Hydrophobic Nitroxide Radicals as Probes to Investigate the Hydrophobic Interaction

The EPR spectrum of 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) has been systematically investigated in several solvents and in aqueous solutions of glycerol, t-BuOH, Bu(4)NBr, n-HexNH(3)Br, n-OctNH(3)Br, NaBPh(4), and Ph(4)AsCl. Most of the results have been obtained at 25 degrees C though the temperature dependence of the linewidths has been examined in water and several organic solvents. Data on the spectrum of ditertiarybutylnitroxide in aqueous solutions of glycerol and Bu(4)NBr are also presented. The spectrum was simulated to determine W(H), the linewidth after allowance for the unresolved proton hyperfine interaction, in each manifold of the triplet due to the nitrogen hyperfine interaction. The linewidth W(H) is analyzed in terms of the reorientational correlation time tau(theta) and the angular velocity correlation time tau(J). In most solutions tau(theta) is determined only by the bulk solution viscosity, except in situations where significant clustering of hydrophobic solute molecules occurs. In pure nonaqueous solvents the temperature dependence results are consistent with tau(J) being determined by the bulk viscosity, while in water or aqueous solutions, a very different behavior is found which is interpreted as a manifestation of clathrate-like hydration of the hydrophobic radical. This interpretation is incorporated in a two-state model developed to account for the W(H) data of TEMPO in aqueous solutions of various hydrophobic solutes. The equilibrium parameters derived from the model, for the association of TEMPO with several hydrophobic solutes, support the concept of hydrophobic interactions as pictured from thermodynamic excess functions.