Kinetic study and UV–Vis spectra of 1:2 complexation of free base para‐substituted meso‐tetraphenylporphyrins with trimethylsilyl chloride

The UV–Vis spectra for 1:2 complexation of four different para‐substituted meso‐tetraphenylporphyrin (H₂t(4‐X)pp) and meso‐tetraphenylporphyrins (H₂tpp) with trimethylsilyl chloride (TMSC) displayed large and different redshifts (28–32.4 nm) of Soret and (15–41.7 nm) Q_(0‐0) bands, whereas 1:2 complexation of the less flexible tetramesitylporphyrin (H₂tmp) with TMSC led to rather small redshift (24.8 nm) of the Soret band and blueshift (−7.4 nm) of the Q_(0‐0) band. The varying spectral behavior for the porphyrins complexation seems to essentially reflect the different extent of π‐interactions between the meso‐aryl groups and the presumably saddled porphyrin macrocycle, through their relative coplanarity. The observed order of the rate constants for the complexation of various para‐substituted porphyrins, H₂t(4‐OCH₃)pp (9.27 ± 0.03) × 10⁻³ > H₂t(4‐CH₃)pp (6.68 ± 0.05) × 10⁻³ > H₂tpp (3.2 ± 0.05) × 10⁻³ > H₂t(4‐Cl)pp (8.36 ± 0.06) × 10⁻⁴, clearly demonstrated a higher reaction rate for the porphyrins containing para‐substituents with stronger electron donor ability. The calculated order for porphyrins (0.9 ± 0.1) and for TMSC (1.0 ± 0.1) suggests rate = K[Por][TMSC] for the complexation. Attempts were made to explain the absence of spectral evidence for the presence of an intermediate 1:1 (TMSC) Por adduct in terms of its high reactivity and/or relative instability. © 2007 Wiley Periodicals, Inc. 39: 231–235, 2007     

Simultaneous spectrophotometric determination of p-benzoquinone and chloranil after microcrystalline naphthalene extraction by using genetic algorithm-based wavelength selection-partial least squares regression.

Microcrystalline naphthalene extraction has been used for the preconcentration of p-benzoquinone and tetrachloro-p-benzoquinone (chloranil), after their reaction by aniline, and later simultaneous spectrophotometric analysis by genetic algorithm-partial least squares (GA-PLS) calibration. The chemical variables affecting the analytical performance of the methodology were studied and optimized. Under the optimum conditions i.e., [aniline] = 0.05 M and [naphthalene] = 2.2% (w/v), preconcentration of 25 ml of sample solution permitted the detection of 0.32 and 0.23 microg ml(-1) for p-benzoquinone and chloranil, respectively. The predictive abilities of partial least squares regression (PLS) and genetic algorithm-partial least squares regression (GA-PLS) were examined for simultaneous determination of two quinones. The GA-PLS shows superiority over other PLS methods due to the wavelength selection in PLS calibration using a genetic algorithm without loss of prediction capacity, provides useful information about the chemical system.     

Tabu Machine: A New Neural Network Solution Approach for Combinatorial Optimization Problems

A new artificial neural network solution approach is proposed to solve combinatorial optimization problems. The artificial neural network is called the Tabu Machine because it has the same structure as the Boltzmann Machine does but uses tabu search to govern its state transition mechanism. Similar to the Boltzmann Machine, the Tabu Machine consists of a set of binary state nodes connected with bidirectional arcs. Ruled by the transition mechanism, the nodes adjust their states in order to search for a global minimum energy state. Two combinatorial optimization problems, the maximum cut problem and the independent set problem, are used as examples to conduct a computational experiment. Without using overly sophisticated tabu search techniques, the Tabu Machine outperforms the Boltzmann Machine in terms of both solution quality and computation time.     

