Photoisomerization of azobenzene from first-principles constrained density-functional calculations

Despite considerable work in the field, the precise mechanism for the photoisomerization of azobenzene, C(12)H(10)N(2), is still an open issue. Early theoretical studies of the problem indicated that isomerization occurs through an in-plane inversion path, and this has been used to explain recent time-resolved UV-visible spectroscopy measurements. On the other hand, a number of recent theoretical studies have concluded that a torsion of the N-N bond ("rotation path") is probably the most favorable mechanism for photoisomerization involving the first excited state. We have performed first-principles calculations using constrained density-functional theory (DFT) and time-dependent DFT in the local-density approximation, with results that also favor the rotation path mechanism. Our results are compared with other analyses, primarily based on configuration interaction.     

In‐situ polymerization of aliphatic‐aromatic polyamide nanocomposites in the presence of Halloysite nanotubes

Nanocomposites consisting of semi‐aromatic polyamide (PA) and pristine or alkali activated halloysite nanotubes (HNT or mHNT, respectively) were synthesized by the in‐situ interfacial polymerization method. The condensation reactions were carried out between isophthaloyl dichloride in hexane and triethylenetetramine in water containing different amounts of HNT or mHNT. The interactions and crystallinity of the nanocomposites were studied by Fourier transform infrared (FTIR) spectroscopy and X‐ray diffraction (XRD) analysis, respectively. Development of hydrogen bonds between the functional groups of PA and hydroxyl groups of nanotubes was indicated by FTIR spectroscopy. According to the XRD analysis, the addition of HNT and mHNT nanotubes increased the crystallinity of the PA. This was ascribed to the role of nanotubes as nucleating agents in the PA matrix. Thermal resistance and char residue of PA, as revealed by thermogravimetric analysis, were enhanced by incorporating both HNT and mHNT and the latter was more effective in this regard. Furthermore, while the addition of pristine HNTs decreased the glass transition temperature (Tg) of the PA, the Tg could be increased by about 5°C, in the presence of 5 wt% of mHNTs. Finally, the facilely activated mHNT nanotubes were found to be highly efficient in improving the thermal and structural properties of semi‐aromatic PAs.     

Synthesis and characterization of biodegradable multicomponent amphiphilic conetworks with tunable swelling through combination of ring‐opening polymerization and “click” chemistry method as a controlled release formulation for 2,4‐dichlorophenoxyacetic acid herbicide

Multicomponent (two, three, and four component) amphiphilic conetworks (APCNs) with tunable swelling behaviors were fabricated through the ring opening polymerization and click chemistry utilizing various combinations of azide and alkyne functionalized poly (ethylene glycol) (PEG) and poly (caprolactone) (PCL) precursors. Prepare azido‐terminated star‐shaped PCL, azido‐terminated PEG, alkyne‐terminated PEG, and propargylated pentaerythritol were characterized by hydrogen‐1 proton nuclear magnetic resonance (¹H NMR) and Fourier‐transform infrared (FT‐IR) spectroscopy. The morphology and thermal behavior of the APCNs were studied by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The swelling behavior of APCNs could be manipulated through an establishment of a balance between hydrophilic segments, hydrophobic segments, and cross‐linking density. The 2,4‐dichlorophenoxyacetic acid (2,4‐D) herbicide was entrapped in APCNs as a model agrochemical to study the release profile from APCNs. The obtained results showed that the release of 2,4‐D could be controlled by the swelling degree of APCNs. Finally, the biodegradability rates of APCNs were investigated in agricultural soil. The results exhibited that the decrease in the swelling degree led to decreased degradation rate of APCNs. According to obtained results, these APCNs could be used as biomaterials for the controlled release of agrochemicals.     

