Rotational Barrier in Phenalenyl Neutral Radical Dimer: Separating Pancake and van der Waals Interactions

Pancake pi-stacking produces shorter contacts than van der Waals bonding but it has strongly preferred configurations. By high-level multireference average quadratic coupled cluster theory for the singlet and triplet, we identify the specific orbital component and the nonspecific vdW contributions in the prototypical pancake-bonded dimer of phenalenyl thereby explaining the configurational preferences.