Anisotropic ESR parameters of Cu(II) complexes in solutions and dynamics of solvent coordination: use of “minimum linewidths”

Abstract

The “minimum linewidths” seen in the ESR linewidths against temperature plots, the dependence of line widths on the⁶³Cu nuclear magnetic quantum numbers and the Hubbard relation provide sufficient number of equations to determine the anisotropic ESR parameters in the case of axially symmetric Cu(II) complexes even when unresolved hyperfine structures make contributions to linewidths. After testing the method by reanalysing the literature data on Cu(II) bis-acetylacetonate, it has been used to obtain the anisotropic ESR parameters in the case of bis-salicylaldehydate of Cu(II). Linewidth contributions from unresolved hyperfine structures associated with the¹H of coordinating CHCl₃ inferred in these studies, were confirmed by comparing the widths in CHCl₃ and CDCl₃ under ideal conditions. The temperature dependence of this contribution and the estimate of rate constant at room temperature (∼ 10¹⁰ s⁻¹) suggest that the coordinating solvent exchange is diffusion controlled.