**14. Survey of experimental results**

#### **14.1. 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 Ti4+ has d0 configuration and hence has no unpaired electron in its outermost orbit. Thus Ti4+ exhibits diamagnetism. Hence no d-d transitions are possible. The ionic radius of Ti3+ 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

#### **14.2. Electronic spectra of titanium compounds**

The electronic configuration of Ti3 is [Ar] 3d1 4s2. It has five fold degeneracy and its ground state term symbol is 2D. In an octahedral crystal field, the five fold degeneracy is split into 2T2g and 2Eg states. Thus only one single electron transition, 2T2g 2Eg, is expected in an octahedral crystal field. The separation between these energies is 10Dq, which is crystal field energy. Normally, the ground 2T2g state is split due to Jahn-Teller effect and hence lowering of symmetry is expected for Ti(III) ion. This state splits into 2B2g and 2Eg states in tetragonal symmetry and the excited term 2Eg also splits into 2B1g and 2A1g levels. Thus, *three bands* are expected for *tetragonal (C4v)* symmetry. Energy level diagram in tetragonal environment is shown in Fig -2.

Electronic (Absorption) Spectra of 3d Transition Metal Complexes 17

**Figure 2.** Energy level diagram of Ti3+ in octahedral and tetragonal fields

The transitions in the tetragonal field are described by the following equations.

$$\mathrm{B}^{2}\mathrm{B}\_{2\_{\mathcal{S}}} \rightarrow \, ^{2}\mathrm{E}\_{\mathrm{g}} : \left[ -4Dq - Ds + 4Dt - \left( -4Dq + 2Ds - Dt \right) \right] = -3Ds + 5Dt \tag{1}$$

$$B\_{2\_{\mathcal{R}}} \rightarrow \,^2B\_{1\_{\mathcal{R}}} : \left[ 6D\eta + 2Ds - Dt - \left( -4D\eta + 2Ds - Dt \right) \right] = 10D\eta \tag{2}$$

$$\mathrm{P}^{2}\mathrm{B}\_{2\_{\mathcal{S}}} \rightarrow \mathrm{^{2}}A\_{\mathrm{1g}} : \left[6D\eta - 2\,\mathrm{Ds} - 6\,\mathrm{Dt} - \left(-4\,\mathrm{Dq} + 2\,\mathrm{Ds} - \mathrm{Dt}\right)\right] = 10D\eta - 4\,\mathrm{Ds} + 5\,\mathrm{Dt} \tag{3}$$

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

#### *14.2.1. EPR spectra of titanium compounds*

16 Advanced Aspects of Spectroscopy

**Figure 1.** EPR signal of 3d ions

**14.1. Titanium** 

shown in Fig -2.

**14. Survey of experimental results** 

**14.2. Electronic spectra of titanium compounds** 

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

Titanium exhibits +1 to +4 ionic states. Among them Ti4+ has d0 configuration and hence has no unpaired electron in its outermost orbit. Thus Ti4+ exhibits diamagnetism. Hence no d-d transitions are possible. The ionic radius of Ti3+ 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

The electronic configuration of Ti3 is [Ar] 3d1 4s2. It has five fold degeneracy and its ground state term symbol is 2D. In an octahedral crystal field, the five fold degeneracy is split into 2T2g and 2Eg states. Thus only one single electron transition, 2T2g 2Eg, is expected in an octahedral crystal field. The separation between these energies is 10Dq, which is crystal field energy. Normally, the ground 2T2g state is split due to Jahn-Teller effect and hence lowering of symmetry is expected for Ti(III) ion. This state splits into 2B2g and 2Eg states in tetragonal symmetry and the excited term 2Eg also splits into 2B1g and 2A1g levels. Thus, *three bands* are expected for *tetragonal (C4v)* symmetry. Energy level diagram in tetragonal environment is

screens (TiCl4 is used), sunscreens. The main sources of Ti are ilmenite and rutile.

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 Ti3+ has spin, s = 1/2. The abundance of isotopes is reported as 46Ti ≈ 87%, 48Ti ≈7.7% and 50Ti ≈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 46Ti 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.

**Figure 3.** RT powered EPR spectrum of Ti(III).

#### *14.2.2. Relation between EPR and optical absorption spectra*

EPR studies for Ti3+ can be correlated with optical data to obtain the orbital reduction parameter.

$$\alpha = K \frac{\mathcal{\lambda}\_{\text{(Coulomb)}}}{\mathcal{\lambda}\_{\text{(ionic)}}} = \frac{\left(\mathcal{g}\_{\epsilon} - \mathcal{g}\_{11}\right) \Delta E}{n \mathcal{\lambda}\_{\text{(ionic)}}} \tag{4}$$

where n is 8 for C4V , *E* is the energy of appropriate transition, λ is the spin-orbit coupling constant for Ti3+, i.e., 154 cm-1 and k is the orbital reduction parameter.

