Isotope effects measuring very small

The organic compounds formed (e.g. thiolanes) may be unstable to pyrolysis and will release the isotopically heavy H20 which will mix with H2S released from the kerogen. The total sulfur isotope effects measure were very small, resulting in a total measured ... 

An Ingenious Method for Measuring Very Small Isotope Effects... 

This all seemed very reasonable at the time, but subsequent work was not consistent with it. A small but measurable amount of 180 exchange was reported for some amides in reasonably concentrated HC1 media,277,278 and for at least one amide the amount of exchange decreased with increasing acidity,277 which is the opposite of what would be expected with the Scheme 14 one-water-molecule mechanism taking over from the equation (74) three-water-molecule mechanism as the acidity increased. Also, the solvent deuterium isotope effect was found to be close to unity for at least one amide,278 a result that has since been confirmed,279 which is not what would be expected on the basis of either a three- or a one-water-molecule process.280 Because of this it was decided to reexamine the lactam hydrolysis data subsequent to the publication of the excess acidity analysis of the H NMR results for these,268 a new study appeared with rate constant data for four of these molecules in aqueous H2S04 media obtained by UV spectroscopy at several temperatures,281 and this was included too.282... 

From his first paper (Mulliken 1925a), Mulliken understood that the band heads did not represent a transition from a non-rotating initial state to non-rotating final state. Yet, he used the band heads to study the vibrational isotope effect since he could measure the band heads more easily and since the rotational energy differences are very small compared to the vibrational energy difference. From the theory, the terms linear in n and n" (ain and bin") arise from the harmonic approximation with the coefficients ai and bi corresponding to the harmonic vibrational frequencies in the... 

Recent experiments have determined that the isotopic fractionation between adsorbed and dissolved Cr(VI) is very small (Ellis et al. submitted). In batch experiments, dissolved Cr(VI) was equilibrated with finely ground alumina or goethite. The results of these experiments should be appficable to natural settings, as oxides chemically similar to alumina and goethite provide the main adsorption sites for Cr(VI) anions. Because the analytical precision of the measurements was 0.2%o, it was necessary to amplify any isotopic shifts so that effects smaller than 0.1 %o could be detected. This was accompfished by repeating the sorption step ten or more times, so that a fractionation between dissolved and adsorbed Cr(VI) of 0.04%o was detectable at the 95% confidence level. 

Equation (16) notes that the difference in measured 5 Fe values for Fe(II)aq and ferrihydrite precipitate is equal to difference in the Fe(III)aq-ferrihydrite and Fe(III)aq-Fe(II)aq fractionation factors, assuming that the proportion of Fe(III)aq is very small (<5%). In cases where the proportion of Fe(III)aq ratio is significant (>5%), the isotopic effects of combined oxidation and precipitation may still be calculated using an incremental approach and Equation (12), along with the pertinent fractionations between components (Eqns. 14 and 17). 

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

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