Isotopic exchange and

This enzyme is of wide occurrence in bacteria where it is concerned with the reduction of nitrate and CO2 as well as sulphur. Methods for its estimation depend on measuring some activity of hydrogenase by (a) dye reduction (benzyl viologen or methylene blue), (b) isotopic exchange and (c) evolution of molecular hydrogen. Interpretation of quantitative results is difficult due to the complex relationship between the enzyme cell structure and the particular method selected. ... 

A. I. Shatenshtein, Isotopic Exchange and the Replacement of Hydrogen in Organic Compounds, Consultants Bureau, New York, 1962, p. 105. 

In this chapter it is clearly impossible to do more than sample the extensive literature on the carbon acidity of sulfinyl and sulfonyl compounds, as it illuminates the electronic effects of these groups, particularly in connection with linear free-energy relationships. There are three main areas to cover first, as already indicated, equilibrium acidities (pKa values) second, the kinetics of ionization, usually studied through hydrogen isotopic exchange and finally, the kinetics of other reactions proceeding via carbanionic intermediates. 

The concept of H2 chemisorbing on Rh as a molecular species may appear at first to be contradictory to other studies e.g.. Isotope exchange and TPD but It Is not, given the existence of klnetlcally distinct sites. An adsorption/desorption process can occur on a specific site via ... 

This discovery was quite unexpected, since iron oxide has been never reported as an active catalyst in either partial or full oxidation. The studies of two simplest reactions, i.e. O2 isotopic exchange and N2O decomposition, revealed a dramatic change of Fe properties in the ZSM-5 matrix compared to Fe203 [4]. Fe atoms lose their ability to activate O2 but gain remarkably in their ability to activate N2O. It gives rise to a great effect of the oxidant nature in the reaction of benzene oxidation over the FeZSM-5 zeolite (Table 1). Thus, in the presence of N2O benzene conversion is 27% at 623 K, while in the presence of O2 it is only 0.3% at 773 K. And what is more, there is a perfect change of the reaction route. Instead of selective phenol formation with... 

Fischer Christian-H, Hart Edwin J, Henglein Amim (1986) Ultrasonic irradiation of water in the presence of 181802 Isotope exchange and isotopic distribution of H202. J Phys Chem 90 1954-1956... 

Roitman and Cram (1971) have shown that the kinetic basicity of potassium alkoxide increases on complexation with dicyclohexyl-18-crown-6 ([20] + [21]). Both the rate of isotopic exchange and the rate of racemization of (—)-4-biphenylylmethoxydeuteriomethane [163] by t-BuOK in t-BuOH were found to increase by factors between 30 and 17 000. Isotopic exchange was... 

Quenched Mixed solutions quenched after a predetermined time controlled by the distance between the mixer and quencher and the flow rate. Tedious but leisurely analysis. Essential for the batch method used in rapid isotopic exchange and low temperature epr monitoring. A 10-20 ms resolution. Large volumes of reactants used ( 5 ml). Commercially available. 

Ripley EM, Li C (2003) Sulfur isotope exchange and metal enrichment in the formation of magmatic Cu-Ni-(PGE)-deposits. Econ Geol 98 635-641 Robert F (2001) The origin of water on Earth. Science 293 1056-1058... 

By using NMR in the presence of the enzyme, an investigator can identify bridged-to-nonbridge (and the reverse) isotope exchanges and thereby identify probable intermediates on the reaction pathway. The procedure is useful for any enzyme-catalyzed reaction in which the individual atoms of a functional group within a substrate, intermediate, or product become torsionally equivalent during the course of a reaction. 

A parameter used as a measure of isotope enrichment in stable isotope exchange and compartmental analysis experiments ... 

Rau, M. Rieck, D. Evans, J.W. (1987) Investigation of iron oxide reduction by TEM. Metallurgical Transactions 188 257-278 Raven, K.P. Jain, A. Loeppert, R.H. (1998) Ar-senite and arsenate adsorption on ferrihy-drite Kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Techn. 32 344-349 Rea, B.A. Davis, J.A. Waychunas, G.A. (1994) Studies of the reactivity of the ferrihydrite surface by iron isotopic exchange and Moss-bauer spectroscopy. Clays Clay Min. 42 23-34... 

Ultraviolet or y irradiation of a number of oxides in the presence of excess oxygen leads to the adsorption of oxygen and the formation of Oj and in some cases OJ ions. A variety of techniques have been used, such as thermal desorption, isotopic exchange, and conductivity measurements, but the principal evidence comes from EPR studies. Both the formation and stability of these species are discussed in the sections dealing with the appropriate oxide, but the overall picture is summarized below. 

Linear equilibrium case For isotopic exchange and finite solution volume, Helfferich (1962) gives the following solution ... 

The various reactions undergone by coordinated amino acids have been the subject of several reviews28,31,32,438,446 and only a brief discussion will be given here. The reactions which occur can be roughly classified under three headings (a) aldol condensations, (b) reactions of complexes of amino acid Schiff bases, and (c) isotopic exchange and racemization at the a-carbon of the amino acid. 

Aromatic enols, that is, phenols, are generally more stable than their ketone tautomers. The pH-rate profile for the enolization reaction of 2,4-cyclohexadienone to parent phenol is shown in Fig. V.44 The rate constant kK of the reverse reaction was determined at pH = 1 by measuring the rate of isotopic exchange and correcting for isotope effects to determine the enolization constant Kb = = 5.4 x 1012, pA E = -12.73. The dashed line in... 

Room Temperature Oxidations, Isotopic Exchanges, and Dehydrogenations over Illuminated Neat or Metal-Supporting Semiconductor Catalysts... 

Shatenshtein, A. I. (1960a). Isotope Exchange and Hydrogen Substitution in Organic Compounds in the Light of the Theory of Acids and Bases ( Izotopnyi obmen i zameshchenie vodoroda v organicheskikh soedineniyakh v svete teorii kislot i osnovanii ), Academy of Sciences U.S.S.R. Press, Moscow. 

Isotope exchange and isotope effects. The vinylic hydrogen is lost and reintroduced from the solvent during the elimination-addition process. Incorporation of deuterium from deuteriated solvents is therefore expected. Depending on the elimination mechanism, hydrogen isotope effects would be also sometimes observed when RCX=CDY is compared with RCX=CHY. 

Mechanisms and rates of electrophilic hydrogen isotope exchange and nitration in heterocyclic series 92H(34)2179. 

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