Hypovalent Radicals. 4. Gas-Phase Studies of the Ion-Molecule Reactions of Cyclopentadienylidene Anion Radical in a Flowing Afterglow

Article abstract of DOI:10.1021/ja00541a018

The carbene anion radical, cyclopentadienylidene^-.(c-C5H4^-., 1), was generated by dissociative electron attachment with diazocyclopentadiene (2) in a flowing afterglow apparatus.The ion-molecule reaction of 1 with 2 produced c-C5H4N=N-c-C5H4^-., c-C5H4=c-C5H4^-., and c-C5H5^- by coupling at N_β and C₁ of 2 and H. abstraction from 2, respectively.The PA(1) = 377 +/- 2 kcal mol^-1 was determined from bracketing reactions of ROH + 1 -> RO^- + c-C5H5., which gives ΔH_f⁰(1) = 70.7 +/- 3.2 kcal mol^-1.Although the H. abstraction process by 1 was observed in most of its ion-molecule reactions, 1 failed to react with CH4, C2H4, and c-C3H6 probably because of an activation barrier of (*) 3 kcal mol^-1 in these cases.With dipolar CH3OH and 1, the only observed reaction was H. abstraction from the O-H bond (shown with CH3OD).This lower limit of the H. affinity of 1 gives ΔH_f⁰(1) (*) 67.7 +/- 3 kcal mol^-1, in excellent agreement with the value derived from protonation studies.The reactions of 1 with CH3X (Cl, Br) occur by H. abstraction and halide ion (S_N2) displacement.Anion radical 1 adds to activated olefins H2C=CHX (CN, CO2CH3, Cl) by a nucleophilic Michael addition mechanism.The EA of the carbene c-C5H4 was bracketed by charge-transfer reactions between 1 and C6F6 and NO2.All of these and certain other results are consistent with the ?¹?² electronic configuration as the ground state of 1.The reactions of 1 with alcohols are postulated to proceed via a hydrogen-bonded complex.