Electronic (Absorption) Spectra of 3d Transition Metal Complexes

Since electronic excited states are sensitive to the local fluid environment, dopant electronic transitions are an appropriate probe to study the structure of near critical point fluids (i.e., perturbers). In comparison to valence states, Rydberg states are more sensitive to their environment [1]. However, high-n Rydberg states are usually too sensitive to perturber density fluctuations, which makes these individual dopant states impossible to investigate. (Nevertheless, under the assumption that high-n Rydberg state energies behave similarly to the ionization threshold of the dopant, dopant high-n Rydberg state behavior in supercritical fluids can be probed indirectly by studying the energy of the quasi-free electron, through photoinjection [2–11] and field ionization [12–19].) Low-n Rydberg states, on the other hand, are excellent spectroscopic probes to investigate excited state/fluid interactions.


Types of spectra
Spectra are broadly classified into two groups (i) emission spectra and (ii) absorption spectra i. Emission spectra Emission spectra are of three kinds (a) continuous spectra,(b) band spectra and (c) line spectra.
Continuous spectra: Solids like iron or carbon emit continuous spectra when they are heated until they glow. Continuous spectrum is due to the thermal excitation of the molecules of the substance.
Band spectra: The band spectrum consists of a number of bands of different colours separated by dark regions. The bands are sharply defined at one edge called the head of the band and shade off gradually at the other edge. Band spectrum is emitted by substances in the molecular state when the thermal excitement of the substance is not quite sufficient to break the molecules into continuous atoms.
Line spectra: A line spectrum consists of bright lines in different regions of the visible spectrum against a dark background. All the lines do not have the same intensity. The number of lines, their nature and arrangement depends on the nature of the substance excited. Line spectra are emitted by vapours of elements. No two elements do ever produce similar line spectra.
ii. Absorption spectra: When a substance is placed between a light source and a spectrometer, the substance absorbs certain part of the spectrum. This spectrum is called the absorption spectrum of the substance.
The resultant L may be once again 0, 1, 2, 3, 4…. which are referred to as S, P, D, F G,… respectively in units of h/2π.The orbital multiplicity is given by (2L+1). The term symbol is given by 2S+1 LJ. The left-hand superscript of the term is the spin multiplicity, given by 2S+1 and the right-hand subscript is given by J. It should be noted that S is used to represent two things-(a) total spin angular momentum and (b) and total angular momentum when L = 0. The above rules are illustrated with examples.
Advanced Aspects of Spectroscopy 6 For d 4  Spin multiplicity indicates the number of orientations in the external field. If the spin multiplicity is three, there will be three orientations in the magnetic field.-parallel, perpendicular and opposed. There are similar orientations in the angular momentum in an external field.
The spectroscopic term symbols for d n configurations are given in the Table-1. The terms are read as follows: The left-hand superscript of the term symbol is read as singlet, doublet, triplet, quartet, quintet, sextet, septet, octet, etc., for spin multiplicity values of 1, 2, 3, 4, 5, 6, 7, 8, etc., respectively. 1 S0 (singlet S nought); 2 S1/2 (doublet S one-half); 3 P2 (triplet P two ); 5 I8 (quintet I eight). It is seen from the It is also found that empty sub -shell configurations such as p 0 , d 0 , f 0 , etc., and full filled subshell configurations such as p 6 , d 10 , f 14 , etc., have always the term symbol 1 S0 since the resultant spin and angular momenta are equal to zero. All the inert gases have term symbols for their ground state 1 S0 .Similarly all alkali metals reduce to one electron problems since closed shell core contributes nothing to L , S and J; their ground state term symbol is given by 2 S1/2. Hence d electrons are only of importance in deciding term symbols of transition metals.

Total degeneracy
We have seen that the degeneracy with regard to spin is its multiplicity which is given by (2S+1). The total spin multiplicity is denoted by Ms running from S to -S. Similarly orbital Electronic (Absorption) Spectra of 3d Transition Metal Complexes 7 degeneracy, ML, is given by (2L+1) running from L to -L. For example, L= 2 for D state and so the orbital degeneracy is (2x2+1) =5 fold. Similarly, for F state, the orbital degeneracy is seven fold. Since there are (2L+1) values of ML, and (2S+1) values of Ms in each term, the total degeneracy of the term is given by: 2(L+1)(2S+1).
Each value of ML occurs (2S+1) times and each value of Ms occurs (2L+1) times in the term. For 3 F state, the total degeneracy is 3x7 =21 fold and for the terms 3 P, 1 G, 1 D, 1 S, the total degeneracy is 9,9,5,1 fold respectively. Each fold of degeneracy represents one microstate.

Number of microstates
The electrons may be filled in orbitals by different arrangements since the orbitals have different ml values and electrons may also occupy singly or get paired. Each different type of electronic arrangement gives rise to a microstate. Thus each electronic configuration will have a fixed number of microstates. The numbers of microstates for p 2 configuration are given in Table-2 (for both excited and ground states). Each vertical column is one micro state. Thus for p 2 configuration, there are 15 microstates. In the above diagram, the arrangement of singlet states of paired configurations given in A (see below) is not different from that given in B and hence only one arrangement for each ml value.
The number of microstates possible for any electronic configuration may be calculated from the formula,

Number of microstates = n! / r! (n -r)!
Where n is the twice the number of orbitals, r is the number of electrons and ! is the factorial.
Advanced Aspects of Spectroscopy 8 Similarly for a d 2 configuration, the number of microstates is given by 10! / 2! (10 -2)!  

Multiple term symbols of excited states
The terms arising from d n configuration for 3d metal ions are given  3 Table 4. Terms arising from d n configuration for 3d ions (n=1 to10) Electronic (Absorption) Spectra of 3d Transition Metal Complexes 9 8. Selection rules 8

.1. La Porte selection rule
This rule says that transitions between the orbitals of the same sub shell are forbidden. In other words, the for total orbital angular momentum is Δ L = ± 1. This is La Porte allowed transitions. Thus transition such as 1 S→ 1 P and 2 D→ 2 P are allowed but transition such as 3 D→ 3 S is forbidden since Δ L = -2 .That is, transition should involve a change of one unit of angular momentum. Hence transitions from gerade to ungerade (g to u) or vice versa are allowed, i.e., u → g or g → u but not u → u or g → g. In the case of p sub shell, both ground and excited states are odd and in the case of d sub shell both ground and excited states are even. As a rule transition should be from even to odd or vice versa.
The same rule is also stated in the form of a statement instead of an equation: Electronic transitions within the same p or d sub-shell are forbidden, if the molecule has centre of symmetry.

