Ab initio studies of RO− … HOR′ complexes. Solvent effects on the relative acidities of water and methanol

Ab initio molecular orbital calculations are reported for complexes of hydroxide and methoxide anions with water and methanol. The basis set dependence of the results is carefully considered for HO^? ? H₂O. 4-31G and 6-31G* calculations yield similar geometrical predictions; however, the 6-31G* basis set is superior for computing dissociation energies. Further extension to the 6-31G** level provides little change. The dissociation energies for the complexes range from 25 to 37 kcal/mole with hydroxide ion and methanol acting as the strongest base and acid. The difference in gas phase acidities of water and methanol is halved by the introduction of one solvent molecule.     

The determination of phosphonate base scale inhibitors in brines by plasma spectrometry

The determination of phosphonate base scale inhibitors in brines by direct current plasma (DCP) and inductively coupled plasma atomic emission spectrometry (ICP-AES) is described. The first method is based on a direct nebulization of the brine samples and plasma using the phosphorus line at 213.618 nm. The second method involves extraction of phosphorus as phospho-antimonyl molybdate complex into methylisobutyl ketone (MIBK) phase and analysis of the extract for molybdenum using the Mo 313.260 nm line. Comparison between the proposed methods and an established recommended method [1] shows excellent agreement between the results in addition to the sensitivity and ease of automation provided by AES.     

Computational evidence for methyl-donated hydrogen bonds and hydrogen-bond networking in 1,2-ethanediol-dimethyl sulfoxide

The 1:1 complex of 1,2-ethanediol with dimethyl sulfoxide was studied using density functional theory. A network of three hydrogen bonds holds the complex together, including two in which each methyl group donates to the same hydroxyl oxygen. Four lines of evidence support the existence of methyl-donated hydrogen bonds. The interaction energy is 36 +/- 5 kJ/mol using Becke's three parameter hybrid theory with the 1991 nonlocal correlation functional of Perdew and Wang, and a moderately large basis set (B3PW91/6-311++G**//B3PW91/6-31+G**). To determine the energy of each hydrogen bond, a relaxed potential energy scan was performed in a smaller basis set to break the weaker hydrogen bonds by forced systematic rotation of the methyl groups. Two cross-checking analyses show cooperative effects that cause individual hydrogen bond energies in the network to be nonadditive. When one methyl hydrogen bond is broken, the remaining interactions stabilize the complex by storing an additional 2-3 kJ/mol. With all hydrogen bonds intact, the O[bond]H...O[bond]S hydrogen bond contributes 26 +/- 2 kJ/mol stability, and each weak methyl bond stores 5 +/- 2 kJ/mol.     

Formation, structure and conformational dynamics of highly substituted diphenylcarbonates

The symmetrical carbonates 5-8 were prepared from ethyl 4-hydroxybenzoates with aryl-, 4-(arylethynyl)-phenyl-, arylethynyl- or arylbutadiynyl-substituents in the 3- and 5-positions, by reaction with triphosgene. The choice of base (pyridine, DMAP, NaH) had a strong influence on the conversion: For the synthesis of the carbonates 5 and 6, it was sufficient to use pyridine as the base. However, for the synthesis of the carbonates 7 and 8, NaH had to be used instead. Single-crystal X-ray structure analysis of these carbonates revealed that the substituents point towards the corners of a distorted tetrahedron with the carbonate group sitting in the middle of the tetrahedron and the two angular phenolic building blocks intersecting with an angle of 51-71 degrees. In solution at room temperature, all four substituents are magnetically equivalent as a consequence of conformational flexibility. The two enantiomeric conformers of carbonate 5a interconvert rapidly, probably via a perfect trans conformation with the plane of the Aralpha perpendicular to the carbonate plane. In the case of carbonates 6-8 this process is inhibited by unfavourable interaction of the long substituents at the benzoate moiety. The dynamic process, which has an energy barrier of 8-10 kcalmol(-1), is described in a simplified manner as a continuous rotation of the angular building blocks around the carbonate unit with the C(aryl)-O bonds as the axes of rotation.     