Voltammetric oxidation of 2-oxo-1,2,3,4-tetrahydropyrimidin-5-carboxamides: substituent effects

Electrochemical oxidation of a series of 20 substituted 2-oxo-1,2,3,4-tetrahydropyrimidin-5-carboxamides (THPMs) in acetonitrile has been studied using voltammetric methods at a glassy carbon electrode to investigate the influence of the substituents on the 4- and 5-positions of the heterocyclic ring. Analysis of the results shows that the electronic nature and steric hindrance of the substituents, especially their orientations toward the heterocyclic ring, determine their effects on the oxidation potential. Analysis of the computational results obtained at the DFT-B3LYP/6-31++G** level of theory suggests a mechanism in which the first electron removal occurs from either the N(1) of the heterocyclic ring or N(17) of the amide substitution. This process is followed by a fast proton removal resulting in the formation of stable allylic and/or benzylic radicals which then undergo further oxidation to the 2-oxo-1,2-dihydropyrimidin-5-carboxamides (DHPMs).     

The synthesis and spectroscopic characterization of nano calcium fluorapatite using tetra-butylammonium fluoride

Pure homogeneous nano sized biocompatible fluorapatite (FAp) particles were synthesized by a wet chemical procedure using water soluble tetra-butylammonium fluoride (TBAF) without using high temperatures and any purification processes. Combination of the Bragg's law and the plane-spacing equation for the two high intensity lines, namely, (002) and (300), gives a=9.3531 ?, c=6.8841 ?, confirms the identity of the highly crystalline synthetic material as well as its purity. The effect of various pH's in crystal formation and on their size was also evaluated. The calculated crystallinities were excellent with a rate around 5.0. The synthesized nano FAp was fully characterized by spectroscopic techniques (XRD, SEM, EDS, BET, FT-IR and ICP-AES). The nitrogen adsorption-desorption isotherm showed a type IV diagram and calculation of the surface area was investigated as well.     

Synthesis, characterization, acetylcholinesterase inhibition, molecular modeling and antioxidant activities of some novel Schiff bases derived from 1-(2-ketoiminoethyl)piperazines

Some novel Schiff bases derived from 1-(2-ketoiminoethyl)piperazines were synthesized and characterized by mass spectroscopy, FTIR, UV-Visible, 1H and 13C-NMR. The compounds were tested for inhibitory activities on human acetylcholinesterase (hAChE), antioxidant activities, acute oral toxicity and further studied by molecular modeling techniques. The study identified the compound (DHP) to have the highest activity among the series in hAChE inhibition and DPPH assay while the compound LP revealed the highest activity in the FRAP assay. The hAChE inhibitory activity of DHP is comparable with that of propidium, a known AChE inhibitor. This high activity of DHP was checked by molecular modeling which showed that DHP could not be considered as a bivalent ligand due to its incapability to occupy the esteratic site (ES) region of the 3D crystal structure of hAChE. The antioxidant study unveiled varying results in 1,1-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays. This indicates mechanistic variations of the compounds in the two assays. The potential therapeutic applications and safety of these compounds were suggested for use as human acetylcholinesterase inhibitors and antioxidants.     

Kinematic analysis of 5-R<Emphasis Type="Underline">P</Emphasis>UR (3T2R) parallel mechanisms

This paper investigates some kinematic properties of a five-degree-of-freedom parallel mechanism generating the 3T2R motion and comprising five identical limbs of the RPUR type. The general mechanism originates from the type synthesis performed for symmetrical 5-DOF parallel mechanism. In this study, two classes of simplified designs are proposed whose forward kinematic problem have either a univariate or a closed-form solution. The principal contributions of this study are the solution of the forward kinematic problem for some simplified designs—which may have more solutions than the FKP of the general 6-DOF Stewart platform with 40 solutions—and the determination of the constant-orientation workspace which is based on the topology of the vertex space (Bohemian dome) and a geometric constructive approach.     

A novel one-pot pseudo-four-component isocyanide-based reaction: an unexpected approach for the synthesis of tetrahydrodiisoindoloquinoxalines and tetrahydrobenzodiisoindoloquinoxalines

A novel isocyanide based one-pot pseudo-four-component strategy for the synthesis of tetrahydrodiisoindoloquinoxaline and tetrahydrobenzodiisoindoloquinoxaline derivatives via the reaction of 1,2-diamines, 2-formylbenzoic acid, and an isocyanide at room temperature in good yields without using any catalyst and activation, is described.