Spin selection rule
The selection Rule for Spin Angular Momentum is Thus transitions such as 2 S→ 2 P and 3 D→ 3 P are allowed, but transition such as 1 S→ 3 P is forbidden. The same rule is also stated in the form of a statement, Electronic Transitions between the different states of spin multiplicity are forbidden.
There is no selection rule governing the change in the value of n, the principal quantum number. Thus in hydrogen, transitions such as 1s → 2p, 1s → 3p, 1s → 4p are allowed.
Usually, electronic absorption is indicated by reverse arrow, ← , and emission is indicated by the forward arrow, → , though this rule is not strictly obeyed.

Spin-orbit coupling
For electronic transition to take place, Δ S = 0 and Δ L= ± 1 in the absence of spin-orbit coupling. However, spin and orbital motions are coupled. Even, if they are coupled very weakly, a little of each spin state mixes with the other in the ground and excited states by an amount dependent upon the energy difference in the orbital states and magnitude of spin -orbit coupling constant. Therefore electronic transitions occur between different states of spin multiplicity and also between states in which Δ L is not equal to ± 1. For example, if the ground state were 99% singlet and 1% triplet (due to spin-orbit coupling) and the excited state were 1% singlet and 99 % triplet, then the intensity would derive from the triplet -triplet and singlet-singlet interactions. Spin-orbit coupling provides small energy differences between degenerate state.
This coupling is of two types. The single electron spin orbit coupling parameter ζ, gives the strength of the interaction between the spin and orbital angular momenta of a single electron for a particular configuration. The other parameter, λ, is the property of the term. For high spin complexes,

  
Here positive sign holds for shells less than half field and negative sign holds for more than half filled shells. S is the same as the one given for the free ion. The λ values in crystals are close to their free ion values. Λ decreases in crystal with decreasing Racah parameters B and C. For high spin d 5 configuration, there is no spin orbit coupling because 6 S state is unaffected by the ligand fields. The λ and ζ values for 3d series are given in Table-

La Porte selection rule
Physically 3d (even) and 4p (odd) wave functions may be mixed, if centre of inversion (i) is removed. There are two processes by which i is removed.
a. The central metal ion is placed in a distorted field (tetrahedral field, Tetragonal distortions, etc.,) The most important case of distorted or asymmetric field is the case of a tetrahedral complex. Tetrahedron has no inversion centre and so d-p mixing takes place. So electronic transitions in tetrahedral complexes are much more intense, often by a factor 100, than in a analogous octahedral complexes. Trans isomer of [Co(en)2Cl2] + in aqueous solution is three to four times less intense than the cis isomer because the former is centro-symmetric. Other types of distortion include Jahn -Teller distortions. b. Odd vibrations of the surrounding ligands create the distorted field for a time that is long enough compared to the time necessary for the electronic transition to occur (Franck Condon Principle).Certain vibrations will remove the centre of symmetry. Mathematically this implies coupling of vibrational and electronic wave functions. Breaking down of La Porte rule by vibrionic coupling has been termed as "Intensity Stealing". If the forbidden excited term lies energetically nearby a fully allowed transition, it would produce a very intense band. Intensity Stealing by this mechanism decreases in magnitude with increasing energy separation between the excited term and the allowed level.
Electronic (Absorption) Spectra of 3d Transition Metal Complexes 11

Splitting of energy states
The symbols A(or a) and B (or b) with any suffixes indicate wave functions which are singly degenerate. Similarly E (or e) indicates double degeneracy and T (or t) indicates triple degeneracy. Lower case symbols, a1g, a 2g, e g, etc., are used to indicate electron wave functions(orbitals) and upper case symbols are used to describe electronic energy levels. Thus 2 T2g means an energy level which is triply degenerate with respect to orbital state and also doubly degenerate with respect to its spin state. Upper case symbols are also used without any spin multiplicity term and they then refer to symmetry (ex., A1g symmetry). The subscripts g and u indicate gerade (even) and ungerade (odd).
Splitting of D state parallels the splitting of d orbitals and splitting of F state splits parallels splitting of f orbitals. For example, F state splits into either T1u, T2u and A2u or T1g, T2g and A2g sub-sets. Which of these is correct is determined by g or u nature of the configuration from which F state is derived. Since f orbitals are u in character 2 F state corresponding to f 1 configuration splits into 2 T1u, 2 T2u, and 2 A2u components; similarly 3 F state derived from d 2 configuration splits into 3 T2g, 3 T1g and 3 A2g components because d orbitals are g in character.

Splitting of energy states corresponding to d n terms
These are given in Table-6.   Energy  Sub-states   S  A1   P  T1   D  E + T2   F  A2+ T1+ T2   G  A1 + E + T1 + T2   H  E + T1 + T1 + T2   I  A1 + A2 + E + T1 + T2 + T2   Table 6. Splitting of energy states corresponding to d n terms The d-d spectra is concerned with d n configuration and hence the crystal field sub-states are given for all the d n configuration in Table -7.  Table 7. Crystal field components of the ground and some excited states of d n (n=1 to 9) configuration

Energy level diagram
Energy Level Diagrams are described by two independent schemes -Orgel Diagrams which are applicable to weak field complexes and Tanabe -Sugano (or simply T-S) Diagrams which are applicable to both weak field and strong field complexes.

Inter-electronic repulsion parameters
The inter-electronic repulsions within a configuration are linear combinations of Coulombic and exchange integrals above the ground term. They are expressed by either of the two ways: Condon -Shortley parameters, F0, F2 and F4 and Racah parameters, A, B and C. The magnitude of these parameters varies with the nature of metal ion.

