Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
EPR and optical studies of Nickel(II) at 300 K
Graphical abstract
Trial and error method has been used to extract the EPR parameters for Nickel(II) in hexaimidazolecobalt(II) dichloride tetrahydrate from the single crystal EPR data obtained at 300 K.
Highlights
► Non Kramer ion have been studied by EPR at 300 K, it is one of the rare observations. ► Single crystal EPR suggests a large D with a rhombic distortion. ► Nickel found to substitute the host ion in the lattice. ► Powder EPR data is comparable with single crystal EPR data. ► Optical study suggests tetragonal distortion supporting EPR data. ► Bonding parameters have been evaluated.
Introduction
Some of the fundamental questions in molecular biology are how proteins fold and how do the molecules assemble in multi-molecular units. Measuring the distance between the sites in protein or a molecule is an ultimate choice to answer these questions. EPR provides an opportunity to measure distance between paramagnetic sites due to the large magnetic moment of electrons. Electron spin relaxation is an important phenomenon in EPR that has wide range of applications including measuring distance [1] in proteins and macromolecules. There are many enzymes and proteins found in biological systems in which Ni(II) is an active centers [2], [3], [4]. So understanding Ni(II) by EPR is significant. However, Ni(II) shows EPR resonance only bellow 77 K [5], [6] due to its peculiar relaxation property. This makes complication in understanding the system. There only few systems reported [7], [8], [9] in which the fine structure of Ni(II) is observed at ambient temperature, which provide an opportunity to understand it better. Imidazole, an organic base, is being biologically important compound, which is often seen to bound with many proteins, enzyme and histidine ligand [10]. Some of the first row transition metal ions in hexaimidazole complexes have been well studied before [9], [11], [12], [13], [14]. Here in we are reporting single crystal EPR and optical studies of Ni(II) ion in hexaimidazolecobalt(II) dichloride tetrahydrate complex, making it one of the very few systems having room temperature EPR data.
Section snippets
Experimental
Single crystal of Ni(II) doped hexaimidazolecobalt(II) dichloride tetrahydrate (abbreviated as HCDT) has been grown at ambient temperature by slow evaporation of aqueous imidazole and cobaltous chloride in the ratio of 6:1 to which dil. HCl (to maintain neutral pH) and 0.05% of Nickel(II) chloride by weight are added. The Ni(II) doped HCDT is crystallized out in 15 days. EPR spectra were recorded on JEOL JES TE 100 EPR spectrometer working at X-band frequency with 100 kHz modulation frequency.
Crystal structure
The IR spectra and powder X-ray data of HCDT is almost identical with that of hexaimidazolezinc(II) dichloride tetrahydrate (HZDT) system. Hence, the crystal structure of isomorphous HZDT is assumed for HCDT lattice, in explaining the EPR analysis, since the crystal structures of HCDT is not available. HZDT belongs to triclinic crystal group with unit cell dimensions: a = 1.07, b = 0.94 and c = 0.84 nm; α = 120, β = 97, γ = 98o and Z = 1 [15]. Zinc ion is surrounded by six imidazole ligands in a distorted
EPR studies
The characteristic EPR spectrum of Ni(II)/HCDT polycrystals is given in Fig. 1. It consists of a strong resonance at 610 mT. This immediately suggests a very large zero field splitting (ZFS) for Ni(II). The approximate ZFS value (D) has been evaluated from the powder spectrum, assuming E = 0 and g = 2.2 [7],here Bo = hν/βg. The D value of ∼910 mT is deduced from the above expression. The deduced D is very large with respect to X-band Frequency and is consistence with absence of other
Conclusions
Single crystal EPR studies of a non-Kramer ion in a paramagnetic host have been undertaken. The interesting observation being that the EPR spectra are observed, even at room temperature. A very few systems are known to be having room temperature EPR spectra. In addition, the analysis has been complicated due to the absence of all allowed transitions in the EPR spectrum. The large zero field splitting term has been analyzed and the g tensor indicates rhombic symmetry, in support of D tensor. The
Acknowledgments
The authors are grateful to Prof. PSRao, Department of Chemistry, Pondicherry University for his valuable suggestions. Velavan Kathirvelu thanks CSIR for Senior Research Associateship (13(8399-A)/Pool/2010).
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