Monoclinic and Triclinic Tetraisopropyl‐ p ‐phenylenediamine: To what Extent do n N /π Interactions Determine Structures?

Two conformations‐two crystal forms! This is the case for the title compound. The crystal of the conformer with n N /π interactions between the nitrogen lone pairs and the benzene π system (left) is monoclinic, whereas that of the conformer without this interaction (right) is triclinic. The dihedral angles ω (C 2 NC 6 ) are 28 and 74°. According to potential energy surface calculations, the conformers have almost identical enthalpies of formation (Δ H   f PM 3= −46 and −47 kJmol −1 ).