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 Co-60 gamma-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 N-14 nuclei similar to that of the parent pyrazine cation and are assigned to n(sigma)-radical cations. The other spectra observed for the tetramethyl, OCH3, SCH3 and NR2 derivatives are assigned to pi-radical cations. These findings confirm that the SOMO has switched from n(sigma) to pi 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 N-14 nuclei. Studies in fluid solution indicate that strong electron donor substituents (OCH3, SCH3, NR2) convert the S-type pi-SOMO of the pyrazine radical anion to a pi-SOMO with predominant A-type character.