A DFT investigation of conformational geometries and interconversion equilibria of phenylthiosemicarbazone and its complexation with zinc.

The geometrical structures of phenylthiosemicarbazone (HAPhTSC) conformers have been obtained by geometry optimizations using density functional theory (DFT) calculations at the B3LYP/6-31G(d) and B3LYP/6-311G(d,p) levels of theory. Six thioamino and 24 thioimino tautomers of HAPhTSC have been found. Six tautomerization reactions between thioamino and thioimino tautomers occurring via transition states and their corresponding activation energies have been obtained. Conformational pathways for tautomerizations and interconversions of HAPhTSC conformers have been presented. Tautomerization between the most stable species of thioamino (Atttcc) and its thioimino (Itttcct) tautomer is an endothermic reaction, Delta H(0)=18.17 kcal mol(-1) and its log K=-13.74, at 298.15 K. Thermodynamic quantities of tautomerizations, interconversions of HAPhTSC conformers and their equilibrium constants are reported. The geometry of the zinc complex with HAPhTSC, found as a Zn(HAPhTSC)2Cl2 structure, has been obtained using B3LYP/6-31G(d) calculations. Binding of the Zn(HAPhTSC)2Cl2 complex is an exothermic and spontaneous reaction. [figure]. Conformational notation defined as a name consisting of a letter "A" for A thioamino tautomer followed by "c" for cis or "t" for trans isomerism of five dihedral angles of chi(C4-C3-C2-N3), phi(C3-C2-N3-N2), psi(C2-N3-N2-C1), theta;(N3-N2-C1-N1) and omega(N2-C1-N1-H2), serially, or a letter "I" for B thioimino tautomer followed by "c" for cis or "t" for trans isomerism of six dihedral angles of chi(C4-C3-C2-N3), phi(C3-C2-N3-N2), psi(C2-N3-N2-C1), theta(N3-N2-C1-N1), omega(N2-C1-N1-H2) and delta (N2-C1-S-H1), serially.