Theoretical characterization of C7, C7-, and C7+.

We present a theoretical investigation of neutral and ionic C7 molecules. Since carbon chains present isomerism and the number of possible structures increases fast with the number of carbon atoms, a B3LYP∕aug-cc-pVTZ search of stationary points has been achieved. For C7, we found twelve minimal structures. Among these forms, eleven C7 isomers are located into the lowest singlet hyper potential energy surface. The most stable form of C7 is linear and possesses a (1)Σg(+) symmetry species. For C7(-), we characterized fifteen stable forms, where twelve are of doublet spin-multiplicity. The global minimum of C7(-) is a (2)Πg doubly degenerate Renner-Teller structure. For C7(+) cation, we found eleven doublet and three quartet isomers with a 7-atom cycle, C7(+) (X(2)A1) ground state. For the most stable forms, explicitly correlated (R)CCSD(T)-F12 calculations have been performed for the determination of equilibrium geometries and for the spectroscopic characterization of C7, C7(-), and C7(+), providing accurate rotational constants and harmonic frequencies. Vertical excitation energies to the lowest electronic states have been computed at the CASSCF∕MRCI∕aug-cc-pVTZ level. Thirty five electronic states of C7, suitable of being involved in reactive processes, lie below 7 eV. Fourteen metastable electronic states of C7(-) have been found below 3.5 eV. For linear-C7, we compute the electron affinity and the ionization energy to be 3.38 eV and 10.42 eV, respectively.