Isotopic labels carbon

It may be concluded that for reactions where the proton is less or more than one-half transferred in the transition state, i.e. the A—H and H—B force constants are unequal, the primary hydrogen-deuterium kinetic isotope effect will be less than the maximum of seven. The maximum isotope effect will be observed only when the proton is exactly half-way between A and B in the transition state. This relationship is also found for carbon kinetic isotope effects where the isotopically labelled carbon is transferred between two atoms in the reaction10,11. This makes interpreting carbon isotope effects difficult. 

Exercise 1.6. Analyze the following sequence from the tricarboxylic acid cycle ( C denotes isotopically labeled carbon) ... 

The Fick s law diffusion coefficient of a permeating molecule is a measure of the frequency with which the molecule moves and the size of each movement. Therefore, the magnitude of the diffusion coefficient is governed by the restraining forces of the medium on the diffusing species. Isotopically labeled carbon in a diamond lattice has a very small diffusion coefficient. The carbon atoms of diamond move infrequently, and each movement is very small—only 1 to 2 A. On the other hand, isotopically labeled carbon dioxide in a gas has an extremely large diffusion coefficient. The gas molecules are in constant motion and each jump is of the order of 1000 A or more. Table 2.1 lists some representative values of diffusion coefficients in different media. 

The polymer of 13C-labeled 2-phenyl-1-methylenecyclopropane by the same catalyst contains the isotope-labeled carbon exclusively at the exo-methylene group. The polymer consists of the 1-phenyl-2-exo-methylenepropan-1,3-diyl structure, exclusively. The 13C 1H NMR signals of the ipso carbon of the phenyl group are split into four due to the four triads, rr, rm, mr, and mm, based on comparison of the spectrum with that of poly(styrene-co-CO) with isotactic, syndiotactic, and atactic arrangement of the monomer units [160] (Scheme 16). Thus, the polymer produced from 2-phenyl-1-... 

Notable exceptions to this generalization, apart from the lack of activity towards derivatives of the phosphonic acids, are isopropyl- and / r/-butyl-phosphonic acids and ben-zylphosphonic acid (2-methylpropyl)phosphonic acid is cleaved to give very low yields of 2-methylpropene and 2-methylpropane Isotopically labelled (carbon or hydrogen) methylphosphonic acid affords methane possessing the identical distribution of the... 

Coccolithophores, in common with other phytoplankton, experience only an ephemeral existence. Typical life spans of phytoplankton in nature are measured in days. Comparison of the rate at which CaCOs is being produced in open ocean waters (as measured by the rate of uptake of isotopically labeled carbon), to the amounts present (the standing-stock ), has led to the calculation that the average turnover (replacement) time for CaC03 averages about 3 days, ranging between a minimum of <1 and a maximum of 7 days at different locations in the Atlantic Ocean. This implies that if a surface-dwelling coccolithophore synthesizes coccoliths on a Monday, the coccoliths are fairly unlikely to still be there on the Friday, either because they have redissolved or else because they have sunk down to deeper waters. 

Isotopically labeled carbons are scrambled to some extent during solvolysis of )8-phenylethyl tosylates. A bridged-ion intermediate or rapid rearrangement of a primary carbonium ion could account for the rearrangement. The extent of rearrangement increases as solvent nucleophilicity decreases. This increase is attributed... 

Both primary and secondary carbocations with )8-phenyl substituents usually give evidence of aryl participation. For example, isotopically labeled carbons are scrambled to some extent during solvolysis of j8-phenylethyl tosylates, A bridged-ion intermediate or rapidly reversible rearrangement of a primary carbocation could account for the randomization of the label. The extent of label scrambling increases as solvent nucleophilicity decreases. The data are shown in Table 5.19. This trend can be attributed to competition between Sn2 displacement by solvent and ionization with participation of the aryl group. While substitution in more nucleophilic solvents such as ethanol proceeds almost exclusively by direct displacement, the non-nucleophilic solvent trifluoroacetic acid leads to complete randomization of the label. 

Photolysis of a number of chromium carbene complexes enriched in 3c at the carbene atom and the carbonyl groups results in products containing two adjacent isotopiCally labelled carbon atoms. Products include 1,2-i>is-enriched amino acid esters (particularly useful as biochemical and biophysical probes), a dipeptide, a cyclobutenone, a P-lactam, and a dioxocyclam.372... 

The production of methanol at industrial scale takes place in the presence of H2, CO, and CO2 mixtures at pressures close to 50 bar in the 550 to 640 K temperature range. The catalyst of choice is copper dispersed on the surface of zinc oxide [20-22]. Recent studies on CO2 hydrogenation have received much attention in an attempt to rekindle interest in the role of CO2 in industrial conditions of methanol synthesis. This issue is closely related to the fact that kinetic experiments, using isotope-labeled carbon oxides [22] and spectroscopic experiments [23], have demonstrated that under industrial conditions methanol is produced by the hydrogenation of CO2, CO merely providing a source of CO2 and acting as... 

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