Correlating EPR Parameters With Structural Anisotropy in Cu(II) Complexes

Quantum chemical calculations offer a legitimate interpretation of EPR parameters and thus helps to correlate the geometry and spectral feature of open-shell molecules. Herein, two pseudo-octahedral Cu(II) systems, one with rhombic and the other with axial symmetry, are chosen as representative systems to understand the electronic structure origin of their spectroscopic behavior. The spin Hamiltonian parameters, g-tensor and hyperfine coupling constant, are computed in the platform of the Density Functional and wave function-based theories. Emphasis is given to explain the shift in the free electron g-value (∆g), which acts almost like a fingerprint of geometry and electronic structure of a transition metal complex. Though, the EPR spectrum of a frozen solution or powder allows derivation of the principal values of anisotropic g-tensor, experimental determination of its direction is not straight forward. Hence, we extend our effort to explore the relative orientation of molecular coordinate frame and g-tensor with the help of multireference configuration interaction (MRCI) calculation. The MRCI calculation helps to determine spin orbit coupling constant (SOC) corresponding to a particular electronic transition and thus turns instrumental in detecting the orientation of g-tensor with respect to the molecular coordinate frame. We find that metal-ligand bond tunes the orbital degeneracy, which in turn influences the SOC and ∆g value. On the other hand, the isotropic part (Fermi Contact term, A_iso) and $A_{\parallel }$ component of the hyperfine coupling for coordinated donor nitrogens reflects the spin distribution pattern. Specifically, the ligand atom, having maximum share of the metallic spin due to high covalency, exhibits the highest value of the A_iso and $A_{\parallel }$ . In essence, this work employs electronic structure information to map the EPR parameters with molecular geometry.