EXPLOITING THE PROPERTIES OF LINE-SHAPE SINGULARITIES IN ORIENTATIONALLY SELECTED ELECTRON-SPIN ECHO ENVELOPE MODULATION SPECTRA OF CU--(2-15-IMIDAZOLE)(4) FOR THE DETERMINATION OF HYPERFINE COUPLING WITH THE REMOTE IMIDAZOLE NITROGEN()(N)

Orientationally selected two-, three-, and four-pulse electron spin ec ho envelope modulation (ESEEM) spectra from remote N-15 nuclei in a Cu 2+-(N-15-imidazole)(4) complex have been obtained in the microwave fre quency region 9.12-9.82 GHz. This range of frequencies has allowed us to reach the conditions corresponding to S and v perpendicular to sing ularities of the lineform of basic transitions. The development of the se lineform singularities in orientationally selected spectra is discu ssed. The knowledge of the magnetic fields corresponding to the singul arities was used to find the hyperfine coupling parameters for the rem ote nitrogen as alpha=2.44 MHz and T-perpendicular to = -0.39 MHz as w ell as to determine the angle between the axis of the complex and the direction of the largest principal hyperfine tenser element as 40 degr ees. Additional geometrical information about the ligand orientation h as been obtained from the analysis of shifts of the sum combination ha rmonic in proton four-pulse ESEEM spectra.