Calculation and Comparison of Energy Interaction and Intensity Parameters for the Interaction of Nd(III) with DL-Valine, DL-Alanine and -Alanine in Presence and Absence of / in Aqueous and Different Aquated Organic Solvents Using 4f-4f Transition Spectra as Probe
Absorption difference and comparative absorption spectrophotometric studies involving 4f-4f transitions of Nd(III) and different amino acids: DL-valine, DL-alanine, and -alanine in presence and absence of Ca(II) and Zn(II) in aqueous and different aquated organic solvents have been carried out. Variations in the spectral energy parameters: Slater-Condon () factor, Racah (), Lande factor (), nephelauxetic ratio (), bonding (), percentage covalency () are calculated to explore the mode of interaction of Nd(III) with different amino acids: DL-valine, DL-alanine, and -alanine. The values of experimentally calculated oscillator strength (P) and computed values of Judd-Ofelt electric dipole intensity parameters, ( = 2,4,6), are also determined for different 4f-4f transitions. The variation in the values of P and parameters explicitly shows the relative sensitivities of the 4f-4f transitions as well as the specific correlation between relative intensities, ligand structures, and nature of Nd(III)-ligand interaction.
Lanthanides ions are often used as spectroscopic probes as surrogates for calcium ions in studies of biological systems as well as promoters in the textile dyeing industry. Lanthanide elements have extensive application in industries, agriculture, and biomolecular reactions, so it has become very important to understand the behaviour of trivalent lanthanide ions in biological system [1, 2]. The co-ordination chemistry of lanthanide in solution state has become more important with the increase use of lanthanides as probes in the exploration of the structural function of biomolecular reactions [3–6] specially due to its ability to replace Ca(II) in a specific manner [7–10]. In our previous study , we have reported the interaction of Pr(III) with amino acids in aqueous and different aquated organic solvents by using 4f-4f absorption spectra as probes and thereby calculating energy interaction parameters like Slater-Condon () factor, Racah (), Lande factor (), nephelauxetic ratio (), bonding (), percentage covalency (), and parameters like calculated values of oscillator strength (P) and computed values of electric dipole intensity parameters, . We also studied  the energy interaction parameters for the complexation of Pr(III) with glutathione reduced (GSH) in presence and absence of Zn(II) in aqueous and aquated organic solvents using 4f-4f transition spectra as probe. The ligands we chose are amino acids, that is, DL-valine, DL-alanine, and -alanine having two binding sites, namely, carboxylic acid group and an amino group.
Ca(II) which is a hard metal ion and Zn(II) which is a soft metal ion are endogenous metal ions that have different co-ordinating behaviour towards biological molecules. For binding, Ca(II) prefers hard donor site like carboxylic acid group while Zn(II) prefers soft donor site like amino group which are found in amino acids. Since Nd(III) resembles Ca(II), its complexation can provide information about the co-ordination characteristics of diamagnetic Ca(II) with biomolecules during biochemical reactions. Hence, paramagnetic lanthanides are good spectral probes to explore the biological roles of Ca(II) by isomorphous substitution. In our present work, the absorption difference and comparative absorption spectroscopy involving 4f-4f transitions of the complexation of different amino acids with Nd(III) in presence and absence of Ca(II)/Zn(II) has been carried out in aqueous and aquated organic solvents. The variation in the energy parameters like Slater-Condon factor (, K = 2, 4, 6), Lande-Spin-Orbit coupling (), nephelauxetic ratio (), bonding (), percentage covalency () is calculated to explain the nature of complexation. The changes in the values of experimentally determined oscillator strength (P) and Judd-Ofelt electric dipole intensity parameter, suggest the specific correlation between relative intensities, ligand structures, and nature of complexation.
Nd(NO3)6H2O of 99.9% purity was purchased from CDH analytical reagent and DL-valine, DL-alanine, and -alanine were purchased from Loba-Chemie Indo-Australian Co. The solvents used are methanol, acetonitrile, dimethylformamide (DMF), and dioxane, and they are of AR grade from Qualigens.
The solutions of Nd(III), DL-valine, DL-alanine , -alanine, Ca(II) and Zn(II) salts were prepared in different solvents in the concentration M. For the present study, Nd(III) : ligand was kept at 1 : 1 molar ratio and in multimetal complexation like Nd(III) : ligand : Ca(II)/Zn(II) was also kept at 1 : 1 : 1 molar ratio. The absorption spectra were recorded at pH 4 on a Perkin Elmer Lambda-35 UV-Vis spectrophotometer upgraded with high resolution and expansion of scale in the region 350–1000 nm. The temperature for all the observations is maintained at 298 K for all the observations by using water circulating thermostat model DS-G HAAKE.
