EPR anisotropy in single crystals of diacetato bis (2,6-dimethylpyridine) copper (II) with a single molecule per unit cell and a weak exchange coupling

Abstract

Single-crystal EPR rotational data for Cu(ac) 2 (2,6-Me 2py) 2 give g | = 2.248, g ⊥ = 2.061, ΔB pp | = 23.6 mT, and ΔB pp ⊥ = 3.5 mT at room temperature. The linewidth ΔB pp was found to be phenomenologically described by a second-rank tensor with angular dependence ΔB pp( θ) = ΔB pp |cos 2 θ + ΔB pp ⊥sin 2 θ. This linewidth behavior is discussed in comparison with other types of ΔB pp( θ) dependences in single crystals. The linewidth value and its angular variations were calculated in terms of the second moment and exchange-narrowing theory using tensor description of both secular and nonsecular dipolar coupling. A dominant contribution to the linewidth anisotropy arises from hyperfine interaction, and exchange coupling is evaluated to be | J| = 0.065 cm −1. However, theoretical and experimental plots of ΔB pp( θ) differ significantly, indicating that existing linewidth theories are not able to reproduce qualitatively linewidth behavior in this crystal. A reason for this is anisotropy of the merging effect between hyperfine lines due to the isotropic exchange interaction. The exchange coupling is comparable with hyperfine splitting, and, as a result, the angular linewidth dependence is dominated by a competition between exchange broadening (near g | where hyperfine splitting is maximal) and exchange narrowing (near g ⊥, where hyperfine splitting is minimal) which is not included in classical linewidth theories.