No abstract

DFT calculations were performed on a biphenyl-based molecule bonded to gold nanoleads in order to evaluate its potential as a reversible molecular switch. The torsion angle (φ) between the aromatic rings may be controlled by means of reducing a disulfide functionality that bridges the two rings, giving rise to a "closed" species (disulfide bridge oxidized, φ ∼ 28°) and an "opened" species (disulfide bridge reduced, φ ∼ 65°). The mechanical properties of the nanojunction formed by this molecular species sandwiched between gold cluster pyramids mimicking metallic electrodes were determined. The thermodynamics of the reduction reaction was studied on the disulfide bridge as well as on the potentially competing anchoring sulfur atoms. A highly favorable product ratio toward the disulfide bridge reduction was found. Conductance values were calculated by means of non-equilibrium Green functions techniques. Interestingly, a significant difference between the closed (high conductance) and opened (low conductance) species was found.

An accelerated dynamics scheme is employed to sample the configurational space of a system consisting of an alkanedithiol molecule confined to the gap between a metal tip and a perfect metal surface. With this information and by means of nonequilibrium green functions techniques (NEGF), conductance calculations are performed. The present results show that even for this system, which is one of the most simple conceivable because of the perfectness of the surface, a complex behavior appears due to the occurrence of an unexpected tip-molecule-surface arrangement, where the insertion of one of the molecular ends into the tip-surface gap generates configurations with strongly enhanced conductance. Estimates are also made for the time required to generate the molecular junction, indicating that it should depend on the tip-surface distance, thus opening the way to new experiments in this direction.

The main question addressed in this paper is whether the nucleophilic substitution of the p-nitrophenoxy group in (CO)5Cr=C(OC6H4-4-NO2)Ph (1-NO2) by a series of substituted phenoxide ions is concerted or stepwise. Rate constants, kArO, for these substitution reactions were determined in 50% MeCN-50% water (v/v) at 25 degrees C. A Brønsted plot of log kArO versus pKa(ArOH) s consistent with a stepwise mechanism. This contrasts with reactions of aryl oxide ions with p-nitrophenyl acetate and with similar acyl transfers which are concerted. The reason for the contrast is that the tetrahedral intermediates formed in the reactions of 1-NO2 are much more stable than those in acyl transfers and the intrinsic barriers to their decomposition are higher than for the ester reactions. The points on the Brønsted plots for which pKa(ArOH) > or = pKa(PNP) define a straight line with beta(nuc) = -0.39, suggesting that bond formation has made very little progress at the transition state and that partial desolvation of the nucleophile is part of the activation process. The hydrolysis of 1-NO2 and of the unsubstituted analogue (1-H) has also been studied over a wide pH range, providing rate constants for nucleophilic attack by hydroxide ion (kOH), by water (kH2O), and by general base-catalyzed reaction with water (kB). Furthermore, kH2O values were obtained for the hydrolysis of (CO)5Cr=C(OC6H4X)Ph (1-X) as a byproduct of the reactions of 1-NO2 with aryl oxide ions. Structure-reactivity relationships for these reactions are discussed in terms of inductive, pi-donor, and steric effects.

Accelerated molecular dynamics and quantum conductance calculations are employed to shed light onto the electrochemical properties of the Au|1,8-octanedithiol|Au junction. Widely different contact geometries with varying degrees of roughness are examined. Strikingly, the two extreme situations considered in this work, tip-tip and tip-perfect surface junctions, give almost indistinguishable conductances. This result contrasts the usual notion that different S-Au bonding geometries combined with molecular torsions provide the explanation for the experimentally observed sets (low, medium, high) of conductance peaks. In this work, we provide an alternative explanation for the occurrence of these sets in terms of the specific anchoring sites of the molecule to the tip, which in turn determines the interaction of a portion of the carbon chain with the tip.

Cyanide-modified Pt(111) surfaces have been recently used to study atomic-ensemble effects in electrocatalysis. These studies have been based on the assumption that cyanide acts as a third body, blocking some surface sites but leaving the electronic properties of the surrounding ones unaltered, although this is in apparent contradiction with the observation of a positive shift of 0.2 V in the onset of hydrogen adsorption on cyanide-modified Pt(111) electrodes, as compared with clean Pt(111). We have performed theoretical calculations in order to provide support to this assumption and explain the positive shift in the onset of hydrogen adsorption, which is shown to be due to the formation of CNH ad .

DFT computational energies for free alkoxy-and thio-substitued free carbenes correlate with experimental steric data (Taft parameters) showing the importance of the steric volume at the expense of the electronic interaction in determining the conformation of free carbenes 1-14. The shorter C-O bond distances in the alkoxy derivatives render alkoxycarbene more sensitive to steric effects than thiocarbenes. This strong dependence of the conformation with the steric hindrance is not applicable to metal-complexed carbenes. Thio-substituted carbene complexes exist exclusively as syn-isomers due to the combination of an interaction of the sulfur lone pair with two CO ligands and the steric repulsion with the "CO-wall". The stronger C carbene -S bond compared to C carbene -O is responsible for the increased syn-anti rotation barrier observed for the alkylthio-substituted metal carbene complexes compared to their oxygen analogues. The differences of polarity between syn-and anti-isomers of alkylthio-substituted metal carbene complexes explain also the increment of the isomerization barrier with the polarity of the solvent. The effect of the substituent attached to the carbene carbon is not decisive in the conformation of these compounds.

We report on the calculated binding energies and rupture forces of a series of ortho-substituted pyrazines bonded to gold nanoclusters. This system is proposed as a model for the 4,4-bipyridine-gold nanojunction to assess the effect of the substituent on the mechanical stability and the rupture modes of such nanojunction. The present results show that although there is a definite trend to a reduced rupture force for a more electron withdrawing substituent on the pyrazine molecule, this dependence is small, allowing for the construction of nanojunctions with different transport properties but similar mechanical stabilities. A clear-cut correlation is found between the rupture force of the nanojunction and the pK a value of the structurally related pyridines, providing with a simple empirical way of predicting the stability of the system. The possibility of attaching the molecule to the cluster through a monatomic gold chain is also explored, with the finding that the Au-Au rupture at the chain is a possible mechanism when the chain is at least two atoms long.