Search results for "Mechanism"

Study of the thermal behaviour of ordered bimetallic EDTA complexes

1986

Abstract The study of the thermal behaviour of a family of isostructural ordered bimetallic MM'(EDTA)·6H 2 O complexes has been performed using TG and DTA. Formal kinetic analysis of the dehydration step has been approached using non-isothermal procedures. The resulting correlation among the calculated E a values and the cationic sizes implies a common dehydration mechanism. Copper-containing complexes show distinctive behaviour during the ligand pyrolysis processes. A mechanistic interpretation involving formation of Cu(I) and Cu(0) intermediates is furnished. The evolution of the inorganic residues in systems containing both Co and Ni is especially considered given the interest in the (Co…

Kinetic study of methoxide-promoted elimination reactions of some 1,1,1-trichloro-2,2-bis(phenyl-substituted)ethanes

2001

The methoxide-promoted elimination reaction of some 1,1,1-trichloro-2,2-bis(phenyl-substituted)ethanes (1) was investigated. The ortho-substituted derivatives were found to be less reactive than the corresponding ortho-unsubstituted derivatives, irrespective of the nature of their substituent. The reactivity data were correlated with the 13C NMR chemical shift values of C-β of either 1,1,1-trichloro-2,2-bis(phenyl-substituted)ethanes or 1,1-dichloro-2,2-bis(phenyl-substituted)ethenes and the better result was obtained for the former correlation. Activation parameters for the methoxide-promoted elimination of 1 show very similar values for ortho-substituted derivatives. The total data set se…

Direct electrochemical detosylation of tetratosylcyclen to cyclen with carbon cathodes

2005

The electrochemical detosylation of tetratosylcyclen to cyclen (1,4,7,10-tetraazacyclododecane) in aprotic solvents at a carbon cathode was investigated both in a bench-scale batch electrochemical reactor and in a continuous batch recirculation reaction system equipped with a parallel-plate electrochemical cell. Yields and faradaic efficiency in cyclen, respectively, of 80 and 55% were obtained in the bench-scale reactor working in acetonitrile. It was found that the faradaic efficiency in cyclen depends on the competition between the reduction of the tetratosylcyclen and parasitic processes as the reduction of the tetraalkylammonium cations present in the supporting electrolyte. We have ob…

Study of the thermal decomposition of bromazepan complexes with Co(II), Ni(II), Cu(II) and Zn(II)

1984

Abstract The thermal behaviour of complexes of bromazepan with Co(II), Ni(II), Cu(II) and Zn(II) was studied by thermogravimetry (TG) and differential thermal analysis (DTA). The complexes decompose in two steps: dehydration and decomposition. A correlation between the dehydration temperatures of MB2Xn and the coordinating ability of X is observed.

Pd-catalyzed hydrodehalogenation of chlorinated olefins: Theoretical insights to the reaction mechanism

2012

Abstract Density functional theory calculations are applied to study energetics of trichloroethene (TCE) hydrodechlorination over pure Pd(1 1 1), chlorine-covered Pd(1 1 1), and Pd island supported by Au(1 1 1). Our results show that in all cases C Cl bond breakings take place more readily than C H bond formations and that TCE dechlorinates fully producing CCH precursors for the hydrogenations. The reaction pathway through radical-like species provides a possible explanation to the experimental product distributions that show a nominal amount of lesser chlorinated species in the presence of excess hydrogen. The surface chlorine resulted from the TCE decomposition weakens the binding of the …

1990

The anionic polymerization of tert-butyl acylate (tBuA) initiated by tert-butyl α-lithioisobutyrate was investigated in THF at 25°C. The individual oligomers were isolated and characterized by UV, IR and NMR spectroscopy. The distribution of products was determined in the range of reaction times 0,02 s ≤ t ≤ 1800 s. The oligomerization is extremely fast. Even after 0,02 s at -30°C, no residual monomer could be detected. After short reaction times the reaction mixture almost exclusively consists of linear oligomers of tBuA. This indicates that there is no termination reaction by Claisen condensation during polymerization. Nevertheless, the molecular weight distribution is rather broad (Mw/Mn…

A DFT Study of Inter- and Intramolecular Aryne Ene Reactions

2015

The molecular mechanisms of inter- and intramolecular aryne-ene reactions have been theoretically studied by DFT methods at the MPWB1K/6-311G(d,p) level. These reactions proceed through a one-step mechanism via nearly asynchronous transition states (TSs), in which the C–C single bond formation is slightly more advanced than the hydrogen transfer process. These ene reactions show very low activation enthalpies (<1 kcal/mol) and are strongly exothermic by more than 73 kcal/mol. An electron localisation function (ELF) topological analysis of the changes of electron density during these ene reactions indicates that the bonding changes are nonconcerted. ELF topological analysis of the electron d…

A combined experimental and theoretical study of the unimolecular elimination kinetics of 2-alkoxypropionic acids in the gas phase

1999

Abstract The reaction mechanism associated with the decomposition of three 2-alkoxypropionic acids (2-methoxy-, 2-ethoxy- and 2-isopropoxypropionic acid) in gas phase to form acetaldehyde, carbon monoxide, and the corresponding alcohol has been analyzed by a combination of experimental and theoretical studies. The kinetics of these systems were determined in a static system over the temperature and pressure range of 301.2–370.7°C and 61–190 Torr, respectively, in seasoned vessel, with the free-radical inhibitor cyclohexene. The experimental data show that these decompositions are homogeneous, unimolecular and follow a first-order rate law. A detailed characterization, at MP2/6-31G** computa…

A DFT study of the polar Diels–Alder reaction between 4-aza-6-nitrobenzofuroxan and cyclopentadiene

2005

Abstract The polar Diels–Alder reaction between 4-aza-6-nitrobenzofuroxan (ANBF) and cyclopentadiene has been studied using DFT procedures at the B3LYP/6-31G* level. Only one highly asynchronous transition state structure associated to the formation of the [4+2] adduct 13 is found. A further [3,3] sigmatropic shift on the [4+2] cycloadduct 13 allows its conversion into the thermodynamically more stable [2+4] cycloadduct 14 . The analysis of the global and local electrophilicities of the reagents correctly explain the behaviour of ANBF as a strong electrophile in polar cycloadditions.

A DFT Study of the Molecular Mechanisms of the Diels−Alder Reaction between Cyclopentadiene and 3-Phenyl-1-(2-pyridyl)-2-propen-1-one − Role of the Z…

2002

The molecular mechanism of the Diels−Alder reaction between cyclopentadiene (1) and 3-phenyl-1-(2-pyridyl)-2-propen-1-one (2) in the absence and in the presence of a Zn2+ Lewis acid catalyst has been studied by quantum mechanical calculations at the B3LYP/6-31G* level of theory. A continuum model was selected to represent the effects of the water as solvent. For the uncatalyzed process, two channels, endo and exo, were characterized, and the mechanism corresponded to an asynchronous concerted reaction associated with a [4+2] process. The presence of a Lewis acid catalyst changed the mechanism drastically, the reaction taking place by a polar stepwise mechanism. In the first step, a C−C sigm…