Electron spin echo envelope modulation studies of water bound to tetracyanonickelate(III)

Electron spin echo envelope modulation studies aimed at characterizing the superhyperfine coupling between protons of axially bound water molecules and the nickel ion in Ni^III(CN)₄(H₂O)₂ ^- were carried out. X-band, two-pulse ESEEM experiments showed that this coupling is characterized by a large anisotropic component resulting in 0.1-1.6-MHz shifts of the v_α + v_β proton sum combination peak from twice the Larmor frequency. A means of reducing these data into a field profile constructed by plotting these frequency shifts as a function of magnetic field strength across the EPR absorption envelope is presented. Computer simulations of typical field profiles are shown to be sensitive to the effective Ni-H dipole-dipole distance, the orientation of the principal axis of the hyperfine coupling tensor with respect to the g-tensor, and to a lesser extent, the Fermi contact or scalar hyperfine coupling term. For water coordinated to Ni^II(CN)₄(H₂O)₂ ^-, analysis of the field profile yields an effective dipole-dipole distance of 2.4 Å, an angle describing the orientation of A_∥ for the proton superhyperfine coupling tensor with respect to the g₃ axis of 12°, and a scalar coupling ≤4 MHz. These results are discussed in the context of recent ENDOR studies of the nickel site in Ni-Fe hydrogenases.