Rapid solvent exchange

Methodologies for the fabrication of biomimetic membranes vary somewhat from one biomimetic membrane to another. However, a number of experimental procedures for the formation of lipid monolayers and bilayers on solid supports are common to several biomimetic membranes. The most popular procedures are vesicle fusion, Langmuir-Blodgett and Langmuir-Schaefer transfers, and rapid solvent exchange. The formation of lipid monolayers and bilayers on gold and... 

Rate constants for the incorporation of several metal ions of the first transition series into N-methyltetraphenylporhyrin in N,N-dimethylformamide solution have been determined by Bain-Ackerman and Lavalle [36], Table 1.8.The rate order parallels that of the solvent exchange of the metal ions. Thus rapid solvent exchange in the d Jahn Teller distorted connerCII) leads to a rapid incorporation rate. 

Generally, however, rapid solvent exchange occurs at room temperature so that the NMR signals from the two solvated forms cannot be separated. Still, the average chemical shifts =/(Xg) can yield information on the preferential solvation of ions P in the solvent mixture Ah-B. The midpoint of the function bet-... 

Buboltz JT, Feigenson GW. A novel strategy for the preparation of liposomes Rapid solvent exchange. Biochim Biophys Acta 1999 1417 232-245. 

One important point to stress from these results is the possibihty of using copper chloride instead of copper triflate to prepare the complexes. It is well known that in organic solvents there is a dramatic counteranion effect on the activity and enantioselectivity of these catalysts. On the other hand, the rapid anion exchange produced in the ionic hquid resulted in better performance of the complexes, as the bis(triflyl)imide behaves in a similar way to the triflate counteranion. 

Where solvent exchange controls the formation kinetics, substitution of a ligand for a solvent molecule in a solvated metal ion has commonly been considered to reflect the two-step process illustrated by [7.1]. A mechanism of this type has been termed a dissociative interchange or 7d process. Initially, complexation involves rapid formation of an outer-sphere complex (of ion-ion or ion-dipole nature) which is characterized by the equilibrium constant Kos. In some cases, the value of Kos may be determined experimentally alternatively, it may be estimated from first principles (Margerum, Cayley, Weatherburn Pagenkopf, 1978). The second step is then the conversion of the outer-sphere complex to an inner-sphere one, the formation of which is controlled by the natural rate of solvent exchange on the metal. Solvent exchange may be defined in terms of its characteristic first-order rate constant, kex, whose value varies widely from one metal to the next. 

In solution, the 170 NMR spectra of tributyltin acetate and dibutyltin diacetate show a single signal in a variety of solvents over the range 200-300 K. This is interpreted to imply the existence, in solution, of a cyclic monomer in which the carboxylate groups ate anisobidentate, and are undergoing rapid intramolecular exchange 34.350... 

Fig. 7.3 Free energy profiles for pianar-octahedral equiiibria. In A, ligand or solvent exchange is more rapid than the planar, octahedral equilibrium as established in (7.7). In B, the formation and dissociation of the octahedral complex is rate-determining and the interconversion of the planar and five-coordinated species is more rapid.

Such contributions are small enough to make them hitherto undetected by spectroscopic methods. Rapid tautomeric exchange of hydrogen atoms between the two nitrogen atoms takes place, however, since NMR spectra in a variety of solvents show that for symmetrically disubstituted dihydrodiazepines the 2 and 3 positions are equivalent as are the 5 and 7 positions (65CB2701 68MI1). /V-substituted dihydrodiazepines must exist in the conjugated form. 

As is to be expected, an alkynic ketone undergoes a Michael addition with a carbanion, leading eventually to a pyranone (50JA1022). Using malonic esters, a 3-alkoxycarbonyl derivative results, which is hydrolyzed to the 2-oxopyran-3-carboxylic acid under alkaline conditions, but to the pyranone by sulfuric acid. Rapid ester exchange is observed with the initial products, the alcohol used as solvent determining the nature of the alkyl group in the 3-carboxylic esters (Scheme 90). 

In some studies an assumption has been made about which of B or C (Fig. 7) is the rate-determining step, based more on prejudice about the role of the spin state change than on other evidence. At present it appears that only mechanism B can be distinguished from A and C. This is because mechanism B provides a pathway for ligand (or solvent) exchange from the six-coordinate complex, which is more rapid than the planar-octahedral interconversion and which can be observed by NMR... 

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