Isotope solvent

However, this procedure would be incorrect. It is possible, for the purpose of representing kinetic orders, to write equation (149) as the first step of the scheme, but we may do so only because the ratio [LA]/ [A-] is proportional to the concentration of hydrogen ion. However, this concentration is much smaller than that of LA, and the relevant pre-equilibrium which is disturbed by the isotopic solvent changes is thus the protonation (deuteronation) of SH by LA and not by L30+. 

Equations (169) to (171) are the analogues of (29) to (31) for aqueous solutions and are different from them. These expressions put in more explicit form the suggestion (Gold, 1960) that the study of acid-base phenomena in non-aqueous isotopic solvent mixtures can give information concerning the formula of the dominant form of Bronsted acid present. 

Weatherly, S.C. Yang, I.V. Armistead, P.A. Thorp, H.H. Proton-coupled electron transfer in guanine oxidation Effect of isotope, solvent, and chemical modification. J. Phys. Chem., B 2003, 107, 372-378. 

The differences in the hydration of a solnte in H2O and D2O have been extensively stndied by measnring their thermodynamic properties, the change of free energy (AG°t), enthalpy (A//°t), and entropy (AY°t) at the transfer of 1 mol of solnte from a highly dilute solution in H2O to the same concentration in D2O under reversible conditions (mostly 25 °C and atmospheric pressure). Greyson measured the electromotive force (emf) of electrochemical cells of several alkali halides containing heavy and normal water solutions. The cell potentials had been combined with available heat of solution data to determine the entropy of transfer of the salts between the isotopic solvents. The thermodynamic properties for the transfer from H2O to D2O and the solubilities of alkali halides at 25° in H2O and D2O are shown in Table 4. 

Isotope discrimination studies were employed to deduce if the double proton transfer of proline racemase is concerted or stepwise [88]. Isotope discrimination is an alternative manifestation of the multiple kinetic isotope effect techniques previously discussed, wherein racemization is conducted in mixed isotopic solvents of H2O and D2O and the discrimination in the incorporation of solvent deuterium is measured. If the double proton transfer is stepwise, deuteration of the substrate... 

The first method is based on the statistical distribution of an iso-topically-labelled solvent such that the ratio of the concentration of isotopic solvent in the coordinated solvent to its concentration in the bulk solvent is equal to the ratio of coordinated solvent molecules to bulk solvent molecules. One obvious requirement is that the half-life of solvent exchange must be considerably longer than the time required for isotopic sampling. Furthermore, there must be an efficient means of separating the coordinated solvent from the bulk solvent. This approach was first used by Hunt and Taube [13] to establish the existence of Cr(H2 0)g as a distinct species in aqueous solution. Although only of limited application to metal ions more labile than Cr the method has been employed to determine the solvation number of the hydrated Al ion using a flow... 

The different scattering patterns correspond to various isotopic solvent mixtures where the proteated PS core (h-PS) is matched shell contrast d-PB shell matched core contrast and an intermediate contrast where the h-alkane/d-alkane mixture corresponds to a scattering length density in between that of the core and shell. Note that, in the latter case, the scattering at low angles Q 0) almost disappears because the scattering contributions from core and shell are almost compensated by the interference term since and Va Fb can be rationalized from... 

The introduction of deuterium in place of proton in water, and its consequent exchange into some positions of enzymes and substrates, produces solvent isotope effect on the kinetic and equilihrium constants associated with the enzymatic reactions. These effects, usually expressed as ratios of the appropriate constants in two isotopic solvents HOH and DOD, are useful in the study of reaction mechanisms (Candour Schowen, 1978 Cook, 1991 Quinn Sutton, 1991). 

Solvent isotope effects, since they refer to rate or equilibrium constants, have to do with the effect of the isotopic solvent on both initial (reactant) stales and final (transition or product) stales. We shall make an allempi lo dissert Ihese effecls and lo deal with the effects on the individual states (Quinn Sutton, 1991). 

It has been demonstrated that halogenation of sulfides proceeds via the Pummerer rearrangement. Isotope, solvent, and reagent effects have been interpreted in terms of the HSAB concept (57). 

Contrast variation methods can sometimes be used to remove a component of the scattering by matching its scattering power with that of the medium in which it is dispersed. This principle can be used in SANS experiments via isotopic solvent mixtures (e.g. H2O/D2O) to adjust the scattering power of the medium, as for example in studies of polymer latexes. Grancio and Williams [35] postulated a polymer-rich spherical core surrounded by a monomer-rich shell which serves as the major locus of polymerization, thus giving rise to core-shell morphology. Thus, the first formed... 

A schematic representation relating ligand binding in isotopic solvents can be created as ... 

Rumpel, H. and Limbach, H.H. (1989) NMR study of kinetic HH/HD/DD isotope, solvent, solid-state effects on double proton transfer in azophenine. J. Am. Chem. Soc., Ill, 5429 — 5441. 

In 1965, the Dubna workers found a longer-lived lawrencium isotope, 256Lr, with a half-life of 35 s. In 1968, Thiorso and associates at Berkeley used a few atoms of this isotope to study the oxidation behavior of lawrencium. Using solvent extraction techniques and working very rapidly, they extracted lawrencium ions from a buffered aqueous solution into an organic solvent — completing each extraction in about 30 s. 

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02" ] > / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the... 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

The plutonium usually contains isotopes of higher mass number (Fig. 1). A variety of industrial-scale processes have been devised for the recovery and purification of plutonium. These can be divided, in general, into the categories of precipitation, solvent extraction, and ion exchange. 

Benzoates. The selective debenzoylation of sucrose octabenzoate [2425-84-5] using isopropylamine in the absence of solvents caused deacylation in the furanose ring to give 2,3,4,6,1/3/6 -hepta- and 2,3,4,6,1/6 -hexa-O-benzoyl-sucroses in 24.1 and 25.4% after 21 and 80 hours, respectively (54). The unambiguous assignment of partially benzoylated sucrose derivatives was accompHshed by specific isotopic labeling techniques (54). Identification of any benzoylated sucrose derivative can thus be achieved by comparison of its C-nmr carbonyl carbon resonances with those of the assigned octabenzoate derivative after benzoylation with 10 atom % benzoyl—carbonyl chloride in pyridine. 

Cesium isotopes can be recovered from fission products by digestion in nitric acid, and after filtration of waste the radioactive cesium phosphotungstate is precipitated using phosphotungstic acid. This technique can be used to prepare radioactive cesium metal or compounds. Various processes for removal of Cs isotopes from radioactive waste have been developed including solvent extraction using macrocycHc polyethers (62) or crown ethers (63) and coprecipitation with sodium tetraphenylboron (64). 

Cya.nideExcha.nge, Acetone cyanohydrin and methyl isobutyl ketone cyanohydrin [4131 -68-4] dissolved in an organic solvent, such as diethyl ether or methyl isobutyl ketone, undergo cyanide exchange with aqueous cyanide ion to yield a significant cyanide carbon isotope separation. The two-phase system yields cyanohydrin enriched in carbon-13 and aqueous cyanide depleted in carbon-13. Fquilibrium is obtained in seconds. 

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