Solvent components, structural changes

Once thermal reactions begin, the coal undergoes structural changes through spontaneous bimolecular reactions between coal constituents or solvent species (75) or through catalytic reactions accelerated by added catalysts or the inherent mineral components (76). Hence, the reactivity changes with the progress of these reactions. 

It is necessary to emphasize that Fig. 6 represents a model for only one particular peridinin conformation. As discussed above, a distribution of peridinin conformers exists in polar solvents. A slight structural change in the excited state is also responsible for the rise component observed in polar solvents, but this dynamic cannot be included in the model depicted in Fig. 6 as it includes a change of the S /ICT potential surface. In protic solvents the dynamics of peridinin are further complicated as hydrogen bonding leads to the formation of a red peridinin form with different properties of both ground and excited states. 

Earlier work at Mobil and other laboratories (4, 5) had identified several processes responsible for solvent degradation (e.g. hydrogen transfer, cracking, isomerization, alkylation and condensation). In general, each of these processes converts a molecule which is useful in the liquefaction process to one which is less useful. Each of these processes can lead to both changes in structure and the physical properties of the solvent components. 

Structural Changes of Solvent Components. Of the several possible reactions which solvent components can undergo we have examined three ... 

Alkylation of solvent represents still another pathway for changing the properties of a recycle solvent. If we consider alkylation in terms of the transfer to methyl groups from coal to solvent components, then there are several structural and physical changes that occur to the solvent. Alkylation will increase the hydrogen content of the solvent at the expense of coal since the solvent molecule will have a C-H replaced by C-CH. This represents an increase in the aliphatic content and conversely a decrease in the aromatic content of the solvent. Kleinpeter (7) has indicated that alkylation of condensed aromatics is a problem. High aliphatic character will decrease the ability of the solvent to act as a physical solvent for coal liquefaction products. 

Actually, V represents the partial specihc volume of the solute, that is, the increase in the volume of the solution caused by the addition of I g of solute. If there are no significant structural changes in the solute as a result of interaction with the solvent, v can be taken as the sum of the fractional specific volumes of the subunits or components of the solute as described in Problem 2-9. 

The same coenzyme binding pattern and no structural changes in the protein component were detectable for the mutant enzymes of transketolase from Saccha-romyces cerevisiae and their complexes with coenzyme analogs studied by X-ray crystallography (Konig et ah, 1994 Wikner et ah, 1994). Summarizing, it can be ruled out that the differences in the H/D exchange rate constants of transketolase from Saccharomyces cerevisiae are a result of a different solvent accessibility of a base involved in the proton abstraction mechanism of ThDP. 

These are older techniques and are no longer much used. A solute alters the viscosity of the solvent. The ion and its solvent molecules exert a viscous drag on the rest of the solvent and the change in viscosity can be used to calculate a total hydration number which is then split up into its individual components. However, viscosity measurements are normally used as an aid to developing a structural interpretation of solvation (see Section 13.8), and to give correction factors in conductance equations (see Section 12.10). 

The extractive chemical disintegration process can be called direct coal liquefaction. Here, solvents rich in hydroaromatic components are especially suited in extracting nearly all of the reactive coal macerals. These types of solvents actively participate chemically in bond breakage and stabilization, are consumed or structurally changed, and are normally used at temperatures considerably in excess of 300°C (570°F). On the other hand, because of the heterogeneous nature of coal it is a distinct possibility there may be/could be no clear operational or mechanistic distinction between extractive disintegration and extractive chemical disintegration processes. 

In contrast with reversible redox switching of precycled films, the initial redox cycle of Prussian Blue yielded more complex mass responses with much greater irreversible solvation changes. This is dramatically illustrated by the data of Fig. 17 [108], which shows the i-E and AM-E responses to the first redox cycle for each of two nominally identical films, one exposed to H2O and the other to D2O. Although the difference in solvent molar masses is quite small (11%), solvent transfer is such a large component of this initial redox-driven structural change that the overall responses are very different. Somewhat reminiscent of the a-//3-Ni(OH)2 case, this underscores the fact that when the observed response is a sum of individual transfer components (commonly in opposing directions, for example, in response to a volume constraint), a relatively small variation in one component... 

We can now make a general statement about the conditions required for a useful TSM. If the curve of Xtv v) is such that one can define two values vi and V2, which are well separated in such a way that x and X2 have comparable magnitudes, then it is expected that a major contribution to the energy of solvation will come from structural changes between the two components this contribution may involve energies of different orders of magnitude compared with the interaction of the solute with the solvent. The shifts in the locations of vi and V2 due to the addition of s can be collected in one frozen-in term so that (3.5.46) is finally written as... 

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