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 exceptiona lly persistent and oxygen-insensitive 2,7-di-tert-butylpyren-1-oxyl ra dical 2 whose EPR and ENDOR spectra give the following proton hyperfin e 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 a ccomplished by measuring the EPR and ENDOR spectra of partly deuteriat ed 2,7-di-tert-butyl(4,5,9,10-H-2(4))pyren-1-oxyl radical. Radical 2 i s 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) (Delta H) and - 35 +/- 15 J mol(-1) K-1 (Delta S), respectively. The very low Delta H and negat ive Delta S values are briefly discussed.