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 an
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
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.