Cu3C4-:: A new sandwich molecule with two revolving C22- units

A combined photoelectron spectroscopy (PES) and ab initio study was carried out on a novel copper carbide cluster in the gas phase: Cu3C4-. It was generated in a laser vaporization cluster source and appeared to exhibit enhanced stability among the Cu3Cn- series. Its PES spectra were obtained at several photon energies, showing numerous well-resolved bands. Extensive ab initio calculations were performed on Cu3C4-, and two isomers were identified: a C-2 structure ((1)A) with a Cu-3(3+) triangular group sandwiched by two C-2(2-) units and a linear CuCCCuCCCu structure (D-infinity/i, (1)Sigma(g)(+)). A comparison of ab initio PES spectra with experimental data showed that the sandwich Cu3C4- cluster was solely responsible for the observed spectra and the linear isomer was not present, suggesting that the C-2 structure is the global minimum in accordance with CCSD(T)/311+G* predictions. Interestingly, a relatively low barrier (0.4-0.6 kcal/mol) was found for the internal rotation of the C-2(2-) units in the sandwich Cu3C4-. To test different levels of theory in describing the CumCn- systems and lay foundations for the validity of the theoretical methods, extensive calculations at a variety of levels were also carried out on a simpler copper carbide species CuC2-, where two isomers were found to be close in energy: a linear one (C-infinityv, (1)Sigma(+)) and a triangular one (C-2v, (1)A(1)). The calculated electronic transitions for CuC2- were also compared with the PES data, in which both isomers were present.