Thermodynamic stability, water

By protodetritiation of the thiazolium salt (152) and of 2 tritiothiamine (153) Kemp and O Brien (432) measured a kinetic isotope effect, of 2.7 for (152). They evaluated the rate of protonation of the corresponding yiides and found that the enzyme-mediated reaction of thiamine with pyruvate is at least 10 times faster than the maximum rate possible with 152. The scale of this rate ratio establishes the presence within the enzyme of a higher concentration of thiamine ylide than can be realized in water. Thus a major role of the enzyme might be to change the relative thermodynamic stabilities of thiamine and its ylide (432). 

Fig. 4.14 Poiemial/pH diagram for (he Pb-H20 system. The area between and corresponds to the thermodynamic stability of water. Light lines represent equilibrium conditions between a solid phase and an ion al activities 1, 10 , 10" and 10 . Heavy lines represent equilibrium conditions between two solid phases. Broken lines represent equilibrium conditions between two ions for a ratio of these ions equal to unity (after Delahay, Pourbaix and van...

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

Matsui and Mochida24) have determined the thermodynamic stabilities (log 1 /Kd) for a- and P-cyclodextrin complexes with a variety of alcohols (Table 2) and analyzed the results in connection with the physicochemical properties of the guest molecules by the multivariate technique. The log 1/Kd values were plotted against log Pe, where Pe is the partition coefficient of alcohol in a diethyl ether-water system. The plots for the a- and P-cyclodextrin complexes with eight 1-alkanols gave approximately straight lines with slopes of around one. 

The thermodynamic stability of a species is a measure of the extent to which this species will be formed from other species under certain conditions, provided that the system is allowed to reach equilibrium. Consider a metal ion M in solution together with a monodentate ligand L, then the system may be described by the following stepwise equilibria, in which, for convenience, coordinated water molecules are not shown ... 

As thermodynamic stability indexes for the hydrocarbon ions, pA R+ and pA a values [(4) and (5)] have been widely applied for the carbocation and carbanion, respectively, in solution. Here K + stands for the equilibrium constant for the reaction (6) of a carbocation and a water molecule stands for the equilibrium constant for the reaction (7) of a hydrocarbon with a water molecule to give the conjugate carbanion. The equilibrium constants are given by (8) and (9) for dilute aqueous solutions. Obviously, the reference system for the pKn+ scale is the corresponding alcohol, and... 

Two lines on the Pourbaix diagrams (Figure 5.2) deserve reference. These are the sloping dashed lines, and they represent the stability limits for water they are essentially the domain of thermodynamic stability of water superimposed on potential-pH diagrams. At the upper line, oxidation of water goes according to ... 

Another elegant example of the thermal generation and subsequent intramolecular cycloaddition of an o-QM can be found in Snider s biomimetic synthesis of the tetracyclic core of bisabosquals.2 Treatment of the starting material with acid causes the MOM ethers to cleave from the phenol core (Fig. 4.3). Under thermal conditions, a proton transfer ensues from one of the phenols to its neighboring benzylic alcohol residue. Upon expulsion of water, an o-QM forms. The E or Z geometry of the o-QM intermediate and its propensity toward interception by formaldehyde, water, or itself, again prove inconsequential as the outcome is decided by the relative thermodynamic stabilities among accessible products. 

In extreme cases where all electron transfer steps are reversible and the water trapping reactions are very slow, the charge is distributed over the guanines according to the thermodynamic stabilization. From these experiments one cannot deduce the influence of the sequence on the hole transfer rate. Therefore, using a chemical assay of this type leads to results that have to be discussed with great care. 

Roux, B. Nina, M. Pomes, R. Smith, J., Thermodynamic stability of water molecules in the Bacteriorhodopsin proton channel a molecular dynamics and free energy perturbation study, Biophys. J. 1996, 71, 670-681... 

Knowledge of the 90 chemical elements and their properties in compounds led to the construction, by man, of a unique table of elements, the Periodic Table, of 18 Groups in six periods in a pattern fully explained by quantum theory, described in Chapter 2. There is then a huge variety of chemical combinations possible on the Earth and limitations on what is observable are related to element position in this Table. It also relates to the thermodynamic and/or kinetic stability of particular combinations of them in given physical circumstances (Table 11.3). The initial state of the surface of the Earth with which we are concerned was a dynamic water layer, the sea, covering a crust mainly of oxides and some sulfides and with an atmosphere of NH3, HCN, N2, C02(C0, CH4), H20, with some H2 but no 02. This combination of phases and their contents then produced an aqueous solution layer of particular components in which there were many concentration restrictions between it and the components of the other two layers due to thermodynamic stability, equilibria, or kinetic stability of the chemicals trapped in the phases. It is the case that equilibrium... 

The fact that SFs does not react with water is not due to thermodynamic stability. Rather, it is because there is no low-energy pathway for the reaction to take place (kinetic stability). Six fluorine atoms surrounding the sulfur atom effectively prevent attack, and the sulfur atom has no unshared pairs of electrons where other molecules might attack. In SF4, not only is there sufficient space for an attacking species to gain access to the sulfur atom, but also the unshared pair is a reactive site. As a result of these structural differences, SF6 is relatively inert, whereas SF4 is very reactive. 

On the other hand, optically active telluroxides have not been isolated until recently, although it has been surmised that they are key intermediates in asymmetric synthesis.3,4 In 1997, optically active telluroxides 3, stabilized by bulky substituents toward racemization, were isolated for the first time by liquid chromatography on optically active columns.13,14 The stereochemistry was determined by comparing their chiroptical properties with those of chiral selenoxides with known absolute configurations. The stability of the chiral telluroxides toward racemization was found to be lower than that of the corresponding selenoxides, and the racemization mechanism that involved formation of the achiral hydrate by reaction of water was also clarified. Telluroxides 4 and 5, which were thermodynamically stabilized by nitrogen-tellurium interactions, were also optically resolved and their absolute configurations and stability were studied (Scheme 2).12,14... 

Two of the water clusters that appear to exhibit particular thermodynamic stability under ambient conditions of the liquid phase are shown in Fig. 5.29 a cyclic pentamer (Wsc) and a cyclic hexamer (W6C). The characteristic factors (F1)-(F3) that distinguish these clusters from other possible isomeric forms can be identified as follows ... 

Interactive Version 2.15.0 (Parkhurst Appelo 1999). The llnl.dat database distributed with PHREEQC was used for all calculations. Geochemical modeling was used to speciate the components of the tailings pore-water at discrete depths and to investigate the thermodynamic stability of mineral phases through out the tailings. 

But time is money. The waste industry, therefore, breaks the colloid artificially to remove the particulate solid from the water. They employ one of two methods. Firstly, they add to the water an inorganic polymer such as silicate. The colloid s thermodynamic stability depends on the surface of its particles, each of which has a slight excess charge. As like charges repel (in consequence of Coulomb s law ... 

As a measure of their thermodynamic stability, the pAfR+ values for the carbocation salts were determined spectrophotometrically in a buffer solution prepared in aqueous solution of acetonitrile. The KR+ scale is defined by the equilibrium constant for the reaction of a carbocation with water molecule (/CR+ = [R0H][H30+]/[R+]). Therefore, the larger p/CR+ index indicates higher stability for the carbocation. However, the neutralization of these cations was not completely reversible. This is attributable to instability of the neutralized products. The instability of the neutralized products should arise from production of unstable polyolefinic substructure by attack of the base at the aromatic core. 

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