Lifetime of ionic intermediates

Lifetimes of the ionic intermediates of nucleophilic substitution are generally correlated to the pathways followed under given reaction conditions. Information on the lifetimes of ionic intermediates formed by bromine addition to olefins in methanol, as determined by the azide clock method, do not allow the different reaction pathways to be distin-... 

The regio-, stereo- and chemoselectivities have been mainly interpreted in terms of bridging of the ionic intermediate and/or ion pair dissociation. Solvent-separated ion pairs and free ions have often been considered to explain the product selectivities of these reactions. Nevertheless, the stereochemical outcomes can also be determined by the relative rates of the ion pair dissociation and of the nucleophilic trapping of the intermediate, i.e. by the lifetime of the intermediate . 

In the case of PMMA dissolved in acetone, the change of LSI could be correlated both with the separation of fragments and, to some extent, with the lifetime of the intermediates which contribute to the main-chain scission [71]. The LSI decreased in two modes, probably due to the two pathways for the main-chain scission. The fast mode with a lifetime of about 20 ps was influenced neither in its extent nor in its rate by the addition of 02 or mercaptane. Therefore the first mode was ascribed to the diffusional separation of fragments which are generated by the main-chain scission through the direct decomposition of electronically excited or ionic intermediates. The slow mode with a lifetime of 6 ms was suppressed, to an extent, depending on the ( -concentration it was attributed to long-lived polymer radicals. The added 02 reacts with lateral polymer radicals to prevent their decomposition. 

The energy dependence of the vibrational excitation cross section depends on the lifetime of the intermediate ionic states (the so-called resonances). First let us consider the so-called short-lifetime resonanees (eg., H2, N2O, H2O, etc), where the lifetime of the autoionization states AB (bi) is much shorter than the period of oscillation (t 10 s). The... 

In molecular solvents, the nature of the ionic intermediate is not dependent on the properties of the solvent, whereas the lifetimes of the ionic intermediates are. In ionic liquids the nature of the intermediate is probably also not affected by the medium, whereas it does affect the lifetime of the intermediate or the rotation around the C-C bond, favoring the nucleophilic trapping of the first formed bromocarbenium ion. 

We turn, now, to the second question. A necessary condition for the application of quasiequilibrium theory is that the lifetime of the intermediate should be long enough for the internal energy to be equilibrated. There will, of course, always be a distribution of lifetimes and no method exists to measure this distribution.f Three means exist to set qualitative lower bounds on the lifetimes randomization of isotopic labels, energy equilibration (as revealed by the translational energy of the ionic products), and the angular distribution of the products. In the absence of any definite information on the time scales for these processes, we present possible order-of-magnitude values for the purpose of this discussion. The first process can be extremely rapid and requires only a few bond vibrations, i.e., 5 X 10 sec. The second process requires more vibrations, but,... 

Both these parameters reflect a moderate stability for BrN3 in methanol. This feature made it impossible to apply the azide clock method to determine the lifetime of the ionic intermediates arising from deactivated alkenes (for example, / ,p -bis(trifluoromethyl)stilbenes). Nevertheless, for a couple of p-methoxy substituted stilbenes and unsubstituted stilbenes the selectivity ratios reported in Table II have been found. (IT)... 

The determination of the lifetime of the ionic intermediates using the azide-clock method has been however useful in showing that electrophilic addition of Br2 can occur, even through a fully concerted mechanism, definable as SN2-like. Bromination of cyclic enol ethers (glycals) 8-10 in methanol in the presence of... 

Thus although ionic intermediates may be formed, their lifetime is too short for them to be chemically significant. Lea12 had previously suggested that the electron could escape from the field of the parent ion. Calculations by Platzman13 indicated that the electron might well travel a distance of 50 A before thermalisation. 

Crystal structures of transient intermediates are accessible to study, provided that substantial concentrations of the desired intermediates can be attained, that their lifetimes are long with respect to the minimum x-ray exposure necessary to obtain a quantifiable diffraction pattern, and that crystallographic isomorphism is maintained throughout the progress of the reaction. Concentrations and lifetimes may be manipulated by adjustment of the chemical nature of the reactants, the pH, ionic strength and nature of the solvent, the presence or absence of activators and inhibitors, by pressure, and of course by temperature. Different intermediates may accumulate to high concentrations, with extended lifetimes, under different experimental conditions but it may be true that some key intermediates may not accumulate under any conditions. 

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