EPR relaxations of aqueous Ni2+ ion

Abstract

The experimental data for proton NMR relaxation and shifts in aqueous Ni²⁺ solutions at 25°C have been extended, especially to high fields (8.4 T). Infinite‐order perturbation theory for the EPR relaxation in the Ni²⁺ and a generalized set of Solomon–Bloembergen equations for the proton–electron spin–spin interaction are applied to deduce the rms value Δ and the correlation time τ_v for the tensor D(t) in the zero field splitting or SDS term of the electron spin Hamiltonian. Both the Brownian rotation of the Ni(H₂O)₆²⁺, with correlation time τ_ϑ, and the mean lifetime τ_p of protons in this complex, before exchanging with the bulk, are very important in determining the functional relation of the proton relation times and shift to the corresponding EPR quantities. The best fit of the theory to the data implies the following values for the parameters for assumed values τ_ϑ=25 psec and τ_p=32 μsec: Δ=2.6 cm⁻¹, τ_v=2.2 psec, T_lS=T_2S=2.9 psec at zero magnetic field, proton contact shift in the complex=14 ppm, proton dynamical shift in the complex at zero field=2.1 ppm. The proton dynamical shift is a second‐order effect in the dipolar electron–proton spin–spin interaction. The discussion includes the comparison of some of these parameters with earlier determinations as well as a summary of additional effects that might be included in a more realistic model.