Gas phase UV spectrum of a Cu(II)–Bis(benzene) sandwich complex: experiment and theory

Photofragmentation with tuneable UV radiation has been used to generate a spectrum for the copper-bis(benzene) complex, [Cu(C6H6)2]2+ in the gas phase. The ions were held in an ion trap where their temperature was reduced to ~150 K, whereby the spectrum revealed two broad features at ~38200 cm-1 and ~45700 cm-1. Detailed calculations using density functional theory (DFT) show the complex can occupy three minimum energy structures with C2v and C2 (staggered and eclipsed) symmetries. Adiabatic time-dependent DFT (TDDFT) has been used to identify electronic transitions in [Cu(benzene)2]2+ and the calculations show these to fall into two groups that are in excellent agreement with the experimental data. However, the open-shell electronic configuration of Cu2+ (d9) may give rise to excited states with double-excitation character and the single-excitation adiabatic TDDFT treatment leads to extensive spin contamination. By quantifying the extent of spin contamination and allowing for the inclusion of a small percentage (~10 %), the theory can provide quantitative agreement with the experimental data.