Pancake-Bonded Dimers of Semiquinone Radical Cations of N,N,N′,N′-Tetramethyl-p-phenylenediamine (Wurster’s Blue)

Pancake bonding in dimers of semiquinoid radical cations in a novel compound, TMPD chloride co-crystal with 2,5-dichlorohydroquinone, is studied by a combination of X-ray charge density and quantum chemical computations. The rings in a pancake-bonded dimer are parallel with a transversal offset, the interplanar separation distance is 3.1191(1) Å, and the centroid distance is 3.2022(2) Å. AIM analysis of experimental charge density revealed four symmetry-independent bonding critical points and a cage critical point between the TMPD cations in a dimer, with a maximum electron density of 0.055 e Å–3. A highest occupied molecular orbital extends between both rings of a dimer. The estimated energy of the covalent component is −3.24 kcal mol–1.