Optical absorption and photoluminescence properties of the PPV nanotubes and nanowires

We measured optical absorption and time resolved photoluminescence decay properties of the PPV nanotubes and nanowires which were prepared by CVD polymerization using templates. When compared with bulk PPV films, their nano objects showed different optical properties, long photoluminescence decay time and higher photoluminescence efficiencies.     

A Hydrogen‐Bonded Organic Framework (HOF) with Type IV NH3 Adsorption Behavior

An S‐shaped gas isotherm pattern displays high working capacity in pressure‐swing adsorption cycle, as established for CO₂, CH₄, acetylene, and CO. However, to our knowledge, this type of adsorption behavior has not been revealed for NH₃ gas. Herein, we design and characterize a hydrogen‐bonded organic framework (HOF) that can adsorb NH₃ uniquely in an S‐shape (type IV) fashion. While conventional porous materials, mostly with type I NH₃ adsorption behavior, require relatively high regeneration temperature, this platform which has significant working capacity is easily regenerated and recyclable at room temperature.     

XRD studies on the femtosecond laser ablated single-crystal germanium in air

Ultrashort laser ablation of single-crystal germanium has been performed in air with femtosecond laser pulses (150 fs, 1 kHz) of 810 nm in the laser fluence range of 0.7–35.4 J/cm². Ablation depth dependence on the laser fluence shows that there are two different processes, which are explained in terms of electronic heating process and the optical penetration one. Structure of ablated region is characterized by means of two different XRD techniques. With increasing the laser fluence higher than 10.2 J/cm², the laser-processed region of germanium exhibits poly-crystalline diffraction peaks in a wide-angle (θ/2θ) scan and a split of diffraction peak of (4 0 0) plane in the rocking curve, which are absent in the lower laser fluence. These observations could be explained in terms of structural changes induced by ultrashort laser irradiation at the higher laser fluence.     

Quercetin 2,4‐Dioxygenase Activates Dioxygen in a Side‐On O2–Ni Complex

Quercetin 2,4‐dioxygenase (quercetinase) from Streptomyces uses nickel as the active‐site cofactor to catalyze oxidative cleavage of the flavonol quercetin. How this unusual active‐site metal supports catalysis and O₂ activation is under debate. We present crystal structures of Ni‐quercetinase in three different states, thus providing direct insight into how quercetin and O₂ are activated at the Ni²⁺ ion. The Ni²⁺ ion is coordinated by three histidine residues and a glutamate residue (E⁷⁶) in all three states. Upon binding, quercetin replaces one water ligand at Ni and is stabilized by a short hydrogen bond through E⁷⁶, the carboxylate group of which rotates by 90°. This conformational change weakens the interaction between Ni and the remaining water ligand, thereby preparing a coordination site at Ni to bind O₂. O₂ binds side‐on to the Ni²⁺ ion and is perpendicular to the C2?C3 and C3?C4 bonds of quercetin, which are cleaved in the following reaction steps.     

How the [NiFe4S4] Cluster of CO Dehydrogenase Activates CO2 and NCO−

Ni,Fe‐containing CO dehydrogenases (CODHs) use a [NiFe₄S₄] cluster, termed cluster C, to reversibly reduce CO₂ to CO with high turnover number. Binding to Ni and Fe activates CO₂, but current crystal structures have insufficient resolution to analyze the geometry of bound CO₂ and reveal the extent and nature of its activation. The crystal structures of CODH in complex with CO₂ and the isoelectronic inhibitor NCO^? are reported at true atomic resolution (d_min≤1.1 Å). Like CO₂, NCO^? is a μ₂,η² ligand of the cluster and acts as a mechanism‐based inhibitor. While bound CO₂ has the geometry of a carboxylate group, NCO^? is transformed into a carbamoyl group, thus indicating that both molecules undergo a formal two‐electron reduction after binding and are stabilized by substantial π backbonding. The structures reveal the combination of stable μ₂,η² coordination by Ni and Fe2 with reductive activation as the basis for both the turnover of CO₂ and inhibition by NCO^?.     

Effect of mixtures of water and organic solvents on the acidities of 5‐membered heteroaromatic carboxylic acids

pKa values of various 5‐membered heterocyclic aromatic carboxylic acids (pKa^s) were determined in solutions of 20.3, 35.2, 50.1, 65.1, and 79.9 weight percent of organic solvents in water. The pKa^s values show good linearity when they are ploted as a function of the dielectric constants of the mixed solvents methanol, ethanol, isopropyl alcohol, and tert‐butyl alcohol. On the other hand, the pKa^s values show a poor correlation with the dielectric constants in aqueous acetonitrile, N,N‐dimethylformamide, dimethyl sulfoxide, and dioxane. The pKa values in pure water and in pure organic solvent could be calculated by extrapolation of the plot of the pKa^s versus percentage of organic solvent. The pKa^s values of 4‐ and 5‐sub‐stituted 2‐thiophenecarboxylic acids were also determined, and the ρ values are calculated in the same series of the solvents.     

Determination of reactivity by MO theory. 27. Molecular orbital study of the gas-phase decarboxylation of but-3-enoic acid

The MINDO /3 calculations were performed on the potential energy profile involved in the equilibrium Optimized structures of stable molecules and transition states have been determined; thermodynamic stabilities of pure acids and barriers indicated that the equilibrium can be set up from any acids. It was argued that direct decarboxylation is only conceivable from (I), since in this process a 1, 5-hydrogen shift is involved, whereas a higher barrier process of 1, 3-hydrogen shift is required in direct decarboxylations from other acids. Direct interconversion of (I) and (III) was found to be unfavorable due to a high barrier involved.     

Primary kinetic isotope effects on hydride transfer from 1,3-dimethyl-2-phenylbenzimidazoline to NAD(+) analogues

Primary kinetic isotope effects (KIE) have been determined spectrophotometrically for the reaction of NAD(+) analogues (pyridinium, quinolinium, phenanthridinium, and acridinium ions) with 1,3-dimethyl-2-phenylbenzimidazoline in a 4:1 mixture of 2-propanol and water by volume at 25 degrees C. The values of KIE varied systematically from 6.27 to 4.06 as the equilibrium constant changed from around 10 to around 10(12). This is consistent with Marcus theory of atom transfer, assuming that there are no high-energy intermediates. Within this theory, the perpendicular effect is responsible for most of the change in KIE. The Marcus theory of atom transfer is consistent with a linear, triatomic model of the reaction. Perpendicular effects arise from the systematic decrease of bond distances and increase of bond orders in the critical complexes of the two related degenerate hydride transfer reactions as their C--H bonds become stronger. The parallel effect (Leffler--Hammond effect) is attenuated by the fairly high intrinsic barrier (lambda/4 is around 92 kJ/mol) and makes a smaller contribution to the change in the KIE.