SIKIE

One of the first chemical reactions for which SIKIE was identified was the association reaction of Oj and O to produce ozone. The initial key observation was that stratospheric ozone was enriched in 0 at levels up to 40% above natural abundance. Following laboratory experiments, which showed that the enhancement occurred independently of isotopic mass (i.e., " O3 and were equivalently... 

Although SIKIE have been observed in additional systems (e.g., the formation of SjFjq from SFj+SFj and COj from CO+O and the photodissociation of Oj)," ... 

A symmetry correlation scheme that is capable of addressing SIKIE must combine the geometry-independent treatment of all the molecular degrees of freedom characteristic of Quack s approach with the connection of reactant and transition-state syrrrmetries characteristic of a Woodward-Hoffrnann-type analysis. To make the problem somewhat less abstract, consider the cartoon in Figure 1... 

Determination of the symmetry properties of the asymptotic wavefunction, however, is not sufficient to treat SIKIE, because the critical question is whether the symmetry of a particular affects the propensity to connect with a particular... 

The experimentally determined EF(7,8) and EF(7,6) as a function of ion source inlet pressure are presented in Figures 3 a and 3b respectively. On a qualitative level, both plots show a similar dramatic increase in EF at low pressure, indicating the occurrence of significant SIKIE in the formation of Hej. The apparent loss of EF at increased pressure is best understood not as a loss of SIKIE but rather as an increase in the importance of reactions such as,... 

From a more general point of view, the trajectory calculations on the Hj/Hj system suggest that SIKIE predictions for ion- nolecule reactions based solely on differing synunetry correlations for even and odd I may not be readily realized under... 

In the limit of high rotation, it is possible to associate these A-doublet components with states where the half-filled orbital is either in, or perpendicular to, the plane of rotation. If the ions are thermally equilibrated, the e and/parity label states would each represent 50% of the population, implying a possible SIKIE on the order of a factor of two. However, there is no fundamental reason why El caimot have a propensity for producing a particular parity label state that could lead to SIKIE considerably larger (or smaller) than a factor of two. 

A summary of the symmetry analysis for the various isotopomers is presented in Table 5 where, in keeping with the conclusions of the general analysis, only ground vibrational states of the reactants are considered. Inspection of Table 5 indicates that isotopic substitution that preserves the CO2 centrosymmetry lifts the restriction based on I while preserving the restriction based on the e parity label state. Because C substitution will always preserve molecular centrosymmetry, the symmetry analysis predicts that ( 02)2 clusters containing a C isotope could show at most a formation-rate enhancement of a factor of two above that of (002)2- Also, because this symmetry restriction is independent of the detailed nature of the quantum states of the COj ions, the C SIKIE is predicted to be independent of the way in which the ion is prepared (i.e., E. Conversely, Table 5 indicates that when the COj centrosymmetry is removed, there are no symmetry restrictions to cluster formation. The extent to which the formation of (002)2 containing a ion will be enhanced above that of ( 62)2 depends on the e/f parity label state distribution of the CO2 ions, which, as was demonstrated in the O2/O2 study,can depend on E. ... 

Two important conclusions can be drawn from the simunary of the symmetry analysis of Ar/CO collisions in Table 6. First, no SIKIE is predicted for C substitution because the symmetry of the system is independent of the isotope of carbon involved. Second, because the predicted a based symmetry restrictions for Ar COj cluster formation are identical to those predicted for (002)2, dependence of the magnitude of observed 0 SIKIE on the conditions of CO2 formation is expected. However, the e/f parity label state propensities for El-produced COJ, inferred from 0 SIKIE in (COj) formation, are not sufficient to predict the magnitude of 0 SIKIE in Ar-COj formation because, for above the threshold for Ar formation, COj ions are also produced by the charge-transfer reaction,... 

Although SIKIE may well occur in neutral chemistry (e.g., O3 formation), gas phase ion chemistry has shown itself to be a valuable arena for exploring the phenomenon and evaluating emerging theories. For example, one theory of non-mass-dependent KIE indicated that isotopic fractionation cannot ensue directly from symmetry alone. However, such a conclusion would appear to be incorrect, because that is exactly what is happening in the several cases discussed. The error in that analysis arises in the statistical thermodynamic treatment of the reversible association reaction ... 

In the development of various equilibrium expressions, it was assumed implicitly that every internal energy state of the reactants (A + B) would have an equal probability of interacting on the particular Bom-Oppenheimer PES relevant to the chemical process of interest. The work described here clearly demonstrates that the validity of such an assumption can depend directly on symmetry considerations. Indeed, it is the propensity of reactants in motional states of a particular symmetry to preferentially interact on a Bom-Oppenheimer PES of a complementary symmetry when identical nuclei are present that is at the heart of the present theoretical description of SIKIE. 

Much of the success in the use of ion-molecule reactions to study SIKIE stems from the relative ease with which the theoretically identified requirement of there being identical reactants in different electronic states, or at least one of the reactants being in a degenerate electronic state, can be experimentally realized. Thus, it is natural to consider possible future directions in which ion-molecule studies might extend the current understanding of SIKIE. Presently, all known systems exhibiting... 

Ibid., p. 928. There is a slight inconsistency when elsewhere (iNd., p. 305) he refers to the (non-market) struggle between peasants sikI manorial lords as a diss conflict. 

For the underi) ing medunism, see Qusttrone end Tversky, "Self-deception sikI the voters illusion". 

---