Rate Constants For Exchange Reactions Research Articles

Кинетика сорбции ионов меди, никеля и кобальта при совместном присутствии из водных растворов цеолитами

Экспериментально исследовано взаимное влияние на кинетику сорбции ионов меди (II), никеля (II) и кобальта (II) при совместном присутствии цеолитом Ягоднинского месторождения Камчатского края. Показано, что лимитирующей стадией сорбции является внутридиффузионный механизм. Для каждого иона рассчитаны величина сорбционной емкости, коэффициент внутренней диффузии, константа скорости реакции обмена по моделям псевдо-первого и псевдо-второго порядков.

Self-Assembled Monolayer Exchange Reactions as a Tool for Channel Interface Engineering in Low-Voltage Organic Thin-Film Transistors

In this work, we compared the kinetics of monolayer self-assembly long-chained carboxylic acids and phosphonic acids on thin aluminum oxide surfaces and investigated their dielectric properties in capacitors and low-voltage organic thin-film transistors. Phosphonic acid anchor groups tend to substitute carboxylic acid molecules on aluminum oxide surfaces and thus allow the formation of mixed or fully exchanged monolayers. With different alkyl chain substituents (n-alkyl or fluorinated alkyl chains), the exchange reaction can be monitored as a function of time by static contact angle measurements. The threshold voltage in α,α'-dihexyl-sexithiophene thin-film transistors composed of such mixed layer dielectrics correlates with the exchange progress and can be tuned from negative to positive values or vice versa depending on the dipole moment of the alkyl chain substituents. The change in the dipole moment with increasing exchange time also shifts the capacitance of these devices. The rate constants for exchange reactions determined by the time-dependent shift of static contact angle, threshold voltage, and capacitance exhibit virtually the same value thus proving the exchange kinetics to be highly controllable. In general, the exchange approach is a powerful tool in interface engineering, displaying a great potential for tailoring of device characteristics.

Weak complexation of nitroxide radicals by cyclodextrins

Electron spin resonance measurements of Heisenberg spin-exchange rates have been used to determine the formation constants (K/sub a/) and thermodynamic parameters for the complexation reactions involving di-tert-butyl nitroxide (DTBN), 3-carbamoylproxyl (CAPRO), 3-(aminomethyl)proxyl (AMPRO), and the complexing agent ..cap alpha..-cyclodextrin (proxyl = 2,2,5,5-tetramethyl-pyrrolidinyl-1-oxy). At 298 K the K/sub a/ values for DTBN, CAPRO, and AMPRO are 6.0 +/- 1, 3.3 +/- 1, and 4.9 +/- 1. The ..delta..H and ..delta..S values determined from the formation constants are as follows: -33 +/- 6 kJ, -94 +/- 18 J K/sup -1/ (DTBN); -22 +/- 7 kJ, -64 +lt. slash- 20 J K/sup -1/ (CAPRO); -33 +/- 8 kJ, -97 +/- 25 J K/sup -1/ (AMPRO). Rate constants for exchange reactions involving radical-radical collisions, radical-complex collisions, and complex-complex collisions are reported. Possible structures for the complexes are discussed in terms of the measured equilibrium and rate constants.

Oxygen isotope exchange rate between dissolved sulfate and water at hydrothermal temperatures

Oxygen isotope exchange rate between dissolved sulfate and water was experimentally determined at 100, 200 and 300°C. The isotope exchange rate is strongly dependent on temperature and pH of the solution. Combining the temperature and pH dependence of the reaction rate, the exchange reaction was estimated to be first-order with respect to sulfate. The logarithm of apparent rate constant of exchange reaction at a given temperature is a function of the pH calculated at the experimental temperatures. From the pH dependence of the apparent rate constant, it was deduced that the isotope exchange reaction between dissolved sulfate and water proceeds through collision between H 2SO 0 4 and H 2O at low pH, and between HSO − 4 and H 2O at intermediate pH. The isotope exchange rate obtained indicates that oxygen isotope geothermometry utilizing the studied isotope exchange is suitable for temperature estimation of geothermal reservoirs. The extrapolated half-life of this reaction to oceanic temperature is about 10 9 years, implying that exchange between oceanic sulfate and water cannot control the oxygen isotope ratio of oceanic sulfates.

Kinetics of ligand-exchange reactions of macrobicyclic cryptands

The rate constants for exchange reactions between metal cryptate complexes, MCryn+1, and a free cryptand, Cry2, have been determined in several solvents. Two mechanisms could be distinguished: a reaction proceeding via the free metal cation following dissociation of MCryn+1, and a direct, bimolecular cation exchange between the two ligands. In solvents such as water, dimethylsulphoxide and dimethylformamide, which interact strongly with cations, the former pathway predominates for alkali-metal and alkaline-earth-metal cryptates. In poorly solvating media such as methanol and propylene carbonate, where dissociation rate constants are very low, bimolecular pathways make an important contribution to the overall exchange pathway. Exchange reactions of Ag+ cryptates show ligand-dependent rates and strong saturation effects in both methanol and dimethylsulphoxide.