The CNDO and INDO MO calculations have been performed on the oxidised and reduced species of rubredoxins using Clack, Hush and Yandle’s parameterisation scheme for the transition metal complexes in conjunction with Pople and Nesbet’s open-shell UHF method in an attempt to elicit information on the geometry of the conformation of the iron environment. The properties like isotropic and anisotropic hyperfine coupling constants and quadrupole splitting have been calculated and compared with the corresponding experimental values. The results suggest that both the oxidised and reduced species have the iron atoms attached to four sulphur atoms situated as pairs in two perpendicular planes as a result of a highly distorted tetrahedral arrangement

Low-dimensional systems are formed by planar metal dithiolene complexes which stack as columnar structures in the solid state. Stronger interactions among units within a chain leads to highly anisotropic magnetic properties. The magnetic effects are a manifestation of exchange interaction,J and can be studied through detailedepr techniques in conjunction with magnetic susceptibility and x-ray crystal structure. A brief review of such studies carried out mostly in our laboratory is presented along with the relevant background materials.

The single crystalEPR results of [NiBr₂(diars)₂]Br, a low spin Ni(III) complex, magnetically diluted in [Co(NO)Br(diars)₂]Br show a very large bromine hyperfine coupling. The experimental g, ligand hyperfine splitting due to arsenic and bromine and spin densities are compared with other analogous diphos and diars complexes. The unpaired electron is essentially in the$$d_{z^2 } $$ orbital and there is extensive delocalization over all the ligand nuclei due to extended 4s and 4p radial wave functions. On the other hand the pure crystal of [NiBr₂(diars)₂]Br shows single exchange averagedEPR line. Linewidth analysis using the existing theories of exchange gives values of roughly 800 G and 380 G for respectively isotropic and intersite exchange interactions in this system.

Studies on reconstituted hemeproteins, viz, hemoglobin and myoglobin, with metal ions such as Co(II), Ni(II) and Cu(II), using a variety of spectroscopic techniques such as EPR, NMR, resonance Raman and optical, indicate three levels at which the metal ion coordinations are influenced by protein conformation. While in monomeric proteins the type of coordination, essentially a five-coordinate one, is determined by the protein conformation in the heme pocket, the sub-unit interactions in tetrameric proteins can produce a mixture of two types of coordination in individual proteins, one similar to that in monomeric proteins and the other a four-coordinate one with a possible weak axial coordination. As a result, tetrameric proteins with Ni(II) as the metal ion reveal the presence of differing spin states. Furthermore, it is proved that the effects on the proximal histidine associated with theR andT quaternary conformation influence the five-coordinated sites. Some results on hybrid hemoglobins as well as those from isolated chains are also reported.

Four biradicals of differing chain lengths have been prepared by spin-labelling the sulphhydryl groups of dithiothreitol. Temperature-dependent EPR spectra of these biradicals indicate that the dynamics of biradicals is associated with both fast and slow exchanges of conformations. The three-conformational model incorporating cage configuration is found to be more appropriate. Certain thermodynamic parameters of conformationalchanges have been calculated from the experimental EPR parameters, obtained by the use of simulation procedures developed recently.

A binuclear copper(II) complex [Cu₂L(OH)](C1O₄)₂·2H₂O has been synthesized and characterized by X-ray crystallography and¹H-NMR studies. The crystal structure shows that the bridging angles between Cu(l)-O(l)-Cu(2) and Cu(l)-O(2)-Cu(2) are 98.9(2)° and 102.2(2)° respectively. The Cu(l)-Cu(2) distance is 3.0097(12)Å. This indicates that the interaction between the two copper atoms is antiferromagnetic in nature. The geometry around Cu(l) is distorted square-pyramid with one water molecule occupying the axial fifth position, whereas, the geometry around Cu(2) is distorted square-planar with weak interactions of one of the perchlorate anion. There are eight molecules present in the unit cell. There is an interdimer interaction between the dimers. The temperature-dependent¹H-NMR chemical shift studies have been performed on six different protons of this complex which reveals that the exchange coupling constant (− 2J) is same for all protons (208 ± 1 cm⁻¹). However, the hyperfine coupling constant (A′) was found to be different in magnitude as well as in sign. We also report solvent dependent NMR properties.