Towards highly powerful neutral organic superacids—a DFT study of some polycyano derivatives of planar hydrocarbons

Density functional theory (DFT) calculations at B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level have been carried out on indene, cyclopentaphenanthrene and 1H-phenalene and their heptacyano and nonacyano derivatives, respectively, in order to examine their acidities in the gas-phase and DMSO. It is found that polycyano derivatives represent powerful organic superacids, the most acidic being nonacyano-1H-phenalene. The origin of the highly pronounced acidity is identified as a strong anionic resonance in the resulting conjugate base. Comparison of the calculated ΔHacid value for 1H-phenalene with the experimental NIST value shows that the latter is too large by 8–11 kcal mol−1. A possible reason for this error is briefly discussed.