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Associative Mechanisms (associative + mechanism)

Distribution by Scientific Domains

Chemistry	60%

Selected Abstracts

Ligand-Exchange Processes on Solvated Lithium Cations: Acetonitrile and Hydrogen Cyanide,

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2007
Ewa Pasgreta
Abstract Solutions of LiClO4 in solvent mixtures of acetonitrile and water, or acetonitrile and nitromethane, were studied by 7Li NMR spectroscopy. Measured chemical shifts indicate that the Li+ cation is coordinated by four acetonitrile molecules. In the binary water/acetonitrile mixture, water coordinates more strongly to Li+ than acetonitrile such that addition of water immediately leads to the formation of [Li(H2O)4]+. The solvent-exchange mechanism for [Li(L)4]+ (L = CH3CN and HCN) was studied by using DFT calculations (RB3LYP/6-311+G**). This process was found to follow a limiting associative mechanism involving the formation of relatively stable five-coordinate intermediates. The suggested mechanisms are discussed with reference to available experimental and theoretical data. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

A Density Functional Theory Study of the Stille Cross-Coupling via Associative Transmetalation.

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2007
Coordinating Solvents, The Role of Ligands
Abstract An associative mechanism has been computationally characterized for the Stille cross-coupling of vinyl bromide and trimethylvinylstannane catalyzed by PdL2 (L=PMe3, AsMe3) with or without dimethylformamide as coordinating ligand. All the species along the catalytic cycles that start from both the cis - and the trans -PdL(Y)(vinyl)Br complexes (Y=L or S; L=PMe3, AsMe3 or PH3; S=DMF) have been located in the gas phase and in the presence of polar solvents. Computations support the central role of species trans -PdL(DMF)(vinyl)Br which react by ligand dissociation and stannane coordination in the rate-limiting transmetalation step via a puckered four-coordinate (at palladium) transition state comprised of Pd, Br, Sn and sp2 C atoms. A donating solvent may enter the catalytic cycle assisting isomerization of cis -PdL2(vinyl)Br to trans -PdL(DMF)(vinyl)Br complexes via a pentacoordinate square pyramidal Pd intermediate. In keeping with experimental observations, the activation energies of the catalytic cycles with arsines as Pd ligands are lower than those with phosphines. Polytopal rearrangements from the three-coordinate T-shaped Pd complexes resulting from transmetalation account for the isomerization and the CC bond formation on the reductive elimination step. [source]

Amniotic fluid can act as an appetitive unconditioned stimulus in preweanling rats

DEVELOPMENTAL PSYCHOBIOLOGY, Issue 2 2007
Carlos Arias
Abstract Studies in humans and animals indicate that exposure to flavors in the amniotic fluid during the later gestational period may induce preferences for those flavors. Considering that during the last prenatal period the amniotic fluid contains substances that activate the opioid system, and that this system plays a critical role in the acquisition of olfactory preferences early in life, it has been hypothesized that the amniotic fluid may acquire appetitive unconditioned properties during this period. This has been tested in an experiment in which preweanling rats were exposed to alcohol odor (CS) paired or unpaired with the intraoral infusion of amniotic fluid (US) collected on gestational day 20. The pairing of these two stimuli induced an enhanced palatability of alcohol's flavor as well an increased intake of the drug. These results support the idea that amniotic fluid acquires appetitive unconditioned properties during the last days of gestation and suggest that associative mechanisms involving the amniotic fluid could be underlying odor and taste preferences acquired through fetal exposure. © 2007 Wiley Periodicals, Inc. Dev Psychobiol 49: 139,149, 2007. [source]

Bayes nets and babies: infants' developing statistical reasoning abilities and their representation of causal knowledge

DEVELOPMENTAL SCIENCE, Issue 3 2007
David M. Sobel
A fundamental assumption of the causal graphical model framework is the Markov assumption, which posits that learners can discriminate between two events that are dependent because of a direct causal relation between them and two events that are independent conditional on the value of another event(s). Sobel and Kirkham (2006) demonstrated that 8-month-old infants registered conditional independence information among a sequence of events; infants responded according to the Markov assumption in such a way that was inconsistent with models that rely on simple calculations of associative strength. The present experiment extends these findings to younger infants, and demonstrates that such responses potentially develop during the second half of the first year of life. These data are discussed in terms of a developmental trajectory between associative mechanisms and causal graphical models as representations of infants' causal and statistical learning. [source]

Theoretical study of acetonitrile-exchange reactions on hexasolvated divalent cations in the first transition series elements

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2009
Hiroaki Wasada
Abstract Penta-, hexa-, and heptaacetonitrile complexes of divalent cations of the first transition series are studied by ab initio molecular orbital calculations. The factors that determine the structural stability and the reaction mechanism of solvent-exchange reactions are discussed. All the penta- and hexaacetonitrile species are at local minima, whereas the geometrical stability of the heptacoordinated species depends on the 3d electron configurations. The structural stability of heptaacetonitrile species is intermediate between those of hydrogen cyanide complexes and hydrates. Acetonitrile exchange reactions have more dissociative character than hydrogen cyanide exchange reactions because the inductive effect of the methyl group in CH3CN destabilizes the heptacoordinated structures. The successive binding energies show that associative mechanisms are favorable for acetonitrile exchange with earlier members of the first transition series, whereas dissociative mechanisms become favorable for later members. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source]