ENDOR and EPR of a Nitroxyl Radical Formed from N,2,4,6-Tetranitro-N-Methylaniline

Abstract

ENDOR (electron-nuclear double resonance) spectra of 2,4,6-tetranitrophenyl-N-methylnitroxylradical (I) in N,2,4,6-tetranitrophenyl-N-methylaniline(tetryl; C₇N₅H₅O₈) are recorded at 120 K and 4 K. At 120 K, an ENDOR signal at 28.3 MHz is observed and assigned to the β-proton couplings of the ‒CH₃ group. At 4 K, matrix ENDOR signals are detected at 14.5 MHz due to an unknown trapped species (possibly •CH₃ or •NO). Also at 4 K, signals are assigned to the ¹H atoms of the nitroaromatic ring. A computer model based on the crystal structure coordinates reveals that (I) retains the parent molecule configuration of tetryl. The principal g-values of (I) determined by a single crystal EPR (electron paramagnetic resonance) study at 298 K are: g _x = 2.0088, g _y = 2.0060, g _z = 2.0020. The principal ¹⁴N hyperfine couplings (hfcs) are: A _x = 0.125 mT, A _y = 0.425 mT, A _z = 2.78 mT. The principal ¹H hfcs are nearly isotropic due to rotation of the ‒CH₃ group with values: A _x = 1.2 mT, A _y = 1.0 mT, A _z = 1.0 mT. The highly stable nature of (I) in tetryl and the insensitivity of tetryl suggests that stable nitroxyl radicals in energetic materials may be indicators of mechanical and thermal sensitivity.