- OVERVIEW OF ELECTRON SPIN RESONANCE AND
ITS APPLICATIONS -
H. A. Farach and C. P. Poole, Jr., Department of Physics
and Astronomy
University of South Carolina, Columbia, South Carolina 29208
USA
The phenomenon
of electron spin resonance (ESR) is based on the fact that an electron
is a charged particle which spins around its axis and this causes it to
act like a tiny bar magnet. In technical language we say that it has a
magnetic moment, the value of which is called the Bohr magneton.
If an external magnetic field Be is impressed on the system, the
electron will align itself with the direction of this field and process
around this axis. This behavior is analogous to that of a spinning top
in the earth's gravitational field. Increasing the applied magnetic field
will induce the electron to process faster and acquire more energy of motion
called kinetic energy. In practice, the magnetic field will divide the
electrons into two groups. In one group the magnetic moments of the electrons
are aligned with the magnetic field, while in the other group the magnetic
moments are aligned opposite or antiparallel to this external field.
Classically the needle of a compass points north, but if disturbed
it could point in any direction. At the microscopic level of the electron,
classical mechanics is replaced by quantum mechanics and one of the consequences
is that the electron can only point either in the same direction as the
external magnetic field or opposite to it. If perturbed only will point
in one of the two directions but not in between. These two possible orientations
in the applied field correspond to the projections Ms = 
1/2
along the magnetic field direction where Ms is a dimensionless number used
to designate the orientation. Each orientation is associated with a different
energy, the one with the spins antiparallel to the external field (Ms =
1/2) being the lower energy state.
If a second weaker alternating magnetic field B1 oscillating
at a microwave frequency is now applied at right angles to the main field
B0, then the electron can be "tipped" over when the
microwave frequency is equal to the precession frequency. Another way to
describe the phenomenon of ESR is to say that the quanta of the incident
microwaves induce transitions between the two states of the unpaired
electron. When the energy hvof these quanta coincides with the energy level
separation :
between the two states then resonance absorption of energy takes
place.
The incoming radiation hv absorbed by the electrons in the lower
energy level will induce these electrons to jump into the higher energy
state. The incoming radiation is, however, also absorbed by the electrons
in the higher energy level causing them to jump down to the lower level,
a phenomenon called stimulated emission. Since the coefficients of absorption
and stimulated emission are equal, no net value would be observed if the
spin population were equally distributed between these two levels. In general,
however, no, the population of the groundstate, exceeds n2,
the population of the excited state, and a net absorption of microwave
radiation takes place.
The population ratio of these two states at the temperature T can
in most cases be described by the Boltzmann distribution
where k is a universal constant of nature called Boltzman's constant.
A material containing atomic magnetic moments satisfying this Boltzmann
distribution is called paramagnetic. Since 
E
= hv at resonance the sensitivity of this technique is improved by using
a high applied frequency. In practice, the most common frequencies in ESR
are those in the welldeveloped radar wavebands. In addition, low operating
temperatures are also beneficial for signal enhancement.
In most substances chemical bonding results in the pairing of the
electrons which are transferred from one atom to another atom to form an
ionic bond; or are shared between atoms to form a covalent bond so these
materials are not magnetic. However, in a paramagnetic substance, i. e.
one which contains unpaired electrons, resonance occurs at definite values
of the applied magnetic field and incident microwave radiation.
At first glance one would expect the resonance spectrum of an unpaired
electron to be always the same, but it is not because the magnetic behavior
of the electron is modified by its surroundings. This modification of the
resonance by the surroundings permits us to learn something about the structure
of the material under study.
In a typical experiment the sample is placed in a resonant cavity
which has a high quality factor or high Q. At a fixed microwave frequency
o, and the magnetic
field is varied until resonance occurs at the value Be given by
hv = g
Bo
(2)
In this equation g is the "gfactor" which is a dimensionless
constant and ,
is the unit magnetic moment of the spinning electron called the Bohr magneton
.
In ESR it is customary to measure the gfactor. Some transition
ions such as cobalt (Co2+) and copper (Cu2+) have gvalues which differ
significantly form g = 2, but free radicals have values that are fairly
close to g = 2.0023 which characterizes a free electron.
Most ESR spectra are more complex than is implied by Equation 2.
The resonant absorption is not infinitely narrow, which would be the case
if absorption occurs at a precise value of the applied magnetic field.
The actual line width ranges from a few milligauss for free radicals in
solution to over 1000 gauss for some transition metal compounds in the
solid state.
The reason for the finite line width B
is that unpaired electrons not only interact with the externally applied
magnetic field but also interact in a more or less random manner with the
magnetic fields in their neighborhood. By observing this line width and
line intensity, one can obtain information about the spin environment.
