Exchange processes in solution

A number of hydrated cations in aqueous solution undergo exchange with the solvent at rates slow enough to be observed on the NMR spectroscopic timescale by using O isotopic labelling O has / =f, while both 0 and O are NMR inactive. Different chemical shifts are observed for the O nuclei in bulk and coordinated water, and from the signal intensity ratios, hydration numbers can be obtained. For example, Al has been shown to be present as [Al(OH2)6].  

Reactions such as that in equation 3.41 are known as redistribution reactions. 

A redistribution reaction is one in which substituents exchange between species but the types and numbers of each type of bond remain the same. 

Mm are potentially suitable as contrast agents, and of these, complexes containing the Gd + ion have so far proved to be especially useful. As a free ion, Gd + is extremely toxic and, therefore, to minimize side-effects in patients. 

The molecular structure of [Gd(DTPA-BMA)(H2Q)] (Omniscan) detenmned by X-ray diftaction [A. Aukrust et al. ( 1) Organic Process Research Development, vol. 5, p. 361]. Colour c Gd, gieoi N, blue O, ted C, gtey H, white. 

The position of equilibrium can be followed by using P NMR spectroscopy, since each of the four species has a characteristic chemical shift. Rate data are obtained by following the variation in relative signal integrals with 

Values of AH° for these types of reactions are almost zero, the redistribution of the groups being driven by an increase in the entropy of the system. 

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A survey of transition-metal polyazacycloalkane complexes in general, with data also for Cd and Hg species in particular, has been published.180 Structural, 3H and 13C NMR, UV/vis, and conductivity data of several transition-metal complexes, including Cd and Hg complexes, with derivatives of the 16-membered ligand l,9-dithia-5,13-diazacyclohexadecane have been compiled and compared. In particular, conversion between configurational isomers and exchange processes in solution have been discussed.181... 

The nickel ) complex of 92 cannot be prepared directly via the template method, but can be prepared by a transmetallation procedure. Synthesis of the macrocycle in the presence of one of the metal ions known to be effective as a template is followed by a metal exchange process in solution to insert the nickel ) ion. This cation exhibits a strong preference for the square planar, square pyramidal, and octahedral geometries 79). Thus the failure of the nickel ) cation to behave as a template ion in the synthesis of 92 is probably due to the disinclination of the metal to accommodate the pentagonal array of donor nitrogen atoms necessary for reaction to occur. 

The ion exchange process in solutions of indifferent electrolytes can be described in the most general way by the Nikolsky equation, which for rather concentrated (non-ideal) solutions can be written as... 

Exchange processes in solutions of AICI3 and A1(N03)3 have been investigated in water-DMSO mixtures by NMR spectroscopy, and in DMF by Al NMR spectroscopy.The encapsulation of [R4N] in [Ga4L6] , H4L = (76), has been investigated by NMR spectroscopy. 

NMR spectroscopy is a most powerful method for the investigation of dynamic processes occurring at the molecular level. In particular, both intramolecular and intermolecular chemical exchange processes in solution may be investigated and for inorganic... 

A metal when immersed in a solution of its cations may take up the reversible potential corresponding with the M exchange process, but whether or not this occurs in practice will depend on the magnitude of its exchange current density in relation to any others that are possible due to other exchange processes in the solution under consideration. In oxygenated solution the and 02 0H equilibria provide possible alterna-... 

Note that the exchange process in benzene solution is significantly slower than in water, which greatly facilitates rate measurements by 170 NMR spectroscopy. 

In considering ionic equilibria, it is convenient to write the exchange process in the form of a chemical equation. For example, a water-softening process designed to remove calcium ions from solution may be written as ... 

In essence, aqueous titration of surface acidity is an ion-exchange process in which hydrated surface protons are replaced by other hydrated cations (e.g., Na+, NIV") during the course of the titration. The procedure is straightforward. It usually consists of the direct titration of an aqueous suspension of the sample of powdered solid with a dilute base (e.g., sodium hydroxide) to a neutral endpoint. Another commonly used procedure consists of noting the pH of an appropriate salt solution (e.g., ammonium acetate), adding the sample, and measuring the amount of di-... 

Some basic concepts developed for vapor adsorption can also be applied to the adsorption from solution. However, there are also differences. In solution, adsorption is always an exchange process. It is an exchange process in two ways First, a molecule adsorbing to a surface has to replace solvent molecules. Second, the adsorbing molecule gives up part of its solvent environment. This has several practical consequences ... 

Unfortunately, the acetone complex did not give clear answers about the validity of a dark, thermal H/D exchange process. Acetone solutions of the complex did not catalyze H2/D2 or D2/p-H exchange. The complex decomposes in the presence of water and in solutions other than acetone. 

K. Bunzl, Kinetics of ion exchange in soil organic matter. III. Differential ion exchange reactions of Pb2+ ions in humic acid and peat, J. Soil Sci. 25 517 (1974). For a general discussion, see K. Bunzl, Kinetics of differential ion-exchange processes in a finite solution volume, J. Chromatogr. 102 169 (1974). 

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