Theoretical studies on structures and electronic spectra of linear carbon chains C2nH+ (n = 1-5).

The density functional theory (DFT) and the complete active space self-consistent-field (CASSCF) method have been used for full geometry optimization of carbon chains C2nH+ (n = 1–5) in their ground states and selected excited states, respectively. Calculations show that C2nH+ (n = 1–5) have stable linear structures with the ground state of X3Π for C2H+ or X3Σ- for other species. The excited-state properties of C2nH+ have been investigated by the multiconfigurational second-order perturbation theory (CASPT2), and predicted vertical excitation energies show good agreement with the available experimental values. On the basis of our calculations, the unsolved observed bands in previous experiments have been interpreted. CASSCF/CASPT2 calculations also have been used to explore the vertical emission energy of selected low-lying states in C2nH+ (n = 1–5). Present results indicate that the predicted vertical excitation and emission energies of C2nH+ have similar size dependences, and they gradually decrease as the chain size increases. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]