Daniel Forrer

Role and effective treatment of dispersive forces in materials: Polyethylene and graphite crystals as test cases.

A semiempirical addition of dispersive forces to conventional density functionals (DFT‐D) has been implemented into a pseudopotential plane‐wave code. Test calculations on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are corrected for the basis set superposition error. By applying the DFT‐D/plane‐wave approach a substantial agreement with experiments is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by standard local and semilocal density functionals.

Tuning the catalytic activity of Ag(110)-supported Fe phthalocyanine in the oxygen reduction reaction.

A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.

Magnetic Properties and Vapochromic Reversible Guest-Induced Transformation in a Bispyrazolato Copper(II) Polymer: an Experimental and Dispersion-Corrected Density Functional Theory Study

Dispersion-corrected density functional theory (DFT-D) calculations, Electron Spin Resonance spectroscopy (EPR), and variable temperature magnetic moment measurements were used to investigate the structure and the electronic/magnetic properties of bispyrazolato-copper(II) coordination polymer and of its hydration product. The Cu(II) ions are antiferromagnetically coupled through the sigma system of the pyrazolate rings in both compounds. Theoretical electron density maps reveal that water molecules interact simultaneously and to a comparable extent with two Cu(II) centers (through the electronegative O end) and two pyrazolate rings (through the partly positively charged H atoms), which is compatible with the observed internuclear distances. DFT-D calculations indicate that low kinetic barriers are involved in the rearrangement of the host structure.

Stereoselective Photopolymerization of Tetraphenylporphyrin Derivatives on Ag(110) at the Sub-Monolayer Level

We explore a photochemical approach to achieve an ordered polymeric structure at the sub-monolayer level on a metal substrate. In particular, a tetraphenylporphyrin derivative carrying para-amino-phenyl functional groups is used to obtain extended and highly ordered molecular wires on Ag(110). Scanning tunneling microscopy and density functional theory calculations reveal that porphyrin building blocks are joined through azo bridges, mainly as cis isomers. The observed highly stereoselective growth is the result of adsorbate/surface interactions, as indicated by X-ray photoelectron spectroscopy. At variance with previous studies, we tailor the formation of long-range ordered structures by the separate control of the surface molecular diffusion through sample heating, and of the reaction initiation through light absorption. This previously unreported approach shows that the photo-induced covalent stabilization of self-assembled molecular monolayers to obtain highly ordered surface covalent organic frameworks is viable by a careful choice of the precursors and reaction conditions.

Molecular, Electronic, and Crystal Structures of Self-Assembled Hydrothermally Synthesized Zn(II)−Mercaptonicotinate: A Combined Spectroscopic and Theoretical Approach

A Zn(II) 2-mercaptonicotinate coordination polymer (Zn1), with Zn(II) ions chelated by both sulfur and oxygen in a distorted square pyramidal environment, and a molecular Zn(II) 2-hydroxynicotinate complex (Zn2) were synthesized by the reaction of zinc acetylacetonate with 2-mercaptonicotinic (Zn1) and 2-hydroxynicotinc (Zn2) acid, respectively, under hydrothermal conditions. The crystal structures of Zn1 and Zn2 were determined by single crystal X-ray diffraction measurements. Dispersion-corrected density functional theory (DFT) calculations reproduce very well the experimental structures and show that Zn1 is stable against hydration, whereas Zn2 is stable against dehydration over wide ranges of temperature and pressure, in agreement with thermogravimetric analysis results. The electronic structure of the two compounds is computed with the DFT+U method. The theoretical valence band agrees well with the X-ray photoelectron spectroscopy experiments. Furthermore, the band gap of Zn1 is found to be narrower than that of Zn1 and is characterized by the presence of sulfur lone pairs at the edge of the valence band.

Density functional theory study of the binding capability of tris(pyrazol-1-yl)methane toward Cu(I) and Ag(I) cations.

Density functional theory (DFT) has been used to look into the electronic structure of [M(tpm)]+ molecular ion conformers (M = Cu, Ag; tpm = tris(pyrazol-1-yl)methane) and to study the energetics of their interconversion. Theoretical data pertaining to the free tpm state the intrinsic instability of its kappa3-like conformation, thus indicating that, even though frequently observed, the kappa3-tripodal coordinative mode is unlikely to be directly achieved through the interaction of M(I) with the kappa3-like tpm conformer. It is also found that the energy barrier for the kappa2-[M(tpm)]+ --> kappa3-[M(tpm)]+ conversion is negligible. As far as the bonding scheme is concerned, the tpm --> M(I) donation, both sigma and pi in character, is the main source of the M(I)-tpm bonding, whereas back-donation from completely occupied M(I) d orbitals into tpm-based pi* levels plays a negligible role.

Vapochromic properties versus metal ion coordination of β-bispyrazolato-copper(ii) coordination polymers: A first-principles investigation.

A series of monoadducts of the β-bispyrazolato–copper(II) one-dimensional coordination polymer is investigated with plane-wave density functional theory–dispersive interactions (DFT-D) calculations. It was found that weak Lewis bases, such as H2O and CH3OH, prefer a symmetric bridging coordination to the Cu(II) ions, which in turn assume a highly distorted 4 + 2 octahedral configuration. Stronger Lewis bases, namely, NH3 and pyridine, prefer instead to bind to a single Cu(II) ion, which adopts a 5-fold coordination in a square-pyramidal environment. A semi-bridging coordination, corresponding to a 5 + 1 distorted octahedral Cu(II) environment, is finally predicted for molecules of intermediate Lewis basicity, such as CH3CN. The soundness of these results is corroborated by a good correlation found between the theoretical coordination number of the Cu(II) ions, the computed spin-down fundamental band gap, and the experimentally observed vapochromic effects.

[Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3] as a molecular model of ZnS surfaces: an experimental and theoretical study

Experimental and theoretical results pertaining to [Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3], a possible molecular model of ZnS S-terminated polar surfaces, as well as a potential source of strictly monodispersed ZnS quantum dots, are presented and discussed. The results of density functional theory (DFT) calculations provided a rationale for the peculiar arrangement of [Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3] clusters in the solid state, contemporarily indicating the unsuitability of the isolated species to mimic whatever (polar or non-polar) ZnS surface. Despite the fact that such a failure is further confirmed by time-dependent DFT and UV–Vis diffuse reflectance spectroscopy, the combined use of theoretical outcomes, DRIFT measurements, and literature data pertaining to the surface chemical properties of ZnS (Hertl in Langmuir 4:594, 1988) ultimately testifies that [Zn10(µ4-S)(µ3-S)6(Py)9(SO4)3] is perfectly suited to model the interaction of pyridine molecules with ZnS surface Lewis acid sites. The herein reported theoretical results are expected to be a useful reference for the interpretation of chemisorption experiments of Py-based Lewis bases on single crystal ZnS surfaces.

Pt(II) and Pd(II) Pyrrolidine-Dithiocarbamates Investigated by XPS

In the present contribution, a series of four metal dithiocarbamates, namely 1-pyrrolidinecarbodithioate methyl ester (PyDTM) of Pd(II) and Pt(II), PtCl2(PyDTM), PtBr2(PyDTM), PdBr2(PyDTM), PdCl2(PyDTM), were analysed by x-ray Photoelectron Spectroscopy (XPS). Besides the wide scan spectra, detailed spectra for the C 1s, O 1s, N 1s, S 2s, S 2p, Pt 4f (for Pt-based compounds), Pd 3d (for Pd-based compounds), Cl 2p (for Cl containing compounds), Br 3p (for Br containing compounds) regions were acquired and the related data are presented and discussed.