Charge-Site Effects on the Radical Reactivity of Distonic Ions ² Christopher J. Petzold, Eric D. Nelson, Harvey A. Lardin, and Hilkka I. Kentta 1 maa* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393 ReceiVed: January 15, 2002; In Final Form: July 29, 2002 The perturbation on the intrinsic reactivity of a radical moiety by the presence of a nearby charged moiety was probed by comparing the reactivity of analogous positively and negatively charged distonic ions. The possible contributions of various factors (collisional encounter probability, ion-molecule solvation effects, reaction exothermicity, polar effects) to the overall perturbation are discussed. The N-(3-dehydrophenyl)- pyridinium and 3-dehydrobenzoate ions were chosen as distonic ion models of the phenyl radical for this study. The significant differences in their reaction rates are examined and the origins of these rate differences are explored. Observations that nucleophilic radicals react more rapidly with electron-deficient reagents and electrophilic radicals react more rapidly with electron-rich reagents can be made slightly more quantitative by comparing certain thermochemical values of each distonic ion (radical moiety IE and EA) with those of each neutral reagent. The smaller the relevant IE-EA energy difference, the lower the transition state energy and the faster the reaction. The overwhelming control of the transition state energy by the charge site of the distonic phenyl radical analogues described here emerges as an important caveat to their use as models for phenyl radical reactivity. Introduction The classification of “distonic ions” is often applied to a special subset of radical ions that possess spatial separation between charged and radical moieties. 1 A radical ion can be considered a distonic ion if it can be formed conceptually by the ionization of a biradical, zwitterion, or ylide. 2 Intuition suggests that, like their neutral forms (i.e., biradicals, zwitterions, and ylides), distonic ions should be high-energy species. Yet, distonic ions can be surprisingly stablesoften lying lower in energy than their more conventional isomers (i.e., ions with the same connectivity as stable neutral molecules). 2 In many cases, this causes distonic ions to be formed by the spontaneous isomerization of such “conventional” radical ions. 3 Distonic ions that possess a rigid bonding framework between their ionic and free radical moieties can often react fairly independently at each moiety. The reactions of these distonic ions can thus be thought of as a combination of the reactivities expected for the analogous ions and free radicals. For this reason, certain distonic ions have been used as models of radical and biradical species that can be studied via powerful mass spectrometric techniques. 4 These techniques often allow studies that would be difficult for the neutral analogues 4a and provide useful information about the reactivity of such species. This work explores the validity of the assumption that carefully selected distonic ions can serve as models for the chemical reactivity of radicals and biradicals. The perturbation on the intrinsic reactivity of the radical moiety by the presence of the nearby charged moiety was probed by comparing the reactivity of analogous positively and negatively charged distonic ions. Two distonic ion models of the phenyl radical were chosen for this studysthe N-(3-dehydrophenyl)pyridinium (1) and 3-dehydrobenzoate (2) ions (Scheme 1). Despite the immediately obvious differences in the identity and charge of the ionic moieties of 1 and 2, these ions also have significant similarities. Both ions have the same radical moiety, and have their charged moiety located at the meta- position with respect to the radical sites. 5 This geometric configuration results in charged and radical moieties being held rigidly apart such that moderately sized neutral reagents will be unable to interact simultaneously with both charged and radical moieties. Reactions involving the participation of both charged and radical moieties will thus not be observed. 6 Further, both the N-phenylpyridinium and benzoate ions (the ionic frameworks that these two distonic ions are built on) are stable ions of low reactivity. Thus, the reactions of both distonic ions should arise exclusively from the phenyl radical moieties without direct participation of the charge sites. The distonic ions 1 and 2 possess similar relative orientations of charged and radical moieties. Since the charged groups that perturb the intrinsic phenyl radical reactivity in these two cases are of opposite polarity, they should exert an opposite perturba- tion on the intrinsic reactivity of the phenyl radical. Thus, comparison of their reactivity serves to define the range of effects caused by the presence of a remote charge site. Both ions 1 and 2 have been generated and their reactivity studied previously. 4c,7 However, these studies were performed under different conditions and without any significant overlap of content. This work represents the first direct comparison of the ² Part of the special issue “Jack Beauchamp Festschrift”. SCHEME 1 9767 J. Phys. Chem. A 2002, 106, 9767-9775 10.1021/jp025521i CCC: $22.00 © 2002 American Chemical Society Published on Web 08/31/2002