Racah parameters
The Racah parameters are A, B and C. The Racah parameter A corresponds to the partial shift of all terms of a given electronic configuration. Hence in the optical transition considerations, it is not taken into account. The parameter, B measures the inter electronic repulsion among the electrons in the d-orbitals. The decrease in the value of the interelectronic repulsion parameter, B leads to formation of partially covalent bonding. The ratio between the crystal B 1 parameter and the free ion B parameter is known as nephelauxetic rato and it is denoted by β. The value of β is a measure of covalency. The smaller the value, the greater is the covalency between the metal ion and the ligands. The B and C values are a measure of spatial arrangement of the orbitals of the ligand and the metal ion.
Racah redefined the empirical Condon -Shortley parameters so that the separation between states having the maximum multiplicity (for example, difference between is a function of 3 F and 3 P or 4 F and 4 P is a function of a single parameter, B. However, separations between terms of different multiplicity involve both B and C Electronic (Absorption) Spectra of 3d Transition Metal Complexes 13

Tanabe -Sugano diagrams
Exact solutions for the excited sate energy levels in terms of Dq, B and C are obtained from Tanabe-Sugano matrices. However, these are very large (10 x 10) matrices and hand calculations are not feasible. For this reason Tanabe-Sugano have drawn energy level diagrams known as T-S diagrams or energy level diagrams. The T-S diagrams are valid only if the value of B, C and Dq ae lower for a complex than for the free ion value.
Quantitative interpretation of electronic absorption spectra is possible by using Tanabe -Sugano diagrams or simply T-S diagrams. These diagrams are widely employed to correlate and interpret spectra for ions of all types, from d 2 to d 8

Electron spin resonance
Electron Spin Resonance (ESR) is a branch of spectroscopy in which radiation of microwave frequency is absorbed by molecules possessing electrons with unpaired spins. It is known by different names such as Electron Paramagnetic Resonance (EPR), Electron Spin Resonance (ESR) and Electron Magnetic Resonance (EMR). This method is an essential tool for the analysis of the structure of molecular systems or ions containing unpaired electrons, which have spin-degenerate ground states in the absence of magnetic field. In the study of solid state materials, EPR method is employed to understand the symmetry of surroundings of the paramagnetic ion and the nature of its bonding to the nearest neighbouring ligands.
When a paramagnetic substance is placed in a steady magnetic field (H), the unpaired electron in the outer shell tends to align with the field. So the two fold spin degeneracy is removed. Thus the two energy levels, E1/2 and E-1/2 are separated by gH, where g is spectroscopic splitting factor and is called gyro magnetic ratio and  is the Bohr magneton.
Since there is a finite probability for a transition between these two energy levels, a change in the energy state can be stimulated by an external radio frequency. When microwave frequency () is applied perpendicular to the direction of the field, resonance absorption will occur between the two split spin levels. The resonance condition is given by, h = gH, where h is Planck's constant.
The resonance condition can be satisfied by varying  or H. However, EPR studies are carried out at a constant frequency (), by varying magnetic field (H). For a free electron, the g value is 2.0023. Since h and  are constants, one can calculate the g factor. This factor determines the divergence of the Zeeman levels of the unpaired electron in a magnetic field and is characteristic of the spin system.
In the crystal systems, the electron spins couple with the orbital motions and the g value is a measure of the spin and orbital contributions to the total magnetic moment of the unpaired electron and any deviation of magnetic moment from the free spin value is due to the spinorbit interaction. It is known that the crystal field removes only the orbital degeneracy of the ground terms of the central metal ion either partially or completely. The strong electrical fields of the surrounding ligands results in "Stark Splitting" of the energy levels of the paramagnetic ion. The nature and amount of splitting strongly depends on the symmetry of the crystalline electric field. The Stark splitting of the free ion levels in the crystal field determines the magnetic behaviour of the paramagnetic ion in a crystal. Whenever there is a contribution from the unquenched orbital angular momentum, the measured g values are isotropic as a result of the asymmetric crystal field since the contribution from the orbital motion is anisotropic. To decide the ultimate ground state of a paramagnetic ion in the crystal, the two important theorems, Kramers and Jahn-Teller, are useful. Using group theory, one can know the nature of the splitting of the free ion levels in the crystal fields of various symmetries.
Jahn-Teller theorem states that any nonlinear molecule in an electronically degenerate ground state is unstable and tends to distort in order to remove this degeneracy. The direction of distortion which results in greatest stabilization can often be deduced from EPR and other spectroscopic data.
Kramers' theorem deals with restrictions to the amount of spin degeneracy which can be removed by a purely electrostatic field. If the system contains an odd number of electrons, such an electrostatic field cannot reduce the degeneracy of any level below two. Each pair forms what is known as a Kramers' doublet, which can be separated only by a magnetic field. For example, Fe(III) and Mn(II) belonging to d 5 configuration, exhibit three Kramers' doublets labeled as 5/2, 3/2 and 1/2.
If the central metal ion also possesses a non-zero nuclear spin, I, then hyperfine splitting occurs as a result of the interaction between the nuclear magnetic moment and the electronic magnetic moment. The measurement of g value and hyperfine splitting factor provides information about the electronic states of the unpaired electrons and also about the nature of the bonding between the paramagnetic ion and its surrounded ligands. If the ligands also contain non-zero nuclear spin, then the electron spin interacts with the magnetic moment of the ligands. Then one could expect super hyperfine EPR spectrum.
The g value also depends on the orientation of the molecules having the unpaired electron with respect to the applied magnetic field. In the case of perfect cubic symmetry, the g value does not depend on the orientation of the crystal. But in the case of low symmetry crystal fields, g varies with orientation. Therefore we get three values gxx, gyy, and gzz corresponding to a, b and c directions of the crystal. In the case of tetragonal site gxx = gyy which is referred to as g  and corresponds to the external magnetic field perpendicular to the Z-axis. When it is parallel, the value is denoted as g  . Hence one can deduce the symmetry of a complex by EPR spectrum i.e., cubic, tetragonal, trigonal or orthorhombic. Anyhow, it is not possible to distinguish between orthorhombic and other lower symmetries by EPR.

EPR signals of first group transition metal ions
Transition metal ions of 3d group exhibit different patterns of EPR signals depending on their electron spin and the crystalline environment. For example, 3d 1 ions, VO 2+ and Ti 3+ have s = 1/2 and hence are expected to exhibit a single line whose g value is slightly below 2.0. In the case of most abundant 51 V, s = 1/2 and I = 7/2, an eight line pattern with hyperfine structure of almost equal intensity can be expected as shown in Fig-1. In the case of most abundant Ti, (s = 1/2 and I = 0), no hyperfine structure exists. However, the presence of less abundant isotopes ( 47 Ti with I = 5/2 and 49 Ti with I = 7/2) give rise to weak hyperfine structure with six and eight components respectively. This weak structure is also shown in Fig-1.
Cr(III), a d 3 ion, with s = 3/2 exhibits three fine line structure. The most abundant 52 Cr has I = 0 and does not exhibit hyperfine structure. However, 53 Cr with I = 3/2 gives rise to hyperfine structure with four components. This structure will be weak because of the low abundance of 53 Cr. Thus each one of the three fine structure lines of 53 Cr is split into four weak hyperfine lines. Of these, two are overlapped by the intense central line due to the most abundant 52 Cr and the other two lines are seen in the form of weak satellites.
Mn(II) and Fe(III) with d 5 configuration have s = 5/2 and exhibit five lines which correspond to a 5/2  3/2, 3/2  1/2 and +1/2  -1/2 transitions. In the case of 55 Mn, which has I = 5/2, each of the five transitions will give rise to a six line hyperfine structure. But in powders, usually one observes the six-hyperfine lines corresponding to +1/2  -1/2 transition only. The remaining four transition sets will be broadened due to the high anisotropy. Fe 3+ yields no hyperfine structure as seen in Fig -1.
Co 2+ , a d 7 configuration, with s value of 3/2 exhibits three fine structure lines. In the case of 59 Co (I = 7/2), eight line hyperfine pattern can be observed as shown in the Fig-1.

