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1 Available online Journal of Chemical and Pharmaceutical Research, 2013, 5(10): Research Article ISSN : CODEN(USA) : JCPRC5 Comparative absorption spectral study for the interactions of Pr(III) with selected urea and thiourea: Energy and electric dipole intensity parameters N. Yaiphaba Department of Chemistry, D. M. College of Science, Imphal, Manipur, India ABSTRACT The complexation of urea and thiourea with Pr(III) has been studied in different organic solvents like methanol (CH 3 OH), acetonitrile (CH 3 CN), dioxane (C 4 H 8 O 2 ) and DMF (C 3 H 7 NO) and their equimolar mixtures by employing absorption difference and comparative absorption spectrophotometry. The complexation of Pr(III) with ureas and thioureas is indicated by the changes in the absorption intensity following the subsequent changes in the oscillator strength of different bands and Judd-Ofelt intensity (T λ ) parameters. The other spectral parameters like energy interaction parameters namely Slater Condon (F k ), Racah (E k ), Lande (ξ ), Nephelauxetic ratio (β) and bonding parameters (b 1/2 ) are further computed to explain the nature of complexation. The difference in the energy parameters with respect to donor atoms and solvents reveal that the chemical environment around the lanthanide ions has great impact on f f transition and any change in the environment result in modification of the spectra. Various solvents and their equimolar mixtures are also used to discuss the participation of solvents in the complexation. ey words: Transition spectra, oscillator strength, pseudohypersensitive, nephelauxetic effect INTRODUCTION The photophysical property of lanthanides has been becoming an active area of research to both chemists and spectroscopists for many years, because of their potential applications in many fields. Most of these applications are based on their luminescent and absorption spectral properties of lanthanides [1-4]. The transition spectra of lanthanide have been found to exhibit significant sensitivity towards even minor change in the immediate coordination environment around lanthanide ion through variation in the absorption spectral pattern, degree of red shift and intensification of bands. The intensities of these hypersensitive transitions [5 9] have also been used extensively between outer and inner sphere coordination, in identifying the coordinating sites of ligands, in determining the immediate coordination environment and in predicting the coordination number of lanthanides. Recently, han et al. [10] have used the intensity and band shapes for both hypersensitive and non-hypersensitive transitions in studying the effect of changes in the environment upon absorption spectra of Ho(III) and Er(III) complexes with thiocyanate and 2,2-bipyridine. Our interest in the present study lies in investigating the interaction behaviour of hard metal ion, Pr(III) with ally urea/thiourea and phenyl urea/thiourea having different donor atom with different π-electron density. The present study deals with the interaction of Pr(III) ions with ureas viz. allyl urea (au), phenyl urea(pu) and thioureas viz. allyl thiourea (atu), phenyl thiourea (ptu) using absorption difference and comparative absorption spectrophotometry. The maor structural difference between these systems are (i) the possible ligand and metal orbital interaction in ureas and thioureas (ii) different π-electron density and (iii) the varying coordination ability of the different solvents involved in our study. Both ureas and thioureas are ambidentate ligand each possessing two 377

