Solubility isotope effect

Table 5.3 Solute and solvent solubility isotope effects for (benzene-water) solutions at 306.2 K obtained from IE s on Henry s Law coefficients, Ki and Kn- [Isotope effects on free energies of transfer, ideal gas to solution in the limit of infinite dilution] (Dutta-Choudhury, M., Miljevic, N.

The largest solubility isotope effects are found for sparingly soluble salts. For example, lead chloride and potassium bichromate are 36% and 33.5% more soluble in H20 than D20 at 298.15 and 278.15 K, respectively. For the more soluble salts, NaCl and KC1, the values are 6.4% and 9.0%. Interestingly LiF and LiCl.aq have inverse effects of 13% and 2%, respectively. Recall that lithium salts are commonly designated as structure makers . Almost all other electrolytes are structure breakers . 

Beck, T. L., Quantum path integral extension of Widom s test particle method for chemical potentials with application to isotope effects on hydrogen solubilities in model solids, J. Chem. Phys. 1992, 96, 7175-7177... 

Because solvent viscosity experiments indicated that the rate-determining step in the PLCBc reaction was likely to be a chemical one, deuterium isotope effects were measured to probe whether proton transfer might be occurring in this step. Toward this end, the kinetic parameters for the PLCBc catalyzed hydrolysis of the soluble substrate C6PC were determined in D20, and a normal primary deuterium isotope effect of 1.9 on kcat/Km was observed for the reaction [34]. A primary isotope effect of magnitude of 1.9 is commonly seen in enzymatic reactions in which proton transfer is rate-limiting, although effects of up to 4.0 have been recorded [107-110]. 

Lectka and co-workers (252) subsequently extended this system to the catalysis of the imino ene reaction. This reaction proceeds in low conversion albeit good selectivity in dichloromethane. The optimal solvent proved to be benzotrifluoride (BTF), possessing solubility properties similar to dichloromethane while accelerating the rate of the ene reaction presumably due to its aromaticity. A variety of 1,1-disubstituted alkenes participated in the ene reaction, providing amino acid derivatives in high yields and selectivities (85-99% ee). Evidence for the concerted nature of this reaction was provided by a high primary kinetic isotope effect (ku/kr) = 4.4). 

Abstract Isotope effects on the PVT properties of non-ideal gases and isotope effects on condensed phase physical properties such as vapor pressure, molar volume, heats of vaporization or solution, solubility, etc., are treated in some thermodynamic detail. Both pure component and mixture properties are considered. Numerous examples of condensed phase isotope effects are employed to illustrate theoretical and practical points of interest. 

At temperatures well below UCST, solubilities of hydrocarbons in water or water in hydrocarbons drop to very low values. The solutions are very nearly ideal in the Henry s law sense, and the isotope effects on solubility can be directly interpreted as the isotope effect on the standard state partial molar free energy of transfer from the Raoult s law standard state to the Henry s law standard state. Good examples include the aqueous solutions of benzene, cyclohexane, toluene,... 

There is a good deal of data which compares the solubilities of H2 and D2 in various solvents, but by far the most extensive information on gas solubility IE s compares gas solubilities in H20 and D20 (Table 5.11). The solubility of D2 is considerably higher than H2 in all solvents investigated (Table 5.12). The isotope effect increases sharply as temperature falls (although not shown in Table 5.12). The isotope effect increases sharply as temperature falls (although not shown in Ta-... 

Table 5.11 H2O/D2O solvent isotope effects on gas solubility. Thermodynamics of transfer at 298.15 K, AY = Y (H2O) — Y (D20) (Scharlin, P. and Battino, R.. /. Solut. Chem. 21, 67 (1992). Fluid Phase Equlib. 94, 137 (1994))...

Table 5.13 Solvent isotope effects on solubility of selected electrolytes in H20 Salt T/K Lh AL, %...

Concentrations of aqueous electrolyte solutions are conventionally expressed using the aquamolality scale (L = moles salt per 55.508 mol solvent (l,000g for H20)). Some typical solubilities (298.15K) are listed in Table 5.13. Almost all salts are less soluble in D20 than in H20. For those salts whose solubility increases with temperature, which is the ordinary behaviour, the isotope effects decrease with temperature. Writing the standard state partial molar free energy of pure solid salt as Pxsalt) and its standard state in solution as p, (HorD) we have on comparing the saturated solutions in H20 and D20,... 

Van Hook, W. A. and Rebelo L. P. N. Isotope effects on solubility, in T. Letcher, T., ed., Developments and Applications in Solubility, Royal Society of Chemistry, Cambridge (2007)... 

Pressure-composition-temperature and thermodynamic relationships of of the titanium-molybdenum-hydrogen (deuterium) system are reported. 0-TiMo exhibits Sieverts Law behavior only in the very dilute region, with deviations toward decreased solubility thereafter. Data indicate that the presence of Mo in the 0-Ti lattice inhibits hydrogen solubility. This trend may stem from two factors for Mo contents >50 atom %, an electronic factor dominates whereas at lower Mo contents, behavior is controlled by the decrease in lattice parameter with increasing Mo content. Evidence suggests that Mo atoms block adjacent interstitial sites for hydrogen occupation. Thermodynamic data for deuterium absorption indicate that for temperatures below 297°C an inverse isotope effect is exhibited, in that the deuteride is more stable than the hydride. There is evidence for similar behavior in the tritide. 

Methane is slightly soluble in HF-SbF5 even at atmospheric pressure (0.005 M), which facilitates direct kinetic studies by NMR. Thus the transition states for methane activation in this medium have been studied experimentally by Ahlberg et al.49 The first-order rate constants [Eqs. (5.9) and (5.10)], determined experimentally on the basis of2H -decoupled 600-MHz 1H NMR time-dependent spectra (Figure 5.2), are on the order of 3.2 x 10 1 s 1 at —20°C and show a secondary kinetic isotope effect (SKIE) of 1 0 02. 

In supercritical HjO it was found that the toluene-soluble products (TS) profile leveled off at about 50%. However in supercritical D2O, liquefaction was consistently superior to that in the protio medium, and the ultimate convertibility was about 60%. This inverse isotope effect can be explained by a model in which the limits to conversion are solely based on the competition between the kinetics of the parallel routes to TS and char, respectively. Conversion is thus limited by the kinetics of the conversion system, this limitation being even more severe in conventional donor systems which are inherently less effective for liquefaction than is water/CO. In principle the conversion of coal virtually completely to TS in a single step is feasible in a system with sufficient reducing potential. 

Stahl, S.S., Fransicsco, W. A., Merkx, M., Klinman, J.P., and Lippard, S.J. (2001) Oxygen kinetic isotope effects in soluble methane monooxygenase, J. Biol. Chem. 276,4549-4553. 

Wilkins, P. C., Dalton, H., Samuel, C. J., and Green, J., 1994, Further evidence for multiple pathways in soluble methane-monooxygenase-catalysed oxidations from the measurement of deuterium kinetic isotope effects, Eur. J. Biochem. 226 5559560. 

In other studies, precipitation of the dihydrate would cranplicate matters and particularly, would give a maximum rate as a function of N which varies according to the solubility of that species in different media, as oteerved. We wish to note here that the interpretation of initiation prcq>osed by Kucera and cdUaborators, based tm direct initiation and solvation by water molecules is untenable. In fact, Dainton et showed irrevocably that the rde of water is cocatdytic (chemical), by proving that when DjO replaces H2O deuterium is found in the polymer and the rate of initiation is lower (kinetic isotope effect). 

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