Polarity of the transition state controls the reactivity of related charged phenyl radicals toward atom and group donors

Polar effects are demonstrated to be a key factor in controlling the reacti vities of related charged phenyl radicals in different exothermic atom and group abstraction reactions in the gas phase. The effects of various meta s ubstituents on the phenyl radicals' reactivity were probed via the measurem ent of bimolecular reaction rate constants by using Fourier transform ion c yclotron resonance mass spectrometry. This approach requires an additional, charged substituent to be present in the phenyl radical to allow mass spec trometric manipulation. The m-pyridinium group was chosen for this purpose. The substrates studied were allyl iodide, dimethyl disulfide, and tert-but yl isocyanide. Two of the reactions of interest, .I and . SCH3 transfer, ar e thought to occur by concerted bimolecular homolytic substitution (S(H)2), and the third one, . CN transfer, by an addition/elimination mechanism. Fo r all three substrates, the reaction rate was found to increase in the foll owing order for the differently substituted phenyl radicals: CH3 approximat e to H < Br approximate to Cl approximate to COOH < NO2 approximate to CN. This trend does not arise from differences in reaction exothermicities or b ond dissociation energies but via lowering the reaction barrier by electron ic effects. The stabilization of the transition state is attributed to its increased polar character. A semiquantitative measure of the barrier loweri ng effect for each substituent is obtained from its influence on the electr on affinity of the charged radical, as the calculated (B3LYP/6-31+G(d)) adi abatic electron affinities of the radical model systems (ammonium instead o f pyridinium charge site) follow the same trend as the reactivities.