Nitroxide Bound β-Cyclodextrin:  Is There an Inclusion Complex?

236CDTIPNO, a derivative of the persistent free radical TIPNO (1,1-dimethylethyl 2-methyl-1-phenylpropyl nitroxide) covalently bound to a permethylated-β-cyclodextrin, was prepared. Self-association of 236CDTIPNO in water was proved by solvent- and competition-dependent EPR spectroscopy experiments with 2,6-di-O-Me-β-cyclodextrin (DIMEB) and permethylated-β-cyclodextrin (TRIMEB) as external hosts competing for accommodation of the TIPNO moiety. Temperature-dependent EPR spectra were simulated with a novel two-dimensional (field-temperature) EPR simulation program that afforded a full determination of the thermodynamic parameters characterizing the rate constants of the self-inclusion reaction derived from Arrhenius and Eyring models. This method allows separating the line broadening effects due to relaxation from a chemical exchange, even if only the fast exchange regime is accessible experimentally. The activation parameters for the forward and backward steps were consistent with an equilibrium between a nonassociated form and a weakly associated form, with activation free enthalpies for each reaction of around 34 kJ·mol^-1.