Isotopic small labeling

In the presence of a small amount of (CF3C0)20, even the economical CaCl2 catalyzes the acetic acid synthesis by K2S2O8 (eq. (34)) (58). The yield based on the methane exceeds 94%. Isotope ( C) labeled experiments clearly showed that the methyl group and the carbonyl group of acetic acid come from methane and CO, respectively. The C—H bond is presumably cleaved by radical mechanism. 

Behind the column, mica solid-state detectors were positioned. They were kept at lower temperatures in order to adsorb the RfCl4 molecules. It was assumed that the produced isotopes of Rf decay at least partly by spontaneous fission. Mica is known to be well suited for identification of latent fission tracks. In a series of experiments that accumulated a total beam dose of 4 x 10 beam particles 65 fission tracks were detected along the mica detectors. These fission events were assigned to a spontaneously fissioning isotope of Rf, presumably Rf. Later, this assignment was questioned since additional measurements proved that this isotope has a half-life of only 20 ms, too short for a chemical study. It was therefore concluded that Rf with a half-life of 3 s and an assumed small fission branch was the isotope that labeled the separated molecule. 

Fig. 12. Isotope distribution pattern resulting from the incorporation of small labelled...

Decay products of the principal radionuclides used in tracer technology (see Table 1) are not themselves radioactive. Therefore, the primary decomposition events of isotopes in molecules labeled with only one radionuclide / molecule result in unlabeled impurities at a rate proportional to the half-life of the isotope. Eor and H, impurities arising from the decay process are in relatively small amounts. Eor the shorter half-life isotopes the relative amounts of these impurities caused by primary decomposition are larger, but usually not problematic because they are not radioactive and do not interfere with the application of the tracer compounds. Eor multilabeled tritiated compounds the rate of accumulation of labeled impurities owing to tritium decay can be significant. This increases with the number of radioactive atoms per molecule. 

A small preference for the involvement of the N-1 nitrogen is observed in the loss of HCN from the molecule of N-labelled indazole (80OMS533). In addition, there is a small isotope effect (1.08). 

When specifically labelled compounds are required, direct chemical synthesis may be necessary. The standard techniques of preparative chemistry are used, suitably modified for small-scale work with radioactive materials. The starting material is tritium gas which can be obtained at greater than 98% isotopic abundance. Tritiated water can be made either by catalytic oxidation over palladium or by reduction of a metal oxide ... 

The symmetrical intermediate 5 can be attacked by the NH3 at either of two positions, which explains why about one-half of the aniline produced from the radioactive chlorobenzene was labeled at the 2 position. The fact that the 1 and 2 positions were not labeled equally is the result of a small isotope effect. Other evidence for this mechanism is the following ... 

In most cases, more 1,4- than 1,2-addition product is obtained. This may be a consequence of thermodynamic control of products, as against kinetic. In most cases, under the reaction conditions, 15 is converted to a mixture of 15 and 16, which is richer in 16. That is, either isomer gives the same mixture of both, which contains more 16. It was found that at low temperatures, butadiene and HCl gave only 20-25% 1,4 adduct, while at high temperatures, where attainment of equilibrium is more likely, the mixture contained 75% 1,4 product. 1,2 Addition predominated over 1,4 in the reaction between DCl and 1,3-pentadiene, where the intermediate was the symmetrical (except for the D label) HjCHC—CH—CHCH2D. Ion pairs were invoked to explain this result, since a free ion would be expected to be attacked by Cl equally well at both positions, except for the very small isotope effect. 

Dichlorodibenzo-p-dioxin was prepared from isotopic potassium 2,4-dichlorophenate uniformly labeled with Ullman conditions gave a 20.5% yield. Small amounts of dichlorophenoxy chlorophenol were removed from the product by extraction with sodium hydroxide before purification by fractional sublimation and recrystallization from anisole. Chlorination of 2,7-dichlorodibenzo-p-dioxin in chloroform solution containing trace amounts of FeCls and 12 yielded a mixture of tri-, tetra-, and pentachloro substitution products. Purification by digestion in boiling chloroform, fractional sublimation, and recrystallization from anisole was effective in refining this product to 92% 2,3,7,8-tetrachloro isomer, which also contained 7% of the tri- and 1% of the penta-substituted dibenzo-p-dioxin. Mass spectroscopy was used exclusively to monitor the quality of the products during the synthesis. 

A small solvent isotope effect was found by Bell and Onwood kuiolkoiO = 1.08) in contradiction to that of only 0.38 reported by Taylor and Halpern . Over one-third of the oxygen present in the carbonate originated from the oxidant when 0-labelled permanganate was used . The reaction is subject to pronounced catalysis by ferric ions . 

Generally, it is most likely that metal NPs are stabilized by the aggregates of the non-functionalized imidazolium ILs rather than by the isolated ions. In addition, the interaction between ILs and the metal NPs have been evidenced by X-ray photoelectron spectroscopy (XPS), small-angle X-ray scattering (SAXS), isotope labeling, and surface-enhanced Raman spectroscopy (SERS) techniques. 

Small angle neutron scattering (SANS) of "marked" (isotopically labeled) chains in unmarked networks has proven to be of potential value in... 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

By hydrolysis under very mild alkaline conditions (with a boiling suspension of barium carbonate), ribonucleic acids have been shown to yield small quantities of cyclic phosphates as well as the normal nucleotides.96 These materials were identical electrophoretically with synthetic cyclic phosphates and were readily hydrolyzed to mixtures of 2- and 3-phosphates. Their formation in this way constitutes strong support for Brown and Todd s theory. The precise way in which the alkaline hydrolysis of the polynucleotide occurs has been studied using isotopically labeled water, and the results are in agreement202 with the scheme outlined above. 

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