Ionization, isotope effect

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

The basicities of some phosphinamides (84) have been measured and the acid-catalysed hydrolysis studied. Unsubstituted and A -alkyl derivatives follow an A2 mechanism of reversible protonation followed by ratedetermining water attack. However, the rates for the A -aryl derivatives follow Hq (but with a slope of 0.5), and an A mechanism was suggested as most consistent with this fact and the solvent isotope effect. The anomalous dependence on Ho, together with the large negative value of A5, while not necessarily excluding an ionization mechanism, leaves the question in some doubt. 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

The use of carbon-14 is bringing to light many unexpected topological complexities. It is a more subtle label, in spite of the existence of small isotope effects, than a substituent alkyl group. Any considerable substitution may alter the mechanism of a reaction from that of the unsubstituted compound, while carbon-14 is unlikely to do this. An example of newly discovered topological complexity is the ionization of... 

It is worth noting that Murr and Donnelly (1970a,b) have demonstrated that the secondary a-deuterium KIE is only approximately 75% of the theoretical maximum kinetic isotope effect when the ionization (ki) step of the reaction (Scheme 1) is fully rate determining, i.e. when the reaction occurs via a limiting SN1 mechanism (Shiner, 1970b Westaway, 1987c). 

Subsequent work confirmed this apparently abnormal behaviour. Deuteriation at remote sites (the S- or e-position) induces small inverse secondary isotope effects in a-cleavages occurring in the ion source, but normal isotope effects in the decomposition of metastable ions in the field-free regions94,95. The time dependence of the isotope effect was also studied by field ionization kinetics, which permit the analysis of fragmentations occurring after lifetimes as short as 10 12 s-1. It was found that the inverse isotope effect favouring loss of the deuteriated radical operates at times shorter than 10 9 s95. 

Rameback H, Berglund M, Kessel R, Wellum R (2002) Modeling isotope fractionation in thermal ionization mass spectrometry filaments having diffusion controlled emission. Int J Mass Spectrom 216 203-208 Roe JE, Anhar AD, Barling J (2003) Nonhiological fractionation of Fe isotopes evidence of an equilibrium isotope effect. Chem Geol 195 69-85... 

Also, a rigorous treatment of isotope effects within the framework of QET reveals that the assumption /muZ/mD = hZZ d represents a simplification. [69] It is only valid for when the species studied populate a small internal energy distribution, e.g., as metastable ions do, whereas wide internal energy distributions, e.g., those of ions fragmenting in the ion source after 70 eV electron ionization, may cause erroneous results. This is because the fc(E) functions of isotopic reactions are not truly parallel, [76] but they fulfill this requirement over a small range of internal energies (Figs. 2.17 and 2.18)... 

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

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