ESR Signal in Azafullerene (C₅₉N)₂ Induced by Thermal Homolysis

Thermal homolysis of biazafullerenyl, (C₅₉N)₂, has been studied by CW ESR in the temperature range 290-829 K. The room-temperature ESR spectrum consists of a single sharp narrow line whose intensity is consistent with the presence of a low concentration of defect spins (1 spin per 600 C₅₉N units). Around 500 K, three sharp lines emerge on top of the impurity signal and are assigned to the ¹⁴N(I = 1) hyperfine spectrum with a hyperfine coupling constant, a_iso = 3.60(1) G and a g-factor of 2.0004(2). The intensity of the observed ¹⁴N hyperfine spectrum directly measures the fraction of the C₅₉N^• radicals formed as the temperature increases, growing from ∼50 ppm at 516 K to ∼300 ppm at 740 K. The temperature evolution of this fraction allows us to extract an estimate of the binding energy as 7(1) kcal/mol. The high temperature treatment of the sample eventually results in the partial decomposition and formation of C₆₀.