Superacid protonation of dihydrocyclobuta[e]pyrene and its C60–o-quinodimethane adduct. An NMR, ab initio/GIAO and AM1/PM3 study

Autor: Laali, Kenneth K., Hollenstein, Sandro, Galembeck, Sergio E., Nakamura, Yosuke, Nishimura, Jun
Zdroj: Journal of the Chemical Society, Perkin Transactions 2; October 1, 1999, Vol. 1999 Issue: 10 p2129-2132, 4p
Abstrakt: Dihydrocyclobuta[e]pyrene 1 is protonated in FSO3H–SO2ClF to give a ca. 2∶1 mixture of two arenium ions of α-protonation 1H+ and 1aH+. Ab initio/GIAO theory at the B3LYP/6-31G(d,p) level was utilized to calculate the 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 exhibit a uniform charge alternation path within a pyrenium moiety. Reasonable predictions 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 C60 on tropicity in an anchored-pyrenium ion, the C60–o-quinodimethane adduct 2 was protonated in FSO3H·SbF5–SO2ClF; formation of regioisomeric bound-arenium ions (2H+, 2aH+) which are very poorly soluble in superacid media was inferred from 1H NMR and AM1/PM3 calculations were used to model these cations. Further model studies using a fragment of C60 bound to regioisomeric pyrenium ions were realized at the LSDA/DN level of theory with electrostatic potential derived charges at the B3LYP/6-31G(d,p) level.
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