widely been used as a detector of microwave radiation. Microwave Bridge such as magic T and hybrid ring variety is most common. It is used to record the derivative spectra.
As a detector usually a silicon crystal detector is used. In order to adjust the spectrometer and to observe the signal, a cathode ray oscilloscope has been employed. For recording the signal, a strip chart or X-Y recorder is used.
EPR spectra are usually displayed in the derivative form to improve the signal-to-noise ratio. To observe the signal a system is connected different devices can be used.
Like most spectroscopic techniques, when the radiation is absorbed, a spectrum is produced similar to the one on the left. In EPR spectrometers a phase-sensitive detector is used. This results in the absorption signal being presented as the first derivative.
Therefore, the absorption of maximum corresponds to the point where the spectrum passes through zero. This is the point that is used to determine the center of the signal.
The spectrum of ESR is obtained by plotting intensity against the strength of a magnetic field in it. The better way is to represent it as a derivative curve in which the first derivative or slope of the absorption curve is plotted against the strength of the magnetic field.
The absorption or derivative curve is proportional to the number of unpaired electrons in the sample either by the total area covered. The comparison is made with a standard sample, to find out the number of electrons in an unknown sample having a known number of unpaired electrons, which possessing the same line shape as the unknown. The most widely used standard for free radical is1,1-diphenyl-2-picrylhydrazyl (DDPH).
The two spin levels are so nearly equally populated, so, the magnetic resonance suffers from a problem not confront in higher energy forms of spectroscopy. An intense radiation field will tend to equalize the populations, leading to a decrease in net absorption, and this effect is called “saturation”.
A spin system returns to thermal equilibrium via energy transfer to the surroundings and the rate process is called spin-lattice relaxation. It consists of a characteristic time, T1, and the spin-lattice relaxation time (rate constant = 1/T1).
Systems with a long T1(i.e., spin systems weakly coupled to the surroundings) will be easily saturated, and those with shorter T1will be more difficult to saturate. As spin-orbit coupling supply an important energy transfer mechanism, so, usually find that odd-electron species with light atoms like- organic radicals having long T1, and those with heavier atoms like-organotransition metal radicals having shorter T1.
It is also known as Spin-spin splitting or denoted by ‘hfs’. It is similar to the chemical shift in the NMR spectra. It is caused by the interaction between the spinning electrons and the adjacent spinning magnetic field.
When a single electron is interacting with one nucleus the number of splitting will be 2I+ 1, where ‘I’ is the spin quantum number of the nucleus. In general, a single electron interacts magnetically with “n” equivalent nuclei the electron signal is split up to (2nI+1) multiple.
Reference standard (internal reference)-
Any one of the following is used as the Reference standard in ESR
1.DPPH- l, I-DI Phenyl-2-PicryI Hydrazyl free radical which has a ‘g’ value of 2.0036.
2.Cr3+ is a tiny chip of ruby crystal with a ‘g’ value of 1.4 that can also be used.
Derivative curve- The derivative curve is used instead of an absorption curve, to get more information. A typical absorption curve and a derivative curve is given as-
Determination of values-
‘g’ value is called as ‘Spectroscopic splitting factor’ or Lande’s splitting factor. Determination of this ‘g value is important as this gives an idea about the electronic environment.
where H = Resonance frequency used
∆H= Field separation between standard and sample
Gstd= ‘g’ value of internal standard
Some common ‘g’ values
Unbound electron- 2.002319
Organic free radicals- 2.0023 (approx.)
Instead of ‘g’ values, hyperfine splitting constants (in gauss) are also used for comparison.
You may read- Nuclear magnetic resonance spectroscopy.
Applications of ESR-
– Study of free radicals, including reaction velocity and mechanism.
-Structure elucidation of organic and inorganic molecules.
-Study of biological systems, using spin labeling techniques,
-To study the quantitative analysis
-To elucidate the electronic structure of free radicals and paramagnetic transition metal complexes with their action.
-Study the reaction with the mechanism of free radicals, photochemical, and polymerization reactions.
-Study the magnetic properties of the materials and their order of orientations.
-To explore the dimensionality of exchange coupling of the magnetically coupled materials.
-To study conducting properties of the organic conducting polymers.
-To verify the active sites on the surface of the catalyst.
-To study the oxidation state and conducting properties of the superconducting materials.
– To determine the presence of oil under the earth.
-To investigate the active sites in the metal enzymes
-To study the structure and activities of hemoglobin and other biological samples
-Analyze the hole and electron centers in the semiconductor materials with their action.
-Study the ground state energy levels of the metal cluster complex with their action.
-For determine the unknown concentration of the metal ion in paramagnetic compounds with their action.
-To determine the presence of trace amounts of metal ions in the polluted samples.