Esr

Electron Spin Resonance Spectrometer (ESR)

It was discovery by Zavoisky in 1944.

Electron Spin Resonance Spectroscopy also called electron paramagnetic resonance spectroscopy [EPR])

ESR has become an essential tool for the study of the structure and dynamics of molecular systems containing one or more unpaired electrons.

Electron Spin Resonance Spectrometer (ESR) ESR spectroscopy is a very powerful and sensitive method for the characterization of the

electronic structures of materials with unpaired electrons.

There is a variety of ESR techniques, each with its own advantages.

In continuous wave ESR (CW-ESR), the sample is subjected to a continuous beam of microwave irradiation of fixed frequency and the magnetic field is swept.

Different microwave frequencies may be used and they 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 electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, record in essence the NMR spectra of paramagnetic species.


The theory of ESR spectroscopy shares much in common with that of nuclear magnetic resonance spectroscopy.

However, the magnetic moment of the electron is about 1000 times as large as the nuclear moment and the constants employed in NMR theory frequently are different in magnitude and sign.

Types of Materials Studied by ESR Inorganic and organic free radicals which possess an odd number of electrons, such as

Fremy's radical Odd-electron molecules such as NO, N02, and Cl02. Many molecules of this type have

been examined by gas-phase ESR techniques. Triplet-state molecules, such as O2 and S2. These systems have two unpaired electrons.

Optical irradiation of solids and solutions can often permit investigation of photo excited triplet states, which are important in photochemistry.

Transition-metal complexes, organometallic compounds, and catalysts containing metal ions with incomplete 3d, 4d, or 5d electron subshells. The detection of V(IV) (which has the Is22s22p63s23p63d1 configuration) in crude petroleum is one notable application of ESR spectroscopy.

Rare earth and actinide compounds containing incomplete 4f, 6d, or 5f subshells. Impurities in solids, such as semiconductor materials. Odd electrons gained by an

acceptor or lost by a donor impurity may be associated with energy bands in crystals.

Metals. The electrons in conduction bands of metals can be examined by ESR spectroscopy.

Comparison between EPR and NMR

EPR is fundamentally similar to the more widely familiar method of NMR spectroscopy, with several important distinctions. While both spectroscopies deal with the interaction of electromagnetic radiation with magnetic moments of particles, there are many differences between the two spectroscopies:

1 EPR focuses on the interactions between an external magnetic field and the unpaired electrons of whatever system it is localized to, as opposed to the nuclei of individual atoms.

2 The electromagnetic radiation used in NMR typically is confined to the radio frequency range between 300 and 1000MHz, whereas EPR is typically performed using microwaves in the 3 - 400 GHz range.

3 In EPR, the frequency is typically held constant, while the magnetic field strength is varied. This is the reverse of how NMR experiments are typically performed, where the magnetic field is held constant while the radio frequency is varied.

4 Due to the short relaxation times of electron spins in comparison to nuclei, EPR experiments must often be performed at very low temperatures, often below 10 K, and sometimes as low as 2 K. This typically requires the use of liquid helium as a coolant.

5 EPR spectroscopy is inherently roughly 1,000 times more sensitive than NMR spectroscopy due to the higher frequency of electromagnetic radiation used in EPR in comparison to NMR.

13.1 THE RESONANCE CONDITION

The electron is a charged particle with angular momentum (orbital and spin) and,as such, it possesses a magnetic moment, /Le' given by

/Le = -gf3J (13.1)Here J (in units of h/27T, where h = Planck's constant) is the total angular momentumvector, g is a dimensionless constant (the g-value, g-factor, or spectroscopic splittingfactor), and f3 is a constant, the Bohr magneton. The negative sign in Equation 13.1is a consequence of negative electronic charge. Neglecting orbital angular momentumand considering only the total spin angular momentum S, Equation 13.1 can bewritten as

/Le = -gf3S (13.2)The g-value for the free electron, ge, is 2.0023. The approximation made in Equa-tion 13.2 is valid for most discussions of the ESR spectra of the organic free radicalsand transition-metal complexes whose orbital angular momentum can be consideredto be "quenched." Treating the g-value as an experimental quantity does not harmthe present discussion, since deviations of g-values from ge can be accounted for byintroduction of spin-orbit coupling.

Magnetic moments can be detected by their interactions with magnetic fields.In zero field, the magnetic moments of unpaired electrons in a sample are randomlyoriented. In the presence of a magnetic field H, electron moments assume orienta-tions with respect to the applied field, giving rise to 2S + 1 energy states (Zeemansplitting). The measurable components of /Le are gf3ms where ms is the magnetic spinquantum number, which can take the values +S, +(S - 1), ... , -(S - 1), -S.The application of a magnetic field to an S = 1/2 (or larger) system is said to removethe spin degeneracy (i.e., the equal energy values of m. in the absence of an appliedmagnetic field).

