Heats of Generation of Substituted Annulene Dianions

Article abstract of DOI:10.1021/ja00278a023

Calorimetric studies have shown that either a phenyl or tert-butoxy substituent on <8>annulene (<18>) increases the heat of generation (ΔH_gen⁰) of the respective dianion <ΔH⁰ for R-<8>(HMPA) + 2Na(s) --> R-<8>^2- + 2Na⁺(HMPA)> in hexamethylphosphoramide relative to that of <8>.The values for ΔH_gen⁰ are -51, -38, and -42 kcal/mol respectively for R=H, C4H9, and C6H5.The destabilization of the dianion relative to the neutral molecule due to the presence of a phenyl group is accounted for by the fact that the phenyl group is nearly orthogonal to the charged eight-member ring system.This is supported by the NMR spectrum of Ph-<8>^2-. 1H NMR studies carried out upon the solvent (HMPA) in the presence of <8>^*- show that the Knight shift is very close to that predicted from changes in bulk paramagnetic susceptibility.This confirms the facts that <8>^*-(HMPA) is free of ion association and that ion association must be present to provide a mechanism of spin transfer from the anion radical to the solvent.The previously reported dissociation enthalpy of <8>^2-,K⁺ in HMPA has been combined with several calorimetrically determined values and an extra thermodynamic parameter to obtain a value of -195 kcal/mol for a single ion heat of solvatation of the dianion of <8>annulene in HMPA.