Electron spin exchange between identical and nonidentical molecules, chemical
exchange between the paramagnetic molecule and its environment, and the
interaction of nearby molecules having unpaired spins are some examples
of environmental effects which can influence line width and intensity in
the ESR spectrum.
An observed spectrum sometimes contains several lines referred to
as hyperfine structure arising from the electrons interacting with nuclear
spins denoted by I. The electronic spin of a transition metal ion usually
interacts with its own nuclear spin, and in aromatic free radicals, the
unpaired electron circulates among several atoms; and the resultant hyperfine
structure is the result of the interaction of this electronic spin with
several atoms such as hydrogen with nuclear spins. An example is the
ethyl radical :
in which the unpaired electron ( 
)
interacts with the nuclear spins of two hydrogen atoms on the same carbon
atom and also with the three hydrogens of the adjacent methyl group (CH3).
The energy levels of an unpaired electron interacting with two nuclear
spins is shown in Figure 1.
The simplest case is one in which there is no hyperfine interaction
I=0). By placing the unpaired electron in a magnetic field, the
number of energy levels is increased from one (E = Eo) to two (E = E0
1/2gH),
as shown in the left portion of Figure 1.
The center and right side of the figure show cases where two nuclear
spins with I=1/2 interact with the electron. A measure for the amount of
interaction is given by the hyperfine coupling constant, Ai
Each nuclear spin I1 and I2 splits each original
level into two levels. Hence, for A1 
A2,
the original two levels are split into 8 levels, as shown. Although there
are 8 levels, only 4 transitions are possible because of the following
selection rules.
MS
= 1
MI
= 0
where Ms is the electron spin quantum number and MI
is the nuclear spin quantum number.
The hyperfine coupling constant Al with the nuclear species, and
hyperfine splittings of the order of 10 millitesla or more can be observed.
In organic compounds the electron may interact with several nuclei, and
the hyperfine interactions may be as low as a few microtesla. The technique
of ESR has been used to study many types of systems and we will comment
on a few of them.
1. Biological Systems
ESR has been applied quite extensively to biological systems. One
can follow the variations that occur under changing environmental conditions
by monitoring the intensity of a free radical signal. For example, the
presence of free radicals has been studied in healthy and diseased tissue.
If a transition metal ion is present, as in case of the Fe ions of hemoglobin,
then changes in its valence state may be studied by ESR. Early concrete
evidence for the role of free radicals in photosynthesis, was provided
by the observation of a sharp ESR resonance line. When incident light was
turned off the resonance soon weakened or disappeared.
An inconvenience associated with the analysis of biological samples
is the presence of water which produces high dielectric losses, making
it necessary to use a flat quartz sample cell. Some typical systems which
have been studied by ESR are hemoglobin, nucleic acids, enzymes, irradiated
chloroplasts, riboflavin (before and after UV irradiation), and carcinogens.
2. Chemical Systems
A number of chemical substances such as synthetic polymers and rubber
contain free radicals. The nature of the free radical depends upon the
method of synthesis and on the history of the substance. Recent refinements
in instrumentation have permitted the detection of free radical intermediates
in chemical reactions.
Typical chemical systems that have been studied by ESR are polymers,
catalysts, rubber, alonglived free radical intermediates, charred carbon,
and chemical complexes with transition metals.
3. Conduction Electrons
Conduction electrons have been detected by both conventional electron
spin resonance methods, and by the related cyclotron resonance technique.
The latter employs and ESR spectrometer and the sample is located in a
region of strong microwave electric field strength. In the usual ESR arrangement,
on the other hand, the sample is placed at a position of strong microwave
magnetic field strength. Conduction electrons have been detected in solutions
of alkali metals in liquid ammonia, alkaline earth metals (fine powders),
alloys (e. g., small amounts of paramagnetic metal alloyed with another
metal), nonresonant absorption of microwaves by superconductors, and graphite.
4. Free Radicals
A free radical is a compound which contains and unpaired spin, such
as the methyl radical CH3 produced through the breakup of methane
CH4
CH3
+ H (5)
where both the hydrogen atom and the methyl radical are electrically
neutral. A charged free radical or radical ion is a neutral molecule which
has gained or lost an electron
C6H6 + eC6H6
-(cation)
(6)
C6H6 e
+ C6H6+ (anion)
Free radicals have been observed in gaseous, liquid, and solid systems.
They are sometimes stable, but can also be shodlived intermediates in
chemical reactions.
Free radicals and radical ions ordinarily have gfactors close to
the free electron value of 2.0023 (e. g., for the stable free radical 
,
' diphenyl-picrylhydrazyl,
referred to as DPPH, g = 2.0036). In lowviscosity solutions, radicals
exhibit hyperfine patterns with a typical overall spread of about 2.5 millitesla.
The scrupulous removal of oxygen often reveals hitherto unresolved structure.
