Entropic Effects Distinguishing the Reactivity of Fullerenes in Diels-Alder Cycloadditions

Fullerenes combine high curvature, symmetry, and multiple reactive sites, making them ideal systems to explore structure-reactivity relationships. This work presents an equilibrium and kinetic study of the Diels-Alder reactions of three fullerenes (C₆₀, C₇₀, and IC₆₀MA) with the dienes anthracene and 9,10-dimethylanthracene in m-xylene solution, using Ultraviolet-visible (UV-Vis) spectroscopy. Equilibrium constants and rate constants for both the forward and reverse reactions were determined at various temperatures, allowing the calculation of standard molar enthalpies and entropies of reaction as well as activation parameters ΔH_m ^‡ and ΔS_m ^‡. The equilibrium constants follow the trend C₆₀> C₇₀∼ IC₆₀MA, with C₆₀ exhibiting higher reactivity due to both a faster forward reaction and a slower reverse reaction. The results reveal that the reactivities of the fullerenes are predominantly distinguished by entropic rather than enthalpic contributions. We show that molecular symmetry, isomerism, and dynamic surface phenomena, such as addend ring walking (its extent depending on fullerene structure), collectively determine the observed reactivity trends. Kinetic results suggest that the transition states of C₆₀ and C₇₀ differ significantly in entropy, a trend consistent with the smoother reactive landscape of C₆₀ relative to the surface-segmented C₇₀.