INTRODUCTION 1.1. Introduction Electron Paramagnetic Resonance (EPR) has been used as a tool to identify paramagnetic transition metal ions, trapped hole centres and electrons close to the conduction states in non-linear optical and photorefractive materials [1-3]. The transition-group, rare earth and actinide ions that are the members of the 3d, 4d, 5d, 4f and 5f groups have been the subject of EPR investigations. Of the approximately 160 known elements, 55 belong to these series. Transition metal ion complexes and salts have played a vital role in many aspects of EPR, including the development of the spin Hamiltonian concept. Their importance was based on: 1. Availability of a number of unpaired electrons per species (total spin S ranges from 1/2 to 7/2) 2. Availability of systems with simple local symmetries (e. g., cubic) and well-characterized neighbors to the central ion 3. Easy preparation, stability and with a variety of oxidation states 4. Availability of reasonably applicable and adequate electronic theory, for example, the crystal-field model. Later on, researchers started investigating systems of lower-symmetry, since these have a tremendous application in chemical catalysis and biomedical. One aspect that makes transition elements interesting subjects for study by EPR or other techniques is their variable valence (Table 1.1). This feature is due to their unfilled electron shells. 1
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INTRODUCTION
1.1. Introduction
Electron Paramagnetic Resonance (EPR) has been used as a tool to identify
paramagnetic transition metal ions, trapped hole centres and electrons close to the
conduction states in non-linear optical and photorefractive materials [1-3]. The
transition-group, rare earth and actinide ions that are the members of the 3d, 4d, 5d, 4f
and 5f groups have been the subject of EPR investigations. Of the approximately 160
known elements, 55 belong to these series.
Transition metal ion complexes and salts have played a vital role in many aspects
of EPR, including the development of the spin Hamiltonian concept. Their importance
was based on:
1. Availability of a number of unpaired electrons per species (total spin S
ranges from 1/2 to 7/2)
2. Availability of systems with simple local symmetries (e. g., cubic) and
well-characterized neighbors to the central ion
3. Easy preparation, stability and with a variety of oxidation states
4. Availability of reasonably applicable and adequate electronic theory, for
example, the crystal-field model.
Later on, researchers started investigating systems of lower-symmetry, since these
have a tremendous application in chemical catalysis and biomedical. One aspect that
makes transition elements interesting subjects for study by EPR or other techniques is
their variable valence (Table 1.1). This feature is due to their unfilled electron shells.
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For example, the easiness of iron to change between +2 and +3 oxidation states provides
sites for electron transfer in biological oxidation-reduction systems. The observation of
hyperfine splitting will help to identify the central nucleus. The nucleus need not be that
of the host ion; it may instead be a foreign nucleus that is present naturally or is
introduced by doping [4].
The structural simplicity, symmetry, wide range of optical transparency and ease
of preparation of ionic crystals has made them the subject of intensive investigation. In
view of the fact that they are intrinsically insulators with a forbidden gap of ∼8 eV, these
crystals have a wide range of optical transparency. At one end, the UV absorption is due
to electronic excitation and at the other end the lattice vibrations are responsible for IR
absorption. Such crystals are good host media for incorporating various paramagnetic
impurities that can be used as probes for investigating the properties of the host material.
In this process, the influence of the host on the electronic and vibrational properties of the
impurity can also be investigated. This property has been used to extract information
about several important parameters of the materials. A number of experimental
techniques have been used in this process. Some of the physical properties of the crystal
are mainly due to the existence of defects in the crystal, while many others are indirectly
influenced by defects. In the first category, one may mention ionic conductivity,
diffusion, dielectric loss and ionic thermo currents. Properties such as thermal
conductivity, mechanical strength, etc. are strongly influenced by defects. Most of these
defects either created artificially or natural to the crystal, give rise to localized energy
levels between the valence band and the conduction band. Such defects are studied by
optical absorption, luminescence, photoconductivity, etc. Other deliberately introduced
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impurities are paramagnetic in nature and in such cases; the powerful tool of EPR can be
used to identify the surroundings and the location of the impurity.
In some cases, when the impurity defect is a molecular entity, vibrational
spectroscopy of the impurity can be used to investigate the properties of the material. It
will obviously be a great advantage to choose an impurity that will give optical
absorption and is paramagnetic, which has the characteristic vibrational frequencies that
affect the conductivity and dielectric loss. In initial stages of solid state research, the
alkali halides received a lot of attention because of their high symmetry, simple crystal
structure and the ease of preparation of single crystals. A vast amount of work has been
done on the study of various low symmetry materials and an exhaustive review is not
possible in this thesis. However, only some representative examples of various low-
symmetry complexes are presented in their respective chapters. Main influence of the
presence of low symmetry is seen in properties such as lifting of the degeneracy of
electronic energy levels, the splitting of the degenerate modes in the infrared, the high
anisotropy in the g and hyperfine interaction tensors, the unusual sequence of spin-
Hamiltonian parameters, the abnormally low values of hyperfine coupling constants,
mixed ground states, occurrence of spin-forbidden transitions, vibronic mixing, Jahn-
Teller distortions, phase transitions and co-operative magnetic phenomena.
