Four-pulse electron spin echo envelope modulation studies of axial water ligation to diaquatetracyanonickelate(III)

Four-pulse electron spin echo envelope modulation (ESEEM) studies aimed at enhancing our previous characterization of the hyperfine interactions between the protons of axially bound water molecules and the nickel ion of Ni^III(CN)₄(H₂O)₂^- were carried out. Because the ligand hyperfine coupling of the strongly bound water protons, is characterized by a large anisotropic interaction, the v_α + v_β sum combination peak, resolved in two- and four-pulse ESEEM measurements, shows a pronounced shift from twice the proton Larmor frequency. In contrast to our previous study where a two-pulse (π/2-τ-π) microwave pulse sequence was used, four-pulse data show deep modulation with reduced damping for the sum combination feature that results in a 10-fold increase in spectral resolution. The corresponding ESEEM spectra provide line shape and frequency constraints that allow for a more accurate and complete characterization of the bound water proton hyperfine coupling tensor. Theoretical simulation of the magnetic field dependence of the v_α + v_β line shapes and frequency shifts from the twice the Larmor frequency gave an effective Ni-H dipole-dipole distance of 2.33 ± 0.03 Å and a θ_n, the orientation for the principal axis system of the ¹H hyperfine coupling tensor with respect to the g₃ axis of the Ni(III) g tensor, of 18 ± 3°. The τ-suppression behavior of the v_α + v_β line shape at a fixed magnetic field position was used to place more exact constraints on the isotropic hyperfine coupling constant than possible with a simple two-pulse approach. An isotropic hyperfine coupling constant of |2.5| ± 0.5 MHz was found for the bound axial water protons.