Superacid protonation of dihydrocyclobuta[e]pyrene and its C-60-o-quinodimethane adduct. An NMR, ab initio/GIAO and AM1/PM3 study

Dihydrocyclobuta[e]pyrene 1 is protonated in FSO3H-SO2ClF to give a ca. 2 : 1 mixture of two arenium ions of alpha-protonation 1H(+) and 1aH(+). Ab in itio/GIAO theory at the B3LYP/6-31G(d,p) level was utilized to calculate th e NPA charges, relative energies and to predict the NMR chemical shifts for 1H(+) and 1aH(+) as a means to enhance the solution NMR-based assignments. The resulting regioisomeric arenium ions are highly delocalized and exhibi t a uniform charge alternation path within a pyrenium moiety. Reasonable pr edictions of their relative energies and the overall charge delocalization mode can also be obtained at the AM1/PM3 levels. In an attempt to determine the influence of C-60 on tropicity in an anchored-pyrenium ion, the C-60-o -quinodimethane adduct 2 was protonated in FSO3H . SbF5-SO2ClF; formation o f regioisomeric bound-arenium ions (2H(+), 2aH(+)) which are very poorly so luble in superacid media was inferred from H-1 NMR and AM1/PM3 calculations were used to model these cations. Further model studies using a fragment o f C-60 bound to regioisomeric pyrenium ions were realized at the LSDA/DN le vel of theory with electrostatic potential derived charges at the B3LYP/6-3 1G(d,p) level.