## *14.2.3. 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).

## *14.2.4. 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 2B2g 2Eg, 2B2g 2B1g, and 2B2g 2A1g 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-8.


**Table 8.** Crystal field parameters of Ti(III)

The magnitude of Dt indicates the strength of the tetragonal distortion. This is more in lamprophyllite when compared to the other samples.


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), *e* 11 *ionic g g E n* . The α value obtained is 0.51, which indicates

higher covalent character between ligand and metal ion.

## **15. Vanadium**

18 Advanced Aspects of Spectroscopy

*14.2.3. Typical examples* 

*14.2.4. Optical absorption studies* 

**Table 8.** Crystal field parameters of Ti(III)

lamprophyllite when compared to the other samples.

tetragonally distorted octahedral site.

tetragonally distorted environment.

 

powdered samples are recorded at room temperature (RT).

constant for Ti3+, i.e., 154 cm-1 and k is the orbital reduction parameter.

 

*Covalency e* 11 *Ionic ionic <sup>g</sup> g E <sup>K</sup> n*

where n is 8 for C4V , *E* is the energy of appropriate transition, λ is the spin-orbit coupling

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

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 2B2g 2Eg, 2B2g 2B1g, and 2B2g 2A1g 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

The magnitude of Dt indicates the strength of the tetragonal distortion. This is more in

i. X-band EPR spectra of the powdered sample of titanite shows a broad resonance line in the centre (335.9 mT). The measured g value is 1.957. Another resonance line is noticed at 341.4 mT with g =1.926. The central eight line transition is superimposed on the spectrum and the components are attributed to VO(II) impurity. The g value of Ti3+ is 1.957 and other g value is due to VO(II). The g value of 1.95 indicates that Ti3+ is in

ii. The EPR spectrum of anatase shows a large number of resonances centered around g value of 2 which is attributed to Ti3+. The additional structures between g values of 2 and 4 are attributed to Fe(III) impurity in the compound. Both the ions are in

octahedral field, Dq and tetragonal field parameters, Ds and Dt, are given in Table-8.

Sample Dq cm-1 Ds cm-1 Dt cm-1 Titanite 1370 -1367 608 Anatase 1205 -1867 268 Lamprophyllite 877 -1426 1525 Benitoite 1050 -1945 485

(4)

Vanadium abundance in earth's crust is 120 parts per million by weight. Vanadium's ground state electron configuration is [Ar] 3d34s2. 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, 51V, natural isotope is stable:

#### **15.1. Electronic spectra of vanadium compounds**

Vanadium in its tetravalent state invariably exists as oxo-cation, VO2+ (vanadyl). The VO2+ ion has a single d electron which gives rise to the free ion term 2D. In a crystal field of octahedral symmetry, this electron occupies the t2g orbital and gives rise to ground state term 2T2g. When the electron absorbs energy, it is excited to the eg orbital and accordingly in octahedral geometry only one band corresponding to the transition, 2T2g → 2Eg, 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, 2T2g splits into 2B2g and 2Eg, whereas 2Eg splits into 2B1g, 2A1g. Hence three bands are expected in C4V symmetry in the range of 11000 –14000, 14500 – 19000 and 20000 – 31250 cm-1. The degeneracy of 2Eg is also removed in C2V symmetry resulting four bands. Energy level diagram of VO2+ in octahedral C4V and C2V symmetries are shown in Fig- 4. In the tetragonal C4V symmetry transitions are described by the following equations.

$$\mathrm{^2B\_{2\_{\mathcal{S}}}} \rightarrow \, ^2E\_{\mathfrak{g}} : \left[ -4Dq - Ds + 4Dt - \left( -4Dq + 2Ds - Dt \right) \right] = -3Ds + 5Dt \tag{5}$$

$$\mathcal{B}\_{2\_{\mathcal{R}}} \rightarrow \,^2 \mathcal{B}\_{1\_{\mathcal{R}}} : \left[ 6D\eta + 2Ds - Dt - \left( -4D\eta + 2Ds - Dt \right) \right] = 10D\eta \tag{6}$$

$$\mathbf{B}^{2}\mathbf{B}\_{2\_{\mathcal{S}}} \rightarrow \,^{2}A\_{1\_{\mathcal{S}}} \cdot \left[ 6D\eta - 2Ds - 6Dt - \left( -4D\eta + 2Ds - Dt \right) \right] = 10D\eta - 4Ds + 5Dt \tag{7}$$

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.

