A density functional theory study of catalytic sites for oxygen reduction in Fe/N/C catalysts used in H2/O2 fuel cells.

The oxygen reduction catalytic activity of carbon-supported FeN4 moieties bridging micropores between two graphene sheets was investigated by density functional theory (DFT). Based on the FeN(2+2)/C structure proposed earlier by our group, two types of FeN(2+2)/C structures were considered: one mostly planar and one in which the Fe ion is significantly displaced out of the graphitic plane. A structure in which the FeN4 moiety is embedded in an extended graphene sheet (FeN/C) was also considered. In addition, we have investigated the influence of an axial pyridine group approaching the Fe centre. The formation energy is lowest for the planar FeN(2+2)/C structure. The overall downhill behaviour of the relative free energy vs. the reaction step suggests that most structures have catalytic activity near zero potential. This conclusion is further supported by calculations of the binding energies of adsorbed O2 and H2O and of the O-O bond lengths of adsorbed O2 and OOH. The side-on interaction of adsorbed O2 is preferred over the end-on interaction for the three basic structures without the axial pyridine. The pyridine coordination produces a stronger binding of O2 for the planar FeN(2+2)/C and the FeN/C structures as well as a dominant end-on interaction of O2. The energy levels of the planar FeN(2+2)/C structure with and without the pyridine ligand are nearly equal for iron spin states S = 1 and S = 2, suggesting that both configurations are formed with similar concentration during the preparation process, as also previously found for two of the iron sites by Mössbauer spectroscopy experiments.

Ullmann-type coupling of brominated tetrathienoanthracene on copper and silver.

We report the synthesis of extended two-dimensional organic networks on Cu(111), Ag(111), Cu(110), and Ag(110) from thiophene-based molecules. A combination of scanning tunnelling microscopy and X-ray photoemission spectroscopy yields insight into the reaction pathways from single molecules towards the formation of two-dimensional organometallic and polymeric structures via Ullmann reaction dehalogenation and C-C coupling. The thermal stability of the molecular networks is probed by annealing at elevated temperatures of up to 500 °C. On Cu(111) only organometallic structures are formed, while on Ag(111) both organometallic and covalent polymeric networks were found to coexist. The ratio between organometallic and covalent bonds could be controlled by means of the annealing temperature. The thiophene moieties start degrading at 200 °C on the copper surface, whereas on silver the degradation process becomes significant only at 400 °C. Our work reveals how the interplay of a specific surface type and temperature steers the formation of organometallic and polymeric networks and describes how these factors influence the structural integrity of two-dimensional organic networks.

Unprecedented transformation of tetrathienoanthracene into pentacene on Ni(111).

The imaging and characterization of single-molecule reaction events is essential to both extending our basic understanding of chemistry and applying this understanding to challenges at the frontiers of technology, for example, in nanoelectronics. Specifically, understanding the behavior of individual molecules can elucidate processes critical to the controlled synthesis of materials for applications in multiple nanoscale technologies. Here, we report the synthesis of an important semiconducting organic molecule through an unprecedented reaction observed with submolecular resolution by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Our images reveal a sulfur abstraction and cyclization reaction that converts tetrathienoanthracene precursors into pentacene on the Ni(111) surface. The identity of the final reaction product was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). This reaction has no known literature analogue, and highlights the power of local-probe techniques for exploring new chemical pathways.

Laterally extended spiral graphite analogue boron-nitrogen helices.

Ab initio self-consistent field molecular orbital and density functional theory calculations have been performed on a series of extended helical boron-nitrogen analogues of a "spiral graphite", the [N]polymethylenylnaphthalenes (N = 6, 8, and 12), with the molecular formula N(x)B(y)H(z) (where x = 28, 37, and 55, y = 27, 36, and 54, z = 23, 29, and 41). Interchanging the positions of the boron and nitrogen atoms in the helix leads to very similar structures N(x-1)B(y+1)H(z) in all three studied cases. The electronic structure and the optimum geometries of these helices were investigated at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. Electron density contours were calculated for the largest helices at the B3LYP/6-31G(d,p) level of theory.

Helices of boron-nitrogen hexagons and decagons. A theoretical study.

Ab initio self-consistent field molecular orbital and density functional theory calculations have been performed on a series of helical boron-nitrogen structures comprised of fused hexagons and larger polygons. The presence of an even number N of rings in the boron-nitrogen [N]helicenes leads to the possibility of angular isomers. The electronic structure and stability of three isomeric nonhydrogenated boron-nitrogen helices were investigated at the HF/6-31G(d) and the B3LYP/6-31G(d) levels of theory. According to this study some of the initially assumed regular helical structures are unstable; two types of the isomeric structures convert to characteristically different equilibrium geometries. Electron density contours were calculated in order to interpret the existing bonding patterns.

Theoretical study on the structure and stability of some unusual boron-nitrogen helices.

Ab initio self-consistent field molecular orbital and density functional theory calculations have been performed on a series of helical structures comprised of boron-nitrogen analogues of extended helicenes, with helically arranged N fused benzene rings, and alternating N benzene units fused to N - 1 cyclobutadiene rings as reference structures. The electronic structure and stability of boron-nitrogen analogues of angular [N]helicenes, [N]phenylenes (N = 5, 6, 7, 12), and [N]methylenylnaphthalenes (N = 6) were investigated at the HF/6-31G(d) and the B3LYP/6-31G(d) levels of theory. The presence of an even number N of rings in the boron-nitrogen [N]helicenes leads to the possibility of angular isomers. Electron density contours were calculated in order to interpret the existing bonding patterns. These structures may provide supramolecular building blocks and macromolecular "springs" with unusual electronic properties.