Titanium
Titanium is the ninth most abundant element in the Earth's crust (0.6%). There are 13 known isotopes of titanium. Among them five are natural isotopes with atomic masses 46 to 50 and the others are artificial isotopes. The synthetic isotopes are all radioactive. Titanium alloys are used in spacecraft, jewelry, clocks, armored vehicles, and in the construction of buildings. The compounds of titanium are used in the preparation of paints, rubber, plastics, paper, smoke screens (TiCl4 is used), sunscreens. The main sources of Ti are ilmenite and rutile.
Titanium exhibits +1 to +4 ionic states. Among them Ti 4+ has d 0 configuration and hence has no unpaired electron in its outermost orbit. Thus Ti 4+ exhibits diamagnetism. Hence no d-d transitions are possible. The ionic radius of Ti 3+ is the same as that of Fe(II) (0.76 A.U). Ti(I) and Ti(III) have unpaired electrons in their outermost orbits and exhibit para magnetism

Electronic spectra of titanium compounds
The electronic configuration of Ti 3 is [Ar] 3d 1 4s 2 . It has five fold degeneracy and its ground state term symbol is 2 D. In an octahedral crystal field, the five fold degeneracy is split into 2 T2g and 2 Eg states. Thus only one single electron transition, 2 T2g  2 Eg, is expected in an octahedral crystal field. The separation between these energies is 10Dq, which is crystal field energy. Normally, the ground 2 T2g state is split due to Jahn-Teller effect and hence lowering of symmetry is expected for Ti(III) ion. This state splits into 2 B2g and 2 Eg states in tetragonal symmetry and the excited term 2 Eg also splits into 2 B1g and 2 A1g levels. Thus, three bands are expected for tetragonal (C4v) symmetry. Energy level diagram in tetragonal environment is shown in Fig -2. The transitions in the tetragonal field are described by the following equations.

 
In the above formulae, Dq is octahedral crystal field and Ds and Dt are tetragonal field parameters. The same sign of Dq and Dt indicates an axial elongation and opposite sign indicates an axial compression

EPR spectra of titanium compounds
When any Ti(III) compound in the form of powder is placed in a magnetic field, it gives a resonance signal. The single d-electron of Ti 3+ has spin, s = 1/2. The abundance of isotopes is reported as 46 Ti ≈ 87%, 48 Ti ≈7.7% and 50 Ti ≈5.5% and have nuclear spin I = 0, 5/2 and 7/2 respectively. Electron spin and nuclear spin interactions give rise to (2I+1) hyperfine lines (0,6 and 8) and appear as satellite. Since 46 Ti abundance is more, the EPR signal contains only one resonance line which is similar to the one shown in Fig-3. The g value for this resonance is slightly less than 2.0.

Relation between EPR and optical absorption spectra
EPR studies for Ti 3+ can be correlated with optical data to obtain the orbital reduction parameter.
where n is 8 for C4V , E  is the energy of appropriate transition, λ is the spin-orbit coupling constant for Ti 3+ , i.e., 154 cm -1 and k is the orbital reduction parameter.

Typical examples
EPR and optical absorption spectral data of selected samples are discussed as examples. The data chosen from the literature are typical for each sample and hence should be considered as representative only. For more complete information on specific example, the original references are to be consulted. X-band spectra and optical absorption spectra of the powdered samples are recorded at room temperature (RT).

Optical absorption studies
Ti(III) ion in solids is characterized by three broad bands around 7000, 12000 and 18000 cm -1 .
These are due to the transitions from 2 B2g  2 Eg, 2 B2g  2 B1g, and 2 B2g  2 A1g respectively. Three bands of titanite at 7140, 13700 and 16130 cm -1 and of anatase at 6945, 12050 and 18180 cm -1 are attributed to the above transitions. The optical absorption spectrum of lamprophyllite is also similar. The optical absorption spectrum of benitoite sample displays three bands at 8260, 10525 and 15880 cm -1 . From the observed band positions, the crystal field parameter in octahedral field, Dq and tetragonal field parameters, Ds and Dt, are given in Table-  iii. X band EPR of polycrystalline lamprophyllite sample indicates a broad resonance line with line width 56.6 mT and a g value of 2.0. This is due to the presence of Ti(III) in the compound. The broad line is due to the dipolar-dipolar interaction of Ti(III) ions. Even at liquid nitrogen temperature, only the line intensity increases indicating that Curie law is obeyed.
Using EPR and optical absorption spectral results of titanite, the covalency parameter is calculated using equation (4), . The α value obtained is 0.51, which indicates higher covalent character between ligand and metal ion.

Vanadium
Vanadium abundance in earth's crust is 120 parts per million by weight. Vanadium's ground state electron configuration is [Ar] 3d 3 4s 2 . Vanadium exhibits four common oxidation states +5, +4, +3, and +2 each of which can be distinguished by its color. Vanadium(V) compounds are yellow in color whereas +4 compounds are blue, +3 compounds are green and +2 compounds are violet in colour. Vanadium is used in making specialty steels like rust resistant and high speed tools. The element occurs naturally in about 65 different minerals and in fossil fuel deposits. Vanadium is used by some life forms as an active center of enzymes. Vanadium oxides exhibit intriguing electrochemical, photochemical, catalytical, spectroscopic and optical properties. Vanadium has 18 isotopes with mass numbers varying from 43 to 60. Of these, 51 V, natural isotope is stable:

Electronic spectra of vanadium compounds
Vanadium in its tetravalent state invariably exists as oxo-cation, VO 2+ (vanadyl). The VO 2+ ion has a single d electron which gives rise to the free ion term 2 D. In a crystal field of octahedral symmetry, this electron occupies the t2g orbital and gives rise to ground state term 2 T2g. When the electron absorbs energy, it is excited to the eg orbital and accordingly in octahedral geometry only one band corresponding to the transition, 2 T2g → 2 Eg, is expected. Because of the non-symmetrical alignment of the V=O bond along the axis, the site symmetry, in general, is lowered to tetragonal (C4V) or rhombic (C2V) symmetry. In C4V site symmetry, 2 T2g splits into 2 B2g and 2 Eg, whereas 2 Eg splits into 2 B1g, 2 A1g. Hence three bands are expected in C4V symmetry in the range of 11000 -14000, 14500 -19000 and 20000 -31250 cm -1 . The degeneracy of 2 Eg is also removed in C2V symmetry resulting four bands. Energy level diagram of VO 2+ in octahedral C4V and C2V symmetries are shown in Fig-4. In the tetragonal C4V symmetry transitions are described by the following equations.

