The pentacyanocyclopentadienyl system: structures and energetics

In light of the recent silylation route to protonated pentacyanocyclopentadienyl (PCCp) anion reported by Richardson and Reed, PCCp anion, neutral radical, and protonated species have been investigated theoretically. The predicted adiabatic electron affinity for PCCp (ZPVE-corrected value in parentheses) is enormous, 5.56 (5.47) eV. As with the unsubstituted cyclopentadienyl radical, the PCCp radical exhibits a Jahn-Teller distortion from the D(5)(h) symmetry of the aromatic anion, yielding five equivalent (2)B(2) minima that should show uninhibited pseudorotation about a D(5)(h) conical intersection. Formation of the conjugate acid occurs via protonation of the anion at a nitrile nitrogen, which is favored over protonation at a ring carbon by 6.5 kcal/mol, with the preference explained by retention of aromaticity upon protonation at the nitrogen. Possible acid dimer structures have been investigated to evaluate the proposed polymeric acid structure of Richardson and Reed. Our predictions confirm their suggested polymeric structure, but we also present an alternative, self-contained dimer that should be competitive kinetically and thermodynamically. Vibrational frequencies and infrared intensities are predicted, to aid in the experimental identification of several of these species.