Calculation of Electron Affinities of Polycyclic Aromatic Hydrocarbons and Solvation Energies of Their Radical Anion

Electron affinities (EAs) and free energies for electron attachment (ΔG°a,298K) have been directly calculated
for 45 polynuclear aromatic hydrocarbons (PAHs) and related molecules by a variety of theoretical methods,
with standard regression errors of about 0.07 eV (mean unsigned error = 0.05 eV) at the B3LYP/6-31+G(d,p)
level and larger errors with HF or MP2 methods or using Koopmans' Theorem. Comparison of gas-phase
free energies with solution-phase reduction potentials provides a measure of solvation energy differences
between the radical anion and neutral PAH. A simple Born-charging model approximates the solvation effects
on the radical anions, leading to a good correlation with experimental solvation energy differences. This is
used to estimate unknown or questionable EAs from reduction potentials. Two independent methods are used
to predict ΔG°a,298K values:  (1) based upon DFT methods, or (2) based upon reduction potentials and the
Born model. They suggest reassignments or a resolution of conflicting experimental EAs for nearly one-half
(17 of 38) of the PAH molecules for which experimental EAs have been reported. For the antiaromatic
molecules, 1,3,5-tri-tert-butylpentalene and the dithia-substituted cyclobutadiene 1, the reduction potentials
lead to estimated EAs close to those expected from DFT calculations and provide a basis for the prediction
of the EAs and reduction potentials of pentalene and cyclobutadiene. The Born model has been used to relate
the electrostatic solvation energies of PAH and hydrocarbon radical anions, and spherical halide anions, alkali
metal cations, and ammonium ions to effective ionic radii from DFT electron-density envelopes. The Born
model used for PAHs has been successfully extended here to quantitatively explain the solvation energy of
the C60 radical anion.