NEXAFS study of a Pt-containing rod-like organometallic polymer (Pt-DEBP): molecular orientation onto HOPG, Au/Si(1 1 1), Cr/Si(1 1 1) and Si(1 1 1) surfaces

The influence of different substrates on the molecular orientation of organometallic polymer Pt-DEBP, [Pt(PBu₃)₂CCC₁₂H₈CC]_n, has been investigated by NEXAFS spectroscopy. Thin films were deposited on HOPG, Au/Si(1 1 1), Cr/Si(1 1 1), Si(1 1 1) and stainless steel. The assignment of the spectral features has been carried out on the basis of previous STEX calculations performed on phenylacetylene model molecule in gas phase and adsorbed on Pt(1 1 1) and Cu(1 0 0). Angular dependent analysis of the π* resonance occurring at 285.50 eV photon energy deriving by the benzene carbon orbitals showed a polarisation effect for all substrates. A preferential molecular orientation at nearly 40° to the surface normal was observed. This result might be explained by the strong interaction between sp and sp² carbons of the organic diethynylbiphenyl DEBP moiety contained in close chains, leading to polymer self-assembling.     

Pyridine 1:1 adducts of urea (Z′ = 1) and thiourea (Z′ = 8)

During our studies of urea and thiourea adducts, we noticed that no adducts with unsubstituted pyridine had been structurally investigated. The 1:1 adduct of pyridine and urea, C₅H₅N·CH₄N₂O, crystallizes in the P2₁/c space group with Z = 4. The structure is of a standard type for urea adducts, whereby the urea molecules form a ribbon, parallel to the a axis, consisting of linked R₂²(8) rings, and the pyridine molecules are attached to the periphery of the ribbon by bifurcated (N—H…)₂N hydrogen bonds. The 1:1 adduct of pyridine and thiourea, C₅H₅N·CH₄N₂S, crystallizes in the P2₁/n space group, with Z = 32 (Z′ = 8). The structure displays similar ribbons to those of the urea adduct. There are two independent ribbons parallel to the b axis at z ≃ 0 and , and three at z ≃ and ; the latter are crosslinked to form a layer structure by additional long N—H…S interactions, which each formally replace one branch of a bifurcated hydrogen‐bond system.     

Anwendung der α-Alkinon-Cyclisierung: Totalsynthese von (±)-Alben

Application of the α-Alkynone Cyclization: Total Synthesis of (±)-Albene A synthesis of the racemic form of the natural tricyclic hydrocarbon albene (1) from the Diels-Alder adduct 2 of tiglyl chloride and cyclopentadiene is described (24% yield). The key step 5→6 involves a thermal α-alkynone cyclization (Scheme 3), which is able to establish a new quarternary C-atom at an unactivated position with a high degree of regiospecificity.     

The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1)

The potential of zero charge (pzc) of Pd(1 1 1) has been determined in dilute NaF solutions by measuring the Gouy–Chapman minimum of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of −0.12 V vs. SCE has been found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) have also been determined. While the pzc of the first, pseudomorphic Pd layer on Au(1 1 1) is −0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/water interface is estimated.     

A 1:1 adduct between di­methyl­tin dibromide and N‐methyl­pyrrolidinone (NMP)

The title compound, di­bromo­di­methyl(N‐methyl­pyrrolidin‐2‐one‐O)­tin(IV), [SnBr₂(CH₃)₂(C₅H₉NO)], exhibits pentacoordination of the Sn atom, with long and short Sn—Br bonds [2.6737 (4) and 2.5256 (4) Å, respectively]. The distorted trigonal–bipyramidal coordination polyhedron has two methyl groups and one Br atom in the equatorial plane, the second Br atom and the N‐methyl­pyrrolidinone (NMP) ligand occupying the apical positions.     

Application of organic monolayers formed on Si(1 1 1): possibilities for nanometer-scale patterning

The modification of hydrogen-terminated Si(1 1 1) wafer surfaces was reproduced by previously reported methods of the electrolysis of para-substituted benzendiazonium salts and the Grignard reaction with various alkyl moieties. The electrolysis methods formed partially ordered two-dimensional monolayers, which were however obscured by precipitation of by-products. The Grignard reaction deposited a monolayer of moieties of alkyl groups randomly arranged, which are more suitable for surface passivation. Aiming for the application to nanometer-scale monolayer patterning of the Si(1 1 1) wafer surface, the organic-monolayer-covered Si(1 1 1) surfaces were subjected to electron beam bombardment. After electron bombardment with ambient O₂ or H₂O introduced, adsorption of oxygen was observed within the beam spot. By immersing the bombarded specimen into an aqueous NiSO₄+(NH₄)₂SO₄ solution, the oxygen-deposited portions selectively included Ni atoms. This will be useful in constructing nanometer-scale metallic structures over Si wafer surfaces.     

The molecular orientation of CO on Pd(1 1 1): a polarization-dependent SFG study

The molecular orientation of carbon monoxide adsorbed on Pd(1 1 1) was examined by sum frequency generation (SFG) vibrational spectroscopy utilizing different polarization combinations of the visible and SFG light. This allows to determine the CO tilt angle with respect to the substrate, provided that a proper optical model for the interface can be defined. It is demonstrated that it is essential to invoke the β_aac hyperpolarizability into the analysis and that polarization-dependent SFG of CO/Pd(1 1 1) yields information on β_aac/β_ccc rather than the tilt angle.     