**Figure 4.** Energy level diagram indicating the assignment of the transitions in octahedral C4V symmetry.

#### **15.2. 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 51V cannot be resolved. The g value for this resonance is less than 2. (ii) VO2+ 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, <sup>11</sup> *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,

$$A\_{11} = P\left[ -\frac{4}{7} - k - \left( \mathbf{g}\_{11} - \mathbf{g}\_{\epsilon} \right) + \frac{3}{7} \left( \mathbf{g}\_{\perp} - \mathbf{g}\_{\epsilon} \right) \right] \tag{8}$$

$$A\_{\perp} = P\left[\frac{2}{7} - k + \frac{11}{14} \mathbb{E}\left[\mathbf{g}\_{\perp} - \mathbf{g}\_{\epsilon}\right]\right] \tag{9}$$

$$\mathcal{g} = \frac{1}{3}(\mathcal{g}\_{11} + \mathcal{2}\mathcal{g}\_{\perp}) \text{ and } A = \frac{1}{3}(A\_{11} + \mathcal{2}A\_{\perp}) \tag{10}$$

Using the EPR data, the admixture coefficients are calculated from the following formulae,

$$\mathbf{g}\_{11} = \mathbf{2}\left(\mathbf{\mathcal{BC}}\_1^2 - \mathbf{C}\_2^2 - \mathbf{2C}\_3^2\right) \tag{11}$$

$$\mathcal{G}\_{\perp} = 4\mathcal{C}\_{1}\left(\mathcal{C}\_{2} - \mathcal{C}\_{3}\right) \text{ and } \mathcal{C}\_{1}^{2} + \mathcal{C}\_{2}^{2} + \mathcal{C}\_{3}^{2} = 1 \tag{12}$$

$$A\_{11} = P\left[\mathcal{g}\_{11} - \left(k + \frac{15}{7}\right)\left(1 - 2\mathcal{C}\_3^2\right) - \frac{3}{7}\left(1 + \mathcal{C}\_1\mathcal{C}\_2\mathcal{C}\_3\right)\right] \tag{13}$$

$$A\_{\perp} = p \left[ \frac{11}{14} \text{g}\_{\perp} - 2 \text{C}\_{1} \text{C}\_{2} \left( k + \frac{9}{7} \right) \right] \tag{14}$$

#### **15.3. 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 .

$$\mathbf{g}\_{11} = \mathbf{g}\_{\epsilon} - \frac{8\lambda\mathcal{J}^{\*2}}{\Delta E\_{\text{xy}}} \tag{15}$$

$$\mathbf{g}\_1 = \mathbf{g}\_\epsilon - \frac{2\lambda \mathbf{e}\_x^{\*2}}{\Delta E\_{xx}} \tag{16}$$

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 2B2g 2B1g and E2 is the energy of 2B2g 2Eg.

λ is the spin-orbit coupling constant(160 cm-1) for the free vanadium(VO2+).

#### **15.4. Typical examples**

20 Advanced Aspects of Spectroscopy

sign indicates an axial compression.

**15.2. EPR spectra of vanadium compounds** 

This is characteristic feature of a tetragonally compressed complex.

Using the EPR data, the value of dipolar term P and k term are calculated,

2 2

2 1 : 6 2 6 4 2 10 4 5 *B A Dq Ds Dt Dq Ds Dt Dq Ds Dt g g*

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

**Figure 4.** Energy level diagram indicating the assignment of the transitions in octahedral C4V symmetry.

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 51V cannot be resolved. The g value for this resonance is less than 2. (ii) VO2+ 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, <sup>11</sup> *g* < *g* < ge which shows the presence of an unpaired electron in the *xy d* orbital.

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.

> <sup>11</sup> <sup>11</sup> 4 3 7 7 *A P kg g gg e e*

> > 2 11 7 14 *AP k gg <sup>e</sup>*

 

> 

 <sup>11</sup> <sup>11</sup> an 1 1 2 2

3 3 *<sup>g</sup> gg AAA* <sup>d</sup> (10)

(8)

(9)

(7)

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).

X-band EPR spectra of the vanadium(IV) complex with DMF recorded in solutions reveal a well-resolved axial anisotropy with 16-line hyperfine structure. This is characteristic of an interaction of vanadium nuclear spin (51V, *I* = 7/2) with S. The observed EPR parameters are *g*11 =1.947, *A*11 = 161.3 x 10-4 cm-1 and *g*=1.978, *A*= 49.0 x 10-4 cm-1. EPR parameters of several samples are available in literature and some of them are given in Table -9.


**Table 9.** Various EPR parameters of VO(II) in minerals

Using the EPR data, the admixture coefficients are calculated for apophyllite and pascoite minerals and are given in the Table -10.


**Table 10.** Admixture coefficients of VO2+ ion

EPR spectrum of polycrystalline sample of wavellite with sixteen line pattern indicates the presence of VO2+ ion as an impurity. The EPR parameters calculated are gzz= 1.933 and gyy=gxx = 1.970 and the corresponding A values are 19.0 and 6.2 mT.