The principle of EPR is based on the possibility of recording unpaired electrons in
atomic, crystalline ionic and molecular structures; in complexes of transition metal ions
such as Cu(II), VO(II), Cr(III), Mo(II), Mn(II), Fe(III), Ni(II), etc.; upon cleavage of
covalent chemical bonds. In the latter case, the unpaired electron(s) appear either at
unoccupied atoms or at molecular and macromolecular fragments, i.e., at free radicals of
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higher activity. Many of the above electronic states have been reported in biological
systems. Over the decades, EPR technique has been constantly improved [5-7]. EPR
spectroscopy enables unpaired electrons to be detected and characterized. This is
possible due to the fact that odd electrons have a magnetic moment caused by spin (S), at
least equal to 1/2.
Molecules with unpaired electrons, like radicals, transition metal complexes, etc.
have a wide range of applications in chemistry, physics, biology and medicine. It may be
used to probe the "static" structure of solid and liquid systems and is also very useful in
investigating dynamic processes.
Structural and spectral investigations on complexes of simple carboxylic acids of
transition metal ions and rare earths, despite a long history, continue to be of current
interest for several reasons. They exist in widely different structures showing the
influence of metal-metal interaction, hydrogen bonding and marked differences in their
thermal decomposition behavior [8-10]. Interest in these salts also stems from the fact
that they lend themselves to a comparative study of salts of the same carboxylic acid with
different metal ions, of different carboxylic acids with the same metal ions and of the
transition metal ions with the rare earths in analogous situations. Published studies on
solids by employing techniques such as X-ray structure determinations, optical spectra,
EPR, Mossbauer spectroscopy and differential thermal and thermo gravimetric studies [9,
10] are numerous.
Coordination compounds containing malonic acid as a ligand have been recently
studied due to their potential application as materials in molecular electronics, catalysts,
biologically active compounds, molecular-based magnetic materials, etc. [11, 12]. On the
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other hand, dicarboxalic acids show additional features such as formation of both normal
and acid complexes, another chemically interesting feature for a comparative study. With
these points of view in mind, EPR studies on single crystals, optical studies, IR behavior
of complexes of simple polycarboxylic acids such as malonic and maleic with transition
metal ions have been undertaken. In the case of malonic acid, detailed crystal structures
have been designed for Cu(II) doped in Diaquamalonatozinc(II) (DAMZ),
Diaquamalanato(1,10-phenanthroline)zinc(II) (DAMPZ) and Fe(III) doped in
Dipotassium diaquabis(malonato-κ2O,O’)zincate(II) dihydrate (PMZD) [13-15]. These
studies show several interesting features. The interest in Cu(II), Fe(III) doped DAMZ,
DAMPZ and PMZD arises from the non isomorphous nature of the metals and zinc
complexes with the difference in the coordination and the interesting options that become
available as a result, regarding the microsymmetry of the guest ion in the host lattice. An
important feature of the malonic bridge is the fact that the magnitude of the exchange
interaction depends on the possible bridging modes that it can adopt. Thus the ferro- or
antiferromagnetic interactions may appear in malonate complexes, governed by the
dimensionality of the structure. In recent years, a vast amount of research has been
devoted to the characterization of dimeric copper(II) complexes. Much of this interest
stems from the fact that copper(II) dimers are excellent model systems for the study of
magnetic interactions [16].
Tutton salts form an isostructural series of hydrated double sulphates which have
been studied by many techniques, because they are easily grown and are interesting for
many subjects. Detailed EPR studies of VO(II)-doped Tutton salts, estimating, among
other things, ligand superhyperfine (SHF) interaction, bonding coefficients, impurity-
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host exchange interaction and host-ion spin-lattice relaxation times. The present thesis
(chapter V and VI) reports an extensive X-band EPR study on a VO(II) doped zinc
sodiumsulphatehexahydrate (ZSSH) and cis-diaquabis(1,10-phenanthroline-N,N')zinc(II)
(1,10-Phenanthroline-N,N') bis(thiosulfato-S)zincate(II) Monohydrate (DPZSZM) single
crystal in the room temperature [17,18].
The motivations for the present studies are: (i) to evaluate accurately the g, A
tensors of the VO(II) ion in ZSSH without the assumption of the coincidences of their
principal axes, using eigen values calculated to the second order perturbation (ii) to
determine correctly the orientations of the VO(II)-O2- bond axes in ZSSH from a
knowledge of the orientations of the principal axes of the g2 tensor (iii) to estimate the
VO(II) impurity ion EPR line width, using the correct expression applicable to the
presence of two different kinds of paramagnetic ions and (iv) to estimate the bonding
coefficients of the [VO(H2O)5]2+ complex in ZSSH using the EPR and optical absorption
data.
