Kinetic isotope effects internal

Secondary kinetic isotope effects are observed if an isotopic label is located adjacent to or remote from the bond that is being broken or formed during the reaction. Again, these depend on the internal energy of the decomposing ions. Secondary kinetic isotope effects, 4ec, are generally much smaller than their primary analogues. 

Example The ratio [M-CH3]V[M-CD3] from isopropylbenzene molecular ions decomposing by benzylic cleavage (Chap. 6.4) varied from 1.02 for ion source fragmentations (70eVEI) over 1.28 for metastable ions in the FFR to 1.56 in the 2 FFR, thus clearly demonstrating the dependence of the secondary kinetic isotope effect on internal energy. [77]... 

The mechanism of cyclopropenations of alkynes with ethyl diazoacetate, catalysed by (AcO)4Rh2 and (DPTI)3Rh2(OAc), has been studied by a combination of kinetic isotope effects and theoretical calculations. With each catalyst, a significant normal 13C KIE was observed for the terminal acetylenic carbon, while a very small 13C KIE was detected at the internal acetylenic carbon. These isotope effects are consistent with the canonical variational transition structures for cyclopropenations with intact tetrabridged rhodium carbenoids but not with a 2 + 2-cycloaddition on a tribridged rhodium carbenoid structure.99... 

With thermal systems either in the gas phase or in solution, it is in ter -molecular isotope effects which are more commonly studied. Intramolecular isotope effects involve distinguishing and measuring two, or more, chemically identical but isotopically different products produced in the same reaction vessel from the same reactant. The situation is different in mass spectrometry. Intramolecular isotope effects are conveniently studied, because the chemically identical products are naturally separated according to their masses. Intermolecular isotope effects on ion abundances are also easily measured, but, as regards kinetics and mechanism of reaction, their value is limited. Whereas an intramolecular isotope effect (on ion abundances) reflects kinetic isotope effects, an intermolecular isotope effect (on ion abundances) reflects kinetic isotope effects, isotope effects on the internal energy distribution, P(E), and other factors as well and the effects cannot be easily separated (vide infra). 

Taken as a whole, the literature on isotope effects in mass spectrometry exhibits two salient features. The values of isotope effects for different molecules and different experimental conditions vary greatly and the isotope effects for some decompositions are very large (>100). These isotope effects are based on ion abundances, as has already been emphasised, but the kinetic isotope effects if measured would show not dissimilar variety and magnitudes. Both features arise because the range of internal energies encountered in reactive ions is very wide and the isotope effects are dependent upon internal energy, usually increasing... 

With FIK and metastable ions, the observation window, A t = t, — t2, is narrow (Sects. 3.2 and 3.3) and the range of internal energies, E, contributing significantly to the measured ion current may, therefore, be narrow. There is, therefore, some possibility that within the narrow energy range the kinetic isotope effect ki(E)/kn(E) will be constant,... 

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. 

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). 

More recently, the internal energy distribution in neutral intermediates was studied using kinetic isotope effects on neutral dissociations [27-30]. It was shown that the internal energy distribution of the ground-electronic state of a neutral intermediate formed by endothermic electron transfer can be expressed by the function shown in Eq. (5) ... 

In Fig. 6.14(b) the two-barrier or stepwise transfer Arrhenius diagrams are plotted, where it was assumed that the secondary isotope effects of dissociation and neutralization are small, i.e. equal to 1. In addition, absence of isotopic fractionation is assumed, i.e. (j> = 1. In this case, k /k is equal to the kinetic isotope effects P(j = Pj, of the dissociation and neutralization steps. When these isotope effects are large, which is the case at low temperatures, kUD DD jg equal to 2. The statistical factor arises from the fact that in the DD reaction D is transferred in both steps. Therefore, when the intermediate is reached, return to the reactant as well as reaction to the product occurs with equal probability. By contrast, there is no internal return in the HD reaction which exhibits only a single rate-limiting... 

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