 
In the above formulae, Dq is octahedral crystal field parameter and Ds, Dt are tetragonal field parameters. The same sign of Dq and Dt indicates an axial elongation and opposite sign indicates an axial compression.

EPR spectra of vanadium compounds
The EPR signal is of three types. (i) is due to high concentration of vanadium. If the vanadium content in the compound is high, it gives a broad resonance line. Therefore the hyperfine line from 51 V cannot be resolved. The g value for this resonance is less than 2. (ii) VO 2+ ion has s= ½ and I = 7/2. The EPR spectrum shows hyperfine pattern of eight equidistant lines. In C4v symmetry two sets of eight lines are expected (sixteen-line pattern) whereas in C2v symmetry three sets of eight lines are expected. Further in tetragonal distortion, 11 g < g  < ge which shows the presence of an unpaired electron in the xy d orbital.
This is characteristic feature of a tetragonally compressed complex.
Further lowering of symmetry gives rise to EPR spectrum which is similar to the one shown in gyy and gzz respectively. The hyperfine constants are designated as A1, A2 and A3 respectively.
Using the EPR data, the value of dipolar term P and k term are calculated, and

Relation between EPR and optical absorption spectra
The optical absorption results and EPR results are related as follows. EPR studies can be correlated with optical data to obtain the orbital coefficients *2  and *2   .
Here g11 and g  are the spectroscopic splitting factors parallel and perpendicular to the magnetic field direction of ge (i.e., 2.0023 for a free electron).
 E1 is the energy of 2 B2g  2 B1g and  E2 is the energy of 2 B2g  2 Eg.

Typical examples
EPR and optical absorption spectral data of certain selected samples are discussed. The data chosen from the literature are typical for each sample. The data should be considered as representative only. For more complete information on specific example, original references are to be consulted. X-band spectra of the powdered samples and optical absorption spectra are recorded at room temperature (RT).  Therefore, the observed bands in electronic absorption spectrum are ascribed to charge transfer bands. These appear around 37000, 45000 cm -1 . These are assigned to transitions from ligand orbitals to metal d-orbitals: A1 → T2 (t1 → 2e) and A1→ T2 (3t2 → 2e) in tetrahedral configuration for the ion 3 4 VO  .

Advanced Aspects of Spectroscopy 24
Vanadium doped silica gel also shows sharp band at 41520 cm -1 and shoulders at 45450 and 34480 cm -1 . These are also assigned to charge transfer transitions in tetrahedral environment of 3 4 VO  . The minimum value of 10Dq for 3 4 VO  is expected at about 16000 cm -1 in octahedral geometry. This is expected because the two bands at 34480 and 45450 cm -1 are from the ligand orbitals to two vacant d orbitals which are 10Dq apart. This would be about twice the energy separation (8000 cm -1 ) observed for tetrahedral 3 4 VO  .Hence the evidence does not satisfy the assignment of bands to d-d transitions. Therefore the bands are due to charge transfer transitions.

Chromium
Chromium is the 6 th most abundant transition metal. Chromium is used in the manufacture of stainless steel and alloys.

Optical spectra a. Divalent chromium(d 2 )
Cr 2+ has a d 4 configuration and forms high spin complexes only for crystal fields less than 2000 cm -1 . The ground state term in an octahedral crystal field is 5 Eg belonging to the 31 2 gg te configuration. The excited state 5 T2g corresponds to promotion of one single electron to give 22 2 gg te configuration. The d 4 electron is susceptible to Jahn-Teller distortion and hence Cr 2+ compounds usually are of low symmetry. In lower symmetry, the excited quintet state of Cr 2+ splits into three levels and the ground level quintet state splits into two levels. In the case of Cr 2+ (H2O)6, the value of Dq is 1400 cm -1 . In spinels, Cr 2+ is in the tetrahedral environment and Dq is about 667 cm -1 only.

b. Trivalent chromium(d 3 ):
In octahedral symmetry, the three unpaired electrons are in 3 2 g t orbitals which give rise to 4 A2g, 2 Eg, 2 T1g and 2 T2g states. Of these 4 A2g is the ground state. If one electron is excited, the configuration is 21 2 gg tewhich gives two quartet states 4 T1g and 4 T2g and a number of doublet states. When the next electron is also excited, the configuration is 12 2 g g tewhich gives rise to one quartet state 4 T1g and some doublet states. The octahedral crystal field parameter Dq is characteristic of the metal ion and the ligands. The Racah parameter, B depends on the size of the 3d orbital; B is inversely proportional to covalency in the crystal.

EPR spectra of chromium compounds
Cr 3+ ion, splits into |1/2 and |3/2 Kramers' doublets in the absence of magnetic field, separated by 2D, D being the zero-field splitting parameter. This degeneracy can be lifted only by an external magnetic field. In such a case, three resonances are observed corresponding to the transitions, |-3/2  |-1/2, |-1/2  |1/2 and |1/2  |3/2 at gB -2D, gB and gB + 2D respectively. In a powder spectrum, mainly the perpendicular component is visible. If all the three transitions are observed, the separation between the extreme sets of lines is 4D [gB + 2D -(gB -2D) = 4D]. If D is equal to zero, a single resonance line appears with g ~ 1.98. If D is very large compared to microwave frequency, a single line is seen around g = 4.0.

Relation between EPR and optical absorption spectra
A comparison is made between the observed geff from EPR results and the calculated one from the optical spectrum. For Cr 3+ , EPR and optical results are related by, Here g11 and g  are the spectroscopic splitting factors parallel and perpendicular to the magnetic field direction, g , the free electron value ge, is 2.0023. These values give, The value of D can also be estimated from the optical absorption spectrum.