Nephelauxetic ratio has 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 [13, 14]
where (K 2, 4, 6) is the Slater-Condon parameters, and the Racah parameters for complex (c) and free ions (f), respectively.
The bonding parameter () is represented by the amount of mixing of 4f-orbital and ligand orbital and is related to the nephelauxetic effect as
The energy of 4f-4f transitions is composed of two main components, that is, the electrostatic and spin-orbit interaction between 4f-electrons
where and are the angular part of electrostatic and spin-orbit interaction, respectively. (Slater-Condon) and (Lande parameter) are radial integrals.
Thus to define energy level scheme of 4 configuration, it is necessary to have four radial integrals F2, F4, and which are evaluated by Hartree-Fock method [13, 15]. Then, energy of the th level is given by
where is the zero order energy of the th level
when and .
The difference between the observed values and the zero order value is evaluated by
By using the zero-order energy and partial derivatives of Nd(III) ion given by Wong , the previous equation can be solved by least squares technique and the value of and can be found out. From these values, the value of F2, F4, and are found out by using (5) and (6). The intensity of the absorption band is measured by the oscillator strength (P), which is directly proportional to the area under the absorption curve. It can be expressed in terms of molar extinction coefficient (), energy of the transition in wave number (), and the refractive index () of the medium by the relationship
where = molar extinction coefficient Absorbance/Concentration path length of the cell in cm (l 1 cm), is the energy of transition in wave number, and is the refractive index of the medium.
The experimental values of oscillator strength () of absorption band were given by Gaussian curve analysis as
where is half band width.
The observed oscillator strength () of the transition energies was expressed in terms of parameters defined by Ofelt  known as T2, T4, T6 parameters which are given by the following equation:
where is the matrix element given by Carnall et al.  and is the frequency of transition.
3. Results and Discussion
Lanthanide complexes have very small crystal field stabilization energy and fast water exchange rate. Therefore, the conversion from one geometry to another is very convenient and facile. The sensitivity of hypersensitive bands in lanthanides towards coordination environment has been recognized since long. A few, however, are very sensitive to the environment and are usually more intense when a lanthanide ion gets complexed than it is in the corresponding aquo ion. Such transitions are called hypersensitive transitions. The transitions 4I9/2 and 4I9/24G7/2 of Nd(III) do not obey the selection rules for hypersensitive transition but have been found to exhibit substantial sensitivity in the complexes. Such transitions are called “Ligand Mediated Pseudohypersentive” or Pseudohypersentive transitions . Karraker [18, 19] showed that the shape, energy, and oscillator strength of hypersensitive or pseudohypersensitive transitions can be correlated with coordination number and are diagnostic of immediate coordination environment around lanthanide ions. The computed and observed values of energy interaction - Slater-Condon (), Lande (), Nephelauxetic ratio (), bonding (b1/2), and percentage covalency () parameters for Nd(III), Nd(III) : DL-valine/DL-alanine/-alanine, Nd(III) : DL-valine/DL-alanine/-alanine : Ca(II)/Zn in aqueous and different aquated organic solvents is given in Table 1.
The comparative absorption spectra of Nd(III), Nd(III) : DL-valine/DL-alanine/β-alanine, Nd(III) : DL-valine/DL-alanine/-alanine : Ca(II), and Nd(III) : DL-valine/DL-alanine/-alanine : Zn(II) in solvent DMF are given in Figures 1, 2, and 3.
From the figures, it is revealed that marginal red shift occurs as different amino acids are added to Nd(III). The wavelength further increases on addition of Ca(II) to Nd(III) : ligand. The same is also observed in case of Zn(II) but the increase in wavelength is more in the case when Zn(II) is added to Nd(III)-ligand. There is a slight decrease in Slater-Condon () and Spin-orbit interaction () which indicates lowering of both coulombic and spin-orbit interaction parameters thus leading to the expansion of the central metal ion orbital as the complexation goes on which lead to increase in the values of nephelauxetic ratio (), bonding parameter (), and percentage covalency (). This is in accordance with the theory of f f transitions reported earlier . Ryan and Jørgensen  noticed the dependence of nephelauxetic effect on the coordination number. It was suggested that shortening in the metal-ligand distance occurs with decrease in the coordination number. Misra et al.  observed a general decrease in the values of Slater-Condon () and Spin-orbit interaction () parameters as compared to the corresponding parameters of the free ion. In all the systems, the values of nephelauxetic effect () range from 1.0057 to 1.0200 and positive values of bonding parameter () indicate covalent bonding between Nd(III) and the ligand in the complexes. There is small variation in the bonding parameter () value and this suggests that the 4f-orbitals are very slightly involved in the bonding of lanthanide complexes. The same trend is also observed in case of Pr(III)in our previous work .