2 donor sites (N & O in ureas and N & S in thioureas) which possess different affinities towards different metal ions. The interaction are studied by computation of various spectral parameters namely energy interaction (Slator Condon (F k s ), nephelauxetic effect (β), bonding (b 1/2 ) and percent covalency (δ) parameters) and intensity parameters (oscillator strength and Judd Ofelt parameters). EXPERIMENTAL SECTION 2.1 Materials Ureas (allyl ureas and phenyl urea) and thiourea (allyl thiourea and phenyl thiourea,) from Sisco Chemical Laboratories, Praseodymium(III) nitrate hexahydrate (99.9%) purity from M/S Indian Rare Earths Ltd. are used for synthesis and spectral analysis. The solvents used are CH 3 OH, CH 3 CN, DMF and Dioxane which are of A/R grade from E. Merk. The absorption spectra of Pr(III) complex with ureas/thioureas were recorded on Perkin Elmer Lambda-35 UV Vis Spectrometer in the range nm. The concentrations of all the solutions were maintained at 0.01 mol L -1. The temperature of all observations was maintained by using Perkin Elmer PTP1 Peltier-temperature system. For a typical absorption study, a solution of Pr(III) nitrate (0.01 mol L -1 ) and urea (0.01 mol L -1 ) in double distilled water were added and the resultant mixture was stirred for sufficient time to form the complex in solution. The solution was scanned for the absorption spectra in the spectral region 350 nm 1000 nm. 2.2 Methods Nephelauxetic ratio has long been regarded as a measure of covalency. The Nephelauxetic effect has been interpreted in terms of Slater-Condon and Racah parameters, by the ratio of the free ion and complex ion [11-12]. F E β = or (1) F E C f C f Where, F (= 2,4,6) is the Slater-Condon parameters and E, the Racah parameters for complex (C) and free ions (f) respectively. The bonding parameter and percent covalency are calculated as b β = (2) 2 1 β δ = 100 β (3) In the electronic transition of -, the energy, E so arises from the most important magnetic interactions, while the spin orbit interactions may be written as, E = A ξ (4) so so Where A so is the angular part of spin orbit interaction and ξ is the radial integral and is known as Lande s parameter. By first order approximation the energy E of the th level is given by Wong[13] as E (F,ξ ) E E o = Eo(F,ξ ) + F + ξ (5) F ξ Where E o is the zero order energy of the th level. The value of F and ξ are given by F F + F and = (6) ξ o = ξ o + ξ The differences between the observed E value and the zero order value E is evaluated by 378

3 E = = 2,4,6 E F E + ξ ξ (7) By using the zero order energy and partial derivatives of Pr(III) ion given by Wong (7), the above equation can be solved by least squares technique and the value of F and ξ can be found out. From these values the value of F 2, F 4, F 6 and ξ are found out by using equation (6). The calculation of the band intensities is based upon the theoretical treatment derived by Judd and Ofelt [14]. They considered the transitions are essentially electric dipole in character and the oscillator strength corresponding to the induced electric dipole transition ΨJ Ψ / J / as given by P = N (λλ N 2 Tσ(f λ ψj U f ψ J ) λ= 2,4,6 (8) Where U (λ) is matrix element of rank λ. The three quantitiest 2,T 4 and T 6 are related to the radial parts of the N wave functions, the wave functions of perturbing configurations of which the nearest is N-1 5d. The measured intensity of an absorption band is related to the probability (P) for the absorption of radiant energy (oscillator strength) by the expression: mc P = εi( σ)dσ 2 Nπe or 9 P = ε ( σ)dσ (9) i Where ε is the molar absorptivity at the energy σ (cm -1 ) and other symbols have their usual meaning. From these values the value of T 2,T 4 and T 6 are calculated by using P ν obs [( U )] T2 + [( U )] T4 + [( U )] T6 = (10) RESULTS AND DISCUSSION Figure 1 shows the comparative absorption spectra of Pr(III) and Pr(III) with different urea and thiourea in DMF as solvent while Figure 2 shows the comparative absorption spectra of Pr(III) with pu in methanol, acetonitrile, dioxane and DMF. Pr(III) ions observed four energy bands ( 3 H 4 3 P 2, 3 P 1, 3 P 0 and 1 D 2 ) in the visible spectral region. These transitions do not obey the selection rules for hypersensitive transition, yet, they have been found to exhibit substantial sensitivity towards even minor coordination changes around Ln(III) ion, due to the difference in the binding behaviour and changes in the immediate coordination environment around Ln(III) ion. Such transitions are regarded as ligand mediated pseudo hypersensitive transitions [15]. arraker [16] shows the interrelation between the coordination number with the shape, energy and oscillator strengths of hypersensitive and pseudohypersensitive transitions. 379

