Electronic spectra of linear isoelectronic species HC6H+, C6H, HC5N+

Abstract

Multireference configuration interaction calculations have been used to determine term energies of low-lying doublet electronic states of isoelectronic carbon chains HC₆H⁺, C₆H and HC₅N⁺. Calculations on relevant excited states in the energy range up to 6 eV show that ²Π and ²Φ excited states due to the π–π excitation are comparable in energy pattern among such species, but that there are significant differences in the properties of ²Σ and ²Δ excited state series of these isoelectronic species. Similarities and discrepancies are discussed based on molecular orbital energy patterns and electron correlation. The strongest transitions for HC₆H⁺, C₆H and HC₅N⁺ are predicted to correspond to X²Π → 2²Π transitions at 2.22, 2.56 and 2.17 eV, respectively. For HC₆H⁺, no ²Σ or ²Δ states arising from σ–π electron promotion are found in the energy range up to 6 eV. However, in C₆H the first excited state 1²Σ⁺ due to the 13σ → 3π excitation is found to be only slightly higher (0.22 eV) than the ground state X²Π. Calculated transition energies in the present study show good agreement with available experimental results.