Département de chimie moléculaire

Buckminsterfullerene C60 is proposed as a radical sponge for scavenging reactive oxygen species such as the hydroxyl radical •OH. Reaction energies are calculated using density-functional theory at the B3LYP-D4/def2-SVP level for successive gas-phase addition reactions of •OH to C60 up through and including six hydroxyl radicals. In total, 285 reactions were investigated yielding minimum energy structures, each of which is not necessarily the lowest energy conformer but is estimated to be within 5kcal/mol of the lowest energy conformer for each new addition. We confirm that the lowest energy isomers form a belt of hydroxyl groups around the equator of C60, but ask the question of what governs the relative stability of subtitutions at different carbons? Factors concerning regioselectivity are analyzed in terms of conceptual density-functional theory, frontier molecular orbital theory, and charge and spin densities, based upon Mulliken population analysis. The complexity of applying such an analysis to such large quasi-degenerate and often open-shell systems results in noisy data, but this is adequately off-set by the quantity of data examined which allow the identification of clear statistical trends. We confirm that •OH is an electrophilic radical whose successive reaction with C60 is under both charge and orbital control. This is seen to be especially the case for addition to odd •C60(OH)2m+1 fullerenols, but is also seen from a Fukui function and dual descriptor analysis for even C60(OH)2m fullerenols. Of particular interest is the ability of the condensed radical Fukui function f0 to provide information about the reactivity of even C60(OH)2m fullerenols with •OH also when the spin density is zero, and the observation that the interpretation of the sign of the dual descriptor changes depending upon whether a spin-restricted calculation is being performed for even C60(OH)2m fullerenols or a spin-unrestricted calculation is being performed for odd •C60(OH)2m+1 fullerenols.