Solute-solvent complex stability

The stability of solute-solvent complexes is governed by the capacity of the solvent to form the specific hydrogen bonds. A comparative study of the chemical shifts of N in adenosine and ATP in dimethylsulfoxide-de and water was performed to gain insight into the HB effects. The N-1 resonance of ATP in water is cu 12 ppm more shielded than the N-1 of adenosine in dimethylsulfoxide-dg. This clearly indicates significant interaction between the N-1 atom and the water molecule. For the protonated molecules, the chemical shift of N-1 for ATP in water is almost equivalent to that for adenosine in DMSO-de-... 

Three emission bands (a, p and y in the order of decreasing energy) are observed in CH2CI2 solution and are found to be the emission from the excited state of 1, from the excited state of a solute-solvent complex and from a relaxed twisted excited state of the solute-solvent complex, respectively. Model compound studies show that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complexation process between 1 and the OH groups in PVF is also shown to occur. A model for the stabilization of particles of 1 in polymer solution is put forward where we propose that the stabilization mechanism is a steric effect achieved by adsorption of PVF macromolecules onto particles of 1 via the formation of the PVF 1 complex. 

Here we report preliminary results on the multiple fluorescence emission of 1 and 2. From structure-property relationships, solvent effect and temperature effect studies, we are able to show that the multiple emission is from the emission of free squaraine in solution, the emission of the solute-solvent complex and the emission of a twisted relaxed excited state. Further solvent effect study using 2 as a model shows that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complex-ation process is also detected between 1 and the hydroxy groups on the macromolecular chains of poly(vinyl formal). The Important role of this complexation process on the stabilization mechanism of particles of 1 in polymer solution is discussed. 

Figure 6 shows the fluorescence excitation and emission spectra of Sq2-Sq5 in CH2CI2. In each case, the excitation spectrum was found to be identical to the absorption spectrum and is independent of the monitoring wavelength. The spectral results are summarized in Table 3. Although the effect of chain length on Xp may be small, it has a profound effect on the composition of the emission band. For example, for N = CH3, the intensities of the a- and p-bands are about the same (Fig. 1). As the chain length is increased, the intensity of the a-band decreases whereas the opposite is observed for the p-band (Fig. 6). The gradual dominance of the P-emission indicates that the equilibrium constant for the solute-solvent complex increases as the chain length increases. This is actually consistent with the solute-solvent complex model discussed above. Namely as the CT D-A-D state of squaraine is stabilized (by the electron-releasing N-alkyl group), the tendency for complexation increases [6].

Papisov et al. (1974) performed calorimetric and potentiometric experiments to determine the thermodynamic parameters of the complex formation of PMAA and PAA with PEG. They investigated how temperature and the nature of the solvent affected the complex stability. They found that in aqueous media the enthalpy and entropy associated with the formation of the PMAA/PEG complex are positive while in an aqueous mixture of methanol both of the thermodynamic quantities become negative. The exact values are shown in Table II. The viscosities of aqueous solutions containing complexes of PMAA and PEG increase with decreasing temperature as a result of a breakdown of the complexes. 

As has been suggested in the previous section, explanations of solvent effects on the basis of the macroscopic physical properties of the solvent are not very successful. The alternative approach is to make use of the microscopic or chemical properties of the solvent and to consider the detailed interaction of solvent molecules with their own kind and with solute molecules. If a configuration in which one or more solvent molecules interacts with a solute molecule has a particularly low free energy, it is feasible to describe at least that part of the solute-solvent interaction as the formation of a molecular complex and to speak of an equilibrium between solvated and non-solvated molecules. Such a stabilization of a particular solute by solvation will shift any equilibrium involving that solute. For example, in the case of formation of carbonium ions from triphenylcarbinol, the equilibrium is shifted in favor of the carbonium ion by an acidic solvent that reacts with hydroxide ion and with water. The carbonium ion concentration in sulfuric acid is greater than it is in methanol-... 

To conduct meaningful mechanistic and kinetic studies in alcohol media reliable and simple measurement and control of the solution jjpH is essential. Potentiometric titration is the method of choice for obtaining acid dissociation constants or metal ion complex stability constants and in favorable cases the speciation of mixtures of metal-ion-containing complexes in solution can be proposed.20 Titrations in non-aqueous solvents are not nearly as widely reported as those in aqueous media, particularly in cases with metal ions21 and determination of pH in a non-aqueous solvent referenced to that solvent is complicated due to the lack of a way to relate the electrode EMF readings to absolute jjpH (see footnote and ref. 6) so non-aqueous solvents are generally inconvenient to use22 for detailed studies of reaction mechanisms where pH control is required. 

The effect of the solvent properties on the polarographic behavior of Cd(II) complexes with glycine. At-acetyl, and N-benzoylglycine was studied in DM SO, acetonitrile (AN), and DMF solutions [90]. The stability constants were found to depend linearly on the acceptor numbers of the solvents. 

The study of ILs in GLC has yielded important information regarding solute-solvent interactions providing valuable insights into their complex solvation interactions and thermodynamic properties for mixed solvent systems. Moreover, ILs have proven to be an important new class of stationary phases for the separation of a wide variety of different analytes. IL stationary phases will soon be commercially available which will inevitably promote further improvements in separation selectivity, thermal stability, immobilization bonding chemistry/stationary phase stability, and will broaden the range of separated compounds. IL-based stationary phases also hold great promise in GC mass spectrometry where the dual-nature selectivity of the stationary phase eliminates the need for frequent changing of columns. 

The first observation of the NMR spectrum of Na- in a metal solution without added cation-complexing agents has recently been reported (65a). The rationale behind this observation was that the solvent HMPA by itself appeared to fulfill many of the requirements generally sought from macrocyclic complexing agents high solubility via cation complexation, stability to electron reduction, weak anion solvation, etc. The NMR spectrum of Na- in fluid Na-HMPA solutions, shown in Fig. 26, exhibits precisely the same chemical shift as that observed for Na-in solutions of Na in anhydrous methylamine and ethylamine in the presence of 2,2,2-cryptand. The metal anion here is truly "gas-like in... 

The proposed mechanism is in accordance with the observed solvent dependence of the reaction. Whereas the dipolar sulfoxide is expected to be more strongly solvated with an increase in solvent polarity, the less dipolar sulfenate should be relatively insensitive to such a solvent change. Stabilization of the sulfoxide, relative to the less dipolar activated complex (which should be similar to the sulfenate intermediate), increases the enthalpy of activation, AH. This is refiected in the necessity of breaking increasingly strong solute-solvent interactions. On the other hand, because desolvation on activation is expected to increase the degrees of freedom in the system, a more positive AS is expected to work in the opposite direction and effect a compensating increase in k with... 

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