Electron Paramagnetic Resonance and the Primarily 3d Wavefunctions of the Tetrachlorocuprate Ion

Abstract

Electron paramagnetic resonance studies of the tetrahedral CuCl₄⁼ ion in single crystals of Cs₂CuCl₄ and Cs₂ZnCl₄ yield the spectroscopic parameters g₁=2.083±0.001, g₂=2.103±0.003, g₃=2.384±0.006 (Cs₂CuCl₄) or 2.446±0.002 (Cs₂ZnCl₄), and | A₁ | = (5.1±0.5)×10⁻³ cm⁻¹, | A₂ | = (4.6±0.5)×10⁻³ cm⁻¹, and | A₃ | = (2.5±0.5)×10⁻³ cm⁻¹. The indicated errors are estimated from the slight asymmetries in the line shapes and from the lack of full resolution of the hyperfine components. The resonances show no resolved superhyperfine structure. It is demonstrated that the linewidths of 2.2×10⁻³T observed between inflections at low temperatures arise from unresolved superhyperfine interaction and are consistent with the presence of an unpaired electron which spends 1.3% of its time occupying 3s orbitals of the Cl ions of the complex. Comparison with the superhyperfine structure observed in octahedral CuCl₆^4— suggests that this electron also spends 16.7% of its time occupying Cl ion 3p orbitals. Insertion of these numbers into an LCAO—MO analysis of the g values in terms of the known energies of the d—d transitions shows that the unpaired electron spends 12% of its time in a 4p(Cu) orbital and 70% of its time in the 3d(Cu) state. These numbers are consistent with the EPR hyperfine splitting and the polarization ratios and extinction coefficients of the d—d bands. Further considerations demonstrate that the wavefunction of the unpaired electron is a nonbonding orbital.