4 Figure 1 Comparative absorption spectra of Pr(III), Pr(III):atu, Pr(III):au:, Pr(III):ptu, Pr(III):pu in DMF Figure 2 Comparative absorption spectra of Pr(III):pu in Methanol, Acetonitrile, dioxane and DMF On complexation of Pr (III) with the ureas and thioureas, we observed reduction in the values of interelectronic repulsion parameters, Slator Condon and spin orbit coupling constant (Table 1). This phenomenon is referred to as nephelauxetic effect. Nephelauxetic effect is taken as a measure of metal ligand covalent bonding. This effect may be visualized to be due to an expansion of the wave functions which results from interaction between the metal cations and neighbouring ligands. The Nephelauxetic effect increases as the co-ordination number decreases. For spectral studies on the structures of coordination compounds of lanthanides in solution, any evidence of the relationship between the Nephelauxetic band shift and the structure is of special interest. The variation in the value of E k (k = 2, 4, 6) corresponds to that in the value of (F k ), since they are interrelated. Misra et al. [17-18] observed a general decrease in the value of F k, E k and ξ parameters as compared to corresponding parameters of the free ion. The values of Nephelauxetic effect (β) in all the system is less than unity and the values of the bonding parameter (b 1/2 ) are positive which indicates covalent bonding. The marginal decrease in the values of F k and ξ indicating complexation leads to the increase in the values of Nephelauxetic ratio and percentage covalency (Table 1). From the table the decrease in the bonding parameter is found to be lowest for phenyl urea(pu) when DMF is the solvent. 380

5 Table 1 Computed value of energy interaction Slater Condon F k (cm -1 ), Spin orbit interaction ξ (cm -1 ), Racah energy E (cm -1 ), Nephelauxetic ratio (β), bonding (b 1/2 ) and covalency (δ) parameters of Pr(III), Pr(III):atu, Pr(III):au, Pr(III) : ptu and Pr(III): pu systems in different organic solvents : System F 2 F 4 F 6 ξ E 1 E 2 E 3 β b 1/2 δ 1. CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3CN + DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3CN:Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu DMF : Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu CH 3OH + CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH+DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH + Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu Tables 2 gives the computed and observed values of energies for the various transition bands and root mean square (RMS) deviation showing the correctness of the various energy parameters. For Pr(III), the values of Eobs is larger than that of Ecal and with the addition of ligands there is decrease in the values of the energies showing there is complexation. The estimated r. m. s values for Pr(III) for these complexes indicated the accuracy of the method to be sufficient for parametric calculation. 381

6 Table 2 Computed and Observed values of Energies(cm -1 ) and R.M.S. values for Pr(III), Pr(III):atu, Pr(III):au, Pr(III) : ptu and Pr(III): pu in different organic solvents System 3 H 4 3 P 2 H 4 3 P 1 H 4 3 P 0 H 4 1 D 2 Eobs Ecal Eobs Ecal Eobs Ecal Eobs Ecal R.M.S 1. CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu DXN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu 5. CH 3CN:DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu 6. CH 3CN:DXN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu 7. DMF : Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH +CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH : DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH : Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu

7 Table 3 Experimental values of oscillator strength (P x 10 6 ) (observed and calculated) and Judd-Ofelt parameter (T λ x ) cm -1 for Pr(III), Pr(III):atu, Pr(III):au, Pr(III) : ptu and Pr(III): pu complexes in different organic solvents System 3 H 4 3 P 2 H 4 3 P 1 H 4 3 P 0 H 4 1 D 2 P obs P cal P obs P cal P obs P cal P obs P cal T 2 T 4 T 6 1. CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu Dioxane+CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu DMF+Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH+DMF Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu DMF+CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH+Dioxane Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu CH 3OH+CH 3CN Pr(III) Pr(III) : atu Pr(III) : au Pr(III) : ptu Pr(III): pu The absorption intensity data for transitions in a series of Pr(III) complexes of ureas/ thioureas in different organic solvents are reported in Table 3. These are analyzed in terms of oscillator strength, Judd Ofelt intensity parameters T λ (λ = 2,4,6) and changes in the ligands and solvent composition. The addition of ligands (urea/thiourea) to Pr(III) brings about a noticeable enhancement in the oscillator strength of pseudohypersensitive 383

