Thermodynamics of Host–Guest Interactions between Fullerenes and a Buckycatcher

Abstract

¹H NMR and isothermal titration calorimetry (ITC) experiments were employed to obtain reliable thermodynamic data for the formation of the 1:1 inclusion complexes of fullerenes C ₆₀ and C ₇₀ with the buckycatcher (C ₆₀H ₂₈). NMR measurements were done in toluene- d ₈ and chlorobenzene- d ₅ at 288, 298, and 308 K, while the ITC titrations were performed in toluene, chlorobenzene, o-dichlorobenzene, anisole, and 1,1,2,2-tetrachloroethane at temperatures from 278 to 323 K. The association constants, K _a, obtained with both techniques are in very good agreement. The thermodynamic data obtained by ITC indicate that generally the host–guest association is enthalpy-driven. Interestingly, the entropy contributions are, with rare exceptions, slightly stabilizing or close to zero. Neither Δ H nor Δ S is constant over the temperature range studied, and these thermodynamic functions exhibit classical enthalpy/entropy compensation. The Δ C _p values calculated from the temperature dependence of the calorimetric Δ H values are negative for the association of both fullerenes with the buckycatcher in toluene. The negative Δ C _p values are consistent with some desolvation of the host-cavity and the guest in the inclusion complexes, C ₆₀@C ₆₀H ₂₈ and C ₇₀@C ₆₀H ₂₈.