Rita Kakkar

Theoretical Studies on Hydroxamic Acids

Hydroxamic acids find many applications in chemistry and biology and have been the subject of many experimental investigations. Theoretical studies are not as frequent. However, the smallest homolog, formohydroxamic acid (FHA), has been studied at various levels, including high-level ab initio and density functional with large basis sets. All studies indicate that it exists as the Z-amide tautomer and deprotonation occurs from the nitrogen. Many combined experimental and theoretical studies confirm these conclusions. The interaction of formohydroxamic acid with solvent molecules and its adducts with various compounds have also been theoretically investigated. The higher homologs have not been studied as much. Acetohydroxamic acid, also known as Lithostat, has also been investigated at various levels of theory and experiment. Interest in this compound arises from the fact that it is a known inhibitor of urease. Other investigated hydroxamic acids include benzohydroxamic acid, whose conformational properties have also been investigated. Because of their association with inhibition of the urease enzyme and matrix metalloproteinases, as well as their application as siderophores, the complexation chemistry of hydroxamic acids is very important. However, very few theoretical studies aimed at deciphering the complexation of hydroxamic acids have appeared in the literature. Studies on metal ion selectivity of hydroxamic acids reveal that the affinity toward Ni(II), the metal ion present in urease, is due to its electrophilic nature. However, several QSAR and docking studies have appeared in the literature relating to applications of hydroxamic acids as inhibitors.

Metal Ion Selectivity of Kojate Complexes: A Theoretical Study

Density functional calculations have been performed on four-coordinate kojate complexes of selected divalent metal ions in order to determine the affinity of the metal ions for the kojate ion. The complexation reactions are characterized by high energies, showing that they are highly exothermic. It is found that Ni(II) exhibits the highest affinity for the kojate ion, and this is attributed to the largest amount of charge transfer from the ligand to the metal ion. The Ni(II) complex has distorted square planar structure. The HOMOs and LUMOs of the complexes are also discussed. All complexes display a strong band at ~1500 cm−1 corresponding to the stretching frequency of the weakened carbonyl bond. Comparison of the complexation energies for the two steps shows that most of the complexation energy is realized in the first step. The energy released in the second step is about one-third that of the first step.

DFT STUDY OF SOME TRIVALENT d- AND f-BLOCK METAL ION COMPLEXES OF ALLOXAN

Density functional calculations have been employed to elucidate the structures of some six coordinated complexes of alloxan monohydrate with some d- and f-block metals. Alloxan monohydrate may exist in the mono-ionized or di-ionized form in its complexes, and both states were investigated. It is found that when the metal ion is coordinated to three bidentate ligands, the structures are nearly trigonal prismatic, but replacement of a bidendate ligand by two monovalent ligands changes the geometry to deformed octahedral. The metal-alloxanate bonding is largely ionic for the lanthanoids. The calculated vibrational frequencies are in agreement with the experimentally determined ones.