8 bands (irrespective of solvent nature). Consequently, marked variation in the experimentally determined Judd Ofelt (T λ ) intensity parameters was found. This intensification of bands can be interpreted in terms of interaction of -orbital with ligand wave function. The intensification of bands and red shift were taken as an evidence for the involvement of ligand in complexation. This intensification of bands became more prominent for phenyl than ally urea/thiourea. In general comparing the variation observed in the energy interaction parameter (Fk and ξ ) with variation of oscillator strength and Judd Ofelt parameter (intensity parameter) towards minor coordination changes induced by the bonding of these different ureas and thioureas, it is because of this reason that the discussion of energy interaction parameter and energy parameter has been reduced merely to a minimum while main emphasis has been given for using intensity data for structural elucidation. The significant changes in the T λ parameters in absence of significant nephelauxetic effect (as observed in present study) also show that the unsymmetrical part of the field has maor influence on these parameters. Regarding this, Reisfeld [19] has argued that an increase in covalency is not the sole factor that influences T λ parameters. The nephelauxetic effect, which is caused by lowering of the excited states of lanthanide ion by the surrounding crystal field, is influenced mainly by the asymmetric part of the crystal field. The values of T λ parameters change significantly on varying the composition of solvents. T 2 is found to be negative so it is meaningless in the present study. T 4 and T 6 are affected significantly. Both parameters are related not only to the immediate coordination changes but also to changes in symmetry properties of the complex species [20]. The different potential donor sites are responsible for the distortion of the polyhedron of oxygen/sulphur around Pr(III), lowering the site symmetry and increasing the probability of the transition. Absorption spectra of Pr(III) complex in different organic solvents (Figure 2) clearly shows the affinity of solvents towards the Pr(III) coordination environment. The small differences in the band shape in the different solvents may be related to differences in how the donor groups interact with f-electrons of the lanthanide ion. DMF appears to have strongest influence on Pr(III):pu followed by dioxane and least in acetonitrile. This means that DMF is effective in promoting electric dipole intensity [21-23], and to cause alternation in strength and symmetry of field about Pr(III). Changes in symmetry around Pr(III) along with invasion of solvent type lead to the intensification of bands. CONCLUSION From the above study it can be concluded that (i) The relative sensitivities of different - transition bands towards these minor coordination changes and their correlation in terms of different T λ -parameters also indicated clearly that all the T λ parameters are exhibiting sensitivities towards the structural changes. In case of Pr(III), T 6 is most significant followed by T 4 while T 2 is invalid because sometimes its values is coming negative. ii) Maximum intensification was induced by phenyl urea while ally thiourea imparted lowest intensity to the bands. They are observed in the order atu < au < ptu < pu. iii) The solvent also induced varying intensification to the pseudohypersensitive bands; DMF induced maximum intensification followed by solvent mixture comprising of DMF and methanol. Urea in general have been found to be better ligand as compared to analogous thiourea ligands, due to the higher affinity of O-donor atom of ureas by the hard metal ion Ln(III) then S-donor atoms of thioureas. Moreover, the π- electron density have also played a maor role towards the intensification of bands. Acknowledgements The author acknowledges UGC, New Delhi for financial assistance. REFERENCES [1] C Jorgensen. Absorption Spectra and Chemical Bonding in Complexes, Pergamon Press, Oxford,1962. [2] AA han: Iftikhar. 1994, Polyhedron, 13, [3] L ostova; N Trendafilova; G Momekov. J. Trace Elem. Med. Biol., 2008, 22, [4] YU Liang-Cai; LIU Han-Wen; LIU Sheng-Li; Chin. J. Chem. 2008, 26, [5] RG Pearson. Coord. Chem. Rev., 1990, 100, [6] DE Henrie; RL Fellows; GR Choppin. Coord. Chem. Rev., 1976, 19, [7] R Reisfield; Y Eckstein. Solid State Commun., 1973, 13, [8] B Yatsimirskii; N Davidenko. Coord. Chem. Rev., 1979, 27, [9] C Jorgensen; BR Judd. Mol. Phys., 1964, 8, [10] AA han; HA Hussain; Iftikhar. Spectrochim. Acta A, 2004, 60, [11] SN Misra. J.Sc. Ind. Res., 1985, 44, [12] SN Misra; Ind. J. Biochem, Biophys., 1990, 27, [13] EY Wong. J. Chem. Phy., 1963, 38,

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