Typical examples
The data chosen from the literature are typical for each sample. The data should be considered as representative only. For more complete information on specific examples, the original references are to be consulted. X-band spectra and optical absorption spectra of the powdered sample are recorded at room temperature (RT). Several examples are available in the literature. Some of them are given in the Table-12.

Tetravalent chromium (d 2 ):
Absorption spectra of Cr 4+ in forsterite and garnet show the absorption band at 9460 cm -1 which is the typical of Cr 4+ ions. It is attributed to the 3 A2g  3 T2g transition. The absorption band at 19590 cm -1 is also attributed to 3 A2g  3 T1g transition. The absorption band at 19590 cm -1 overlaps with the bands at 16130 and 23065 cm -1 .

Manganese
The atomic number of manganese is 25 and its outermost electronic configuration is [Ar] 3d 5 4s 2 . It exhibits several oxidation states, +2, +3, +4, +6 and +7, of which the most stable are +2 +4 and +7. The ionic radii of Mn 2+  Octahedral complexes of Mn(III) are prone to Jahn-Teller distortion. It is of interest, therefore, to compare the structures of Cr(acac)3 with Mn(acac)3 since the former is a regular octahedron while the latter is prone to dynamic Jahn-Teller distortion.

EPR spectra of manganese compounds
1. Manganese(II): Manganese(II), being a d 5 ion, is very sensitive to distortions in the presence of magnetic field. Mn(II) has a total spin, S = 5/2. The six spin states labeled as ±5/2>, ±3/2> and ±1/2> are known as the three Kramers' doublets; in the absence of external magnetic field ,they are separated by 4D and 2D respectively, where D is the zero-field splitting parameter. These three doublets split into six energy levels by the application of an external magnetic field. Transitions between these six energy levels give rise to five resonance lines. Each of these resonance lines, in turn, splits into a sextet due to the interaction of the electron spin with the nuclear spin of 55 Mn, which is 5/2. Thus one expects a 30-line pattern. However, depending on the relative magnitudes of D and A (hyperfine coupling constant of manganese), these 30 lines appear as a separate bunch of 30 lines or 6 lines (if D = 0). The separation between the extreme set of resonance lines is approximately equal to 8D (first order). If D is very small compared to hyperfine coupling constant (A), the 30 lines are so closely packed that one could see only six lines corresponding -1/2 to +1/2 transition. If D = 0, the system is perfectly octahedral. Deviation from axial symmetry leads to a term known as E in the spin-Hamiltonian. The value of E can be easily calculated from single crystal measurements. A non-zero value of E results in making the spectrum unsymmetrical about the central sextet. Further, the g value for the hyperfine splitting is indicative of the nature of bonding. If the g value shows a negative shift with respect to the free electron g value (2.0023), the bonding is ionic and conversely, if the shift is positive, then the bonding is said to be more covalent in nature.

Typical examples
1. Manganese(II): The EPR spectrum of clinohumite contains a strong sextet at the centre corresponding to the electron spin transition +1/2> to -1/2>. In general, the powder spectrum is characterized by a sextet, corresponding to this transition. The other four transitions corresponding to ±5/2> ↔ ±3/2> and ±3/2>↔ ±1/2> are not seen due to their high anisotropy in D. However, in a few cases only, all the transitions are seen. Moreover, the low field transitions are more intense than the high field transitions. In addition, if E ≠ 0, the EPR spectrum will not be symmetrical about the central sextet. In clinohumite, the spectrum indicates the presence of at least three types of Mn(II) impurities in the mineral.
The extra set of resonances within the main sextet is due to the forbidden transitions. From the powder spectrum of the mineral, the following parameters are calculated: Site I: g = 2.000 (1) te. It gives a single spin-allowed transition 5 Eg→ 5 T2g corresponding to one electron transition. This should appear around 20000 cm -1 . Mn 3+ cation is subject to Jahn-Teller distortion. The distortion decreases the symmetry of the coordination site from octahedral to tetragonal (D4h) or by further lowering the symmetry to rhombic (C2v). Under the tetragonal distortion, the t2g orbital splits into eg and b2g orbitals whereas the eg orbital splits into a1g and b1g orbitals. Hence in a tetragonal site, three absorption bands are observed instead of one. Further distortion splits the eg orbital into singly degenerate a1g and b1g orbitals. Thus four bands are observed for rhombic symmetry (C2v).
The transitions in the tetragonal field are described by the following equations:   In the above equations, Dq is octahedral crystal field and Ds and Dt are tetragonal field parameters. The same sign of Dq and Dt indicates an axial elongation and opposite sign indicates an axial compression.
The optical absorption bands observed for Mn(III) in octahedral coordination with rhombic distortion (C2h) in montmorillonite are given in Table -

Iron
The atomic number of iron is 26 and its electronic configuration is [Ar]4s 2 3d 6. . Iron has 14 isotopes. Among them, the mass of iron varies from 52 to 60 Pure iron is chemically reactive and corrodes rapidly, especially in moist air or at elevated temperatures. Iron is vital to plant and animal life. The ionic radius of Fe 2+ is 0.76 A.U. and that of Fe 3+ is 0.64 A.U. The Electronic (Absorption) Spectra of 3d Transition Metal Complexes 33 most common oxidation states of iron are +2 and +3. Iron(III) complexes are generally in octahedral in shape, and a very few are in tetrahedral also.

EPR spectra of iron compounds
The EPR spectra of powdered Fe 3+ compounds may be described by the spin-Hamiltonian, The second and third terms in the equation (33) represent the effects of axial and rhombic components of the crystal field respectively. When D=E=0, it corresponds to a free ion in the magnetic field, H and if E= 0, it implies a field of axial symmetry. If λ (E/D) increases, it results in the variation of rhombic character. Maximum rhombic character is seen at a value of λ=1/3 and further increase in λ from 1/3 to 1 results in the decrease of rhombic character. When λ =1, the axial field situation is reached. When λ=1/3, the g value is around 4.27 and when λ is less than 1/3, g value is 4. Hence, the resonance is no longer isotropic and the powder spectrum in that region is a triplet corresponding to H along each of the three principle axes. For Fe 3+ , in fields of high anisotropy, the maximum g value is 9. If g values are limited to 0.80 to 4.30, the Fe 3+ ion is under the influence of a strong tetragonal distortion.

