Interplay of hydrogen-bond and coordinate covalent-bond interactions in self-assembly of NH3 molecules on the Si(001) surface

An exchange of hydrogen-bond and coordinate covalent-bond (dative-bond) interactions is found to play a critical role in the self-assembly of NH3 molecules on the Si(001) surface. An NH3 molecule in the height of approximately 3-10 A above the surface is attracted toward the preadsorbed NH2 moiety through the long-range H-bond interaction. Within approximately 3 A, the H-bond interaction becomes repulsive, and instead the dative bond with the buckled-down Si atom governs the adsorption process. The interplay of the two interactions induces the clustering and the zigzag feature of the dissociatively adsorbed NH3 molecules on the Si(001) surface.     

Segregated reduced graphene oxide polymer composite as a high performance electromagnetic interference shield

Herein, we report the synthesis of a graphene/polymer composite via a facile and straightforward approach for electromagnetic interference (EMI) shielding applications. Polystyrene (PS) beads were added in graphene oxide (GO)/water solution followed by the addition of hydroiodic acid (HI) for in situ reduction of GO. The composite solution (rGO/PS) was filtered, hot compressed and tested for EMI shielding and dielectric measurements. A 2-mm thick segregated rGO/PS sample with 10 wt% filler loading delivered a high EMI shielding effectiveness (SE) of 29.7 dB and an AC electrical conductivity of 21.8 S m^?1, which is well above the commercial requirement for EMI shielding applications. For comparison with the segregated rGO/PS composite, a control polymer composite sample utilizing a thermally reduced graphene oxide was synthesized by following a conventional coagulation approach. The as-synthesized conventional rGO/PS yield an EMI SE of 14.2 dB and electrical conductivity of 12.5 S m^?1. The high EMI shielding of segregated rGO/PS is attributed to the better filler-to-filler contact among graphene layers surrounded by PS beads and also to the better reduction and preservation of graphene structure during reduction process that makes the low temperature chemically reduced segregated rGO/PS approach a viable route compared to high temperature thermally reduced conventional rGO/PS approach.     

A theoretical study of the low-lying excited electronic states of thiocarbonyl chlorofluoride and their dissociation pathways

The spectroscopic constants for the ground (X (1)A(1)) and low-lying triplet and singlet excited states (a (3)A("),A (1)A("),B (1)A(')) of thiocarbonyl chlorofluoride (ClFCS) were obtained using the equation-of-motion coupled-cluster singles and doubles method. The calculated vibrational frequencies of the electronic states were within 4% of the experimental values for 21 of the frequencies, but four calculated frequencies were 20%-40% away from the corresponding experimentally reported values, suggesting the need to reexamine previous experimental spectra. The spectroscopic properties of the radical fragments (FCS, ClCS, and CClF) were also studied, and the correlation diagram between the excited electronic states of ClFCS and possible combinations of dissociation fragments were obtained. The potential energy surfaces (PESs) of the excited electronic states of ClFCS along possible dissociation pathways were also studied. The main qualitative dynamical features of the S(1)(A (1)A("))<--S(2)(B (1)A(')) fluorescence of ClFCS, which may occur in spite of the small barrier (8 kcalmol) on the S(2) PES to the dissociation of C-Cl bond, are discussed.     

On the relevance of an antiferromagnetic dimer model for the spin-gapped magnetic solids Cu(terpy)Mo2O7 and Cu(OH)(p-pyc)H2O

The magnetic solids Cu(terpy)Mo2O7 (terpy = terpyridine) and Cu(OH)(p-pyc)H2O (p-pyc = p-pyridinecarboxylate) have a spin gap and possess chains of Cu2+ ions in which two different Cu...Cu distances alternate. On the basis of their reported crystal structures, the spin-exchange interactions of these compounds were examined by performing spin dimer analysis to determine whether an antiferromagnetic dimer or an alternating antiferromagnetic chain model is appropriate for their magnetic properties. Our analysis shows that an antiferromagnetic dimer model is correct for both compounds because of the anisotropic overlap between the magnetic orbitals of their Cu2+ sites.     

