Acidity of hydrofullerenes: A quantum chemical study

Guided by the idea that the acidity of hydrofullerenes might decrease with increasing level of hydrogenation and the possibility of using a pH-dependent regime to separate the various hydrogenated species (as predicted by Taylor and Walton), a theoretical pK(a) scale for a series of hydrofullerenes has been set up. Owing to the lack of experimental data on the acidity of most of these hydrofullerenes, this acidity scale has been established via test compounds using a correlation of experimental pK(a) values in DMSO with calculated deprotonation energy values Delta E in the gas phase. Besides the deprotonation energy, Delta E, the charge on the acidic hydrogen, the molecular electrostatic potential (MEP) and the electronic delocalization Delta are also calculated at the 3-21G level for the test compounds as well as for the hydrofullerenes, for interpretational purposes. The interpretation of the calculated acidity sequence shows that besides a localized effect such as charge on the acidic proton (and the MEP in its neighbourhood), the electronic delocalization in the conjugate base, quantified via a Mulliken population analysis, is also highly important. The dual correlation between the experimental pK(a) values and the q(H) and Delta values shows that the delocalization term accounts for about 75% of the calculated acidity. The calculated acidity sequences are in agreement with the Taylor and Walton prediction pointing out an acidity decrease with increasing number of hydrogen atoms. For a given q(H), the acidity is lower than in the model systems, indicating that the delocalization effects are of special importance in the hydrofullerenes. The calculation of the Delta quantity indeed shows a high electronic delocalization of the conjugate bases of the hydrofullerenes, which is almost the same (approximate to 1 electron) throughout the series considered, The calculations on the global softness of the hydrofullerenes and their conjugate bases using Koopmans' theorem show a decrease with increasing number of hydrogens in both cases. In general all tendencies obtained indicate that the cage as a whole influences the acidity of the hydrofullerenes.