Acidity of substituted hydrofullerenes: An ab initio quantum-chemical study

A study is made of the acidity of a series of substituted hydrofullerenes, all resulting from the substitution of one hydrogen at the 1, 2-C60H2 hydrofullerene by a functional group (6, 6-ring fusion). Acidities are obtained via a previously set up correlation with calculated gas-phase deprotonation energies, and correlated with the properties of the functional group and the cage. All energy and property calculations have been performed at ab initio HF/3-21G level on AM1 fully optimised structures, for the acidic as well as for the conjugate base forms. Besides the deprotonation energy, Delta E, the charge on the acidic hydrogen and the electronic delocalisation, Delta, the global hardness and softness are also calculated for each system. Electronic delocalization in the conjugate base plays an important role in the fairly high acidity of these systems. The interplay of group softness and electronegativity on the acidity sequence is shown, yielding a correlation with the: charge acceptance of the functional group upon deprotonation. (C) 1997 Elsevier Science Ltd.