ENDOR study of nitronylnitroxides in frozen solution

Abstract

ENDOR spectra of differently α-substituted nitronylnitroxides in a frozen solution of n-butylphthalate have been analysed to evaluate the hyperfine coupling tensor A of the radical protons. The EPR spectra in frozen solution showed a good resolution of the spectral features corresponding to the g_x, g_y, and g_z values. Therefore different ENDOR spectra were recorded by saturating the EPR signals at different magnetic fields. The tensor components were obtained by computing the ENDOR spectra, and the relative signs were determined by means of TRIPLE experiments. Also ENDOR spectra of liquid solutions were performed to obtain the isotropic coupling constants. Two different sets of A_{i, H} components and director cosines were evaluated for the methyl protons at 2,3-positions of the nitronylnitroxide ring. The discrepancies between the two sets diminished on going from the α-CH₃ radical to the other α-substituted radicals because of structural effects of the large substituents. The A_{i, H} components of the α-CH₃ protons were also obtained. Only the 2,6-protons of the α-phenyl or α-pyridine contributed to the ENDOR signal and the A_{i, H} components of the pyridine protons were all greater because of the inductive effect of the nitrogen in the pyridine ring.