Exceptionally persistent and oxygen-insensitive 2,7-di-tert-butylpyren-1-oxyl radical: synthesis, dimerization, EPR and ENDOR spectra

Oxidation of 2,7-di-tert-butyl-1-hydroxypyrene 1 yields the exceptionally persistent and oxygen-insensitive 2,7-di-tert-butylpyren-1-oxyl radical 2 whose EPR and ENDOR spectra give the following proton hyperfine coupling (hfc) constants: 0.526, 0.445, 0.426, 0.386, 0.157, 0.107, 0.088 and 0.0054 mT (g= 2.0037). The assignments of the protons are accomplished by measuring the EPR and ENDOR spectra of partly deuteriated 2,7-di-tert-butyl(4,5,9,10-2H4)pyren-1-oxyl radical. Radical 2 is isolated as a dimer which is in equilibrium with 2 in solution, even at low temperatures. The thermodynamic parameters for the equilibrium are determined to be 7.2 ± 2.0 kJ mol–1(ΔH) and –35 ± 15 J mol–1 K–1(ΔS), respectively. The very low ΔH and negative ΔS values are briefly discussed.