Spectroscopic and quantum chemical study on electronic and geometric properties of free and embedded dithizone molecules

The molecular and electronic structure of the three tautomeric forms of dit hizone has been calculated by using semiempirical, density functional theor y (DFT), and ab initio methods. Comparison of ground-state energies shows t he symmetric form most stabilized, but there is only a small barrier (<3 kc al/mol) for the hydrogen transfer from N-H toward H-S (enol form). For unde rstanding the origin of the optical transitions intermolecular interactions have to be taken into account. By using the supermolecule method, the abso rption band pattern can be rationalized already on the level of the PM3 mod el. The nuclear magnetic resonance (NMR) solution spectrum is interpreted i n terms of an equilibrium between the symmetric and the enol forms of dithi zone. The appearance of strong EPR signals only for the solid state reflect s a considerable lowering of the triplet state (symmetric form). Experiment al features are discussed in view of calculated energies (stabilization), c hemical shifts (NMR), and SOMO orbitals (EPR). (C) 2002 John Wiley & Sons, Inc.