Search results for "Onium"
showing 10 items of 2091 documents
Synthesis of cyanoacetic acid by carbon dioxide and electrogenerated acetonitrile anion in undivided cells equipped with sacrificial anodes
2008
The electrochemical synthesis of cyanoacetic acid was performed by cathodic reduction of tetraalkylammonium salts (R4NX) in acetonitrile in undivided cells equipped with sacrificial anodes with concomitant or subsequent addition of carbon dioxide. These syntheses avoid the use of dangerous reagents, yield cyanoacetic acid in moderate faradic yields and confirm that the cathodic reduction of tetraalkylammonium salts in the presence of acetonitrile leads to the formation of the anion CNCH2-.
Speciation of chitosan-phosphate and chitosan-nucleotide systems in an NaCl aqueous solution
2010
AbstractThe speciation of chitosan (310 kDa) with organic (adenosine 5’-monophosphate, AMP, and adenosine 5’-triphosphate, ATP), and inorganic phosphorus containing ligands (phosphate and pyrophosphate) was investigated in NaCl aqueous solutions at I = 0.1mol L−1 and T = 25°C. For all the systems, the investigated results obtained gave evidence for the formation of (chitosan)LHi complex species (L = nucleotides, phosphate and pyrophosphate; i = 1 to 4, but for AMP, i = 1 to 3). The stability data of complex species were used to calculate the sequestering ability of chitosan towards phosphorus compounds considered here, expressed as pL50 i.e., – log(total chitosan concentration) necessary to…
Isolation of enantiomerically pure organometallic palladium compounds: synthesis of the triangles prepared from enantiopure [cis-Pd2(C6H4PPh2)2(NCCH3…
2009
Reaction of the racemic [Pd(C(6)H(4)PPh(2))Br](4) () with the silver salt of 1R-(1alpha,2beta,3alpha)]-3-methyl-2-(nitromethyl)-5-oxocyclopentaneacetate, (R)-AgO(2)CR*, results in the formation of a mixture of diastereoisomers (RRR)- and (SRR)- of the formula Pd(2)(C(6)H(4)PPh(2))(2)(O(2)CR*)(2) that were separated by standard chromatographic methods. Each diastereoisomer was readily converted into the tetrametallic stereoisomers (SS)- and (RR)-, of the formula [Pd(C(6)H(4)PPh(2))Br](4) that were isolated and characterized by X-ray crystallography. The R enantiomer of the solvated cationic species [cis-Pd(2)(C(6)H(4)PPh(2))(2)(NCCH(3))(4)](2+), obtained from (RR)-, was reacted with ammonium…
Der (2-Phenyl-2-trimethylsilyl)ethyl-(PTMSE)-Ester – eine neutral spaltbare Carboxyschutzgruppe/The (2-Phenyl-2-trimethylsiylyl)ethyl (PTMSE) Ester –…
2002
(2-Phenyl-2-trimethylsilyl)ethyl (PTMSE) esters of aminoacids and peptides are stable under the conditions of hydrogenolytic cleavage of benzoxycarbonyl(Z) and benzyl ester groups, base-induced removal of Fmoc groups, palladium(0)-catalyzed removal of allyloxycarbonyl (Aloc) and even acidolytic cleavage of Boc groups. PTMSE esters are also stable under the conditions of peptide condensation reactions. The PTMSE ester is selectively cleaved by treatment with tetrabutylammonium fluoride (TBAF) trihydrate in dichloromethane, i. e. under almost neutral conditions, within a few minutes and, therefore, considered a valuable novel carboxy protecting group.