## **15.5. Typical examples**

a. (i) Divalent vanadium (V2+) of d3 configuration, containing halide and other ions in aqueous solutions, gives three transitions, i.e., 4A2g→ 4T2g, 4A2g → 4T1g(F) and 4A2g → 4T1g(P) in an octahedral geometry. In <sup>2</sup> <sup>2</sup> <sup>6</sup> *V HO* , the three bands are observed

at 11400, 17100 and 24000 cm-1 along with some weak shoulders at about 20000 and 22000 cm-1. The bands observed at 11400, 17100 and 24000 cm-1 are assigned to the transitions 4A2g → 4T2g, 4T1g(F) and 4T1g(P) respectively. 10Dq is 11400 cm-1. For divalent vanadium ion, Racah parameters are B = 860 and C = 4165 cm-1. Calculated Racah parameters are expected to be less than the one in the free ion value. Accordingly, the weak shoulders observed at 20000 and 22000 cm-1 are assigned to 4A2g→ 2T2g, and 4A2g → 2T1g, 2E transitions.

(ii) The optical absorption spectrum of vanadium carboxylate tetrahydrate sample displays three bands at 11400, 17360 and 23920 cm-1. These are assigned to the transitions, 4A2g → 4T2g, 4T1g(F) and 4T1g(P) in an octahedral geometry.


(ii) The electronic absorption spectrum of the VO2+ in CdSO4.8H2O recorded at room temperature shows bands at 12800, 13245, 14815, 18345 cm-1. These bands are assigned to 2B2g→ 2Eg, 2B2g→ 2B1g and 2B2g→ 2A1g transitions. The band observed at 12500 cm-1 is the split component of the band at 13245 cm-1. The crystal field octahedral parameter, Dq (1465 cm-1) and tetragonal field parameters, Ds (-2290 cm-1) and Dt (1126 cm-1) are evaluated.


Several examples are found in the literature. Some of them are given in the Table-11.

**Table 11.**

22 Advanced Aspects of Spectroscopy

Kainite

Apophyllite

Pascoite site I siteII

CAPH

minerals and are given in the Table -10.

**Table 10.** Admixture coefficients of VO2+ ion

**15.5. Typical examples** 

4A2g → 2T1g, 2E transitions.

**Table 9.** Various EPR parameters of VO(II) in minerals

Mineral name 11 *g g A*11 mT *A* mT

1.983

17.7

6.9

6.02

7.6 8.2

4A2g → 4T1g(F) and

, the three bands are observed

18.02

18.50 20.00

1.982

1.988 1.976

1.993

0.7090 0.7285 0.03174 0.34 143

<sup>2</sup> <sup>6</sup> *V HO*

1.932

1.933

1.933 1.946

1.933

Using the EPR data, the admixture coefficients are calculated for apophyllite and pascoite

Sample C1 C2 C3 *K P* (x 10-4 cm-1) Apophyllite 0.7083 0.7124 0.0028 0.86 122.7 Pascoite 0.7010 0.7116 0.0035 0.36 118.4

EPR spectrum of polycrystalline sample of wavellite with sixteen line pattern indicates the presence of VO2+ ion as an impurity. The EPR parameters calculated are gzz= 1.933 and

a. (i) Divalent vanadium (V2+) of d3 configuration, containing halide and other ions

at 11400, 17100 and 24000 cm-1 along with some weak shoulders at about 20000 and 22000 cm-1. The bands observed at 11400, 17100 and 24000 cm-1 are assigned to the transitions 4A2g → 4T2g, 4T1g(F) and 4T1g(P) respectively. 10Dq is 11400 cm-1. For divalent vanadium ion, Racah parameters are B = 860 and C = 4165 cm-1. Calculated Racah parameters are expected to be less than the one in the free ion value. Accordingly, the weak shoulders observed at 20000 and 22000 cm-1 are assigned to 4A2g→ 2T2g, and

(ii) The optical absorption spectrum of vanadium carboxylate tetrahydrate sample displays three bands at 11400, 17360 and 23920 cm-1. These are assigned to the

gyy=gxx = 1.970 and the corresponding A values are 19.0 and 6.2 mT.

4A2g → 4T1g(P) in an octahedral geometry. In <sup>2</sup>

in aqueous solutions, gives three transitions, i.e., 4A2g→ 4T2g,

transitions, 4A2g → 4T2g, 4T1g(F) and 4T1g(P) in an octahedral geometry.

d. Pentavalent vanadium has no d electron and hence d-d transitions are not possible. 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 <sup>3</sup> *VO*<sup>4</sup> .

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 <sup>3</sup> *VO*<sup>4</sup> . The minimum value of 10Dq for <sup>3</sup> *VO*<sup>4</sup> 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 <sup>3</sup> *VO*<sup>4</sup> .Hence the evidence does not satisfy the assignment of bands to d-d transitions. Therefore the bands are due to charge transfer transitions.