Iron (III):
The iron (III) samples exhibit a series of g values ranging from 0 to 9. This is due to the fact that the three Kramers' doublets of S=5/2 are split into S5/2, S3/2 and S1/2 separated by 4D and 2D respectively where D is the zero field splitting parameter.
Depending on the relative populations of these doublets, one observes g value ranging from 0 to 9.0. The line widths are larger in low magnetic field when compared to high magnetic field. If the lowest doublet, S1/2 is populated, it gives a g value of 2 to 6 whereas if the middle Kramers' doublet S3/2 is populated, a g value 4.30 is expected. If the third doublet S5/2 is populated, it gives a g value of 2/7 to 30/7. A few systems are known which exhibit resonances from all the three Kramers' doublets.
The iron(III) in the natural sample enters the lattice in various locations which may not correspond to the lowest energy configuration. After heating the sample, the impurity settles in the lowest energy configuration and the EPR spectrum is simplified. Thus, it is observed that heating the sample results in a simplification of the EPR spectrum and gives a g value of around 2.

Typical examples
1. The EPR spectrum of powdered red sandal wood obtained at room temperature contains a series of lines of various intensity and width. The g values obtained for these are 6.52, 2.63 and 1.92. These three peaks are attributed to Fe(III) impurity in the compound. 2. The EPR spectrum of prehnite at room temperature consists of two parts. The first part consists of the two strong lines (absorption and dispersion) and the second part λ calculated for each g tensor is 32.18.
For axial symmetry, λ is zero. If rhombic character in the crystal field is increased, it results in the increase of λ upto a maximum of 1 3 . In the present case, the observed λ is 1 3 (32.18%).
Thus the EPR studies indicate that the iron oxalate nano-crystal is in orthorhombic structure.

Trivalent iron
Trivalent iron has the electronic configuration of 3d 5 which corresponds to a half-filled dsub-shell and is particularly most stable. In crystalline fields, the usual high spin iii) The unsplit ground state term behaves alike in both octahedral and tetrahedral symmetries and gives rise to same energy level for octahedral, tetrahedral and cubic coordination with usual difference,

Divalent iron
In divalent iron (d 6 ), the free ion ground term is 5 D and the excited terms are triplet states ( 3 H, 3 P, 3 F, 3 G, 3 D) and singlet states ( 1 I, 1 D). In an octahedral field, the 5 D term splits into an upper 5 Eg level and a lower 5 T2g level of which the latter forms the ground state. The only allowed transition is 5 T2g → 5 Eg which gives an intense broad absorption band. This band splits into two bands due to Jahn-Teller effect. The average of these two bands is to be taken as 10Dq band. The transitions arising from the excited triplet states are spin forbidden and hence are weaker than the 10Dq band.  Table 15. A comparison is also made between the calculated and observed energies of the bands and these are presented in Table -

Nickel
Nickel is the 7 th most abundant transition metal in the earth's crust. The electronic configuration of nickel is [Ar]4S 2 3d 8 . Nickel occurs in nature as oxide, silicate and sulphide.
The typical examples are garnierite and pentlandite. Nickel exhibits +1 to +4 oxidation states. Among them divalent state is most stable. Nickel compounds are generally blue and green in color and are often hydrated. Further, most nickel halides are yellow in color. The primary use of nickel is in the preparation of stainless steel. Nickel is also used in the coloring of glass to which it gives a green hue.

Electronic spectra of nickel compounds
The electronic distribution of Ni(II) ion (d 8  Here μ is of the order of 0.01. Dq and B are of similar magnitude. The spin allowed bands are calculated using the above equations whereas the spin forbidden bands are assigned using Tanabe-Sugano diagrams.

Typical examples
The data chosen from the literature are typical and representative for each sample. For more complete information on any specific case, original references are to be consulted. X-band spectra and optical absorption spectra of the powdered sample are recorded at room temperature (RT) only.

EPR spectra
Ni 2+ (d 8 ) has no unpaired electron (square planer) in its orbit. Therefore it does not exhibit EPR signal at room temperature.
But in certain conditions, it shows EPR signal. The EPR data could be related with the optical data by the following equation where ∆ is the energy of the transition of the perfect octahedral site. λ is 324 cm -1 for free Ni 2+ ion.

Copper
Copper is one of the earliest known elements to man. The average percentage of copper in the earth's crust is 0.005%. Pure copper is soft and malleable. An important physical property of copper is its color. Most people refer copper colour as reddish-brown tint. Copper-63 and copper-65 are two naturally occurring isotopes of copper. Nine radioactive isotopes of copper are also known. Among them two radioactive isotopes, copper-64 and copper-67 are used in medicine. Copper easily reacts with oxygen and in moist air, it combines with water and carbon dioxide forming hydroxy copper carbonate (Cu2(OH)2CO3 ).
Animals like crustaceans (shellfish like lobsters, shrimps, and crabs) do not have hemoglobin to carry oxygen through the blood but possess a compound called hemocyanin. This is similar to hemoglobin but contains copper instead of iron. Copper is an essential micronutrient for both plants and animals. A healthy human requires not more than about 2 mg of copper for every kg weight of the body. The main body parts where copper is found in animals are the tissues, liver, muscle and bone.

Copper compounds
Copper exists in two ionic states, Cu(I) and Cu(II). The ionic radius of Cu(II) is 0.73 A.U. The electronic configuration of Cu(I) is [Ar] 3d 10 and hence has no unpaired electron in its outermost orbit. Hence it exhibits diamagnetism. The electronic configuration of Cu(II) is [Ar]3d 9 and has one unpaired electron which is responsible for its para magnetism. The main resources of copper are its minerals. Structural properties could be explored using electronic and EPR spectra which provides information on bonding between ligands and metal ion.

Electronic spectra of copper compounds
In optical spectroscopy, transitions proceed between the split orbital levels whereas in EPR spectroscopy they occur between spin sub-levels that arise due to the external magnetic field. Thus EPR spectroscopy is a natural sequel to optical spectroscopy.

Optical spectra
In octahedral crystal field, the ground state electronic distribution of Cu 2+ is t2g 6 eg 3 which yields 2 Eg term. The excited electronic state is t2g 5 eg 4 which corresponds to 2 T2g term. Thus only one single electron transition, i.e., 2 Eg  2 T2g, is expected in an octahedral crystal field. The difference is 10Dq. Octahedral coordination is distorted either by elongation or compression of octahedron leading to tetragonal symmetry.
Normally, the ground 2 Eg state is split due to Jahn-Teller effect and hence lowering of symmetry is expected for Cu(II) ion. This state splits into 2 B1g(dx 2 -y 2 ) and 2 A1g(dz 2 ) states in tetragonal symmetry and the excited term 2 T2g also splits into 2 B2g(dxy) and 2 Eg(dxz,dyz) levels.
The transitions in the tetragonal field are described by the following equations:    The Jahn-Teller distortion is either tetragonal elongation along the Z axis or contraction in the equatorial xy plane which may ultimately result in a square planar environment in extreme cases as in D4h.