A novel semi-inverse solution method for elastoplastic torsion of heat treated rods

Torsion rods are a primary component of many power transmission and other mechanical systems. The behavior of these rods under elastoplastic torsion is of major concern for designers. Different methods have so far been proposed which deal with the elastoplastic torsion of rods, most of which assume constant yield stress. This assumption produces rough and inaccurate results when the rods are heat treated, since in the process of heat treatment the form of yield stress distribution across the rod cross section changes. We propose a new method for calculating elastoplastic torsion of rods of simply connected cross section which is based on heat treatment observations. In our method the full plastic stress function is obtained by using the semi-inverse method. Elastoplastic stress function is obtained by generalizing the idea of the membrane analogy and using a piecewise continuous stress function. Since the proposed form of yield stress distribution can not be handled by the current Finite Element packages, we produce a computer package with a 3D graphical interface capable of calculating and displaying the 3D elastoplastic stress function, shear stress contours, and torque-angle of rotation per unit length. We show that our method produces excellent agreement for several known cross sections in comparison to methods which assume constant yield stress.     

Alumina-based fiber for solid phase microextraction of alcohols from gaseous samples.

On the basis of the affinity of alumina towards hydroxyl functional groups, a solid phase microextraction (SPME) fiber was constructed using alumina powder and poly(vinyl chloride) (PVC). Different compositions were tested and alumina:PVC (97:3) was selected as an optimun composition. A piece of silver wire was used as fiber support. The fiber producing was simple and could be completed in a few minutes. The mechanical durability of the fiber was so good that only a single fiber was used during this study. Ethanol, 1-butanol, 2-butanol, 1-pentanol, 2-pentanol and cyclohexanol were selected as target analytes. Finally, ethanol concentration in the fruit juices and vinegar was measured using SPME-capillary gas chromatography followed by flame ionization detection.     

The Semileptonic Decay Modes {\bar{B} \rightarrow D\ell \bar{\nu}} and {\bar{B}_{s} \rightarrow D_{s} \ell \bar{\nu}}: A New Analysis in Potential Model

We consider the Schrödinger equation with a combination of Deng–Fan-type and harmonic terms. To solve the corresponding differential equation, we split the equation to two parts: the parent and the perturbation terms. We use the Nikiforov–Uvarov technique to solve the parent part. For the perturbation part, we apply the series expansion method. Next, using the calculated wave function, we investigate some bottom and charm mesons within the Isgur–Wise function formalism. We present especially semileptonic \({\bar{B} \rightarrow D\ell \bar{\nu}}\) and \({\bar{B}_{s} \rightarrow D_s \ell \bar{\nu }}\) decay widths, branching ratios and \({|V_{cb}|}\) (element of the CKM matrix). Masses of some pseudoscalar mesons are also indicated. Comparisons of our results with experimental values and other approaches are included.     

Synthesis,characterization, and crystal structures of α, α′-bis(substituted-benzylidene)cycloalkanone derivatives by nano-TiO<Subscript>2</Subscript>/HOAc

A new and economical synthesis of α, α′-bis(substituted-benzylidene)cycloalkanones has been achieved by the reaction of cycloalkanones with different aromatic aldehydes using nano-TiO₂/acetic acid as a catalyst in ethanol under reflux conditions with excellent yields. Five new products and three new single crystal structures are reported.     

A New Turbidimetric Method for Determination of Melamine Based on its Reaction with Mercury(II)

A turbidimetric technique based on a reaction between Hg(II) and melamine is described for the measurement of melamine content of wastewater of a petrochemical company. The present technique is rapid and simple, uses a small amount of reagent and is set up with low‐cost equipment, making this system economically viable. The experimental conditions such as volume of reagent, pH of buffer, stirring time, stirring speed, ionic strength, etc. were optimized and used in plotting the calibration curve. The linear dynamic range was 1–70 mg/L with a limit of detection of 0.3 mg/L. Repeatability of the proposed method is good, and the relative standard deviation (RSD%) is less than 1.5%. Interferences studies showed that the common compounds existing in wastewater have no negative effect on the efficiency of the method at relatively high concentrations. Finally, the proposed method has been successfully used in determination of melamine in wastewater of Urmia Petrochemical Company.