A Comparative ab Initio Multireference Single and Double Excitation Configuration Interaction Study of the Electronic Spectra of Low-Lying Linear and Cyclic C5H Isomers

Abstract

Multireference configuration interaction (MRD-CI) calculations are performed to compute the electronic spectra of the three low-lying isomers of C5H, a molecule of radio-astronomical interest. The electronic spectrum of linear C5H is dominated by two transitions computed at 4.13 eV (2Π ← X2Π) and 6.11 eV (2Π ← X2Π). Τhe first dipole-allowed transition is calculated at 2.54 eV (2Δ ← X2Π). Further relatively strong excitations are obtained at 2.71 eV (2Σ- ← X2Π) and 4.00 eV (2Π ← X2Π). The first dipole-allowed excitation of a C2v symmetric ring-chain isomer is calculated at 1.27 eV (2A1 ← X2B2). Up to 3.61 eV (2A2 ← X2B2), no other dipole-allowed transitions are obtained, whereas the spectrum of a Cs symmetric ring-chain structure shows several dipole-allowed excitations in the energy interval between 0.85 and 2.53 eV, including a dominating transition computed at 1.67 eV (2A‘ ← X2A‘).