Intermolecular kinetic isotope effects

In the mechanism study of /V-benzyl-/V -alkyl hydroxylamines, regarding oxidation with HgO and p-benzoquinone, it has been proposed on the basis of intra- and intermolecular kinetic isotope effects that, initially, there takes place a one-electron transfer from a nitrogen atom to the oxidant, with a subsequent proton abstraction (106—108). 

Schneider, F.W. and Rabinovitch, B. S., The unimolecular isomerization of methyl-d3 isocyanide. Statistical-weight inverse secondary intermolecular kinetic isotope effects in nonequilibrium thermal systems. J. Am. Chem. Soc. 85, 2365 (1963). 

The intermolecular kinetic isotope effect for the competition of 20 with its deuteriated analogue 22 in chloroform was negligible (Scheme 10, h/ d = 1-00 0.02). Like in other trisubstituted alkenes, this result was interpreted in terms of irreversible formation of a perepoxide intermediate. 

SCHEME 44. Intermolecular kinetic isotope effect in the intrazeolite photooxygenation of 1-phenyl-3 -methyl- 2-butene... 

RELATIONSHIP BETWEEN INTERMOLECULAR ISOTOPE EFFECTS ON ION ABUNDANCES AND INTERMOLECULAR KINETIC ISOTOPE EFFECTS... 

Equation (27) indicates how the intermolecular isotope effect, /j//n, on ion abundances depends on a number of factors in addition to the intermolecular kinetic isotope effect ki(E)/kn(E). The two most important of these additional factors are the internal energy distributions and the competing reactions. 

Because of these various factors, determination of intermolecular kinetic isotope effects ftj(E)/ku(E) from intermolecular isotope effects Ij/In on ion abundances (or rates) is, in general, not possible. 

Intermolecular isotope effects upon ion abundances are, in general, not reliable guides as to intermolecular kinetic isotope effects. Intermolecular kinetic isotope effects kl(E)/ku(E) could be determined by PIPECO, although few measurements have been reported [210]. It would be necessary to conduct measurements at two different photon energies since, in general, the molecules Mj and Mn would not have the same ionization energies (IEs). In this way, the molecular ions Mj" and M could be formed with the same internal energy (E = hv — IE). 

Suppose that an intermolecular kinetic isotope effect kx(E)/kn(E) has been determined for a particular reaction. This isotope effect is related to the numbers and densities of states of reactants and transition states by the expression [see eqn. (1)]... 

Intermolecular kinetic isotope effects depend strongly upon reactant properties. Consider the extreme case of a primary deuterium isotope effect in a molecule with only one hydrogen replaced by deuterium, e.g. the rate coefficient for (CDCl3)f - (CC13)+ + D compared with that for... 

The important conclusion is that intermolecular kinetic isotope effects are not a sensitive probe of transition state properties in the way that intramolecular kinetic isotope effects are, indeed the intermolecular effects can be effectively independent of transition state characteristics. 

Intermolecular kinetic isotope effects are likely to follow the same qualitative trends as the intramolecular, but as has been emphasised, reliable intermolecular kinetic isotope effects are rarely available in mass spectrometry. 

In this special case, therefore, the intermolecular kinetic isotope effect is obtained exactly. 

Fig. 29. Intra- vs. intermolecular kinetic isotope effects in a rate limiting ETPT mechanism showing KIEintra KIEinter- See text.

These results suggest that the chemoselectivity-determining step does not proceed via a classical electrophilic aromatic substitution. These data together with other mechanistic studies including an intermolecular kinetic isotope effect (KIE) (kjk = 1.07 0.03) as well as the kinetic order of the C—H... 

Intermolecular kinetic isotope effect has been estimated by using both of non-deuterated and per-deuterated hydrocarbons, and intramolecular kinetic isotope effect by using hydrocarbons containing both hydrogen and deuterium in a molecule. Intermolecular kinetic isotope effects for sMMO from M.trichosporium OB3b were hardly observed at nearly saturating concentrations of methane, propene, cyclohexane. 

The intermolecular kinetic isotope effect kn /ko for the loss of HCl or DCl from C2H5CP and C2D5CP respectively lies between 10 and 10 . It has been well accounted for by RRKM calculations including tunnelling (Booze and co-workers). 

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