Counter ions, solvated

Water hydration and counter-ion solvation of biological macro molecules considerably alter the translational and rotational dynamics ofbiological macromolecules. The effect can be correlated with a number of different solution properties of a given biological macromolecule such as the hydrated volume Vh- The term Vh is given by... 

However, the energy difference between N- and S-bonded thiocyanate is very small and is influenced by an interplay of several factors steric effects, solvent and the counter-ion in ionic complexes. To illustrate the last point, in complexes [Pd[Et2N(CH2)2NH(CH2)2NH2]NCS]+, the PFg salt is N-bonded, as it is in the unsolvated BPhg salt. However, though the acetone solvate of the tetraphenylborate is N-bonded, the methanol solvate is S-bonded [126],... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

With radical ions, the dimerization equilibrium is strongly influenced by the solvation and association of radical ions with counter ions. It has been shown that the free ions dimerize much more slowly than do the respective contact ion pairs e.g., the quinoline radical anion does not dimerize in the powerfully solvating hexamethylphosphoramide, but it does dimerize rapidly in tetra-hydrofuran (160). Thus, two equilibria should be distinguished (160), viz. 

The former, which occurs in tetrahydrofuran, favors dimerization, while the latter, which takes place in hexamethylphoshoramide, is shifted far to the left. In spite of the complicating effects of solvation and association with counter ions, it appears that within a reaction series of conjugated radical ions, the following relation holds... 

Hence, the solvation energy change accompanying the disproportionation can compensate the repulsion term. In weakly solvating media, association with counter ions occurs and the disproportionation equilibrium should be considered, for example, as follows ... 

As described above, the electrolyte usually contains anions and cations, which are partially or fully solvated, water molecules and various species being involved in electrocatalytic reactions. The excess charge on the electrode surface is compensated by an accumulation of corresponding electrolyte counter-ions, leading to overall charge neutrality. 

Compared to the structures of Li+-water solvates, the structures of Li+-acetonitrile solvates are in general less studied. The Li+ ion was found to be four coordinate with the use of different techniques, e.g., by NMR where acetonitrile was gradually replaced by water in a 1.6 M solution of LiC104 (130), or based on IR intensities measured for the acetonitrile C-N stretching vibration (131,132). Even mixed coordination of a counter ion and acetonitrile were reported to be four coordinate, viz., in [Li(CH3CN)3Br] for 0.58 M LiBr in CH3CN (133). Extensive... 

The complex of protein, crystallographic water and the counter ions are treated as a fully solvated system. Two key developments were made to treat the system in a more realistic manner during molecular dynamics (i) water molecules were placed as a spherical shell with a radius 34 A from the center of the protein. The outer boundary of the spherical shell was defined by means of an artificial wall with a potential of the type W defined as ... 

Fig. 19 The structure of 35 2Zn(II) OH (CF3SO3 )3 CH3OH as determined by X-ray diffraction. For clarity, the triflate counter ions and methanol solvate are not shown.

The second example concerns the lithium ion, either considered in a cluster of water molecules or in aqueous solution. The idealized solution at infinite dilution of a lithium ion (without counter-ion) predicts six molecules of water in the first solvation shell if one uses pair-wise 2-body interactions, but the same type of computation predicts four molecules of water when 3-body effects are included. The computations were performed at room temperature. We have performed cluster computations for the Li fTO), system, with n = 1,2,3,4,5 and 6, using a density functional program developed in our laboratory. When we compute the most stable configuration for the pentamer complex Li+( starting from the most stable config-... 

Nuclear magnetic resonance spectroscopy is often used to quantify the ratio of API and counter-ion in a pharmaceutical salt, together with the type and quantity of hydrate or solvate molecules. 

Variations in the absolute concentration of the carbocation solutions and temperature had minor effects on chemical shifts. The counter ion effect and the equilibrium could be minimized by going to higher superacidity systems with lower nucleophilicity counter ions. Resonances due to the PAH itself were considerably shielded. Solvation by FSO3H and the formation of ion pair-molecule clusters were suggested as possible reasons. 

The N,N -diphenylguanidinium (dpg+) has been foimd to adopt different conformations both in aqueous solutions [9] and in several salts that are being reviewed in this paper. The conformation of dpg+ is very sensitive to the counter-ion, and this effect has been the subject of ab-initio quantum mechanical and molecular mechanics calculations [10]. Stabilization of a particular conformation depends critically on intermolecular interactions with the solvent, since the energetic cost of rotation of the phenyl rings is much lower than typical solvation energies. 

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