Search results for "Anion"

Determination of the lowest-energy oxidation site in nucleotides: 2'-deoxythymidine 5'-monophosphate anion.

2006

High level ab initio computations anticipate nucleobases as the most favorable sites for oxidation in nucleotides. At the CASPT2 level, the lowest ionization channel for the 2'-deoxythymidine 5'-monophosphate anion is related to a pi-orbital of the thymine base. The present findings lead to revision of the recent assignments of the photodetachment photoelectron spectra of mononucleotide anions in the gas phase and support the classical view of the nucleobase being the main actor in the oxidation process of both nucleosides and nucleotides.

Kinetic Investigation on Metal Free Anionic Polymerization of Methyl Methacrylate Using Tetraphenylphosphonium as the Counterion in Tetrahydrofuran

1997

The anionic polymerization of methyl methacrylate using tetraphenylphosphonium triphenylmethanide as an initiator proceeds in a living manner even at room temperature. The rate constants of propagation were measured between -20 and +20 °C using a flow tube reactor. At 0 °C the reaction half-lives range from 0.3 to 1 s. The polymerization follows first-order kinetics with respect to monomer conversion (with a short induction period) and shows a linear dependence of the number-average degree of polymerization on conversion with high initiator efficiencies and narrow molecular weight distributions (M w /M n < 1.1). The dependence of the measured rate constants on the active center concentratio…

A chalcone-based highly selective and sensitive chromofluorogenic probe for trivalent metal cations

2015

[EN] Anew chalcone-based probe for the chromofluorogenic sens-ing of trivalent (Al3 +,Fe3+,Cr3+,Ga3+,In3+and As3 +)overmono-and divalent cationsand anionsisreported. In the presence of trivalent metal cations,the probe was able to displayaremarkable color change from yellow to colorless that wasclearly visible to the naked eye. Also, the initial strongyellowemission was gradually quenched and substituted by aweaklyshifted band.

Macrocyclic peptides in anionic polymers of amino acid N-carboxylic anhydrides.

1976

Controlling the Polymer Microstructure in Anionic Polymerization by Compartmentalization.

2018

An ideal random anionic copolymerization is forced to produce gradient structures by physical separation of two monomers in emulsion compartments. One monomer (M) is preferably soluble in the droplets, while the other one (D) prefers the continuous phase of a DMSO-in-cyclohexane emulsion. The living anionic copolymerization of two activated aziridines is thus confined to the DMSO compartments as polymerization occurs selectively in the droplets. Dilution of the continuous phase adjusts the local concentration of monomer D in the droplets and thus the gradient of the resulting copolymer. The copolymerizations in emulsion are monitored by real-time 1 H NMR kinetics, proving a change of the re…

Configuration of Enolates of β-Ketocarboxylic Acid Esters — Carbocation-Carbanion Salts

1996

Deprotonation of (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoic acid methyl ester (1a) with tetrakis(dimethylamino)-methane (3) led to stable salt (E)-2a, which belongs to a very rare species of salts that consist of a heteroatom-stabilized carbocation and a heteroatom-stabilized carbanion. An analogous guanidinium salt (E)-2b was formed in the reaction of 3-oxo-2,3-diphenylpropanoic acid methyl ester (1b) and 3. The molecular structures of these enolates were studied by NMR spectroscopy and an X-ray analysis of (E)-2a {MC[N(CH3)2]3} and compared with those of the corresponding alkali enolates.

Protein Flexibility and Preorganization in the Design of Enzymes. The Kemp Elimination Catalyzed by HG3.17

2015

A recently designed enzyme, HG3.17, obtained by directed evolution, has shown a catalytic activity close to natural enzymes. Hybrid QM/MM molecular dynamics simulations for the Kemp elimination in this new enzyme have provided a deep insight into the origin of its catalytic efficiency. In this case, we have first demonstrated the presence of different conformations with significantly different reactivity. The larger reactivity is related with a better electrostatic preorganization of the environment that creates a more favorable electrostatic potential for the reaction to proceed. In HG3.17, efforts to improve the catalytic properties must be focused in possible mutations increasing the pre…

Nucleophilic benzoylation using lithiated methyl mandelate as a synthetic equivalent of the benzoyl carbanion. Oxidative decarboxylation of α-hydroxy…

2001

Abstract The synthesis of alkyl aryl ketones using lithiated methyl mandelate as a synthetic equivalent of the benzoyl carbanion is reported (Umpolung). The methodology involves alkylation of methyl mandelate, hydrolysis of the ester group and oxidative decarboxylation of the resulting α-hydroxyacids. The last step is carried out in a catalytic aerobic way using a Co(III) complex in the presence of pivalaldehyde under very mild and advantageous conditions. The procedure is also applied to methyl mandelates substituted on the aromatic ring.

Blockcopolymere aus 2-Isopropenylnaphthalin und Hexamethylcyclotrisiloxan, 7

1985

Anionic polymerization of 2-isopropenylnaphthalene (2-IPN), with butyllithium in THF at −78°C was terminated with ethylene oxide and the resulting terminal alkoxide was used to initiate the polymerization of hexamethylcyclotrisiloxane (D3) at +40°C. This led to the formation of AB-block copolymers which were coupled to ABBA-block copolymers by addition of dichlorodimethylsilane. ABBA-blockcopolymers were obtained with contents of polymethylsiloxane (poly(DMS)) between 77 and 84% of weight and block molecular weights between 1 700 and 27 000 (poly(2-IPN)) and 13 000 and 240 000 (poly(DMS)). Phase separation in polymer films casted from different solvents was studied by electron microscopy an…

Über den mechanismus der anionischen polymerisation von acrylnitril mit triphenylphosphin. 7. Mitt. über makrozwitterionen

1972

Die anionische Polymerisation von Acrylnitril mit Triathylphosphin als Initiator in Dimethylformamid wurde bei verschiedenen Temperaturen und unterschiedlichem Verhaltnis der Monomer/Initiator-Konzentrationen untersucht. Erganzt durch spektroskopische Endgruppenbestimmung der Polymeranionen konnte mittels 1H-NMR-Spektroskopie und Gelchromatographie (GPC) sowie emissionsspektralanalytischer Phosphorbestimmung an den rohen und den durch Umfallungen bzw. GPC fraktionierten Polymerproben gezeigt werden, das hier neben einer ubertragung teilweise eine Polymerisation Uber Makrozwitterionen auftritt. Fur die Bildung von Makrozwitterionen, , sind jedoch Mindestwerte des Monomer/Initiator-Verhaltnis…