The energy of an electron moment in a magnetic field is given byE = -/LeH

Upon combining Equation 13.2 and 13.3, the expressionE = gf3Hm.

(13.3)

(13.4)results (assuming that the direction of the applied field defines the z-axis). WhenS = 1/2, there are two energy levels

Em.= +1/2 = +tgf3HFIGURE 13.1. Energy levels and spectra in ESR spectroscopy. A: Energylevels for an unpaired electron in a magnetic field. B: ESR absorptionpeak: RF power (P) absorbed vs. magnetic field. C: ESR first-derivativepresentation-change of power absorbed per unit change in magneticfield vs. magnetic field. /1H is the peak-to-peak line width. The first-derivative spectrum is the usual form obtained using ESR spectrometers,since phase-sensitive crystal detection of the microwave power absorbedby the sample is usually employed.

368 CHAP.13 Electron Spin Resonance Spectroscopy

(13.5)








(13.3)




(13.5)








(13.3)




(13.5)

TABLE 13.1. Spectrometer Frequencies and g., Resonance Field Strength

Spectrometer Frequency

Designation v(Hz)

X-band x 109K-band x 109Q-band x 109

A(cm)

3.1561.3030.856

0.3170.7671.168

33908207

12,489

a. For the purposes of magnetic resonance spectroscopy, Oersteds (oe) andGauss (G) are effectively the same and are employed interchangeably.

and(13.6)

whose energy is linearly dependent on H. The separation between these energylevels (Fig. 13.1) at a particular value of the magnetic field, H R , is

(13.7)In an ESR experiment, an oscillating magnetic field perpendicular to H R inducestransitions between the m. = -1/2 and m. = +1/2 levels, provided the frequency,v, is such that the resonance condition

dE = hv = gf3HR (13.8)is satisfied. The frequency is held constant and the magnetic field is varied. At aparticular value of the magnetic field, H R , resonance absorption of energy occurs,resulting in a peak in the spectrum (Fig. 13.1B). The frequencies commonly employedin ESR experiments are in the microwave region; these frequencies and magnetic fieldstrengths for g. resonance absorption signals are given in Table 13.1.

13.2 ESR INSTRUMENTATION

As in most other types of spectroscopy, the instrumentation employed in ESRspectroscopy consists of a source of electromagnetic radiation, a sample holder, andappropriate detection equipment for monitoring the amount of radiation absorbedby the sample. In ESR spectroscopy a magnetic field provided by an electromagnetis also required. Monochromatic radiation of the various frequencies employed inESR work (Table 13.1) is obtained from klystrons, which are electronic oscillatorsproducing microwave energy. Spectrometers operating at X-band (3-cm wavelength)are the ones most commonly employed. The microwave radiation is transmittedalong hollow rectangular metal pipes called waveguides.

Figure 13.2 gives a block diagram of a simple ESR spectrometer. The sampleis placed at the center of the sample cavity where the magnetic vector is at a maximum.Quartz tubes mm o.d.) are generally employed to contain solid and solutionsamples. Unlike the NMR technique, the sample tubes are not rotated. The mag-netic field is slowly and linearly increased until the resonance condition (Eqn. 13.8)


Magnetic FieldA

HRIIIIIII

Magnetic FieldB

Magnetic Fieldc



Designation v(Hz)


A(cm)

3.1561.3030.856

0.3170.7671.168

33908207

12,489


and(13.6)










Designation v(Hz)


A(cm)

3.1561.3030.856

0.3170.7671.168

33908207

12,489


and(13.6)









Electron Spin resonance spectroscopy is based on the absorption of microwave radiation by an unpaired electron when it is exposed to a strong magnetic field.

Species that contain unpaired electrons (namely free radicals, odd-electron molecules, transition metal complexes, rare earth ions, etc.) can therefore be detected by ESR.

WORKING PRINCIPLE :

When an atomic or molecular system with unpaired electrons is subjected to a magnetic field, the electronic energy levels of the atom or molecule will split into different levels.

The magnitude of the splitting is dependent on the strength of the applied magnetic field.

The atom or molecule can be excited from one split level to another in the presence of an external radiation of frequency corresponding to the frequency obtained from the difference in energy between the split levels. Such an excitation is called a magnetic resonance absorption.

The atom or molecule under investigation may be in different environments in an actual sample.

The magnetic resonance frequency will hence be influenced by the local environment of the atom or molecule.

The electron spin resonance technique is therefore, a probe for a detailed identification of the various atomic and molecular systems and their environments and all associated parameters.

302 B.M. Weckhuysen et al.

Fig. 4ac. a General layout of an ESR spectrometer; b Block diagram of an ESR spectrometerand c Magnetic and electric field patterns in a standard ESR cavity (reprinted from reference[15]. Copyright 1992 Bruker Instruments, Inc.)

is satisfied, at which point power is absorbed by the sample and a change in currentin the detector crystal is monitored. A pair of Helmholtz coils are mounted aroundthe cavity to increase sensitivity. Feeding the coils from an oscillator superimposesa variable amplitude sinusoidal modulation on the slowly varying magnetic field.The signal detected by the phase-sensitive detection system is proportional to theslope of the ESR absorption as the magnetic field passes through resonance. Therecorder then presents the first-derivative spectrum. Many spectrometers are alsoequipped to present second-derivative spectra.

