Synchrotron X-ray Electron Density Analysis of Chemical Bonding in the Graphitic Carbon Nitride Precursor Melamine

Melamine is a precursor and building block for graphitic carbon nitride (g-CN) materials, a group of layered materials showing great promise for catalytic applications. The synthetic pathway to g-CN includes a polycondensation reaction of melamine by evaporation of ammonia. Melamine molecules in the crystal organize into wave-like planes with an interlayer distance of 3.3 Å similar to that of g-CN. Here we present an extensive investigation of the experimental electron density of melamine obtained from modelling of synchrotron radiation X-ray single-crystal diffraction data measured at 25 K with special focus on the molecular geometry and intermolecular interactions. Both intra- and interlayer structures are dominated by hydrogen bonding and π-interactions. Theoretical gas-phase optimizations of the experimental molecular geometry show that bond lengths and angles for atoms in the same chemical environment (C−N bonds in the ring, amine groups) differ significantly more for the experimental geometry than for the gas-phase-optimized geometries, indicating that intermolecular interactions in the crystal affects the molecular geometry. In the experimental crystal geometry, one amine group has significantly more sp³-like character than the others, hinting at a possible formation mechanism of g-CN. Topological analysis and energy frameworks show that the nitrogen atom in this amine group participates in weak intralayer hydrogen bonding. We hypothesize that melamine condenses to g-CN within the layers and that the unique amine group plays a key role in the condensation process.     

The hydrogen chemisorption on TiC(111) surface studied by high resolution electron energy loss spectroscopy

At room temperature, hydrogen chemisorbs dissociatively on the three-fold hollow site of the TiC(111) surface with a sticking probability of ～ 0.5. The frequency of the stretching vibration of H is 1009 cm^-1, and the effective charge e¹ of hydrogen has been estimated to be 0.08 from the cross section of dipole scattering.     

The virtual k-factorability of graphs

It is shown that if a connected graph G admits a finite covering that has a k-factor, then there is a 1- or 2-fold covering of G that has a k-factor and that such a covering can be given as a canonical bipartite graph associated with G.     

Evaporation of VC and TiC in floating zone melting

The evaporation of vanadium carbide during floating zone systems was examined. The evaporation composition and rate were measured as a function of the melt composition. At a composition of C/V = 0.72, the dependence of evaporation rate on ambient gas pressure (P) was measured at the range of 3.5–40 atm of helium and argon. The evaporation rate was proportional to P^-0.70 in helium and P^-0.31 in argon. The same results were obtained with TiC. These pressure dependences are shown to be expected for laminar and turbulent free convective mass transport.     

Uniqueness and faithfulness of embedding of toroidal graphs

A topological generalization of the uniqueness of duals of 3-connected planar graphs will be obtained. A graph G is uniquely embeddable in a surface F if for any two embeddings $\text{?}{}_1$, $\text{?}{}_2\text{:G → F}$, there are an autohomeomorphism h:F → F and an automorphism σ:G → G such that $\text{h°?}{}_1\text{= ?}{}_2\text{°σ}$. A graph G is faithfully embedabble in a surface F if there is an embedding $\text{?:G → F}$ such that for any automorphism σ:G → G, there is an autohomeomorphism h:F → F with $\text{h°? = f°σ}$. Our main theorems state that any 6-connected toroidal graph is uniquelly embeddable in a torus and that any 6-connected toroidal graph with precisely three exceptions is faithfully embeddable in a torus. The proofs are based on a classification of 6-regular torus graphs.     

An evidence of spin flopping in V5S8 by magnetoresistance experiments

An anomalous field dependence of the magnetoresistance of V₅S₈ is observed at 42 kOe at liquid helium temperature. This is attributed to the spin flopping transition predicted by our torque experiments.     

Chromatic numbers of quadrangulations on closed surfaces

It has been shown that every quadrangulation on any nonspherical orientable closed surface with a sufficiently large representativity has chromatic number at most 3. In this paper, we show that a quadrangulation G on a nonorientable closed surface N_k has chromatic number at least 4 if G has a cycle of odd length which cuts open N_k into an orientable surface. Moreover, we characterize the quadrangulations on the torus and the Klein bottle with chromatic number exactly 3. By our characterization, we prove that every quadrangulation on the torus with representativity at least 9 has chromatic number at most 3, and that a quadrangulation on the Klein bottle with representativity at least 7 has chromatic number at most 3 if a cycle cutting open the Klein bottle into an annulus has even length. As an application of our theory, we prove that every nonorientable closed surface N_k admits an eulerian triangulation with chromatic number at least 5 which has arbitrarily large representativity. © 2001 John Wiley & Sons, Inc. J Graph Theory 37: 100–114, 2001     

Columnar nanostructured polymer films containing ionic liquids in supramolecular one-dimensional nanochannels

Ionic liquids have attracted a considerable attention as the next generation electrolytes for energy devices. We have developed new free-standing and nanostructured polymer films in which ionic liquids are confined into one-dimensionally ordered nanochannels. These polymer films have been obtained by photopolymerization of hydrogen-bonded supramolecular columnar liquid-crystalline self-assemblies of an imidazolium-based ionic liquid and a wedge-shaped diol compound containing polymerizable groups. The macroscopically parallel alignment of the columnar structures on a glass substrate has been achieved by the application of mechanical shearing, and subsequently fixed into polymer films by UV irradiation. This ionic liquid-containing polymer film exhibits higher ionic conductivity than that of the previously reported one-dimensional polymer film obtained by in situ photopolymerization of a covalent-type columnar liquid-crystalline imidazolium salt. The noncovalent supramolecular approach to one-dimensionally ion-conductive polymer films has led to improvement on conductive properties. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 366–371