In a highconcentration solid a single exchange narrowed resonance appears
(B~0.3 millitesla
for DPPH prepared from benzene solution), whereas DPPH in a dilute oxygen
free solution of the solvent tetrahydrofurane produces a spectrum with
more than 100 hyperfine components.. In irradiated single crystals the
free radicals may have strongly directionally dependent or anisotropic
hyperfine interactions and slightly anisotropic gfactors.
Radical ions of many organic compounds can be produced in an electrolytic
cell which is usually a flat quartz cell with a mercury pool cathode and
a platinum anode. This electrolytic cell may be mounted in a flat measuring
cell located in the resonance cavity. When the applied voltage in the electrolytic
system is increased, the current will first increase, but soon levels off
to a plateau. Radical ions are formed in this plateau region. Radical formation
can sometimes be observed visually because of color changes in the solution.
Since oxygen is also paramagnetic, dissolved air must be scrupulously removed
prior to the experiment. The best method is to use the freezepumpthaw
technique where the sample is first frozen and then connected to a high
vacuum source. After closing off the vacuum pump, the sample is melted
and refrozen. Several cycles are sufficient to remove the oxygen.
5. Irradiated Substances
A considerable amount of work has been done on radiation induced
free radicals in organic compounds and paramagnetic color centers in ionic
solids. Most irradiations are carried out with xrays, gammarays, or electrons
whose energies far exceed chemical bond energies, although paramagnetic
spins can also be produced by less energetic ultraviolet light or neutrons.
Most of these spectra are obtained after the sample is irradiated, and
many paramagnetic centers are found to be sufficiently longlived to warrant
such a
procedure. More sophisticated experimental techniques entail simultaneous
irradiation and ESR detection. Lowtemperature irradiation and detection
can reveal the presence of new centers which can be studied at gradually
increasing temperatures to elucidate the kinetics of their recombination.
6. Naturally Occurring Substances
Most of the systems studied by ESR are synthetic or manmade. Nevertheless,
from the beginning of the field, various naturally occurring substances
have been studied, such as: (1) minerals with transition elements [e. g.,
ruby (Cr/Al2O3), dolomite Mn/(Ca, Mg(Co3)]; (2) minerals with defects (e.
g., quartz); (3) hemoglobin (Fe); (4) petroleum; (5) coal; (6) rubber;
and (7) various biological systems.
7. Spin labels
There are a number of free radicals called spin labels which can
attach themselves to particular sites in biological systems and produce
spectra which provide information on changes in the chemical and physical
characteristics in the neighborhood of the site. An example of a spin label
has its unpaired electron on the NO group of a nitroxyl compound
such as 2, 2, 6, 6tetramethyl-4piperadone1oxyl
The unpaired electron shown on the lower oxygen, spends time on
and strongly interacts with the I = 1 nuclear spin of the nitrogen atom
to produce a 3line hyperfine spectrum. When the solvent has a low viscosity
(flows very freely) the radical tumbles rapidly to average out the anisotropies,
and a spectrum of three narrow lines is obtained. At lower temperatures
the viscosity increases and the solvent materials flows very slowly so
the nitrogen radical can no longer tumble rapidly, and a smeared out spectrum
is produced.
When the spin label attaches itself to a particular site in a biological
molecule its spectrum reveals the extent to which free motion or very restricted
motion occurs at the site. Different spin labels have been synthesized
which attach themselves to very specific sites. For example the spin label
ndoxylstearic has the nitroxide doxyl group attached to different positions
n between 5 and 16 on the long stearic acid molecule. This spin label can
be inserted into a cell membrane so each value of n corresponds to a different
depth in the membrane, and a sequence of labels provides information on
the activities of the membrane at various depths below the surface.
BlBLOGRAPHY
Two chapters in Ref. 1 recount the history of ESR, Chap. 1 of Ref.
2 serves as an entree to the extensive literature, and Chap. 1 of Ref.
3 provides a more extensive introduction to the subject. The theoretical
underpinning is surveyed in Refs. 4 and 5.
1. G. R. Eaton and S. S. Eaton, EPR at 50, in press. See Chapters
"Preparing the way for Paramagnetic Resonance" and "The
First Sesquidecade of Paramagnetic Resonance" by C. P. Poole, Jr.
and H. A. Farach.
2. C. P. Poole, Jr. and H. A. Farach, Handbook of Electron Spin Resonance,
American Institute of Physics Press, NY, 1994, see Chap. 2. Data Sources.
3. F. J. Owens, C. P. Poole, Jr., and H. A. Farach, Magnetic Resonance
of Phase Transitions, Academic Press NY 1979; see Chap. 1 Magnetic
Resonance as a Probe of Phase Transitions.
4. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition
Ions, Dover NY 1986.
5. C. P. Poole, Jr. and H. A. Farach Theory of Magnetic Resonance
2nd Edition Wiley NY 1987.