Oscillator

AmplifierCrystalDetector

Sample==TI== 1--.."...-----"-.-1Cavity

ElectromagnetModulationCoils

Attenuator

Waveguide

Klystron

FIGURE 13.2. Block diagram ofa simple ESR spectrometer.

The sample tube must be chosen with careful attention to the physical, chemical,and magnetic properties of each sample. For instance, when the sample is dissolvedin a polar solvent with an appreciable dielectric constant, quartz sample tubes areusually not suitable and capillaries or thin rectangular cells of glass are used. Again,when studying free radicals with g ge, Pyrex tubing can be used, whereas onlyquartz can be used for triplet (S :;::: 1) compounds because of paramagnetic impuritiessuch as Fe3 + in most laboratory-grade glassware. Finger sized Dewar flasks (of theappropriate materials) and other cryogenic equipment permit ESR spectra to be ob-tained as a function of temperature.

The spectral sensitivity of f:SR depends on a variety of factors, but with aresponse time of I sec, as few as 1011 spins ("" 10-12 moles) can be detected withcurrently available spectrometers. This sort of sensitivity suggests that ESR spec-troscopy would be useful for trace analysis. A minimum detectable concentrationis perhaps 10- 9 M in samples with very small dielectric loss. For qualitative meas-urement in aqueous solutions, 10- 7 M is more reasonable, while for quantitativemeasurements the sample concentration should be greater than about 10- 6 M.Unfortunately, ESR spectra are more applicable to qualitative and semiquan-

SEC.13.2 ESR Instrumentation 371

ESR spectroscopy consists of a source of electromagnetic radiation, a sample holder, and appropriate detection equipment for monitoring the amount of radiation absorbed by the sample.

In ESR spectroscopy a magnetic field provided by an electromagnet is also required.

Monochromatic radiation of the various frequencies employed in ESR work is obtained from klystrons, which are electronic oscillators producing microwave energy.

Spectrometers operating at X-band (3-cm wavelength) are the ones most commonly employed.

The microwave radiation is transmitted along hollow rectangular metal pipes called waveguides.

The sample is placed at the center of the sample cavity where the magnetic vector is at a maximum.

Quartz tubes (~3 mm o.d.) are generally employed to contain solid and solution samples.

Unlike the NMR technique, the sample tubes are not rotated.

The magnetic field is slowly and linearly increased until the resonance condition is satisfied, at which point power is absorbed by the sample and a change in current in the detector crystal is monitored.

A pair of Helmholtz coils are mounted around the cavity to increase sensitivity.

Feeding the coils from an oscillator superimposes a variable amplitude sinusoidal modulation on the slowly varying magnetic field.

The signal detected by the phase-sensitive detection system is proportional to the slope of the ESR absorption as the magnetic field passes through resonance.

The recorder then presents the first-derivative spectrum. Many spectrometers are also equipped to present second-derivative spectra.

The sample tube must be chosen with careful attention to the physical, chemical, and magnetic properties of each sample.

For instance, when the sample is dissolved in a polar solvent with an appreciable dielectric constant, quartz sample tubes are usually not suitable and capillaries or thin rectangular cells of glass are used.

Again, when studying free radicals with g = ge, Pyrex tubing can be used, whereas only quartz can be used for triplet (S =1) compounds because of paramagnetic impurities such as Fe3 + in most laboratory-grade glassware.

Finger sized Dewar flasks (of the appropriate materials) and other cryogenic equipment permit ESR spectra to be obtained as a function of temperature.

In any ESR experiment it is important to monitor the microwave frequency at which the spectrometer operates and the magnetic field range swept during the experiment.

Although the frequency is constant during an experiment, the frequency available from a given klystron will vary a little with tuning of the instrument.

The frequency can be determined using a built-in frequency meter or appropriate transfer oscillators and frequency counters.

The magnetic field can be monitored using an NMR gaussmeter or by using samples of known g-value; for instance, the DPPH free radical for which g = 2.0036.

The magnetic field sweep can also be checked using Fremy's radical or oxobis(2,4-pentanedionato)vanadium(IV).

APPLICATIONS :

Electron Spin Resonance, ESR, is a powerful non-destructive and non-intrusive analytical method.

ESR yields meaningful structural information even from ongoing chemical or physical processes, without influencing the process itself.

It is the ideal technique to complement other analytical methods in a wide range of application areas. One can perform studies related to :

Molecular structure Crystal structure Reaction kinetics Valence electron wave functions Molecular motion Relaxation properties Electron transport Crystal / ligand fields Reaction mechanisms etc.

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