The observed and computed values of oscillator strengths and Judd-Ofelt parameters for Nd(III), Nd(III) : DL-valine/DL-alanine/-alanine, Nd(III) : DL-valine/DL-alanine/-alanine : Ca(II)/Zn in aqueous and different aquated organic solvents are given in Table 2.
On complexation the intensities of most of 4f-4f transition change slightly but the intensity of the hypersensitive transition 4I9/24G5/2 of neodymium shows significant intensification and large variation with slight change in the immediate coordination environment around neodymium. This clearly suggests a significant change, when Nd(III) interacts with the ligand in the solution. The comparative absorption spectra shown in Figures 1–3 clearly predict that the addition of ligand to Nd(III) results in significant enhancement in the oscillator strengths of different 4f-4f transitions. As a consequence, we have observed noticeable increase in the magnitude of Judd-Ofelt () intensity parameters. These suggest the binding of the ligand to Nd(III) in solution. The introduction of Zn(II) in the Nd(III)-ligand yields noticeable changes in the oscillator strengths and in the magnitude of Judd-Ofelt () intensity parameters. This result shows the enhancement in the interaction of Nd(III)-ligand in the solution. Among the five transitions of Nd(III) ion , that is, 4I9/2 → 4F3/2, 4F5/2, 4F7/2, 4G5/2, 4G7/2 , we observed the highest oscillator strength value for the hypersensitive transition (4G5/2) and the lowest oscillator strength value for the pseudohypersensitive transition (4F3/2). From these we can conclude that 4G5/2 is the most sensitive transition and 4F3/2 is the least sensitive transition.
The intensification of 4f-4f band specially hypersensitive 4I9/24G5/2 and pseudohypersensitive 4I9/24F7/2, 4I9/24F5/2 transitions is reflected in the magnitude of parameters. Intensification of the bands is due to the introduction of covalency in the metal-ligand bond as the oscillator strength of intra 4f-4f transitions and magnitude of increase with the increase in nephelauxetic effect. In general the oscillator strength for Nd(III) complexes can be expressed in terms of T4 and T6 parameters but the effect of solvent or ligand on the intensity of a hypersensitive transition can be explained by T2 parameters only. The value of T2 parameter which increased in the presence of organic solvents suggests that organic solvents have a better coordinating power in comparison with water as a result of solvation. T4 and T6 parameters which are related to changes in symmetry properties of the complex species are slightly affected in comparison with T2 parameter. At the same time, T6 parameter is also influenced by the extent of mixing of 4 and 5d orbital. It is noted that T4 and T6 parameters are affected significantly in the presence of different solvents. These suggest that symmetry of the complex species is changed significantly and not only the immediate coordination environment of Nd(III). These changes are considered to be good evidence for the involvement of amino acids (ligand) in the inner sphere coordination of Nd(III). Amino acids are expected to form inner sphere complexes by forming an ionic linkage with carboxylic oxygen and an additional linkage due to the amino group. All the results obtained clearly suggest that minor coordination changes in the Nd(III) complexes are caused by the coordinating sites of amino acid, nature of solvent, coordination number, nature of Nd(III)-amino acid bond, which induces significant variation in the intensity of 4f-4f transitions.
The comparative absorption spectra of Nd(III) : DL-alanine : Zn(II), given in Figure 4, shows the effect of solvents.
When the different ligands, that is, DL-valine, DL-alanine, and -alanine are added to Nd(III) in different solvents, maximum intensity is observed when the solvent is DMF. Again, when Ca(II) is added to Nd(III) : DL-valine, Nd(III) : DL-alanine, Nd(III) : -alanine in different solvents, maximum intensification of bands is observed when the solvent is DMF. Similarly, when Zn(II) is added to Nd(III) : DL-valine, Nd(III) : DL-alanine, Nd(III) : -alanine in different solvents, maximum intensity is observed when the solvent is DMF. From these observation, it can be concluded that intensification is maximum in the case of DMF because of the participation of N-donor site of DMF in the complexation of Nd(III) and the amino acid ligands.
From the present studies, it has been observed the involvement of Ca(II)/Zn(II) in the complexation of Nd(III) with amino acids (ligand). There is a slight decrease in the values of and which leads to an increase in the values of nephelauxetic ratio (), bonding parameter (), and percentage covalency () indicating stronger binding of metal ions, that is, Nd(III) with Ca(II)/Zn(II) and amino acids. Same changes are also observed in intensity parameters—oscillator strength (P) and Judd-Ofelt parameter, . The variation of the solvent has significant effect on the oscillator strength (P) and Judd-Ofelt parameter, . Among all the solvents used, maximum intensification is observed in the case of DMF and this is because of the participation of N-donor site of DMF in the complexation.
The authors are thankful to the University Grant Commission (UGC), New Delhi, for their research grant.
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