Journal of the American Chemical Society p. 5509 - 5513 (1986)
Update date:2022-08-11
Topics:
Stevenson, Gerald R.
Nebgen, Mark A.
Calorimetric studies have shown that either a phenyl or tert-butoxy substituent on <8>annulene (<18>) increases the heat of generation (ΔHgen0) 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 ΔHgen0 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.
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