Stabilization of polynuclear plutonium(IV) species by humic acid
2014
Abstract Although the formation of tetravalent plutonium (Pu(IV)) polymers with natural organic matter was previously observed by spectroscopy, there is no quantitative evidence of such reaction in batch experiments. In the present study, Pu(IV) interaction with humic acid (HA) was investigated at pH 1.8, 2.5 and 3, as a function of HA concentration and for Pu total concentration equal to 6 × 10−8 M. The finally measured Pu(IV) concentrations ([Pu(IV)]eq) are below Pu(IV) solubility limit. Pu(IV)–HA interaction can be explained by the complexation of Pu(IV) monomers by HA up to [Pu(IV)]eq ∼ 10−8 M. However, the slope of the log–log Pu(IV)–HA binding isotherm changes from ∼0.7 to ∼3.5 for hi…
Hydrophobicity of Ionizable Compounds. A Theoretical Study and Measurements of Diuretic Octanol−Water Partition Coefficients by Countercurrent Chroma…
1999
Countercurrent chromatography was used to determine the octanol−water partition coefficients (Po/w) of 23 diuretic drugs. The measured Po/w values ranged over 4 orders of magnitude from 0.05 to 550 (−1.3 < log Po/w < +2.7). All the compounds, except spironolactone, were ionizable. The Po/w values were strongly depending on the aqueous-phase pH. A theoretical model linking these values with the pH was derived for four cases: (i) molecular acid−anionic base, (ii) cationic acid−molecular base, (iii) biprotic systems with two acidic or basic groups showing the same charge sign, and (iv) biprotic systems with ionizable groups showing different charges with special attention to the amino acid ca…
Influence of the presence of surfactants and humic acid in waters on the indophenol-type reaction method for ammonium determination.
2005
Abstract This work has evaluated the influence of humic acid and/or surfactants in the quantification of ammonium in waters with the indophenol-type reaction method. Thymol has been employed with the colorimetric method for sample ammonium concentrations between 0.25 and 1 mg L −1 . In addition, SPE/diffuse reflectance method has been used for quantification of ammonium at low concentrations (between 0.025 and 0.25 mg L −1 ). Matrix effect owing to humic substances were observed with the colorimetric method when the concentration was equal or higher than 25 mg L −1 . Lower concentrations of humic compounds produced matrix effects with the more sensitive SPE/diffuse reflectance method. Gener…
Effect of the hydrophobic spacer in bonded [Cu(l-hydroxyprolyl)alkyl]+ silicas on retention and enantioselectivity of α-amino acids in high-performan…
1983
Abstract The following chiral ligands were bonded to silica: [Cu( l -hydroxyprolyl)methyl) + on LiChrosorb Si 60 (1), [Cu( l -hydroxyprolyl)methyl] + on LiChrosorb Si 100 (2) and [Cu( l -hydroxyprolyl) n -octyl] + on LiChrosorb Si 100 (3). The packings contained residual iodomethyl- and ω-iodooctyl groups at the surface. Studies on packing 1 and 2 under comparable conditions in eluents containing 10 -4 M copper acetate showed a higher retention on 1 than on 2 but a much better enantioselectivity in the latter case. The retention of enantiomers on all packings examined was found to be governed by the eluent pH and methanol content as well as by the concentration of ammonium acetate. The vari…
Determination of the four major surfactant classes in cleaning products by reversed-phase liquid chromatography using serially connected UV and evapo…
2016
Abstract A method for the simultaneous determination of the most frequently used surfactant families –linear alkyl benzenesulphonates (LAS), alkyl ether sulphates (AES), fatty alcohol ethoxylates (FAE) and oleins (soaps, fatty acid salts) – in cleaning products, has been developed. The common reversed phase octyl (C8), pentafluorophenyl and biphenyl columns were not capable of separating the anionic LAS and AES classes; however, since only LAS absorbs in the UV, these two classes were independently quantified using a C8 column and serially connected UV and ELSD detection. The best compromise to resolve the four surfactant classes and the oligomers within the classes was achieved with a C8 c…
Single-pump bi-dimensional LC applied to the characterization of derivatized fatty alcohol ethoxylates.
2010
A column selection valve and a 6-port 2-position injection valve were combined to implement bi-dimensional LC in a system driven by a single pump. The system was applied to the separation of linear fatty alcohol ethoxylates with 12-18 carbon atoms in the hydrocarbon moiety, previous derivatization with diphenic anhydride. Separation on the first separation dimension (C(1)) was carried out with a propyl-diol column and an acetonitrile/aqueous ammonium acetate gradient. Fatty alcohol ethoxylate hydrocarbon series (oligomers with the same number of carbon atoms in the alkyl chain but differing in the number of ethylene oxide units) were resolved without distinguishing the oligomers within the …