Density Functional Study of Molecular Recognition and Reactivity of Thiourea Derivatives Used in Sensors for Heavy Metal Polluting Cations

The geometry and electronic structure of four thiourea derivatives, 1,3-diphenylthiourea, 1-furoyl-3-phenylthiourea, 1-furoyl-3-bencyl-3-phenylthiourea, and 1-furoyl-3-hydroxyethylthiourea, were determined by means of the DGauss program, which is a density functional theory based method. Calculations performed were of the all-electron type at the local spin density level of theory. Orbital basis sets of DZVP2 quality were used for the H, C, N, O, and S atoms. The frontier molecular orbitals were characterized. They account for the observed selectivity of these molecules toward the following cations: Pb2+, Hg2+, Cd2+, Ag+, Cu2+, Ni2+, Zn2+, and Mn2+. Moreover, the localization of the highest occupied molecular orbital coupled with a high negative charge over the sulfur center accounts for the softness of the active sulfur site. Indeed, a metal-sulfur interaction is mainly responsible for the observed electrochemical behaviors in ion-selective electrodes toward the different cations. These results agree with the estimated softness reported previously for these cations. Our calculations explain the observed differences of reactivity for these neutral carriers through their interactions with heavy metal cations. 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 609?622, 2000