EPR spectra of copper compounds
When any Cu(II) compound in the form of powder is placed in a magnetic field, it gives a resonance signal. The signal is of three types. They are shown in Fig.-7: whereas if g  >g11 or g11 = 2.00, the ground state is 2 A1g [ fig.7(ii).]. The highest-energy of the half occupied orbital is dx _ 2 y 2 as it has the largest repulsive interaction with the ligands in the equatorial plane. Here g11(corresponding to the magnetic field oriented along the z axis of the complex) > g  > 2.00. This is a characteristic feature of dx 2 -y 2 ground state. Additionally, copper has a nuclear spin of (I)) 3/2 which couples with the electron spin to produce a four line hyperfine splitting of the EPR spectrum. This is shown in Fig-7(ii) and 7(v). Tetragonal cupric complexes generally have large A11 value than those of complexes with D4h symmetry. If g11 > g  , the ground state is 2 B1g whereas if g  >g11 or g11 = 2.00, the ground state is 2 A1g. EPR results give rise to a new parameter, G which is defined as If G value falls in between 3 and 5, the unit cell contains magnetically equivalent ions. If G value is less than 3, the exchange coupling among the magnetically non-equivalent Cu(II) ions in the unit cell is not very strong. If G is greater than 5, a strong exchange coupling takes place among the magnetically non -equivalent Cu(II) ions in the unit cell. Truly compressed structures are relatively rare when compared to elongated structures. In other words, g  > g11, is an unusual observation and this implies two possibilities: i. The concentration of copper in the complex is very high which results in the interaction between Cu(II) ↔ Cu(II) ions. ii. The Cu(II) ion is a compressed octahedron. If the complex contains low copper content, it is assumed that Cu(II) ion is a compressed octahedron. Hence the ground state is 2 A1g 2 () z d .
iii. Further lowering of symmetry gives rise to EPR spectrum which is similar to the one shown in Fig. 9(iv). This spectrum consists of three sets of resolved four lines in low, medium and high fields corresponding to g1, g2 and g3 respectively. The hyperfine structure constants (A values) are designated as A1, A2 and A3 respectively. Line width is estimated for simple cubic lattice using dipole-dipole equation;   where β is the Bohr magneton, s = spin, gO = average value of g factor, ρ = density (2.22 x 10 21 spins/cc).
The calculated g values provide valuable information on the electronic ground state of the ion. If g1> g2 > g3, the quantity R value is given by (g2 -g3) / (g1-g2) which is greater than unity and the ground state is 2 A2g(dz 2 ); if it is less than unity, the ground state is 2 A1g(dx 2 -y 2 ). A large value of g1 is indicative of more ionic bonding between metal and ligand. Further the structure of the compound is an elongated rhombus. From the spin -Hamiltonian parameters, the dipolar term (P) and the Fermi contact term (k) are calculated using the following expressions: Here γCu is the magnetic moment of copper, βo is the Bohr magneton, βN is the nuclear magneton and r is the distance from the central nucleus to the electron, Ao is the average A value and ∆go = go -ge where go is the average g value and ge is the free electron g-value (2.0023). The Fermi contact term, k, is a measure of the polarization produced by the uneven distribution of d-electron density on the inner core s-electron and P is the dipolar term. By assuming either the value of P or k, the other is calculated. Using these values, the hyperfine constant is calculated. This is the average value of g1, g2 and g3.
Using the data of EPR and dipolar term P, the covalency parameter (α 2 ) is calculated .

Relation between EPR and optical absorption spectra
The optical absorption and EPR data are related as follows. In tetragonal symmetry, EPR studies are correlated with optical data to obtain the orbital reduction parameter in rhombic compression.
1. The EPR spectrum of covellite is shown in Fig-9. It is similar to the Fig 8(i). It consists of a broad line with a small sextet. The g value for the broad line is 2.24 which is due to the presence of Cu(II) in the sample. The hyperfine line from either 63 Cu or 65 Cu could not be resolved since the copper content (Cu = 66 wt%) in the mineral is very high. Several copper compounds exhibit this type of EPR spectra. 2. Beaverite [Pb(Fe 3+ ,Cu,Al)3(SO4)2(OH)6]: X-band EPR spectrum of powdered sample recorded at RT is shown in Fig-9. This is similar to Fig-7(ii). The g values are: g11 = 2.42 and g⊥ = 2.097. In addition to the above, a g value of 2.017 is observed which is due to Fe(III) impurity. Fig.9 indicates expanded form of EPR spectrum of Cu(II) and is not resolved because of high copper percentage. Tetragonal cupric complexes with D4h symmetry, possessing axial elongation have ground state 2 B1g (dx 2 −y 2 ).The EPR results are in the order of g11 > g⊥ > ge and hence the ground state is 2 B1g. Though the optical absorption spectrum shows two sites for Cu(II) with same ground state, the same is not noticed in the EPR spectrum because the percentage of copper is high in the sample.
A typical EPR spectrum of enargite is shown in Fig.10. The spectrum is symmetric with g11 = 2.289 and g⊥ = 2.048 which are due to Cu(II). Since g11 > g⊥ > ge, the ground state for Cu(II) is 2 B1g (dx 2 −y 2 ). Using EPR and optical absorption results, the orbital reduction parameters are evaluated, i.e., K11 = 1.03 cm -1 and K  = 1.93 cm -1 . Also G seems to be 5.0 which indicates that the unit cell of the compound contains magnetically equivalent ions. CuO-ZnO nano composite: EPR spectrum of CuO-ZnO nano composite recorded at room temperature is shown in Fig-11. The calculated g values are 1.76, 2.31 and 2.05. The g value of 1.76 is assigned to free radical of O 2-. Further gII value of 2.31, g┴ value of 2.05 are due to Cu(II) in tetragonal distortion. Also it has A11 =13.3 mT. These results show that the ground state of Cu(II) as dx 2y 2 . Further, the covalency parameter, α 2 (0.74) suggests that the composite has some covalent character.