Tri­phenyl­phosphine oxide–1‐naphthoic acid (1/1)

In the title compound, C₁₈H₁₅OP·C₁₁H₈O₂, co‐crystallization of tri­phenyl­phosphine oxide with 1‐naphthoic acid yields a supramolecular structure held together by one O—H⋯O and three C—H⋯O hydrogen bonds. The O—H⋯O hydrogen bond [O⋯O = 2.592 (2) Å] has little effect on the O=P bond distance.     

Solution conformations of mycothiol bimane, 1-D-GlcNAc-alpha-(1 --> 1)-D-myo-Ins and 1-D-GlcNAc-alpha-(1 --> 1)-L-myo-Ins

Mycothiol is an abundant small molecular weight thiol found only in actinomycetes, which include mycobacteria. Mycothiol biosynthetic and detoxification enzymes are novel and unique to actinomycetes, thereby representing potential antimycobacterial targets. To better guide inhibitor design, we have determined by NMR the solution conformations of mycothiol bimane (MSmB) and the pseudodisaccharide 1-D-GlcNAc-alpha-(1 --> 1)-D-myo-Ins (D-GI), molecules that represent the natural substrates for the mycothiol-dependent detoxification enzyme mycothiol-S-conjugate amidase (MCA) and the mycothiol biosynthetic enzyme D-GlcNAc-alpha-(1 --> 1)-D-myo-Ins deacetylase (AcGI deacetylase), respectively. Comparison of the mean structure of MSmB and the energy-minimized structures of two competitive spiroisoxazoline-containing MCA inhibitors shows striking similarities between these molecules in the region of the scissile amide bond of MSmB and provides structural evidence that those inhibitors are substrate mimics. Owing to our earlier finding that AcGI deacetylase will not deacetylate the unnatural isomer 1-d-GlcNAc-alpha-(1 --> 1)-L-myo-Ins (L-GI), the solution conformation of L-GI was also determined. The interglycosidic bond angles for all three compounds are comparable. When considered together with the observation that a simplified cyclohexyl thioglycoside mycothiol analogue is a good substrate for MCA, it appears that the stereochemistry of the inositol ring is critical for deacetylase function, superceding the importance of the full complement of hydroxyl groups on the "nonreducing" ring.     

Comparative theoretical study of the structure and bonding of propyne on the Pt(1 1 1) and Pd(1 1 1) surfaces

The interaction of propyne with the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied by means of the generalised gradient approach of density functional theory using periodic slab models. For both surfaces, the most stable adsorption mode of propyne is di-σ/π mode where the hydrocarbon is σ-bonded to two metal atoms with some additional π bonding to a third adjacent surface atom. The adsorption geometry is a highly distorted propyne with the C₁ and C₂ in a nearly sp² hybridisation. Two equivalent surface structures have been found on Pt and Pd. These correspond to the adsorption on the fcc or hcp hollow sites. The adsorption energies on Pt(1 1 1) and Pd(1 1 1) are predicted to be ∼−197 and −161 kJ mol⁻¹, respectively. The electronic factors that control the chemisorption have been analysed by means of the projected density of states.     

Three‐dimensional hydrogen‐bonded structures in the guanidinium salts of the monocyclic dicarboxylic acids rac‐trans‐cyclohexane‐1,2‐dicarboxylic acid (2:1, anhydrous), isophthalic acid (1:1, monohydrate) and terephthalic acid (2:1, trihydrate)

The structures of bis(guanidinium) rac‐trans‐cyclohexane‐1,2‐dicarboxylate, 2CH₆N₃⁺·C₈H₁₀O₄²⁻, (I), guanidinium 3‐carboxybenzoate monohydrate, CH₆N₃⁺·C₈H₅O₄⁻·H₂O, (II), and bis(guanidinium) benzene‐1,4‐dicarboxylate trihydrate, 2CH₆N₃⁺·C₈H₄O₄²⁻·3H₂O, (III), all reveal three‐dimensional hydrogen‐bonded framework structures. In anhydrous (I), both guanidinium cations form classic cyclic R₂²(8) N—H...O,O′_carboxylate and asymmetric cyclic R₂¹(6) hydrogen‐bonding interactions, while one cation forms an unusual enlarged cyclic interaction with O‐atom acceptors of separate ortho‐related carboxylate groups [graph set R₂²(11)]. Cations and anions also associate across inversion centres, giving cyclic R₄²(8) motifs. In the 1:1 guanidinium salt, (II), the cation forms two separate cyclic R₂¹(6) interactions, one with a carboxyl O‐atom acceptor and the other with the solvent water molecule. The structure is unusual in that both carboxyl groups form short interanion O...H...O contacts, one across a crystallographic inversion centre [O...O = 2.483 (2) Å] and the other about a twofold axis of rotation [O...O = 2.462 (2) Å], representing shared sites on these elements for the single acid H atom. The water molecule links the cation–anion ribbon structures into a three‐dimensional framework. In (III), the repeating molecular unit comprises a benzene‐1,4‐dicarboxylate dianion which lies across a crystallographic inversion centre, two guanidinium cations and two solvent water molecules (each set related by twofold rotational symmetry), and a single water molecule which lies on a twofold axis. Each guanidinium cation forms three types of cyclic interaction with the dianions: one R₂¹(6), the others R₃²(8) and R₃³(10) (both of these involving the water molecules), giving a three‐dimensional structure through bridges down the b‐cell direction. The water molecule at the general site also forms an unusual cyclic R₂²(4) homodimeric association across an inversion centre [O...O = 2.875 (2) Å]. The work described here provides further examples of the common cyclic guanidinium–carboxylate hydrogen‐bonding associations, as well as featuring other less common cyclic motifs.