Quantum mechanical investigation of thiourea adsorption on Ag(111) considering electric field and solvent effects

The adsorption of different thiourea species was investigated on the 111 surface of silver considering electric field and solvent effects with the objective of (a) elucidating the nature of the adsorbed species, mainly in ail electrochemical environment, and (b) understanding the energetics and mechanisms of the surface reactions, particularly the oxidative adsorption of thiourea. We first considered the adsorption in vacuum of molecular species such as thiourea (TU), canonical thiourea, and formamidine disulfide as well as the adsorption of the canonical thiourea radical resulting from the cleavage of the S-H bond. The molecular adsorption of TU in vacuum has the highest binding energy (-33.2 kcal/mol) when the molecular plane is parallel to the surface. However, the strongest surface bond was observed for the canonical thiourea radical (-42.2 kcal/mol) whose nature is the same as that of the methanethiol radical. Externally applied electric fields perpendicular to the surface have an important effect on the orientation and surface bonding of thiourea, leading to an upright configuration of the molecule and to a strengthening of the sulfur-surface bond. Solvent effects were first considered to investigate the equilibria of thiourea in solution with the purpose of elucidating the nature of the predominant species which, in turn, constitutes the initial adsorbate on the surface. In neutral and basic pHs, the thiourea molecule is the predominant species, whereas in acidic electrolytes, the thiouronium ion readily forms. The microsolvation of thiourea with up to 16 water molecules was investigated to determine the structure of the hydration shell and the interaction energy between TU and water molecules. Water molecules form a compact cage around thiourea characterized by linked tetramer and pentamer structures. The NH2 or groups form hydrogen bonds with water molecules, whereas the sulfur atom is poorly hydrated. The implications of the structure of water around thiourea on the adsorption geometry of the molecule are discussed. The oxidative adsorption mechanism of thiourea resulting in chemisorbed canonical thiourea was investigated considering charge-transfer processes, solvent effects, and the presence of a counterion.