Catecholase biomimetic catalytic activity of copper(II) complexes with 2-methyl-3-amino-(3H)-quinazolin-4-one

The oxidation of catechol by molecular oxygen in the presence of a catalytic amount of copper(II) complex with 2-methyl-3-amino-(3H)quinazoline-4-one (MAQ) and various anions (Cl⁻, Br⁻, ClO ₄ ⁻ , SCN⁻, NO ₃ ⁻ and SO ₄ ^– ) was studied. The catecholase biomimetic catalytic activity of the copper(II) complexes has been determined spectrophotometrically by monitoring the oxidative transformation of catechol to the corresponding light absorbing o-quinone (Q). The rate of the catalytic oxidation reaction was investigated and correlated with the catalyst structure, time, concentration of catalyst and substrate and finally solvent effects. Addition of pyridine or Et₃N showed a dramatic effect on the rate of oxidation reaction. Kinetic investigations demonstrate that the rate of oxidation reaction has a first order dependence with respect to the catalyst and catechol concentration and obeying Michaelis–Menten Kinetics. It was shown that the catalytic activity depends on the coordination environment of the catalyst created by the nature of counter anions bound to copper(II) ion in the complex molecule and follows the order: Cl⁻ > NO ₃ ⁻ > Br⁻ > SO ₄ ^– > SCN⁻ > ClO ₄ ⁻ . To further elucidate the catalytic activity of the complexes, their electrochemical properties were investigated and the catecholase mimetic activity has been correlated with the redox potential of the Cu²⁺/Cu⁺ couple in the complexes.     

Uncatalyzed asymmetric diels-alder addition of cyclopentadiene to (E)- and (Z) - (R) -4,5-di-O-isopropylidene-pent-2-enonates

The uncatalyzed Diels-Alder addition of cyclopentadiene to the acrylic ester derivatives {=1} proceeds with high diastereo- and enantioface selectitivity.     

Synthesis and structure of 6- and 7-(acylmethyl)pteridines

6-(Acylmethyl)-7-hydroxypteridines 7-14 as well as the isomeric 7-(acylmethyl)-6-hydroxypteridines 15-22 were prepared by condensation of 5,6-diaminouracils 1 and 2 with ethyl aroylpyruvates 3-6 in pyridine and hydrochloric acid, respectively. The structures of the newly synthesized compounds were confirmed by their hydrolysis into the 7-hydroxy-6-methyl- 23 , 24 and 6-hydroxy-7-methylpteridines 25 and 26 . The synthesis of the 2-(methylthio) derivative 28 is also described.     

Reactivity of 3-amino-, 3-halogeno-, and 3-nitro-2-pyrones and thiopyrones

Substituted pyrrole and furan-2-carboxylic acids were obtained from 3-amino- and 3-halogeno-2-pyrones. The reactions of several substituted 2-pyrones and thiopyrones with different amines and carbonyl reagents were studied.     

The electrochemical behavior and characterization of the anodic oxide film formed on titanium in NaOH solutions

Titanium dissolution and passivation were studied in NaOH aqueous solution using open-circuit potential, potentiodynamic and potentiostatic techniques. Potentiodynamic data showed that the active-passive transition involves active metal dissolution followed by formation of a poorly conducting passive oxide film that passivates the electrode. The critical current density varied with pH as d log<I> j</I><SUB>m</SUB>/d pH=-0.098 in the pH range 11.00–14.00, while the passivation potential is changed according to the following two features: at pH 10.55–13.00, d<I>E</I><SUB>m</SUB>/d pH=-0.06 V; and at pH 13.50–14.00, d<I>E</I><SUB>m</SUB>/d pH=-0.40 V. The apparent activation energy, E*, was calculated from the slope of the Arrhenius plot and was found to be 12.6 kJ mol^–1. Current-time transients showed that the growth of titanium oxide passive film is a diffusion-controlled process. XPS measurements indicated that the passive oxide film consists mainly of TiO₂ and a mixture of suboxides of Ti₂O₃ and TiO. Electronic Publication     

A simple and selective flow-injection spectrophotometric determination of copper(II) by using acetylsalicylhydroxamic acid

A new simple, and rapid flow-injection spectrophotometric method is developed for the determination of trace amounts of Cu(II) by using a new chromogenic reagent acetylsalicylhydroxamic acid (AcSHA). The method is based on the formation of colored Cu(II)-(AcSHA)₂ complex. The optimum conditions for the chromogenic reaction of Cu(II) with AcSHA is studied and the colored (green) complex is selectively monitored at λ_max 700 nm. With the reagent carrier solvent (dimethylsulfoxide (DMSO) and acetate buffer, pH 4.2) flow-rate of 1 ml min⁻¹, a detection limit (2S) of 1 μg l⁻¹ Cu(II) was obtained at a sampling rate of 80 sample h⁻¹. The calibration graph was linear in the Cu(II) concentration range 5-120 μg l⁻¹. The relative standard deviation (n=10) was 0.64% for a sample containing 60 μg l⁻¹ Cu(II). The detailed study of various interferences confirmed the high selectivity of the developed method. The method was successfully applied to determine trace amounts of copper(II) in river and seawater samples. The accuracy of the method was demonstrated by the analysis of standard reference materials C12X3500 and C14XHS 50.