ELECTRON-SPIN ECHO ENVELOPE MODULATION ANALYSIS OF SEO3- RADICAL IN (NH4)3H(SEO4)2 SINGLE-CRYSTAL

A detailed computer analysis of the electron spin echo envelope modula tions of SeO3- radical in a suitable orientation of (NH4)311(SeO4)2 si ngle crystal is presented. It was found that the modulations are due t o a weak dipolar coupling with nitrogens and protons of the only two n eighbouring NH4 groups among the five NH4 groups surrounding SeO3- cen ter. Isotropic dipolar coupling constant is 1.7 MHz for nitrogens and 0.9 MHz for protons. It was shown that thermal reorientations of NH4 g roups observed by NMR have a negligible effect on the electron spin ec ho envelope modulation pattern but can be responsible for the same val ue of a(iso)H for all protons in a NH4 group. A good fit obtained betw een experimental spectra and theoretical calculations assuming nondist urbed crystal geometry indicates a small damage of the crystal by X-ra ys during the radical center formation.