Polar effects on iodine atom abstraction by charged phenyl radicals

The ability of differently substituted charged phenyl radicals (a class of distonic radical cations) to abstract an iodine atom from allyl iodide was systematically examined in the gas phase by using Fourier transform ion cyc lotron resonance mass spectrometry. The reaction products and second-order reaction rate constants were determined for several radicals that differ by the type and/or number of substituents located in the ortho- and/or meta-p osition with respect to the radical site. All the radicals also carry a par a-pyridinium group needed for mass spectrometric manipulation. These electr on-deficient phenyl radicals react with allyl iodide by predominant iodine atom abstraction. The reaction is facilitated by the presence of neutral el ectron-withdrawing substituents, such as F, CF3, Cl, or CN. The extent of r ate increase depends on the type and number of the substituents, as well as their location relative to the radical site. Based on molecular orbital ca lculations (PM3 and Becke3LYP/6-31G(d)+ZPVE), the indicated variations in t he transition state energy are not related to enthalpic factors. Instead, t he results are rationalized by polar effects arising from a variable contri bution of a stabilizing charge transfer resonance structure to the transiti on state. A semiquantitative measure for the barrier-lowering effect of eac h substituent is provided by its influence on the electron affinity of the radical (the electron affinities were calculated by Becke3LYP/ 6-31-G(d) an d AM1, which were found to produce similar values). Methyl substitution doe s not significantly affect the electron affinity, and accordingly, does not have a detectable effect on reactivity. Methyl groups located at ortho-pos itions are an exception, however. o-Methyl-substituted phenyl radicals unde rgo exothermic rearrangement to a benzyl radical in competition with iodine abstraction from allyl iodide.