Resistive switching behavior in a Ni–Ag<Subscript>2</Subscript>Se–Ni nanowire

Hysteretic resistive switching behavior in a silver selenide (Ag₂Se) nanowire, which had a diameter of about 200 nm and a length of about 10 μm, was studied using scanning probe microscopy. Electrical current measurements were carried out in a range from 0 to −10 V and in temperatures below and above the phase transition of Ag₂Se. ON/OFF switching times were measured with pulsed voltages. They displayed different characteristics at low and high temperatures. The results confirm that Ag₂Se nanowires have applications in nanoscale switching devices.     

Direct electrical junction formation of highly vertical aligned zinc oxide nanorods

A two dimensionally assembled monolayer of hexagonal convexo-convex β-cobalt hydroxide nanoplates as a self-disposable sacrificial epi-template leads to a highly vertical alignment of zinc oxide nanorods array having a good electrical contact with metal or semiconductor layer on a substrate in a hydrothermal process.     

Spin dimer analysis of the magnetic structures of Ba3Cr2O8, Ba3Mn2O8, Na4FeO4, and Ba2CoO4 with a three-dimensional network of isolated MO4 (M = Cr, Mn, Fe, Co) tetrahedra

The spin exchange interactions of the magnetic oxides Ba3Cr2O8, Ba3Mn2O8, Na4FeO4, and Ba2CoO4 with a three-dimensional network of isolated MO4 (M = Cr, Mn, Fe, Co) tetrahedra were examined by performing spin dimer analysis on the basis of tight-binding electronic structure calculations. Although the shortest O...O distances between adjacent MO4 tetrahedra are longer than the van der Waals distance, our analysis shows that the super-superexchange interactions between adjacent MO4 tetrahedra are substantial and determine the magnetic structures of these oxides. In agreement with experiment, our analysis predicts a weakly interacting isolated AFM dimer model for both Ba3Cr2O8 and Ba3Mn2O8, the (0.0, 0.5, 0.0) magnetic superstructure for Na4FeO4, the (0.5, 0.0, 0.5) magnetic superstructure for Ba2CoO4, and the presence of magnetic frustration in Ba2CoO4. The comparison of the intra- and interdimer spin exchange interactions of Ba3Cr2O8 and Ba3Mn2O8 indicates that orbital ordering should be present in Ba3Cr2O8.     

Benzylic C(sp3)–C(sp2) cross-coupling of indoles enabled by oxidative radical generation and nickel catalysis

A mechanistically unique functionalization strategy for a benzylic C(sp³)–H bond has been developed based on the facile oxidation event of indole substrates. This novel pathway was initiated by efficient radical generation at the benzylic position of the substrate, with subsequent transition metal catalysis to complete the overall transformation. Ultimately, an aryl or an acyl group could be effectively delivered from an aryl (pseudo)halide or an acid anhydride coupling partner, respectively. The developed method utilizes mild conditions and exhibits a wide substrate scope for both substituted indoles and C(sp²)-based reaction counterparts. Mechanistic studies have shown that competitive hydrogen atom transfer (HAT) processes, which are frequently encountered in conventional methods, are not involved in the product formation process of the developed strategy.

A mechanistically distinct Ni-catalysed benzylic functionalization of indoles is developed by the facile oxidation of arenes. The method exhibits a wide substrate scope and pronounced chemoselectivity that cannot be accessed via known protocols.     

Bi-level optimizing control of a simulated moving bed process with nonlinear adsorption isotherms

A bi-level optimizing control scheme originally proposed for a simulated moving bed (SMB) with linear isotherms has been extended to an SMB with nonlinear isotherms. Cyclic steady state optimization is performed in the upper level to determine the optimum switching period and time-varying feed/desorbent flow rates, and repetitive model predictive control is run in the lower level for purity regulation, taking the decision variables from the upper level as feed-forward information. Experimental as well as numerical study for an SMB process separating a high-concentration mixture of aqueous L-ribose and L-arabinose solutions showed that the proposed scheme performs satisfactorily against various disturbances. In contrast, an alternative scheme based on an SMB model with linear isotherms showed a limitation in the control performance; this scheme was apt to fail in purity regulation.