Synthesis,reactivity and X-ray structure of the complex [(depe)2Fe(S2CPEt3)](BPh4)2 (depe = 1,2-bis(diethylphosphino)ethane)

The reaction of the zwitterion Et₃PCS₂, with iron (II) tetrafluoroborate in the presence of 1,2-bis(diethylphosphino) ethane (depe) and NaBPh₄ gave the unusual complex [(depe)₂Fe(S₂CPEt₃)] (BPh₄)₂ (I), whose structure has been determined by X-ray analysis. A dithioformatoiron(II) complex was made by treating I with NaBH₄.     

Electronic structure of metal/molecule//metal junctions: a density functional theory study of the influence of the molecular terminal group

We report on density functional theory calculations of the electronic structure of Au(111)/molecule//Au(111) junctions in which thiol molecules are chemically bound at one end to a gold electrode (the "substrate"), while the other end has a separation of a few to several angstroms from a second gold electrode (the "tip"). Our goal is to investigate the role of different molecular terminal groups and of the tip-molecule distance either on the spatial dependence of the local density of states (LDOS) at the Fermi energy E(f) or on the energy dependence of the projected density of states onto different molecular subunits. We consider conjugated diphenylthiol (SPh2R) molecules with terminal groups R = H, SH, CH3, or CF3 as well as "mixed" conjugated-saturated phenylthiol-pentane (SPhC4CH3) and butanethiol-toluene (SC4PhCH3) molecules. For SPh2R molecules, the LDOS at E(f) exhibits an oscillatory exponential decay along the molecule, with an average decay constant that depends weakly on the R terminal group. For the mixed aromatic-aliphatic molecules instead, there are large differences in the LDOS at E(f), with SC4PhCH3 showing a much larger LDOS in the proximity of the terminal CH3 group than SPhC4CH3.     

Influence of end group and surface structure on the current-voltage characteristics of alkanethiol monolayers on Au(111)

We present density functional theory calculations of the electronic structure and tunneling characteristics of alkanethiolate monolayers on Au(111). We systematically analyze radical3 x radical3 full coverage monolayers of SC6H12X molecules with different terminal groups, X=CH3, NH2, SH, OH, COOH, OCH3, on defect-free ("perfect") Au(111). We also study the influence of the surface-molecule bonding structure by comparing the properties of monolayers of SC6H12CH3 molecules on the perfect surface and on Au(111) surfaces with vacancies or adatoms. The tunneling currents (I) through the adsorbed monolayers with a single chemical contact have been calculated within the Tersoff-Hamann approach for voltages between -1 and +1 V. Computed currents are found to depend linearly on V at low voltage, with typical values of approximately 60 and 150 pA/molecule at 0.2 and 0.5 V, respectively, in good agreement with several experimental data. Computed tunneling currents show also a significant dependence on both the terminal group X and the surface structure. In particular, in order of decreasing intensities, currents for the different end groups are NH2 approximately SH>CH3>OH>OCH3>COOH. The relationships between the tunneling current, the work function of the surface+SAM, and the lineup of the HOMO with respect to the Fermi energy of the metal surface are examined.     

Testing the TPSS meta-generalized-gradient-approximation exchange-correlation functional in calculations of transition states and reaction barriers

We have studied the performance of local and semilocal exchange-correlation functionals [meta-generalized-gradient-approximation (GGA)-TPSS, GGA-Perdew-Burke-Ernzerhof (PBE), and local density approximation (LDA)] in the calculation of transition states, reaction energies, and barriers for several molecular and one surface reaction, using the plane-wave pseudopotential approach. For molecular reactions, these results have been compared to all-electron Gaussian calculations using the B3LYP hybrid functional, as well as to experiment and high level quantum chemistry calculations, when available. We have found that the transition state structures are accurately identified irrespective of the level of the exchange-correlation functional, with the exception of a qualitatively incorrect LDA prediction for the H-transfer reaction in the hydrogen bonded complex between a water molecule and a OH radical. Both the meta-GGA-TPSS and the GGA-PBE functionals improve significantly the calculated LDA barrier heights. The meta-GGA-TPSS further improves systematically, albeit not always sufficiently, the GGA-PBE barriers. We have also found that, on the Si(001) surface, the meta-GGA-TPSS barriers for hydrogen adsorption agree significantly better than the corresponding GGA-PBE barriers with quantum Monte Carlo cluster results and experimental estimates.     

Excess electrons in reduced rutile and anatase TiO2

As a prototypical photocatalyst, TiO₂ is a material of scientific and technological interest. In photocatalysis and other applications, TiO₂ is often reduced, behaving as an n-type semiconductor with unique physico-chemical properties. In this review, we summarize recent advances in the understanding of the fundamental properties and applications of excess electrons in reduced, undoped TiO₂. We discuss the characteristics of excess electrons in the bulk and at the surface of rutile and anatase TiO₂ focusing on their localization, spatial distribution, energy levels, and dynamical properties. We examine specific features of the electronic states for photoexcited TiO₂, for intrinsic oxygen vacancy and Ti interstitial defects, and for surface hydroxyls. We discuss similarities and differences in the behaviors of excess electrons in the rutile and anatase phases. Finally, we consider the effect of excess electrons on the reactivity, focusing on the interaction between excess electrons and adsorbates.