1. 2. Electron Paramagnetic Resonance
Electron Paramagnetic Resonance (EPR), often called Electron Spin Resonance
(ESR), is a branch of spectroscopy in which electromagnetic radiation (usually of
microwave frequency) is absorbed by molecules, ions, or atoms possessing electrons with
unpaired spins, i.e., electron spin S > 0. EPR was discovered by Zavoisky in 1945 [19].
EPR spectroscopy is closely related to the probably better known nuclear magnetic
resonance (NMR). The NMR technique deals with nonzero nuclear spins, I > 0. In both
EPR and NMR, the sample material is kept in a strong static magnetic field and subjected
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to an orthogonal low- amplitude high-frequency field. EPR usually requires microwave-
frequency radiation (GHz), while NMR is observed at lower radio frequencies (MHz).
With EPR, energy is absorbed by the sample when the frequency of the radiation is
appropriate to the energy difference between the two states of the electrons in the sample,
but only if the transition satisfies the appropriate selection rules. Some of the
paramagnetic transition metal ions V, Fe, Mn, Co, Cu, Mo, and Ni are biologically
important elements also.
EPR spectra are routinely obtained for paramagnetic transition ions in crystals,
chemical complexes and bio-molecules. Samples may be in the form of crystal, powder,
solution or frozen solution. Most commonly, these systems should have electron spin (S)
and S can be any value from 1/2 to 7/2 in increments of 1/2. When the spin is odd, i.e.,
for example, S = 1/2, 3/2, 5/2 or 7/2, spectra are easily obtained at room temperature.
However if the spin is even, i.e., for example, S = 1, 2 or 3, then the possibility of
obtaining spectra depends upon a number of circumstances that are usually not met; thus
EPR of even spin systems is a very specialized one. EPR spectroscopy is capable of
providing molecular structural details inaccessible by other analytical tools. EPR has
developed into a potent, multipurpose, non-destructive and non-intrusive analytical
method. Hence, EPR has become a powerful tool in the hands of chemists, physicists,
geologists, mineralogists, biologists, etc.
EPR technique detects unpaired electrons, identity of the molecule and
information of the molecular structure (structure, dynamics, bounding), molecular
environment (< 0.8 nm for nuclear spins and up to 50 nm for other electron spins). These
capabilities of EPR are a result of the unpaired electron’s spin magnetic moment being
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very sensitive to local magnetic fields within the sample. These fields often arise from
the nuclear magnetic moments of various nuclei that may be present within the bulk
medium. Examples of such nuclei are interstitial atoms (or ions) within a crystal or glass
matrix, nuclei (such as nitrogen) within the molecular structure that also contains the
unpaired electron, and so on. Thus EPR provides a unique means of studying the internal
structures in great detail.
EPR can provide valuable information on structural and dynamical aspects, even
from current chemical and/or physical processes without influencing the process itself.
Therefore, EPR is considered as an ideal complementary technique for other methods in a
wide range of studies in various branches of science and humanities [20-26].
1. 3. EPR applications
ESR can be used to obtain structural information of molecules together with
details about their electron density distributions. In solutions and solids, the dynamics of
molecules can be determined and the kinetics of chemical reactions can be studied.
Quantitatively, analytical applications such as dosimetry can be mentioned, as well as
characterization of the redox-active centers in proteins. All of these properties can be
observed at room temperature, but often lower temperatures are used.
1.3.1. Biomedical EPR spectroscopy
As the spin of an unpaired electron can sense its nearby environment, each free
radical or transition metal ion will have slightly different properties. Therefore EPR can
be used to identify biological molecules that contain free radicals or transition metal ions
in their structure. EPR is a quantitative technique, i.e., the concentration of unpaired
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electrons present in a sample can be estimated, even if the exact nature of the free radical
being observed is not known. This property is very important to distinguish between
reactive free radicals that are present in high concentrations and may be damaging and
those that may be present in only very low concentrations and may not be. EPR
spectroscopy has also find its application to understand the pathophysiology and
underlying chemical mechanism of a wide range of diseases such as Parkinson's disease,
birth asphyxia, stroke, septic shock, kidney damage and coronary heart disease. Two
major types of EPR signals of biomedical relevance are unpaired electrons in free
radicals and metalloproteins.
In summary, various paramagnetic systems present in different branches of
sciences that can be studied by EPR technique are:
1.3.2. Chemistry
Atoms or ions with partially filled inner electron shells (transition metals)
Measurement of magnetic susceptibility
Molecules with odd number of electrons
Molecules with even number of electrons (triplet states)
Oxidation and reduction processes
Biradicals and triplet state molecules
Electron transfer reaction kinetics
Structure, dynamics and reaction of polymers
Short-time behavior of organic free radicals produced by radiation