Dynamic processes of 1,1′-dihydroxy-2,2′,6,6′-tetra-tert-butylbiphenyl radical cation in sulfuric acid as studied by two-dimensional FT-EPR spectroscopy

Slow exchange processes in the solution of the 1,1′-dihydroxy-2,2′,6,6′-tetra-tert-butylbiphenyl cation radical (tBBP^̇+) in concentrated sulfuric acid were investigated by two-dimensional (2D) exchange FT-EPR spectroscopy (EXSY) in the temperature range of 281-310 K. The radical was obtained by dissolving 2,2′,6,6′-tetra-tert-butyldiphenoquinone (tBDP) in concentrated sulfuric acid. The EPR spectrum of the radical cation, tBBP^̇+, showed that the four aromatic protons and the two hydroxyl protons are magnetically equivalent. The 2D EXSY spectra exhibited cross peaks between hyperfine components with ΔM₁^a = ±1 and ΔM₁^b = ±1, where a and b correspond to the aromatic and hydroxyl protons, respectively. Based on this selective cross-peak pattern, the change in the nuclear quantum number of the aromatic protons was attributed to proton spin-lattice relaxation. In contrast, the change in the nuclear quantum number of the hydroxyl protons could arise from proton exchange with the solvent and/or nuclear spin-lattice relaxation. Temperature-dependent measurements showed that the intensity of the cross peaks decreased with increasing temperatures, indicating that in the case of the hydroxyl protons the cross peaks are also a consequence of nuclear relaxation and not proton exchange. The nuclear spin-lattice relaxation rates of the two types of protons and the electron spin-lattice relaxation, T₁, were determined from simulations of experimental 2D spectra recorded with different mixing times. The values obtained at 291 K were T_1a^-1 = (9 ±1) × 10⁵ s^-1 and T_1b^-1 = (4 ±1) × 10⁵ s^-1 and T₁ = (0.83 ±0.06) × 10⁵ s^-1. Using the nuclear relaxation rate of the aromatic protons and assuming that the nuclear relaxation is dominated by the hyperfine anisotropy mechanism, a correlation time of 0.67 × 10^-9 s was obtained. This value was further used to account for the M₁ dependence of the line width. Similar temperature-dependent 2D EXSY measurements were carried out on the dihydroduroquinone radical cation in sulfuric acid. In this case, the cross peaks observed were attributed to both proton exchange and nuclear spin relaxation of the hydroxyl protons. The proton exchange in the dihydroduroquinone radical cation was faster than in tBBP^̇+.