Hydration model, four-state

Figure 12. Four state model of the hydration-mediated counterion—side chain association—dissociation equilibrium, of Mauritz. (Reprinted with permission from ref 107. Copyright 1982 American Chemical Society.)...

It is useful to divide the methods of investigating hydration into four groups. First of all, there are the many methods in which one looks at the ionic solution in the equilibrium state for example, one can study its compressibility or one can study heats and entropies of hydration and fit the values to various model systems. 

Molecular Model of Counterion Dissociation Equilibrium. The following molecular concept is supported, or suggested, by both these spectroscopic observations and past ultrasonic investigations of simple aqueous electrolytes. In particular, a four-state model reminiscent of the multistep ionic dissociation mechanism of Eigen et al., (22, 23) was adopted (24). With regard to Figure 3, tFie states are classified as 1) completely dissociated hydrated ion pairs, 2) ion pairs at the contact of undisturbed primary hydration shells, and 3) outer and 4) inner sphere complexes. The relative populations of these states, (P ... 

Figure 5. Four-state model of hydration-mediated equilibrium between unbound and sidechain-associated counterions in ionomeric membranes (9,10, 13).

We present experimental results on photophysical deactivation pathways of uracil and thymine bases in the gas phase and in solvent/solute complexes. After photoexcitation to the S2 state, a bare molecule is tunneled into and trapped in a dark state with a lifetime of tens to hundreds of nanoseconds. The nature of this dark state is most likely a low lying nn state. Solvent molecules affect the decay pathways by increasing IC from the S2 to the dark state and then further to the ground state, or directly from S2 to S0. The lifetimes of the S2 state and the dark state are both decreased with the addition of only one or two water molecules. When more than four water molecules are attached, the photophysics of these hydrated clusters rapidly approaches that in the condensed phase. This model is now confirmed from other gas phase and liquid phase experiments, as well as from theoretical calculations. This result offers a new interpretation on the origin of the photostability of nucleic acid bases. Although we believe photochemical stability is a major natural selective force, the reason that the nucleic acid bases have been chosen is not because of their intrinsic stability. Rather, it is the stability of the overall system, with a significant contribution from the environment, that has allowed the carriers of the genetic code to survive, accumulate, and eventually evolve into life s complicated form. 

While the thermodynamic evidence may favor the mixture models, the diffraction studies from the static crystalline state tend to support the continuum model. The water molecules in the ices and high hydrates are always four-coordinated. 

In the present study, we synthesized dibromo(l,4,8,ll-tetraazacyclotetradecane)copper(II) ([CuBr2(cyclam)]) and diaqua(l,4,8,ll-tetraazacyclotetradecane)copper(II) difluoride four hydrate ([Cu(cyclam)-(H20)2]F2 4H20) complexes and performed single crystal structure analysis and X-ray absorption near-edge structure (XANES) measurements in crystals and in aqueous solution. Furthermore, DV-Xa molecular orbital calculations have been made for models based on these results, and the structures and electronic states of the [Cu(cyclam)] complexes in crystals and in aqueous solution are discussed, in particular, on the axial coordination to Cu(II). 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

The semiconductor and the electrolyte phases are conveniently characterized through macroscopic relations. A microscopic model is required for the interface between the bulk phases. This model can be arbitrarily complex but is restricted by the requirement that thermodynamic relationships among the bulk phases hold. A convenient model for the interfacial region is represented in Fig. 2. The interface is represented by four planes, inner and outer Helmholtz planes on the electrolyte side of the interface and inner and outer surface states on the semiconductor side. The outer Helmholtz plane (OHP) is the plane of closest approach for (hydrated) ions associated with the bulk solution. The inner Helmholtz plane (IHP) passes through the center of ions specifically adsorbed on the semiconductor surface. The outer surface state (OSS) represents the plane of closest approach for electrons (and holes) associated with the bulk of the semiconductor. The inner surface state (ISS) is a plane of surface sites for adsorbed electrons. If surface sites are neglected, the ISS and the OSS are coincident. 

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