Radical cations and anions of pyrazines: an electron paramagnetic resonance study
Dilute solutions of a series of pyrazines (1–13) in CFCl3 at 77 K were exposed to 60Co γ-rays, generating the corresponding radical cations which were identified by their EPR spectra. Two distinct types of spectra were observed. Those obtained from mono- and di-methyl derivatives showed large hyperfine coupling to two 14N nuclei similar to that of the parent pyrazine cation and are assigned to n(σ)-radical cations. The other spectra observed for the tetramethyl, OCH3, SCH3 and NR2 derivatives are assigned to π-radical cations. These findings confirm that the SOMO has switched from n(σ) to π between the dimethyl derivatives 4, 6 and 8 and the tetramethyl compound 9, respectively. The NR2-substituted radical cations 12˙+ and 13˙+ were also observed in fluid solution. The corresponding radical anions 1˙––13˙–, studied in solid solution, show major hyperfine coupling to the two ring 14N nuclei. Studies in fluid solution indicate that strong electron donor substituents (OCH3, SCH3, NR2) convert the S-type π-SOMO of the pyrazine radical anion to a π-SOMO with predominant A-type character.