Molecular Criteria of Solid-State Structure in Organic Charge-Transfer Salts. 2. Effect of Methyl Substitution on Stacking Interactions of p-phenylenediamine Cation Radicals

The ability of p-phenylenediamine cation radicals to self-associate and form sandwich dimers or higher aggregates was found to be a function of methyl substitution, TMPD·+ (N,N′,N′,N-tetramethyl-p-phenylenediamine) <DMPD·+ (N,N-dimethyl-p-phenylenediamine) ≈ PPD·+ (p-phenylenediamine). The gas-phase ionization potentials or electrochemical disproportionation (2D+ ⇆ D° + D2+) energies showed no correlation with the observed association properties. However, the enthalpy of dimerization (ΔH) was found to be proportional to the experimental unpaired -electron spin density at the terminal ring carbons of the ion radicals. Steric considerations were shown to be relatively unimportant in the self-association processes. The relationship between these molecular data and the solid-state structure of salts formed by these cation radicals was discussed. © 1976, American Chemical Society. All rights reserved.