Isotopic exchange deuterium

Isotopic Exchange Reactions. Exchange reactions between the isotopes of hydrogen are well known and well substantiated. The equihbrium constants for exchange between the various hydrogen molecular species have been documented (18). Kinetics of the radiation-induced exchange reactions of hydrogen, deuterium, and tritium have been critically and authoritatively reviewed (31). The reaction T2 + H2 — 2HT equiUbrates at room temperature even without a catalyst (30). 

There have been numerous studies of the rates of deprotonation of carbonyl compounds. These data are of interest not only because they define the relationship between thermodynamic and kinetic acidity for these compounds, but also because they are necessary for understanding mechanisms of reactions in which enolates are involved as intermediates. Rates of enolate formation can be measured conveniently by following isotopic exchange using either deuterium or tritium ... 

It is also possible to measme the rate of enolization by isotopic exchange. NMR spectroscopy provides a very convenient method for following hydrogen-deuterium exchange, and this is now the preferred method. Data for several ketones are given in... 

Only within the past few years have serious attempts been made to estimate quantitatively the differences in reactivity between thiophene and benzene and between the 2- and 3-position of thiophene. Careful investigation on the acid-induced exchange of deuterium and tritium have shown that the ratios of the exchange rates in the 2- and 3-positions are 1045 61 for deuterium and 911 60 for tritium in 57% by weight aqueous sulfuric acid at 24.6°C. A kinetic isotope effect in the isotopic exchange has been found to be k-r/kr, = 0.51 0.03 in the 2-position and kr/kjy — 0.59 0.04 in the... 

When the reaction was run at 100°C in MeOH H20, isotopic exchange was observed (the product from PhCDO had lost some of its deuterium) Swain, C.G. Powell, A.L. Lynch, T.J. Alpha, S.R. Dunlap, R.P. J. Am. Chem. Soc., 1979, 101, 3584. Side reactions were postulated to account for the loss of deuterium. See, however, Chung, S. J. Chem. Soc., Chem. Commun., 1982, 480. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

The kinetics of deuterium isotope exchange between diphenyl phosphine and t-butylthiol have been studied by H n.m.r. spectroscopy.274 A negative temperature coefficient was observed for the reaction of a perf1uoroalky1 phosphite with a fluorinated aldehyde.275 The kinetics for the reaction of alcohols with phosphoryl trichloride bore strong similarities to those of carboxylic acid derivatives.276 An interesting report desribed the solvolysis of ary 1 hydroxymethyl-phosphonates. It was shown that a phosphoryl group does not prevent carbocation formation on an immediately adjacent carbon atom.277... 

Tetralin. Therefore, either trans-Decalin was formed on the surface of the coal with deuterium from the solvent or gas, or it was formed by isomerization from cis-Decalin with accompanying isotopic exchange with a deuterium source. Because the protium content of the cis-Decalins is much greater than the trans-Decalins, cis-trans isomerization is not very important. 

Many ionic liquids are based on N,N-dialkylimidazolium cations (BMI) which form salts that exist as liquids at, or below, room temperature. Their properties are also influenced by the nature of the anion e. g. BF T PFg. The C-2(H) in imidazole is fairly labile but the C-4(H) and the C-5(H) are less so. Under microwave-enhanced conditions it is therefore possible to introduce three deuterium atoms (Scheme 13.4). As hydrogen isotope exchange is a reversible reaction this means that the three deuterium atoms can be readily exchanged under microwave irradiation. For storage purpose it might be best to back-exchange the C-2(D) so that the 4,5-[2H2] isotopomer can be safely stored as the solid without any dangers of deuterium loss. The recently... 

It is instructive to calculate the anharmonic correction to the zero point energy contribution to fractionation factors for isotope exchange equilibria involving hydrogen and deuterium. For example consider the exchange... 

The value of the fractionation factor for any site will be determined by the shape of the potential well. If it is assumed that the potential well for the hydrogen-bonded proton in (2) is broader, with a lower force constant, than that for the proton in the monocarboxylic acid (Fig. 8), the value of the fractionation factor will be lower for the hydrogen-bonded proton than for the proton in the monocarboxylic acid. It follows that the equilibrium isotope effect on (2) will be less than unity. As a consequence, the isotope-exchange equilibrium will lie towards the left, and the heavier isotope (deuterium in this case) will fractionate into the monocarboxylic acid, where the bond has the larger force constant. 

Recently, Rooney (80) expressed the view that the a,j8 exchange process involved tt olefin complexes and asserts that this explains the pattern of exchange on a palladium film of deuterium with 1,1-dimethylcyclo-butane. Its failure to exhibit appreciable multiple isotopic exchange was attributed to the difficulty of forming a tt olefin complex because of the strain in cyclobutene. 

The thermal decomposition, at least in the early stages, is reversible, as shown by the higher decomposition temperature in the presence of hydrogen than in vacuum or inert gas, and also by the existence of a rapid isotopic exchange between tetrahydroboride and deuterium at T > 625 K (Ziittel et al., 2003). 

The C3—H is labile and can be replaced by deuterium from D2O or from MeOD. [36,55,94,95]. The fact that the C3 proton can undergo facile isotopic exchange in nonacidic conditions is further evidence for the negative charge on C3. Protonation of the spiropyran merocyanine forms appears to occur at C3... 

Two mechanisms have been proposed to account for the deuterium enrichment (1) for organic molecules, high D/H ratios can be explained by ion molecule reactions that occur in interstellar space and (2) for the phyllosilicates the enrichment can be produced via isotope exchange between water and hydrogen (Robert et al. 2000). 

Most hot spring waters have deuterium contents similar to those of local precipitation, but are usually emiched in as a result of isotopic exchange with the country rock at elevated temperatures. The magnitude of the oxygen isotope shift depends on the O-isotope composition of both water and rock, the mineralogy of the rock, temperature, water/rock ratio, and the time of interaction. 

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