Exploiting the properties of line-shape singularities in orientationally selected electron spin echo envelope modulation spectra of Cu_- ²⁺(¹⁵N-imidazole)₄ for the determination of hyperfine coupling with the remote imidazole nitrogen

Orientationally selected two-, three-, and four-pulse electron spin echo envelope modulation (ESEEM) spectra from remote ¹⁵N nuclei in a Cu²⁺-(¹⁵N-imidazole)₄ complex have been obtained in the microwave frequency region 9.12-9.82 GHz. This range of frequencies has allowed us to reach the conditions corresponding to S and v _⊥ singularities of the lineform of basic transitions. The development of these lineform singularities in orientationally selected spectra is discussed. The knowledge of the magnetic fields corresponding to the singularities was used to find the hyperfine coupling parameters for the remote nitrogen as a =2.44 MHz and T_⊥ = -0.39 MHz as well as to determine the angle between the axis of the complex and the direction of the largest principal hyperfine tensor element as 40°. Additional geometrical information about the ligand orientation has been obtained from the analysis of shifts of the sum combination harmonic in proton four-pulse ESEEM spectra.