Electronic Properties of Dilute Solutions

The overall changes in magnetic and transport properties of metal-ammonia solutions from the dilute to concentrated regimes are shown in Fig. 4. 

As with any electrolyte, various aggregate species are expected to form as the concentration of solute increases. In particular, both the electrical conductivity and (metal) NMR data (Fig. 4) signal the appearance of neutral species at metal concentrations in excess of 103 MPM (37), the conductivity via a Morse-like behavior in the equivalent conductance, the magnetic resonance via a finite Knight (contact) shift 

Another important property, the apparent molar volume of the solvated electron, is essentially unaffected by the electron-cation and, indeed, the electron-electron interaction (37). 

In summary then, the appellation (53,84, 85) loose ion-pair for the solvated electron-metal cation association complex in dilute metal-ammonia solutions is therefore a particularly apt description. 

In the weak-interaction model (85) developed in the previous section to explain ion-pairing in metal-ammonia solutions, aggregation interactions involving Ms+ and es are relatively weak, and leave the isolated solvated electron properties virtually intact. However, a major difficulty (29,54,134) arises with the type of model when one considers the precise nature of the corresponding electron spin-pairing interaction in ammonia solutions. It is worth expanding on this issue because it probably remains one of the fundamental dilemmas of metal-ammonia solutions in the dilute range (54). 

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In this study we use electron microscopy (EM) to study xanthan strandedness and topology both in the ordered and disordered conformation. Correlation of data obtained from electron micrographs to physical properties of dilute aqueous solution on the same sample will be used to provide a working hypothesis of the solution configuration of xanthan. Electron micrographs obtained from xanthan of different origins will be compared to assess similarities and differences in secondary structure at the level of resolution in the used EM technique. 

The unique properties of dilute metal-ammonia solutions depend not upon the nature of the metal species, but upon the solvated electron common to all these solutions. Thus, the electron-in-a-cavity model (17, 19, 21) seems best suited to describe the species present in these solutions since the model is independent of the type of cation present. Jortner and his associates (15, 16) have extended this model by assuming that the cavity arises from polarization of the medium by the electron. The energy levels of the bound electrons are obtained by using a potential function containing the static and optical dielectric constants of the bulk medium as parameters. Using one-parameter hydrogen-like wave functions for the first two bound states of the electron, the total energy of the ith state is expressed as... 

The proceedings of a conference on metal-ammonia solutions have been published, featuring reviews of the physical properties of dilute and concentrated solutions, electrical, n.m.r., i.r., and Raman spectroscopic studies of diffusion, the solvated electron, kinetics, and solution structure."" Electron spin resonance in metallic Li-NHa systems has been investigated from 12 to 296 K. In the liquid solutions and in the cubic phase of Li(NH3)4 the conduction e.s.r. lineshapes are in agreement with theory. To a good approximation the solvated ions are the only spin scatterers in the liquid state. The paramagnetic susceptibility of liquid Li(NH3)4 indicates that the concentration of localized moments is low and they order antiferromagnetically below 20 K." ... 

The study of a dilute solution of a polymer allows one to find the structure of an isolated chain. Therefore, it provides the unique opportunity to relate the conformational disorder, curvature of the chain and twist between monomer units, to the electronic properties of the conjugated backbone. From that viewpoint, several theoretical works paid special attention to the effects of twist fluctuations, emphasizing their influence on the n electron delocalization [25—29]. Besides, as some of these solutions exhibit spectacular colour changes as a function of the temperature or solvent quality, they were considered as the relevant systems to characterise the relationship between the n electron delocalization and the local structure of the chain. 

Introduction of meta substituents breaks conjugation efficiently, so that meta PPEs can appear colorless with Xmax of 288—388 nm (Table 12, entries 2—4 Table 11. entries 14. 16, 26, and 28), depending strongly upon the nature of their substituent. An alternating polymer with para-linked 2,5-dialkoxy-benzene and m-phenylene units shows a Xmax of 37 5 nm in solution (Table 11, entry 13). Most of the absorption data obtained for PAEs in dilute solution can be extrapolated from the knowledge of the electronic properties of its constituents. Until now no... 

Since then, the generality and importance of solvent effects on chemical reactivity and physical properties of species in dilute solutions has been widely acknowledged. Solvent-solute interactions for reactants and for products account for observed shifts in chemical equilibria those involving reactants and transition states determine changes in the rates of elementary processes. Shifts of the absorption and/or fluorescence maxima originate in differential solvent-solute interactions of the ground and electronically excited states of a dissolved species. The perturbations induced by the solvents are reflected by concurrent variations of such physical properties of the solute as ir, nmr, and epr spectra and partial molar properties. 

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