Tomohiro Hayashi

Adsorption state of dimethyl disulfide on Au(111): Evidence for adsorption as thiolate at the bridge site

We studied the adsorption state of dimethyl disulfide and methylthiolate on the Au(111) surface by means of the density functional theory (DFT) within a generalized gradient approximation and experimental high-resolution electron energy loss spectroscopy (HREELS) techniques. It turns out that the methylthiolate adsorption is more stable than the dimethyl disulfide adsorption and that the most stable adsorption site for the methylthiolate is the bridge site slightly off-centered towards the fcc-hollow site with its S–C bond tilted from the surface normal by 53°. HREELS results are in excellent agreement with the DFT results, providing very strong support to the depicted adsorption scenario.

First-principles theoretical study of alkylthiolate adsorption on Au(1 1 1)

We have studied methylthiolate (MeS), ethylthiolate (EtS), and butylthiolate (BuS) adsorption on the Au(111) surface using density functional theory within a generalized gradient approximation (GGA). EtS and BuS are also adsorbed at the bridge site slightly off-centered towards the fee-hollow site and the S-C bond is tilted from the surface normal by 52°. The S 2p core level shift of MeS adsorbed at the bridge site agrees quite well with the experimental results, further supporting the bridge configuration. Finally, we have examined several possible MeS configurations in the c(43 × 23) superstructure. At present, however, we cannot obtain consistent results with the experimental ones, presumably due to the limited accuracy of the present GGA functional and/or substrate reconstructions.

Grand canonical Monte Carlo simulation of hydration forces between nonorienting and orienting structureless walls

The hydration forces between structureless model walls were calculated using the grand canonical Monte Carlo technique. Several wall–water interaction potentials were tried, including both orientation independent and strongly directional potentials which reflected the preference of water for tetrahedral hydrogen bonding coordination. Primary attention was given to large wall-to-wall separations (4 nm and more), where the oscillations of the hydration force due to layering effects decayed. The hydration force was found to be highly sensitive to the presence of orientation-dependent terms in the wall–water interaction potential. Strongly directional potentials led to hydrophobic attraction of the walls even when the wall–water interaction was substantially stronger than the water–water interaction. The reason had to do with the orientational ordering induced by the walls in the adjacent water layers.

The interaction of oligo(ethylene glycol) with water: Testing an atomistic force field for transferability

An atomistic force field fitted to ab initio MP2 level energies and geometries of 1,2-dimethoxyethane (EG1) and EG1–water complexes (D. Bedrov, M. Pekny and G. D. Smith, J. Phys. Chem. B, 1998, 102, 996) is tested for transferability to methoxy terminated tri(ethylene glycol) (EG3) and its complexes with water. The test is based on a comparison of the force field predictions for EG3 and EG3–water complexes with the respective results of recent ab initio calculations using density functional theory (DFT) (R. L. C. Wang, H. J. Kreuzer and M. Grunze, Phys. Chem. Chem. Phys., 2000, 2, 3613). The force field well reproduces the structure and stability sequence of EG3 and EG3–water conformers but yields noticeably higher EG3–water binding energies. Most of the difference in binding energy is associated with differences between the ab initio methods used in parametrization (MP2) and testing (DFT) of the force field. The agreement between the force field and ab initio DFT predictions can be improved by introducing an additional fitting parameter, like dielectric permittivity, which attenuates the electrostatic terms of the EG3–water potential.

Superperiodic assembly of 2,6-diethynylpyridine through weak hydrogen bonds at the 1-phenyloctane/HOPG interface.

Regardless of the absence of alkyl chains and conventional hydrogen bonding sites as well as its small size, 2,6-diethynylpyridine forms an ordered array at the interface between 1-phenyloctane and highly oriented pyrolytic graphite (HOPG) under room temperature conditions, as revealed by scanning tunneling microscopy. We propose a model for the superperiodic molecular arrangement with reference to the bulk crystal structure, in which the surface pattern is governed by weak C-H...N and C-H...pi hydrogen bonds as well as the periodic potential of the underlying graphite surface.

Well-Ordered Domains of 4-Fluorobenzenethiol Self-Assembled Monolayers on Au(111) Guided by a Displacement Reaction

Displacement processes of pre-covered cyclohexanethiol (CHT) self-assembled monolayers (SAMs) by 4-fluorobenzenethiol (4-FBT) on Au(111) were examined as a function of displacement time by scanning tunneling microscopy (STM) and water static contact angle (CA) measurements. STM imaging revealed that the adsorption of 4-FBT on Au(111) in a 1 mM ethanol solution at room temperature for 24 h generated disordered SAMs, whereas well-ordered 4-FBT SAMs with a (4√6 × √3)R5° packing structure were formed over the entire Au(111) surfaces via the displacement of pre-covered CHT SAMs by 4-FBT molecules. The CA measurements also showed that CA values increase with increasing displacement time, reflecting that the displacement reaction took place and the resulting SAMs had greater hydrophobicity compared with CHT SAMs. In this study, we found that the displacement technique using CHT SAMs as a molecular template is very useful in obtaining 4-FBT SAMs with a high degree of structural order and large ordered domains.